-THE SCOPE OF ECOLOGY

-FACTORS AFFECTING THE DISTRIBUTION OF ORGANISMS

-AQUATIC AND TERRESTRIAL BIOMES

-THE SPATIAL SCALE OF DISTRIBUTIONS

CHAPTER 52INTRODUCTION TO ECOLOGY

AND THE BIOSPHERE

Organisms are open systems  that interact continuously with their environments--a theme that has already surfaced many times in this book. The scientific study of the interactions between organisms and their environments is called ecology (from the Greek oikos, home, and logos, to study). It is these interactions that determine both the distribution and abundance of organisms, resulting in the two questions that ecologists so often ask about organisms: Where do they live? And how many are there? Ecology is an enormously complex and exciting area of biology that is also of critical practical importance. Photographs of our planet taken by Apollo astronauts, such as the one on this page, remind us that Earth is a finite home in the vastness of space, not an unlimited frontier for human activity. The science of ecology can provide us with the basic understanding that we need to manage our planet’s limited resources .

THE SCOPE OF ECOLOGY Humans have always had an interest in the distribution and abundance of other organisms.

As hunters and gatherers, prehistoric people had to learn where game and edible plants could be found in greatest abundance.

Naturalists, from Aristotle to Darwin, made the process of observing and describing organisms in their natural habitats an end in itself, rather than simply a means of survival.

Because extraordinary insight can still be gained through this descriptive approach of discovery science, natural history remains a fundamental part of ecology.

Although ecology has a long history as a descriptive science, it is becoming increasingly experimental.

In spite of the difficulty of conducting field experiments over large areas and over many years, many ecologists are testing hypotheses in the field.

For example, in 1993, oceanographers added dissolved iron to a 64-km2 area of open ocean in the equatorial Pacific to test on a large scale the hypothesis that a shortage of iron as an inorganic nutrient was limiting production in this ecosystem.

The experiment was repeated in 1995 on an even larger scale with similar results--greatly increased phytoplankton growth when the iron was added.

These kinds of field experiments make ecology a rapidly growing and exciting science.

Since 1993, these 12 small-scale open ocean experiments (red dots) have shown that iron additions do indeed result in phytoplankton blooms, thereby drawing carbon dioxide out of the atmosphere and into the ocean

PhytoplanktonBloom’s

The interactions between organisms and their environments determine the distribution and abundance of organisms

Ecologists use observations and manipulative experiments to test hypotheses aimed at such questions as, Why don’t sequoia trees grow in Colorado? Why are there no malaria-carrying mosquitoes in Minnesota? Why are there so many deer in Ohio? What caused the extinction of the passenger pigeon?

This figure shows the geographic range of the red kangaroo in Australia and graphically illustrates the basis for two major questions ecologists try to answer. What factors limit the geographic range, or distribution , of a species? And what factors determine its abundance ? Given a hypothesis, or suggested explanation, for one of these questions, ecologists make predictions of what should be observed in nature or what the outcome of an experiment should be. In some cases, they can devise mathematical models that enable them to simulate the possible results of large-scale experiments that are impossible to conduct in the field. In this approach, important variables and their hypothetical relationships are described through mathematical equations. The potential ways in which the variables interact can then be studied. For example, many ecologists and climatologists use sophisticated computer programs to develop models that predict the effects that human activities will have on climate and how the resulting climatic changes will affect geographic distributions of life-forms during the next century. Of course, such simulations are only as good as the basic information on which the models are based, and obtaining that information requires extensive laboratory work and fieldwork.

Distribution and abundance of the red kangaroo in Australia, based on aerial surveys. This kangaroo species occurs throughout arid regions of the continent.

The environment of any organism includes abiotic components (nonliving chemical and physical factors), such as temperature, light, water, and nutrients, and biotic (living) components--all the organisms that are part of any individual’s environment. Other organisms may compete with an individual for food and resources, prey upon it, or change its physical and chemical environment. As we will see, questions about the relative importance of various environmental components are frequently at the heart of ecological studies--and some accompanying controversies.

Ecology and evolutionary biology are closely related sciences

Many biologists recognize Charles Darwin as an able naturalist whose observations laid the groundwork for the later development of ecology. Indeed, it was the geographic distribution of organisms and their exquisite adaptations to specific environments that provided Darwin with evidence for evolution. An important cause of evolutionary change is the interaction of organisms with their environment. Thus, events that occur in the framework of ecological time (minutes, months, years) translate into effects over the longer scale of evolutionary time (decades, centuries, millennia, and longer). For instance, hawks feeding on field mice have an immediate (ecological) impact on the prey population by killing certain individuals, thereby reducing population size and altering the gene pool. One long-term evolutionary effect of this predator-prey interaction may be selection for mice with fur coloration that camouflages the animals.

Ecological research ranges from the adaptations of individual organisms to the dynamics of the biosphere

Because there are many levels and types of interactions between organisms and their environments, the questions ecologists address are wide-ranging and complex. Ecology can be divided into four increasingly comprehensive levels of study, from the ecology of individual organisms to the dynamics of ecosystems.

Sample questions at different levels of ecology. (a) Organismal ecology. How do diving whales select their feeding areas? (b) Population ecology. What factors limit the number of striped mice that can inhabit a particular area? (c) Community ecology. What factors influence the diversity of tree species that make up a particular forest? (d) Ecosystem ecology. What processes recycle vital chemical elements, such as nitrogen, within a savanna ecosystem?

Organismal ecology is concerned with the morphological, physiological, and behavioral ways in which individual organisms meet the challenges posed by their biotic and abiotic environments. The geographic distribution of organisms is often limited by the abiotic conditions they can tolerate.

American Elk (Cervus elaphus)

A population is a group of individuals of the same species living in a particular geographic area. Population ecology concentrates mainly on factors that affect how many individuals of a particular species live in an area.

A community consists of all the organisms of all the species that inhabit a particular area; it is an assemblage of populations of many different species. Thus, community ecology deals with the whole array of interacting species in a community. This level of research focuses on the ways in which interactions such as predation, competition, and disease affect community structure and organization.

An ecosystem consists of all the abiotic factors in addition to the entire community of species that exist in a certain area. An ecosystem--a lake, for example--may contain many different communities. In ecosystem ecology, the emphasis is on energy flow and the cycling of chemicals among the various biotic and abiotic components.

Looking beyond the four basic levels of ecology, we come to landscape ecology, which deals with arrays of ecosystems and how they are arranged in a geographic region. A landscape or seascape consists of several different ecosystems linked by exchanges of energy, materials, and organisms. The landscape level of research focuses on the ways in which interactions among populations, communities, and ecosystems are affected by the juxtaposition of different ecosystems, such as streams, lakes, old-growth forests, and forest patches that have had their trees removed by clear-cut logging.

The biosphere is the global ecosystem--the sum of all the planet’s ecosystems. The most extensive level in ecology, the biosphere includes the atmosphere to an altitude of several kilometers, the land down to and including water-bearing rocks at least 3,000 m belowground, lakes and streams, caves, and the oceans to a depth of several kilometers. An example of research at the biosphere level is the analysis of how changes in atmospheric CO2 concentration affect global climate.

Much of our current awareness about the environment had its beginnings in 1962 with Rachel Carson’s Silent Spring. In that now-classic book, Carson warned that the widespread use of pesticides such as DDT was causing population declines in many nontarget organisms. Today, acid precipitation, localized famine aggravated by land misuse and population growth, the poisoning of soil and streams with toxic wastes, and the growing list of species extinct or endangered because of habitat destruction are just a few of the problems that threaten the home we share with millions of other forms of life.

Rachel Carson. Although her Silent Spring , a book seminal to the modern environmental movement, focused on the biosphere’s hangover from the pesticide DDT, Carson’s message was much broader: "The 'control of nature' is a phrase conceived in arrogance, born of the Neanderthal age of biology and philosophy, when it was supposed that nature exists for the convenience of man."

The science of ecology should be distinguished from the informal use of the same word to refer to environmental concerns. And yet, we need to understand the often complicated and delicate relationships between organisms and their environments in order to address environmental problems.

Ecology provides a scientific context for evaluating environmental issues

Many influential ecologists, including David Schindler, the scientist you met in this unit’s interview, recognize their responsibility to educate legislators and the general public about decisions that affect the environment. An important part of this responsibility is communicating the scientific complexity of environmental issues. Politicians and lawyers often want definitive answers to such environmental questions as, How much old-growth forest is needed to save spotted owls? While ecological studies can certainly provide essential information for making policy decisions on habitat preservation, responses to such questions often include further questions: How many owls must be saved? With what certainty must they be saved? How long can they survive in this amount of forest? Ecologists can help answer these questions so that the public can make informed decisions about environmental concerns.

Although our ecological information is always incomplete, we cannot abstain from making decisions until all the answers are known. A guiding principle here is the precautionary principle, which can be expressed simply as "Look before you leap" or "An ounce of protection is worth a pound of cure." Aldo Leopold, the famous wildlife conservationist, expressed the precautionary principle well when he wrote, "To keep every cog and wheel is the first precaution to intelligent tinkering."

FACTORS AFFECTING THE DISTRIBUTION OF ORGANISMS Ecologists have long recognized striking global and regional patterns in the distribution of organisms within the biosphere. Kangaroos occur in Australia but not in North America, while pronghorn antelope occur in the western United States but not in Europe or Africa. More than a century ago, Darwin, Wallace, and other naturalists began to recognize broad patterns of geographic distribution by naming biogeographic realms. We now associate these realms with patterns of continental drift that followed the breakup of Pangaea.

Biogeographic realms. Continental drift and barriers such as deserts and mountain ranges all contribute to the distinctive floras and faunas found in Earth’s major regions. The realms are not sharply delineated but grade together in zones where taxa from adjacent realms coexist.

Biogeography is the study of the past and present distribution of individual species. The field of biogeography provides a good starting point to understanding what limits geographic distributions. To determine what limits the geographic distribution of any particular species, ecologists ask a series of questions. Let’s work our way through this flowchart of inquiry.

Flowchart of factors limiting geographic distribution.

Species dispersal contributes to the distribution of organisms

Why are there no kangaroos in North America? The biogeographer answers with the simplest response: They could not get there because of barriers to dispersal; the area was inaccessible to kangaroos. The dispersal of organisms is a critical process for understanding both geographic isolation in evolution and the broad patterns of current geographic distributions.

Outcome Interpretation

Transplant successful Distribution limited because the area is inaccessible, time has been too short to reach the area, or the species fails to recognize the area as suitable living space

Transplant unsuccessful Distribution limited either by other species or by physical and chemical factors

If a transplant is successful, then the potential range of the species is larger than its actual range, as FIGURE 50.6 illustrates for a hypothetical species. If a species does not occupy all of its potential range, we must determine why. Does the species lack suitable means of dispersal to reach new areas? Some animal species can in fact move into new areas but "choose" not to do so. For these species, we must study their mechanisms of habitat selection.

Set of transplant experiments for a hypothetical species. The results of many separate transplant experiments would be needed to define the limits of the potential range; only four are shown here.

One way to determine if dispersal is a key factor limiting distribution is by observing the results when humans have accidentally or intentionally transplanted a species to areas where that species was previously absent. Some organisms can survive in new areas but cannot reproduce there, so we cannot determine the success of a transplant until at least one life cycle is complete. The two possible outcomes of the transplant direct further research.

Species Transplants

If the species cannot survive and reproduce in the transplant areas, we must ask whether biotic or abiotic factors exclude it from these areas. Limits imposed by other species (biotic factors) may involve the negative effects of predators, parasites, pathogens, or competitors. Or the transplant area could lack required positive effects of interdependent species, such as pollinators that are present within the actual range of a transplanted angiosperm. If other species do not set limits on the range, we are left with the last possibility that physical or chemical factors (abiotic factors) set the geographic range limits. For example, many tropical plant species cannot withstand freezing temperatures, and the frost line effectively limits their distribution.

Colocasia esculentaNo natural pollinator for sexual reproduction.Only propagation throughroots/corm

A proper transplant experiment should have a control , transplants done within the existing distribution to provide data on the effects of handling and transplanting the individual organisms. However, ecologists rarely conduct transplant experiments today. Instead, they document the outcomes when species have been transplanted for other purposes, such as to introduce game animals, or when species have been accidentally transplanted.

Axis deer (Cervus axix) introduced to Texas

The African Honeybee. The African honeybee is a good example of how the human introduction of a species to new areas can have unpredicted and undesirable consquences. The African honeybee (Apis mellifera scutellata) is a very aggressive subspecies of honeybee that was brought to Brazil in 1956 to breed a variety that would produce more honey in the tropics than the standard Italian honeybee (Apis mellifera ligustica) . The African bees escaped by accident in 1957 and have been spreading throughout the Americas ever since. Because African bees are aggressive, they may drive out the established colonies of Italian honeybees. In other situations, hybrids form between the African and Italian subspecies. In 1982, the African honeybee crossed the Panama Canal. It reached Mexico in 1985 and southern Texas in 1990. Moving roughly 110 km per year, it crossed the border into California in 1994 and is currently spreading into the southern United States. By December 2000, the African bee had reached northern Texas, southern California, southern Nevada, and all of Arizona. Unfortunately, African bees are aggressive toward humans and domestic animals, and accounts of severe stinging and even deaths have served to map the spread of the African bee. By 2000, ten people had been killed by these bees in the United States, and beekeepers are understandably worried that the African bee will damage the established honeybee industry. What factors limit the distribution of the African honeybee in North America? Will this species be able to live as far north as northern California and North Carolina? Will cold winters prevent it from moving farther north? Ecologists do not yet know the answers to these questions.

Spread of the African honeybee in the Americas since 1956.

Problems with Introduced Species Although many species are naturally restricted to particular biogeographic realms by their dispersal abilities, humans have managed to move species around the globe, particularly during the last 200 years. In fact, the most spectacular examples of dispersal affecting distribution occur when species that have been deliberately or accidentally introduced by humans explode to occupy a new area. Let’s examine a couple of examples.

The Zebra Mussel. In 1988, the zebra mussel (Dreissena polymorpha) , a fingernail-sized mollusk native to the freshwater Caspian Sea of Asia, was discovered in Lake St. Claire near Detroit. No one knows for sure how the species got transplanted there. The best guess is that around 1985, a ship carried larvae of the mussel in its ballast water from a freshwater port in Europe to the Great Lakes, where the ballast water was emptied with no concern about what organisms it might contain.The zebra mussel quickly became a pest in North America. It reproduces rapidly and forms dense clusters several layers thick on hard surfaces. The mussels were first noticed when they reached densities of 750,000 per square meter in water pipes in Lake Erie and clogged the water intakes of city water systems, electrical power stations, and other industrial facilities in the Great Lakes.Since 1988, zebra mussels have spread rapidly in the river systems of the central United States. Because they are such efficient suspension feeders, zebra mussels actually make the water much clearer, but they alter the native communities of organisms in the process. By feeding on phytoplankton, the mussels depress populations of zooplankton; and the clearer water admits more sunlight, increasing the growth of rooted aquatic plants in shallow waters. In the Hudson River in New York, phytoplankton biomass was reduced about 85% after zebra mussels invaded; zooplankton, which feed on phytoplankton, declined by more than 70% . Some fish and ducks eat zebra mussels, but these predators are too few to slow down population growth of the mussels. Zebra mussels crowd out native mollusk species by colonizing all hard surfaces, including the shells of freshwater clams. The result can be local extinction of the native species.

Expansion of the geographic range of the zebra mussel (Dreissena polymorpha ) since its discovery near Detroit in 1988.

The Tens Rule. Not all introduced species thrive in their new homes, and many of the species humans have moved around the globe have failed to colonize new areas. For example, bird introductions into continental areas are usually failures. In North America, for example, only 13 species of introduced birds are common, although 98 species have been introduced. In Europe, only 13 successful establishments of birds are recorded out of 85 species introduced. A rough generalization for the success of introduced species is the tens rule , which makes the statistical prediction that an average of one out of ten introduced species become established, and one out of ten established species become common enough to become pests.

The ability of species to disperse is important on a global scale but rarely an important factor limiting the local distribution of organisms. Species have many special adaptations for dispersal and often colonize nearby areas. On a global scale, barriers to dispersal help to determine the biogeographic distribution patterns among continents and islands. The ability of humans to overcome these barriers and move species is just one example of how we are altering the biosphere.

Common Water HyacinthEichornia crassipes

Behavior and habitat selection contribute to the distribution of organisms

Some organisms do not occupy all their potential range, even though they are physically able to disperse into the unoccupied areas. In these cases, individuals seem to avoid certain habitats, even when the habitats are suitable. Thus, the distribution of a species may be limited by the behavior of individuals in selecting habitat. Habitat selection is typically thought of only with respect to animals, but plant species can also select their habitats, even though individual plants cannot move. How organisms select the type of habitat to occupy is one of the least understood ecological processes.

Insects often have very stereotyped oviposition (egg-depositing) behavior, and this may restrict their local distribution to certain host plants. Consider the European corn borer, for example, whose larvae will feed on a wide variety of plants but occur almost exclusively on corn because the ovipositing females are attracted by volatile odors produced by the corn plant. The complex chemical signaling between plants and herbivorous insects and pollinators is an important research area in the study of plant-herbivore interactions.

Anopheline mosquitoes are important carriers of disease, and their ecology has been studied extensively because of the difficulty of malaria eradication in tropical areas. Each mosquito species is usually associated with a particular type of breeding place, and one of the striking observations that a student of malaria first makes is that large areas of water in the tropics are completely free of dangerous mosquitoes. Habitat selection for oviposition sites by female mosquitoes appears to restrict their distribution. Larvae can develop successfully over a much wider range of conditions than those in which eggs are laid.

The key point in these examples is that evolution does not produce perfect organisms for every suitable habitat. Adaptation can never be exact and instantaneous, and we must be careful not to expect perfection in organisms (see Chapter 23). We may judge a tropical mosquito deficient for not laying eggs in all suitable rice field habitats, but this failing may only reflect the fact that rice fields are a recent habitat in evolutionary time. In addition, not all behavior that has evolved remains adaptive, particularly in ecosystems modified by humans. Environmental conditions may change such that behaviors that were formerly adaptive are now maladaptive. Ground-nesting birds on islands, for example, are threatened if new ground predators (rats, for example) colonize the island. Populations cannot evolve overnight. Even with suitable genetic variation in a population, natural selection may not be able to operate quickly enough to adjust habitat selection behavior to some abrupt environmental change.

Biotic factors affect the distribution of organisms

Frequently, a species cannot complete its full life cycle if transplanted to an area it did not originally occupy. One reason for this inability to survive and reproduce may be negative interactions with other organisms through predation, disease, and competition. Or there may be an absence of other species upon which the transplanted species depends, such as the absence of specific pollinators in the case of a transplanted plant species.

In predation we find some of the clearest cases of biotic limitation on geographic distribution. Carnivorous predators, such as wolves, kill their prey. And herbivores, such as grazing mammals and most insects, also function as predators, eating parts of plants or whole plants.

Let’s examine a specific case of a predator limiting the distribution of a prey species. In certain marine ecosystems, there is often an inverse relationship between the abundance of sea urchins and the abundance of seaweeds (large algae, such as kelp). Where sea urchins that graze on kelp are common, kelp cannot become established. Thus, the local distribution of kelp can be limited by sea urchins. This kind of interaction can be tested by "removal and addition" experiments. If the hypothesis that sea urchins are a biotic factor limiting kelp distribution is correct, then kelp should invade an area from which sea urchins have been removed. Conversely, if urchins are added to an area rich in kelp, the kelp should be eliminated. One complication is that there are often several other herbivores in addition to sea urchins; thus, careful manipulative experiments are needed to determine the reasons for the kelp being absent. You can see the results of some careful predator-removal experiments in the figure below.

Predator-removal experiments. Researchers tested the effects of two herbivores, sea urchins and limpets (mollusks), on the abundance of seaweeds in adjacent subtidal areas near Sydney, Australia. In areas where both sea urchins and limpets were present (red line), there was virtually no algal cover present. Predator-removal experiments in areas adjacent to the control site supported the hypothesis that sea urchins are the main herbivores limiting distribution of the seaweeds.

Many organisms are limited in their local distribution by the presence of food resources, predators, diseases, and competitors. Unfortunately, some of the most dramatic cases occur when humans introduce (either accidentally or intentionally) exotic predators or diseases into new areas and wipe out native species. We will see examples of these impacts in Chapter 55 when we discuss conservation ecology.

Abiotic factors affect the distribution of organisms

Patterns of geographic distributions of organisms on a global scale are a broad reflection of the influence of abiotic factors. These patterns mainly mirror regional differences in temperature, rainfall, salinity (saltiness), and light. Throughout this discussion, it is important to remember that the environment varies in both space and time. Although two regions of Earth may experience different conditions at any given time, daily and annual fluctuations of abiotic factors sometimes blur or accentuate the distinctions between those regions.

Temperature

Environmental temperature is an important factor determining the distribution of organisms because of its effect on biological processes and the inability of most organisms to regulate body temperature precisely. Cells may rupture if the water they contain freezes (at temperatures below 0°C), and the proteins of most organisms denature at temperatures above 45°C. In addition, few organisms can maintain a sufficiently active metabolism at very low or very high temperatures. Within a suitable range, however, most biochemical reactions and physiological processes occur more rapidly at higher temperatures. Extraordinary adaptations enable some organisms, including thermophilic prokaryotes, to live outside the temperature range habitable by other life.

The internal temperature of an organism is affected by heat exchange with its environment, and most organisms cannot maintain tissue temperatures more than a few degrees above or below the ambient temperature. As endotherms, mammals and birds are the major exceptions, but even endotherms function best within certain environmental temperature ranges, which vary with the species.

Water

Water is essential to life, but its availability varies dramatically among habitats. Freshwater and marine organisms live submerged in an aquatic environment, but they face problems of water balance if their intracellular osmolarity does not match that of the surrounding water. Organisms in terrestrial environments encounter a nearly constant threat of desiccation, and their evolution has been shaped by the requirements for obtaining and conserving adequate supplies of water.

Sunlight

Sunlight provides the energy that drives nearly all ecosystems, although only plants and other photosynthetic organisms use this energy source directly. Light intensity is not the most important factor limiting plant growth in many terrestrial environments, but shading by a forest canopy makes competition for light in the understory intense. In aquatic environments, the intensity and quality of light limit the distribution of photosynthetic organisms. Every meter of water depth selectively absorbs about 45% of the red light and about 2% of the blue light passing through it. As a result, most photosynthesis in aquatic environments occurs relatively near the surface. And the photosynthetic organisms themselves absorb some of the light that penetrates, further reducing light levels in the waters below.

Light is also important to the development and behavior of the many organisms that are sensitive to photoperiod, the relative lengths of daytime and nighttime. Photoperiod is a more reliable indicator than temperature for cuing seasonal events, such as flowering by plants or migration by animals.

Wind

Wind amplifies the effects of environmental temperature on organisms by increasing heat loss due to evaporation and convection (the wind-chill factor). It also contributes to water loss in organisms by increasing the rate of evaporation in animals and transpiration in plants. In addition, wind can have a substantial effect on the morphology of plants by inhibiting the growth of limbs on the windward side of trees; limbs on the leeward side grow normally, resulting in a "flagged" appearance.

Rocks and Soil The physical structure, pH, and mineral composition of rocks and soil limit the distribution of plants and of the animals that feed upon them, thus contributing to the patchiness we see in terrestrial ecosystems. In streams and rivers, the composition of the substrate can affect water chemistry, which in turn influences the resident algae, plants, and animals. In marine environments, the structure of the substrates in the intertidal zone and on seafloors determines the types of organisms that can attach to or burrow in those habitats.

Now that we have surveyed the various factors that affect the distribution of organisms, let’s focus on the major role that climate plays in structuring the biosphere.

Temperature and water are the major climatic factors determining the distribution of organisms

Four abiotic factors--temperature, water, light, and wind--are the major components of climate, the prevailing weather conditions at a locality. Temperature and water are especially important as factors determining the geographic range of organisms.

Climate and Biomes

We can see the great impact of climate on the distribution of organisms by constructing a climograph, a plot of the temperature and rainfall in a particular region. For example, the graph below shows a climograph denoting annual mean temperature and precipitation for some of the biomes of North America. Biomes are major types of ecosystems, those that occupy broad geographic regions. Examples of biomes are coniferous forests, deserts, and grasslands. Notice in graph below that the range of rainfall occurring in northern coniferous forests is similar to that of temperate forests, but the temperature ranges are different. Grasslands are generally drier than either kind of forest, and deserts are drier still.

A climograph for some major kinds of ecosystems (biomes) in North America. The areas plotted here encompass the range of annual mean temperature and precipitation occurring in the biomes.

Annual means for temperature and rainfall are reasonably well correlated with the biomes we find in different regions. However, we must always be careful to distinguish correlation from causation , a cause-and-effect relationship. Although our climograph provides circumstantial evidence that temperature and rainfall are important to the distribution of biomes, it does not prove that these variables govern their geographic location. Only a detailed analysis of the water and temperature tolerances of individual species could establish the controlling effects of these variables.

We can also see in our climograph that factors other than mean temperature and precipitation must play a role in determining which biomes are found where, because there are regions where biomes overlap. For example, certain areas in North America with a particular temperature and precipitation combination support a temperate forest, but other areas with the same values for these variables support a coniferous forest; still others support a grassland. How do we explain this variation? First, remember that the climograph is based on annual averages . Often it is not only the mean or average climate that is important but also the pattern of climatic variation. For example, some areas may get regular precipitation throughout the year, whereas others with the same annual amount of precipitation have distinct wet and dry seasons. A similar phenomenon may occur with respect to temperature. Other factors, such as the bedrock in an area, may greatly affect mineral nutrient availability and soil structure, which in turn affect the kind of vegetation that will develop.

With these complex considerations in mind, let’s take a closer look at how climate affects the geographic distribution of organisms.

Global Climate Patterns

Earth’s global climate patterns are largely determined by the input of solar energy and the planet’s movement in space. The sun’s warming effect on the atmosphere, land, and water establishes the temperature variations, cycles of air movement, and evaporation of water that are responsible for dramatic latitudinal variations in climate. Because solar radiation is most intense when the sun is directly overhead, the shape of the Earth causes latitudinal variation in the intensity of sunlight.

Solar radiation and latitude. Because sunlight strikes the equator perpendicularly, more heat and light are delivered there per unit of surface area than at higher northern and southern latitudes, where sunlight has a longer path through the atmosphere and strikes the curved surface of Earth at an oblique angle.

However, the planet is also tilted on its axis by 23.5° relative to its plane of orbit around the sun, and this tilt causes seasonal variation in the intensity of solar radiation. The tropics (those regions that lie between 23.5° north latitude and 23.5° south latitude) experience the greatest annual input and the least seasonal variation in solar radiation of any region on Earth. The seasonality of light and temperature increases steadily toward the poles; polar regions have long, cold winters with periods of continual darkness and short summers with periods of continual light.

What causes the seasons? The permanent tilt of the Earth on its axis causes seasonal variation in temperature and light intensity as the planet revolves around the sun.

Intense solar radiation near the equator initiates a global circulation of air, creating precipitation and winds. High temperatures in the tropics evaporate water from Earth’s surface and cause warm, wet air masses to rise and flow toward the poles. The rising air masses release much of their water content, creating abundant precipitation in tropical regions. Thus, high temperatures, intense sunshine, and ample rainfall are all characteristic of a tropical climate, fostering the growth of lush vegetation in some tropical forests and the development of coral reefs. The high-altitude air masses, now dry, descend toward Earth at latitudes around 30° north and south, absorbing moisture from the land and creating an arid climate conducive to the development of the deserts that are common at these latitudes. Some of the descending air then flows toward the poles at low altitude, depositing abundant precipitation (though less than in the tropics) where the air masses again rise and release moisture in the vicinity of 60° latitude. Broad expanses of coniferous forest dominate the landscape at these fairly wet but generally cool latitudes. Some of the cold, dry rising air then flows to the poles, where it descends and flows back toward the equator, absorbing moisture and creating the comparatively rainless and bitterly cold climates of the Arctic and Antarctica.

Global air circulation, precipitation, and winds.

Local and Seasonal Effects on Climate

Proximity to bodies of water and topographic features such as mountain ranges create a climatic patchiness on a regional scale, and smaller features of the landscape contribute to local climatic variation. These regional and local variations in climate create a number of ecosystems that are less widely spread than the major biomes.Ocean currents influence climate along the coasts of continents by heating or cooling overlying air masses, which may then pass across the land. Evaporation from the ocean is also greater than it is over land, and coastal regions are generally moister than inland areas at the same latitude. The cool, misty climate produced by the cold California current that flows southward along the western United States supports a rain forest ecosystem dominated by large coniferous trees in the Pacific Northwest and large redwood groves farther south. Similarly, the warm Gulf Stream flowing north out of the Gulf of Mexico and across the North Atlantic tempers the climate on the west coast of the British Isles, making it warmer during winter than the coast of New England, which is actually farther south but is cooled by a current flowing south from the coast of Greenland.

As every vacationer knows, oceans (and large inland bodies of water) generally moderate the climate of nearby terrestrial environments. During a warm summer day, when the land is hotter than a large lake or the ocean, air over the land heats and rises, drawing a cool breeze from the water across the land. At night, by contrast, air over the warmer ocean or lake rises, establishing a circulation that draws cooler air from the land out over the water, replacing it with warmer air from offshore. Proximity to water does not always moderate climate, however. Several regions (including the coast of central and southern California) have a Mediterranean-like climate; in summer, cool, dry ocean breezes are warmed when they contact the land, absorbing moisture and creating hot, rainless summers just a few miles inland.

Mountains also have a significant effect on solar radiation, local temperature, and rainfall. South-facing slopes in the Northern Hemisphere receive more sunlight than nearby north-facing slopes and are therefore warmer and drier. In many of the mountains of western North America, spruce and other conifers occupy the north-facing slopes, whereas shrubby, drought-resistant vegetation inhabits many south-facing slopes. In addition, at any particular latitude, air temperature declines approximately 6°C with every 1,000-m increase in elevation, paralleling the decline of temperature with increasing latitude. In the north temperate zone, for example, a 1,000-m increase in elevation produces a temperature change equivalent to that over an 880-km increase in latitude. This is one reason mountain communities are similar to those at lower elevation farther from the equator.

When warm, moist air approaches a mountain, it rises and cools, releasing moisture on the windward side of the range. On the leeward side of the mountain, cooler, dry air descends, absorbing moisture and producing a rain shadow. Deserts commonly occur on the leeward side of mountain ranges, a phenomenon evident in the Great Basin and Mojave Desert of western North America, the Gobi Desert of Asia, and in the small deserts that characterize the southwest corners of some Caribbean islands.

How mountains affect rainfall. This drawing represents major landforms across the state of Washington. As moist air moves in off the Pacific Ocean and encounters the westernmost mountains (the Coast Range), it flows upward, cools at higher altitudes, and drops a large amount of water. Some of the world’s tallest trees, the Douglas firs, thrive here. Farther inland, precipitation increases again as the air moves up and over higher mountains (the Cascade Range). On the eastern side of the Cascades, there is little precipitation. As a result of this rain shadow, much of central Washington is very arid, almost qualifying as desert.

Seasonality generates local environmental variation in addition to the global changes in day length, solar radiation, and temperature described earlier. Because of the changing angle of the sun over the course of the year, the belts of wet and dry air on either side of the equator undergo slight seasonal shifts in latitude that produce marked wet and dry seasons around 20° latitude, where tropical deciduous forests grow. In addition, seasonal changes in wind patterns produce variations in ocean currents, sometimes causing the upwelling of nutrient-rich, cold water from deep ocean layers, thus nourishing organisms that live near the surface.

Ponds and lakes are also sensitive to seasonal temperature changes (FIGURE 50.15). During the summer and winter, many lakes in temperate regions are thermally stratified--that is, layered vertically according to temperature. Such lakes undergo a biannual mixing of their waters as a result of changing water temperature profiles. This turnover, as it is called, brings oxygenated water from the surface of lakes to the bottom and nutrient-rich water from the bottom to the surface in both spring and autumn (see FIGURE 50.15). These cyclic changes in the abiotic properties of lakes are essential for the survival and growth of organisms at all levels within this ecosystem.

Lake stratification and seasonal turnover. Lakes in temperate zones tend to stratify by temperature and density in winter and summer. The biannual mixing of lake waters occurs because water is most dense at 4°C, and water at that temperature sinks below water that is either warmer or colder.

Microclimate

Climate also varies on a very fine scale, called microclimate. For example, ecologists often refer to the microclimate on a forest floor or under a rock. Many features in the environment influence microclimates by casting shade, affecting evaporation from soil, and changing the patterns of wind. Forest trees frequently moderate the microclimate below. Cleared areas generally experience greater temperature extremes than the forest interior because of greater solar radiation and wind currents that are established by the rapid heating and cooling of open land; evaporation is generally greater in clearings as well. Low-lying ground is usually wetter than high ground and tends to be occupied by different species of trees within the same forest. If you have ever lifted a log or large stone in the woods, you are well aware that there are organisms (such as salamanders, worms, and insects) that live in the shelter of this microenvironment, buffered from the extremes of temperature and moisture. Every environment on Earth is similarly characterized by a mosaic of small-scale differences in the abiotic factors that influence the local distributions of organisms.

Long-Term Climate Change

If temperature and moisture are the master limiting factors for the geographic ranges of plants and animals, the climatic warming that is under way during the 21st century will have profound effects on the biosphere. (The causes and consequences of global warming are discussed in detail in Chapter 54.) One way to get a glimpse of the kinds of changes that may occur is to look back at the changes that have occurred in temperate regions since the end of the last ice age.

The last continental glaciers began retreating in North America and Eurasia about 16,000 years ago. The northward expansion of tree distributions lagged behind the retreat of the ice. A detailed record of these migrations is captured in fossil pollen deposited in lakes and ponds. (It may seem odd to think of trees "migrating," but recall from Chapter 38 that wind and animals can disperse seeds, sometimes over great distances.) In North America, oaks and maples moved rapidly in a northeastward direction from the Mississippi Valley, while hickories advanced more slowly. Hemlocks and white pines moved northwestward from refuges along the Atlantic coast. The important conclusion from this research is that various tree species advanced their range at different rates. If you were sitting in New Hampshire and lived a very long time, you would have seen sugar maple trees arrive 9,000 years ago, hemlock 7,500 years ago, and beech trees 6,500 years ago.

If we can determine the climatic limits of current geographic distributions for organisms, we can make predictions about how distributions will change with climatic warming. A major question when applying this approach to plants is whether seed dispersal is rapid enough to sustain the migration of each species as climate changes. For example, fossils suggest that eastern hemlock was delayed nearly 2,500 years in its movement north at the end of the Ice Age partly because of relatively slow seed dispersal.

Let’s look at a specific case of how the fossil record of past tree migrations can inform predictions about the biological impact of the current global warming trend. The diagram below shows the current and potential geographic range of the American beech (Fagus grandifolia) under two climate-change models. These models predict that the potential northern limit of the beech’s range will move 700-900 km north in the next century, and the southern range limit will shrink to the north as well. If left to natural processes, the beech must move 7-9 km per year to the north to keep pace with the warming climate. By contrast, since the end of the Ice Age, the beech migrated into its present range at a rate of only 0.2 km per year. If these predictions are even approximately correct, migrating species such as beech trees will require human assistance to move into new ranges where they can survive as the climate warms. If this does not occur, beech and many other species may become extinct.

Current geographic range and predicted future range for the American beech (Fagus grandifolia) under two climate-change scenarios.

AQUATIC AND TERRESTRIAL BIOMES

Having examined some of the factors that determine the distribution of organisms on Earth, we now turn to a brief survey of the major types of ecosystems, the biomes, beginning with the aquatic biomes (FIGURE 50.17).

The distribution of major aquatic biomes.

Aquatic biomes occupy the largest part of the biosphere Aquatic biomes account for the largest part of the biosphere in terms of area. Ecologists distinguish between freshwater biomes and marine biomes on the basis of physical and chemical differences. For example, marine biomes generally have salt concentrations that average 3% , whereas freshwater biomes are usually characterized by a salt concentration of less than 1% .

Covering about 75% of Earth’s surface, oceans have always had an enormous impact on the biosphere. The evaporation of seawater provides most of the planet’s rainfall, and ocean temperatures have a major effect on world climate and wind patterns. In addition, marine algae and photosynthetic bacteria supply a substantial portion of the world’s oxygen and consume huge amounts of atmospheric carbon dioxide.

Freshwater biomes are closely linked to the soils and biotic components of the terrestrial biomes through which they pass or in which they are situated. The particular characteristics of a freshwater biome are also influenced by the patterns and speed of water flow and the climate to which the biome is exposed.

Mississippi River Delta

Vertical Stratification of Aquatic Biomes

Many aquatic biomes exhibit pronounced vertical stratification of physical and chemical variables. Light is absorbed by both the water itself and the microorganisms in it, so that its intensity decreases rapidly with depth. Ecologists distinguish between the upper photic zone, where there is sufficient light for photosynthesis, and the lower aphotic zone, where little light penetrates. Water temperature also tends to be stratified, especially during summer and winter. Heat energy from sunlight warms the surface waters to whatever depth the sunlight penetrates, but the deeper waters remain quite cold. In the ocean and in many temperate-zone lakes, a narrow stratum of rapid temperature change called a thermocline separates a more uniformly warm upper layer from more uniformly cold deeper waters. At the bottom of all aquatic biomes, the substrate is called the benthic zone. Made up of sand and organic and inorganic sediments ("ooze"), the benthic zone is occupied by communities of organisms collectively called benthos. A major source of food for the benthos is dead organic matter called detritus. In lakes and oceans, detritus "rains" down from the productive surface waters of the photic zone.

Freshwater Biomes

There are two general categories of freshwater biomes: standing bodies of water (such as lakes and ponds) and moving bodies of water (rivers and streams). Standing bodies of water range from small ponds that are a few square meters to large lakes that are thousands of square kilometers in area. In most lakes, communities of organisms are distributed according to the depth of the water and its distance from shore. Rooted and floating aquatic plants flourish in the littoral zone, the shallow, well-lit waters close to shore. In a lake, the well-lit, open surface waters farther from shore, called the limnetic zone, are occupied by a variety of phytoplankton consisting of algae and cyanobacteria. These organisms photosynthesize and reproduce at a high rate during spring and summer. Zooplankton, mostly rotifers and small crustaceans, graze on the phytoplankton. The zooplankton are consumed by many small fish, which in turn become food for larger fish, semiaquatic snakes and turtles, and fish-eating birds.

Most of the small organisms of a lake’s limnetic zone are short-lived, and their remains continually sink into a deep aphotic region, called the profundal zone, and down to the benthic zone. Microbes in the profundal and benthic zones use oxygen for cellular respiration as they decompose this detritus.

Lakes are often classified according to their production of organic matter. Oligotrophic lakes are deep and nutrient-poor, and the phytoplankton in the limnetic zone are relatively sparse and not very productive. Eutrophic lakes, in contrast, are usually shallower, and the nutrient content of their water is high. As a result, the phytoplankton are very productive, and the waters are often quite murky. Between the oligotrophic and eutrophic extremes are lakes with a moderate amount of nutrients and phytoplankton productivity; these lakes are said to be mesotrophic.

Freshwater biomes. (a) Oligotrophic lakes, such as Lake Baikal (shown here), in Siberia, are nutrient-poor with a small surface area relative to depth. The bottom sediments are low in decomposable organic matter, limiting bacterial populations in the benthos. The shortage of nutrients in the water limits photosynthesis by plankton in the limnetic zone; as a result, the water is clear and oxygen-rich and usually supports diverse populations of fish and invertebrates. (b) Eutrophic lakes, such as this one called Peck’s Pond, in the Pocono Mountains of eastern Pennsylvania, are nutrient-rich and generally have a larger surface area relative to depth. The availability of nutrients supports a high rate of photosynthesis, leading to murkier water than that of an oligotrophic lake. High organic content in the benthos leads to high decomposition rates and potentially low oxygen supplies in the profundal and benthic zones.

Over long periods of time, oligotrophic lakes may become mesotrophic and eventually eutrophic as runoff from the surrounding land brings in additional sediments and nutrients. Unfortunately, human activities often speed this natural process dramatically. Runoff from fertilized lawns and agricultural fields and the dumping of municipal wastes enrich the lakes with excessive amounts of nitrogen and phosphorus; the normally low concentrations of these nutrients limit the growth of algae, including phytoplankton. The result of such pollution is often a population explosion of algae, the production of much detritus, and the eventual depletion of oxygen supplies. Such "cultural eutrophication" makes the water unusable and degrades the lake’s aesthetic value.

Streams and rivers are bodies of water moving continuously in one direction. At the headwaters of a stream (perhaps a spring or snowmelt), the water is often cold and clear, and it carries little sediment and relatively few mineral nutrients. The channel is usually narrow, with a swift current passing over a rocky substrate. Farther downstream, where numerous tributaries may have joined to form a river, the water may be more turbid, carrying substantially more sediment (from the erosion of soil) and nutrients. The channel near the mouth of a river is relatively wide, and the substrate is generally silty from the deposition of sediments over long periods of time.

(c) Rivers and streams support substantially different biological communities than those in ponds and lakes. The photograph shows the mouth of a stream, flowing into the much larger Snake River, in Idaho.

The nutrient content of flowing water biomes is largely determined by the terrain and vegetation through which the water flows. Fallen leaves from dense, overhanging vegetation can add substantial amounts of organic matter, and the weathering of rocks can increase the concentration of inorganic nutrients. The often turbulent flow of a stream constantly oxygenates the water, whereas the sometimes murkier, warmer waters of a large river may contain relatively little oxygen. Stream- and river-dwelling animals exhibit evolutionary adaptations that enable them to resist being swept away. The smaller ones are typically flat in shape and can attach to rocks temporarily. Many arthropods live on the underside or downstream side of rocks, thereby exploiting a microhabitat that is relatively free of turbulent flow.

Many streams and rivers have been polluted by human activities. For centuries, humans have used streams and rivers as depositories of waste, thinking that these materials would be diluted and carried downstream. While some pollutants are carried far from their source, many settle to the bottom, where they can be taken up by aquatic organisms. Even the pollutants that are carried away contribute to estuary, ocean, and lake pollution. Humans have also changed flow patterns, using stream channelization to speed water flow and dams to restrain it. In many cases, dams have completely changed the downstream ecosystems, altering the intensity and volume of water flow and affecting fish and invertebrate populations.

Damming the Columbia River Basin. If Lewis and Clark lived today, they would have a hard time navigating the Columbia. This map shows only the largest of the 250 dams that have altered freshwater ecosystems throughout the Pacific Northwest. The great concrete obstacles make it difficult for salmon to swim upriver to their breeding streams, though many dams now have "fish ladders" that provide detours.

Wetlands

In the simplest sense, a wetland is an area covered with water that supports aquatic plants. In fact, wetlands range from periodically flooded regions to soil that is permanently saturated during the growing season. These conditions favor the growth of specially adapted plants called hydrophytes ("water plants"), which can grow in water or in soil that is periodically anaerobic due to the presence of water. Hydrophytes include floating pond lilies and emergent cattails, many sedges, tamarack, and black spruce. Both the hydrology and the vegetation of an area are important determinants of its classification as a wetland--a classification that can be critical when federal, state, and local governments are making preservation decisions based on rigorous, and often conflicting, definitions.

There are different types of wetlands, ranging from marshes to swamps to bogs. All these varieties, however, generally develop in one of three topographic situations. Basin wetlands develop in shallow basins, ranging from upland depressions to filled-in lakes and ponds. Riverine wetlands develop along shallow and periodically flooded banks of rivers and streams. Fringe wetlands occur along the coasts of large lakes and seas, where water flows back and forth because of rising lake levels or tidal action. Thus, fringe wetlands include both freshwater and marine biomes. Marine coastal wetlands are closely linked to estuaries, which we examine shortly.

Wetlands and estuaries. (a) Wetlands. This marsh in Pennsylvania is an example of a basin wetland. Marshes are usually covered with water year-round. Predominant plants are emergent, with stems and leaves extending above the water surface; they include the pond lilies, reeds, and cattails visible in this photograph. Other kinds of wetlands include swamps (dominated by woody plants), bogs (dominated by sphagnum mosses), and seasonal pools.

Ecologically, wetlands are among the richest of biomes. They contain a diverse community of invertebrates, supporting a wide variety of birds. Herbivores from crustaceans to muskrats consume algae, detritus, and plants. In addition to the rich diversity of species that is supported by wetlands, the ecological and economic value of wetlands exceeds that expected from their geographic extent alone; they provide water-storage basins that reduce the intensity of flooding, and they improve water quality by filtering pollutants. In the past, humans have often regarded wetlands as wastelands--as sources of mosquitoes, flies, and bad odors--and have destroyed many wetlands, mostly filling in with earth to provide land for agriculture and development. Both governments and private organizations are now attempting to protect remaining wetlands through acquisition, economic incentives, and regulation. A great deal of research is under way to determine how wetlands can be created or restored.

Estuaries

The area where a freshwater stream or river merges with the ocean is called an estuary; it is often bordered by extensive coastal wetlands called mudflats and salt marshes. Salinity varies spatially within estuaries, from nearly that of fresh water to that of the ocean; it also varies over the course of a day with the rise and fall of the tides. Nutrients from the river enrich estuarine waters, making estuaries one of the most biologically productive biomes on Earth.

(b) An estuary. This view of an estuary that is part of Chesapeake Bay, in Maryland, shows the intimate association of river mouths and the marine environment into which they carry water. Unfortunately, the land surrounding Chesapeake Bay is heavily populated and industrialized, and pollution that enters the bay through four major rivers has made it unsuitable for many plant and animal species. What was once a bountiful natural source of seafood and other resources has been degraded and rendered less productive by human activity.

Salt marsh grasses, algae, and phytoplankton are the major producers in estuaries. This environment also supports a variety of worms, oysters, crabs, and many of the fish species that humans consume. Many marine invertebrates and fishes use estuaries as a breeding ground or migrate through them to freshwater habitats upstream. Estuaries are also crucial feeding areas for many semiaquatic vertebrates, particularly waterfowl.Although estuaries support a wide variety of commercially valuable species, areas around estuaries are also prime locations for commercial and residential developments. In addition, estuaries are unfortunately at the receiving end for pollutants dumped upstream. Very little undisturbed estuarine habitat remains, and a large percentage has been totally eliminated by landfill and development. Many states have now--rather belatedly--taken steps to preserve their remaining estuaries.

Zonation in Marine Communities Similar to the communities in freshwater lakes, marine communities are distributed according to depth of the water, degree of light penetration, distance from shore, and open water versus bottom. Marine communities illustrate graphically the limitations on distributions that result from these abiotic factors. There is a photic zone where phytoplankton, zooplankton, and many fish species occur, and an aphotic zone below. Because water absorbs light so well and the ocean is so deep, most of the ocean volume is virtually devoid of light, except for tiny amounts produced by a few luminescent fishes and invertebrates. The zone where land meets water is called the intertidal zone; beyond the intertidal zone is the neritic zone, the shallow regions over the continental shelves; and past the continental shelf is the oceanic zone, reaching very great depths. Finally, open water of any depth is the pelagic zone, at the bottom of which is the seafloor, or benthic zone.

Zonation in the marine environment. The marine environment can be classified on the basis of three physical criteria: light penetration (photic and aphotic zones), distance from shore and water depth (intertidal, neritic, and oceanic zones), and whether it is open water or bottom (pelagic and benthic zones). The abyssal zone is the benthic region in deep oceans. Ecologists often combine two designations, such as the oceanic pelagic zone, to identify the location of a biome.

Intertidal Zones

An intertidal zone is alternately submerged and exposed by the twice-daily cycle of tides. Intertidal communities are therefore subject to huge daily variations in the availability of seawater (and the nutrients it carries) and in temperature. Perhaps most significant of all, intertidal organisms are subject to the mechanical forces of wave action, which can dislodge them from the habitat.

The rocky intertidal zone is vertically stratified and provides excellent examples of distributional limitations over short distances. Most of the organisms have structural adaptations that enable them to attach to the hard substrate in this physically tumultuous environment. On sandy substrates (beaches) or mudflats, the intertidal zone is not as clearly stratified. Wave action and tides constantly move the particles of mud and sand, and few large algae or plants occupy these habitats. Many animals, such as suspension-feeding worms and clams and predatory crustaceans, bury themselves in sand or mud, feeding when the tides bring sources of food. Other surface-dwelling organisms, such as crabs and shorebirds, are scavengers or predators on these organisms.

Examples of marine biomes. Intertidal zones. This photograph of rocky intertidal zones on the Oregon coast was taken at low tide to illustrate the vertical zonation of algae and animals. The density of organisms in each of the three major zones is roughly proportional to the percentage of time the zone is submerged. Organisms in the uppermost zone--grazing mollusks, suspension-feeding barnacles, and a few algae--are submerged only during the highest tides and have numerous adaptations that prevent dehydration and overheating. The middle zone, generally submerged at high tide and exposed at low tide, is inhabited by a diverse array of algae, sponges, sea anemones, mollusks, crustaceans, echinoderms, and small fishes. The bottom intertidal zone is exposed only during the lowest tides. A dense cover of seaweeds in this zone often harbors a diversity of invertebrates and fishes.

Partly because of our strong attraction to the seashore, humans have had a long-term impact on intertidal ecosystems. The recreational use of ocean shores has caused a severe decline in the numbers of many beach-nesting birds and sea turtles. Incoming tides carry in polluted water and old fishing lines and plastic debris that can harm wildlife. The most dramatic intertidal pollutant is probably oil, which harms not only birds and marine mammals but also intertidal algae and invertebrates. The ultimate outcome of oil pollution in intertidal zones is reduced species diversity, with increases in the populations of a few oil-resistant species.

Coral Reefs

In warm tropical waters in the neritic zone, coral reefs constitute a conspicuous and distinctive biome. Currents and waves constantly renew nutrient supplies to the reefs, and sunlight penetrates to the ocean floor, allowing photosynthesis.Coral reefs are dominated by the structure of the coral itself, formed by a diverse group of cnidarians that secrete hard external skeletons made of calcium carbonate (see Chapter 33). These skeletons vary in shape, forming a substrate on which other corals, sponges, and algae grow (FIGURE 50.23b). Red algae encrusted with calcium carbonate also add large amounts of limestone to most reefs, as do bryozoans. The coral animals themselves suspension-feed on microscopic organisms and particles of organic debris. They also obtain organic molecules from the photosynthesis of symbiotic dinoflagellate algae that live in their tissues. Coral animals can survive without the dinoflagellates, but their rate of calcium carbonate deposition is much slower without them; thus, reef formation by corals depends on this symbiotic association.

Coral reefs. This coral reef in Fiji illustrates some of the immense variety of microorganisms, invertebrates, and fishes that live among the coral and algae, making coral reefs one of the most diverse and productive biomes on Earth.

Some coral reefs cover enormous expanses of shallow ocean, but this delicate biome is easily degraded by pollution and development, as well as by souvenir hunters who gather the coral skeletons. High water temperatures (greater than 30°C) cause corals to "bleach"--to expel their symbiotic dinoflagellates and die. In 1998, coral reefs around the world suffered moderate to severe bleaching, and there is great concern that global warming could destroy many coral reefs. Corals are also subject to damage from native and introduced predators, such as the crown-of-thorns sea star, which has undergone a population explosion in many regions and destroyed a number of coral reefs in parts of the western Pacific Ocean. Reef communities are very old and grow very slowly; they may not be able to withstand continued human encroachment and dramatic climate changes.

Crown-of-thorns sea star

The Oceanic Pelagic Biome

Most of the ocean’s water lies far from shore in the oceanic pelagic biome, constantly mixed by ocean currents. Nutrient concentrations are generally lower in the open ocean than in coastal areas because the remains of plankton and other organisms sink below the photic region into the dark, lower benthic zone. In some tropical areas, surface waters are lower in nutrients than the surface waters of temperate oceans because a year-round thermal stratification prevents an exchange of nutrients between the surface and the deep. Temperate oceans generally are more productive, because, like temperate lakes, they experience a nutrient turnover in the spring and, to a limited extent, in the fall. The springtime recirculation of nutrients from the depths stimulates a surge of photosynthetic plankton growth.

Photosynthetic plankton grow and reproduce rapidly in the photic region of the oceanic biome. Modern sampling methods, which take bacterial photosynthesis into account, show that the plankton’s rate of organic food production is higher than formerly thought. Nonetheless, photosynthetic plankton account for less than half the photosynthetic activity on Earth. Zooplankton, including protozoans, worms, copepods, shrimplike krill, jellies, and the small larvae of invertebrates and fishes, graze on the photosynthetic plankton. Most plankton exhibit morphological structures, such as bubble-trapping spines, lipid droplets, gelatinous capsules, and air bladders, that help them stay within the photic zone.

Copepods

Krill

The oceanic pelagic biome also includes free-swimming animals, called nekton, that can move against the currents to locate food. Examples of nekton are large squids, fishes, sea turtles, and marine mammals. These animals feed either on plankton or on each other. Although many of these animals feed in the photic region of the pelagic zone, others live at great depths, where fish may have enlarged eyes, enabling them to see in the very dim light, and luminescent organs that attract mates and prey. Many pelagic birds, such as petrels, terns, albatrosses, and boobies, catch fish in the surface waters.

Benthos The ocean bottom below the neritic and pelagic zones is the benthic zone, as in other aquatic biomes. Nutrients reach the seafloor from the waters above by "raining down" in the form of detritus. Although the benthic zone in shallow, near-coastal waters may receive substantial sunlight, light and temperature decline dramatically with depth.

Neritic benthic communities are extremely productive, consisting of bacteria, fungi, seaweeds and filamentous algae, numerous invertebrates, and fishes. Species composition of these communities varies with distance from the shore, water depth, and composition of the bottom.

Organisms in the very deep benthic communities, or abyssal zone, are adapted to continuous cold (about 3°C), extremely high water pressure, near or total absence of light, and low nutrient concentrations. However, oxygen is usually present in abyssal waters, and a fairly diverse community of invertebrates and fishes occupies this region. Marine scientists have also discovered a unique assemblage of organisms associated with deep-sea hydrothermal vents of volcanic origin in midocean ridges. In this dark, hot, oxygen-deficient environment, the food producers are not photosynthesizing organisms but chemoautotrophic prokaryotes. The organic molecules they synthesize support a food chain that includes giant polychaete worms, arthropods, echinoderms, and fishes.

Benthos: a deep-sea vent community. The species composition of benthos varies dramatically with water depth. Pictured here is a vent community, first discovered at a depth of 2,500 m in the late 1970s. These communities are found at spreading centers on the seafloor, where hot magma superheats the water. About a dozen species of prokaryotes identified near the vents are chemoautotrophic producers that obtain energy by oxidizing H2S formed by a reaction of the hot water with dissolved sulfate (SO4

2-). Among the animals in these communities are giant tube-dwelling worms (pictured here), some more than 1 m long. They are apparently nourished by chemo-synthetic prokaryotes that live as symbionts within the worms. Many other invertebrates, including arthropods and echinoderms, are also abundant around the vents.

The geographic distribution of terrestrial biomes is based mainly on regional variations in climate

All the abiotic factors we covered earlier in the chapter, but especially climate, are important in determining why a particular terrestrial biome is found in a certain area. Because there are latitudinal patterns of climate over Earth’s surface, there are also latitudinal patterns of biome distribution. For example, coniferous forests extend in a broad band across North America, Europe, and Asia.

The distribution of major terrestrial biomes. Although terrestrial biomes are mapped here with sharp boundaries, biomes actually grade into one another, sometimes over relatively large areas. The tropics are the low-latitude regions bordered by the Tropic of Cancer and the Tropic of Capricorn.

Terrestrial biomes are often named for major physical or climatic features and for their predominant vegetation. For example, temperate grasslands are dominated by various grass species and are generally found in middle latitudes, where the climate is more moderate than in the tropics or polar regions. Each biome is also characterized by microorganisms, fungi, and animals adapted to that particular environment. Temperate grasslands, for example, are more likely than forests to be populated by large grazing mammals.

Vertical stratification is an important feature of terrestrial biomes, and the shapes and sizes of plants largely define the layering. For example, in many forests, the layers consist of the upper canopy, then the low-tree stratum, the shrub understory, the ground layer of herbaceous plants, the forest floor (litter layer), and finally the root layer. Other (nonforest) biomes have similar, though usually less pronounced vertical strata. For instance, grasslands have a canopy formed by an herbaceous layer of grass species, a litter layer, and a root layer. The root layer in arctic tundra is shallower than in most other biomes because a permanently frozen stratum called permafrost underlies it.

Vertical stratification of a biome’s vegetation provides many different habitats for animals, which often occupy well-defined feeding groups, from the insectivorous and carnivorous birds and bats that feed above canopies to the small mammals, numerous worms, and arthropods that forage the litter and root layers for food.

Terrestrial biomes usually grade into each other, without sharp boundaries. The area of intergradation may be wide or narrow and is called an ecotone .

Saltwater Marsh/ Forest Ecotone

The actual species composition of any one kind of biome varies from one location to another. For instance, in the northern coniferous forest (taiga) of North America, red spruce is common in the east but does not occur in most other areas, where black spruce and white spruce are abundant. Although the vegetation of African deserts superficially resembles that of North American deserts, the plants are in different families. Such "ecological equivalents" can arise because of convergent evolution.

North America Grasslands/ Prairies Africa Savannah

Biomes are dynamic, and natural disturbance rather than stability tends to be the rule. As a result of disturbance, biomes usually exhibit extensive patchiness, with several communities represented in any particular area. Hurricanes create openings in tropical and temperate forests. In northern coniferous forests, old trees die and fall over, or snowfall may break branches and small trees, producing openings or gaps that allow deciduous species, such as aspen and birch, to grow. In many biomes, even the dominant plants depend on periodic disturbance. For example, fire is an integral component of grasslands, savannas, chaparral, and many coniferous forests. Before agricultural and urban development, much of the southeastern United States was dominated by a single conifer species, the longleaf pine. Without periodic burning, deciduous trees tended to replace the pines. Forest managers now use fire as a tool to help maintain many coniferous forests.

In many biomes today, extensive human activities have radically altered the natural patterns of periodic physical disturbance. Most of the eastern United States, for example, is classified as temperate deciduous forest, but human activity has eliminated all but a tiny percentage of the original forest. Fires, which used to be part of life on the Great Plains, are now controlled for the sake of agricultural land use. Humans have altered much of Earth’s surface, replacing original biomes with urban and agricultural ones.

Tropical forest. The photograph shows a tropical rain forest in Costa Rica. Tropical rain forests have pronounced vertical stratification. Trees in the canopy make up the topmost stratum. The canopy is often closed, so that little light reaches the ground below. When an opening does occur, perhaps because of a fallen tree, other trees and large woody vines grow rapidly, competing for light and space as they fill the gap. Many of the trees are covered with epiphytes (plants that grow on other plants rather than in soil), such as orchids and bromeliads. Rainfall, which varies from region to region in the tropics, is the prime determinant of the vegetation growing in an area. In lowland areas that have a prolonged dry season or scarce rainfall at any time, tropical dry forests predominate. The plants found there are a mixture of thorny shrubs and trees and succulents. In regions with distinct wet and dry seasons, tropical deciduous trees are common.

Savanna. This Kenyan savanna is a showcase of large herbivores and their predators. Actually, the dominant herbivores here and in other savannas are insects, especially ants and termites. Grasses and scattered trees are the dominant plants. Fire is an important abiotic component, and the dominant plant species are fire adapted. The luxuriant growth of grasses and forbs (small broadleaf plants) during the rainy season provides a rich food source for animals. However, large grazing mammals must migrate to greener pastures and scattered watering holes during regular periods of seasonal drought.

Desert. Sparse rainfall (less than 30 cm per year) largely determines that an area will be a desert. Some deserts have soil surface temperatures above 60°C during the day. Other deserts, such as those west of the Rocky Mountains and in central Asia, are relatively cold. The Sonoran Desert of southern Arizona (shown here) is characterized by giant saguaro cacti and deeply rooted shrubs. Evolutionary adaptations of desert plants and animals include a remarkable array of mechanisms that store water. The "pleated" structure of saguaro cacti enables the plants to expand when they absorb water during wet periods. Some desert mice never drink, deriving all their water from the metabolic breakdown of the seeds they eat. Many desert plants also rely on CAM photosynthesis, a metabolic adaptation that conserves water in this arid environment. Protective adaptations that deter feeding by mammals and insects, such as spines on cacti and poisons in the leaves of shrubs, are also common in desert plants.

Chaparral. Dense, spiny, evergreen shrubs dominate chaparral biomes, midlatitude coastal areas with mild, rainy winters and long, hot, dry summers. Plants of the chaparral, such as those in this California scrubland, are adapted to and dependent on periodic fires. The dry, woody shrubs are frequently ignited by lightning and by careless human activities, creating summer and autumn brushfires in the densely populated canyons of southern California and elsewhere. Some of the shrubs produce seeds that will germinate only after a hot fire. Food reserves stored in their fire-resistant roots enable them to resprout quickly and use nutrients released by fires.

Temperate grassland. The veldts of South Africa, the puszta of Hungary, the pampas of Argentina and Uruguay, the steppes of Russia, and the plains and prairies of central North America are all temperate grasslands. The key to the persistence of grasslands is seasonal drought, occasional fires, and grazing by large mammals, all of which prevent establishment of woody shrubs and trees. Temperate grasslands, such as the tallgrass prairie in Kansas (shown here), once covered much of central North America. Because grassland soil is both deep and rich in nutrients, these habitats provide fertile land for agriculture. Most grassland in the United States has been converted to farmland, and very little natural prairie exists today.

Temperate deciduous forest. Dense stands of deciduous trees are the trademark of temperate deciduous forests, such as this one in Great Smoky Mountains National Park in North Carolina. Temperate deciduous forests occur throughout midlatitudes where there is sufficient moisture to support the growth of large trees. More open than rain forests and not as tall, a mature temperate deciduous forest has distinct vertical layers, including one or two strata of trees, an understory of shrubs, and an herbaceous stratum. Deciduous forest trees drop their leaves before winter, when temperatures are too low for effective photosynthesis and water lost through transpiration is not easily replaced from frozen soil. Many temperate deciduous forest mammals also enter a dormant winter state called hibernation, and some bird species migrate to warmer climates. Virtually all the original deciduous forests in North America were destroyed by logging and land clearing for agriculture and urban development. In contrast to drier biomes, these forests tend to recover after disturbance, and today we see deciduous trees dominating undeveloped areas over much of their former range.

Coniferous forest. Cone-bearing trees, such as pine, spruce, fir, and hemlock, dominate coniferous forests. Coastal coniferous forests of the U.S. Pacific Northwest, such as the one shown here in Olympic National Park in western Washington, are actually temperate rain forests. Warm, moist air from the Pacific Ocean supports these unique communities, which like most coniferous forests are dominated by one or a few tree species. Extending in a broad band across northern North America and Eurasia to the southern border of the arctic tundra, the northern coniferous forest, or taiga, is the largest terrestrial biome on Earth. Taiga receives heavy snowfall during winter. The conical shape of many conifers prevents too much snow from accumulating on and breaking their branches. Coniferous forests are being logged at an alarming rate, and the old-growth stands of these trees may soon disappear.

Tundra. Permafrost (permanently frozen subsoil), bitterly cold temperatures, and high winds are responsible for the absence of trees and other tall plants in this arctic tundra in central Alaska (photographed in autumn). Although the arctic tundra receives very little annual rainfall, water cannot penetrate the underlying permafrost and accumulates in pools on the shallow topsoil during the short summer. Tundra covers expansive areas of the Arctic, amounting to 20% of Earth’s land surface. High winds and cold temperatures create similar plant communities, called alpine tundra, on very high mountaintops at all latitudes, including the tropics.

THE SPATIAL SCALE OF DISTRIBUTIONS Throughout this chapter, we have examined abiotic factors, such as climate, and biotic factors, such as predation, that contribute to the distribution of the biosphere’s diverse species. We have assumed that it is reasonably straightforward to map the geographic range of a species. But this assumption breaks down as we map the detailed distribution of a species in a local area. No species occurs everywhere throughout its range.

The diagram below illustrates the problem of describing a species’ geographic range on different scales. At one extreme, the range of a species is defined by the worldwide extent of occurrence, a line drawn on a map around the outermost points at which the species has been observed. This is the definition of geographic range used in bird field guides and other natural history guidebooks. At the other extreme, we could measure a much smaller area within the larger geographic range and map the location of each individual. If a particular habitat is not occupied by the species, this region would not be included in its geographic range. Ecologists would like to know the actual areas occupied by each species throughout its range, but such detailed data are not available for most organisms. The important point of this diagram is that we can measure geographic ranges on several spatial scales and that even for a single species, there may be several answers to the question: What limits geographic distribution? Abiotic influences, such as climate, are paramount on the global scale. However, at the local level, more subtle biotic interactions, such as symbiosis, may be the major explanation for why a species occurs in one locale and not another.

A hierarchy of scales for analyzing the geographic distribution of the moss Tetraphis . The question: What limits geographic distribution? There may be different answers, depending on the scale of our analysis.

Different factors may determine the distribution of a species on different scales

Most species have small geographic ranges

Most species in all taxonomic groups have small geographic ranges; only a small minority of species are widespread. The diagram below illustrates this for North American birds and British vascular plants. Ecologists do not know the reason for this pattern, even though it occurs in plants and animals, in aquatic and terrestrial groups, in invertebrates and vertebrates. This pattern of small ranges is related to the observation that most species are relatively rare in nature, and only common organisms tend to have widespread geographic ranges. But this observation simply begs the question: Why are some organisms rare and others common? To explain these patterns of life in the biosphere is one of the research challenges ecologists face. In Chapters 52 and 53, we’ll examine how ecologists are trying to answer such questions about abundance and distribution in the biosphere. And throughout this ecology unit, we’ll see the impact that we humans, by far the most abundant and widely distributed of all large animals, are having on the entire biosphere.

Most species have small geographic ranges. This concept is illustrated by (a) 1,370 species of North American birds and (b) 1,499 species of British vascular plants.