advanced biology: bahe & deken - john burroughs...

Advanced Biology: Bahe & Deken

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CommunityInteractions& Energy

Flow

Chapter 4


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4.1 – Biotic & Abiotic Interactions

An ecosystem has two main parts, a biotic(living) part and an abiotic (nonliving) part.The new John Burroughs School “Green”pond is an ecosystem. Its biotic partincludes the living things -- plants, animals,fungi, protists, and bacteria -- living in thepond. It also includes animals such asraccoons and ducks that visit the pond onoccasion to feed. Its abiotic part includes allthe non-living factors in the pond such astemperature, depth profile, nature of thebottom substrate, dissolved oxygenconcentration, sunlight, nitrates and others.Here we introduce both the biotic andabiotic factors and show how they interact togive an ecosystem its structure.

The nonliving, or abiotic, components of anecosystem are the chemical and physicalfactors that impact living organisms.Significant abiotic factors in terrestrialecosystems include:

sunlight temperature precipitation wind fire soil

Important abiotic factors in aquatic lifezones include:

light penetrationwater currents

dissolved oxygendissolved nutrient concentrations

suspended solidssalinity

Different species thrive under differentphysical conditions. Some require intensesunlight and others can only thrive in shade.Some require a hot environment whileothers need a cool or even cold one. Someflourish under wet conditions and othersdemand dry conditions.

Each population in an ecosystem has a rangeof tolerance to variations in its physical andchemical environment. Small differences in

genetic makeup, health, and age giveindividuals within a species differenttolerance ranges for temperatures and otherfactors. Thus, although a trout populationmay do best within a narrow band oftemperatures (the optimal range), a fewindividuals can survive above and belowthat band. Tolerance does have its limits,beyond which none of the trout can survive.

Ecologists summarize this phenomenon asthe Law of Tolerance. The existence,abundance and distribution of a species in anecosystem are determined by whether thelevels of physical or chemical factors fallwithin the range tolerated by that species.One species can have a wide range oftolerance to some factors and a narrow rangeof tolerance to others. The young of manyspecies are often least tolerant during theirjuvenile or reproductive stages. Generalistsare species that are able to thrive in a widevariety of environmental conditions.Specialists are species that can only thrivein a narrow range of environmentalconditions and/or that have a limited diet.

Sometimes one factor, known as a limitingfactor, is more important than others inregulating population growth. This principlehas become known as the Limiting FactorPrinciple. Too much or too little of anyabiotic factor can limit or prevent growth ofa population, even if other factors are at ornear the optimum range of tolerance.

In terrestrial ecosystems, precipitation isoften the limiting factor. Lack of water in adesert limits plant growth. Soil nitrogen isoften in short supply and limits plantgrowth. If a farmer plants corn with optimallevels of water, phosphorus, and potassium,but in nitrogen poor soil the corn will stopgrowing when it uses up the availablenitrogen. Too much of an abiotic factor canalso be limiting. Too much water or toomuch fertilizer can kill plants.


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4.2 Community Interactions

Organisms in a community can’t help but tointeract. These interactions, along with theabiotic factors, shape an ecosystem.Interactions may include competition forbasic needs such as food, shelter, and water,as well as relationships in which organismsdepend on each other for survival.

If you were to study a forest you would findthat within a forest community differentbirds use the resources of a tree in differentways. For example, one bird species mighteat insects on the leaves while anotherspecies of bird eats the ants found on thebark. The chance of survival for the birdsincreases because they are using differentresources.

A habitat is an area where an organismlives. The birds in the above example wouldboth share one habitat – the tree. If theorganism moves from tree to tree, its habitatwould be a grove of trees.

Organisms not only have a habitat but alsohave a job in that habitat. A niche is therole or position that an organism has in itsenvironment. An organism’s niche is how itmeets its needs for food, shelter, andreproduction.

Competition occurs when more than oneorganism uses a resource at the same time.For example, during a drought water mightbe in limited supply for many organisms.The strong organisms compete with theweaker organisms for this valuable resourceand for survival. Usually the strong surviveand the weak die or move to anotherlocation. At times when water is plentiful,all organisms share the resources andcompetition is not as fierce.

Many, but not all, species get their food byeating other organisms. The act of one

organism consuming another organism forfood is predation. The organism thatpursues another organism is the predator,and the organism that is pursued is the prey.

Some species that interact as predator andprey experience cyclic changes in theirpopulation sizes. Steep increases arefollowed by periodic crashes. A well-known example is the snowshoe harepopulation that is preyed upon by theCanadian lynx.

It seems logical that as the size of preypopulation increases, in this example thehare, more food is available for the predator,the lynx, and thus its numbers also increase.As more lynx consume more hares, the harepopulation must decrease in size which isaccompanied by a corresponding decrease inthe lynx population. The fewer number oflynx means fewer hares will be lost topredation and the hare population willincrease. Now more lynx can be supportedby the increased hare population and thus,the cyclical nature of predator and preypopulations is well illustrated. Besides thisexample, well-documented cases forpredators controlling prey population sizesare found with wolves who eat moose andsharks and alligators who control some fishpopulations.

A symbiotic relationship is an interactionbetween two or more species in which onespecies lives in or on another species. Thereare three main types of symbioticrelationships: parasitism, commensalismand mutualism. All three are important tocommunity structure.

Parasitism is a kind of predator-preyrelationship in which one organism, theparasite, derives its food at the expense ofits symbiotic associate, the host. Parasitesare usually smaller than their hosts. Anexample of a parasite is a tapeworm that


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lives inside the intestines of a human andabsorbs nutrients from its host. Anotherexample is the parasitic wasp (Perilituscoccinellae) that develops inside a SpottedLady Beetle. The beetle is not quite dead,and can still twitch enough to scare off otherinsects that might eat the parasitic wasp’scocoon.

Another type of parasitism is broodparasitism. The brown-headed cowbirdsdemonstrate brood parasitism because theyallow other bird species to raise their young.A brown-headed cowbird lays its eggs intoanother bird’s nest and then abandons iteggs. The host bird will incubate and feedthe cowbirds young. Often the babycowbirds will push the host’s eggs out of thenest until the host is only raising cowbirdyoung.

In contrast to parasitism, in commensalism,one partner benefits without significantlyaffecting the other. Few cases of absolutecommensualism exist, because it is unlikelythat one of the partners will be completelyunaffected, either positively or negatively.Commensal associations sometimes involveone species obtaining food that isinadvertently exposed by another. Forexample, several species of birds feed oninsects flushed out of the grass by grazingcattle. It is difficult to image how this couldaffect the cattle, but the relationship mayhelp or hinder them in some way not yetdiscovered.

In another example of commensalism, thedorsal fin of the remora is modified into asucker with which it forms a temporaryattachment to a shark. When the shark feeds,the remora picks up its scraps. The sharkmakes no attempt to prey on the remora. Fora third example, some species of barnaclesare found only as commensals on the jaws ofwhales. And there are other species of

barnacles found only as commensals onthose barnacles!

The third type of symbiosis, mutualism,benefits both partners in the relationship.Plants are mutualistic with their insectpollinators. A classic example of mutualismcomes from tropical rain forests in Centraland South America. The bull’s horn acaciatree provides room and board for ants of thegenus Pseudomyrmex. The ants live inhollow, large thorns and eat sugar secretedby the tree in special nectaries. In addition,the tree manufactures yellow structures withprotein on the tips of their leaflets. Theseyellow swellings serve no function otherthan to attract and feed the ants. The antsbenefit the host acacia tree by attackingvirtually anything that touches it. They stingother insects and large herbivores and evenclip surrounding vegetation that grows nearthe tree. When the ants are removed, thetrees usually die, probably becauseherbivores damage them so much that theyare unable to compete with surroundingvegetation for light and growing space.

The complex interplay of species insymbiotic relationships highlights animportant point: a web of diverseconnections among organisms structurescommunities.

4.3 Community Interactions for Food

All organisms in an ecosystem depend onone another in many ways. One way is forfood. Organisms are usually classified aseither producers or consumers based onhow they get food.

All organisms need energy to support lifeprocesses. Plants, many protists, and manybacteria contain chlorophyll. This allowsthem to capture sunlight and store it in thechemical bonds of carbohydrates such as


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glucose by the process of photosynthesis.Because they make or produce their ownfood, they are called producers and/orautotrophs (self-feeders). Photosynthesis isa very complex process involving numerouschemical changes. The overall reaction canbe summarized as:

carbon dioxide + water glucose + oxygen

or

6CO2 + 6H2O C6H12O6 + 6O2

Producers occupy what is called the firsttrophic level. The diatoms of Sinkin’Creek, the oak trees of the Drey Land forest,the dandelions of the baseball field, and thegrass of the Burroughs’ prairie are allproducers, providing energy to the rest ofthe ecosystem.

Some organisms cannot make their ownfood. They must feed on or consume otherorganisms to get their energy and nutrients.Therefore they are called consumers and/orheterotrophs (other feeders). Mostconsumers are animals that feed on plants orother animals. Based on how they get theirprimary source of food, consumers areclassified as either herbivores, carnivores,omnivores, scavengers, detritus feeders, ordecomposers.

Herbivores (plant eaters), or primaryconsumers, are animals that feed directly onproducers. Squirrels feed directly on acornsand other nuts; they are herbivores.

Carnivores (meat eaters) eat other animals.Those feeding only on herbivores are calledsecondary consumers. Raccoons eat snailsthat are herbivores and when carnivores feedon first-order carnivores they are called

second-order carnivores (and also thirdorder consumers).

Omnivores (all-feeders) are animals that areboth herbivores and carnivores. Forexample, carp feed on insect larvae, smallcrustaceans, small snails and worms. Theyalso eat algae and pieces of plants. Pigs,rats, foxes, bears, cockroaches and humansare all omnivores.

Scavengers feed on organisms that arealready dead. The bald eagle, for example,eats mainly fish. Sometimes it makes itsown kill, sometimes it robs an osprey of itskill but most often it eats fish that arealready dead. Eagles that live along theseacoast eat dead fish cast up on the beachby waves. Like the eagle, an animal may bea predator at one moment and a scavengerthe next. The dead organisms need not beanimals for the consumer to be called ascavenger. Crayfish in ponds eat both deadplants and animals.

Detritus feeders (pron. di-trite-us) likecrabs, carpenter ants, termites andearthworms extract nutrients from partlydecomposed organic matter in leaf litter,plant debris and animal dung.

Decomposers are composed mainly ofbacteria and fungi, such as yeasts and molds,and get their nutritional requirements bybreaking down dead organic matter and inthe meantime recycle organic matter backinto ecosystems. All organisms producewastes. If decomposers were not present,aquatic ecosystems would soon clog up withwastes and dead organisms and terrestrialecosystems would be knee deep in organicdebris. Without decomposers, life as weknow it would not exist.


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Study Guide Questions for 4.1 – 4.3

The structure of a community is shaped by interactions among the populations making up the community.The most important kinds of interactions are competition, predation, herbivory, and three kinds ofsymbiosis – parasitism, commensalisms, and mutualism. State which of these five interactions isdescribed in each of the examples below.

_______________ 1. Small fish called remoras accompany sharks and dine on scraps left over when thesharks feed.

_______________ 2. Sheep liver flukes feed on bile and can weaken or kill their hosts. They are passedon to other sheep in the animals’ droppings.

_______________ 3. Grazing by introduced mountain goats has reduced the numbers of alpinewildflowers in Olympic National Park.

_______________ 4. Pest-control specialists have brought in a destructive moth to eat tansy ragwort, apoisonous weed.

_______________ 5. Mistletoe obtains nutrients from a tree host.

_______________ 6. A small shrimp takes shelter inside a sponge, which is apparently unaffected by itstenant.

_______________ 7. Mycorrhizal fungi associated with roots obtain sugars from a tree, while enablingthe tree to absorb water and minerals more efficiently.

_______________ 8. A bee pollinates a tropical orchid by being tricked into “mating” with the flower;the bee uses a perfume from the flower to attract a mate.

_______________ 9. Lions hunt large herbivorous mammals such as zebras and wildebeest.

_______________ 10. The influenza virus attacks the lining of the respiratory tract and is passed fromperson to person by contact or airborne droplets.

You can think of an organism’s ecological niche as its “role” or “job” in the community. the nicheincludes the sum of the organism’s functions, abilities, and tolerances. It is possible to describe the nicheas a sort of “job description” for a species, as you might see in a classified ad: “Applicant will be requiredto travel in a herd, drink through nose, and knock down trees for food” could only describe the job of anelephant! Identify the organism whose niche is outlined in each of the following job descriptions. Somewill be in the reading and others should be familiar to you.

_______________ 11. “Ability to attract bees and persuade them to mate with you and each other”.

_______________ 12. “Must collect, eat and store acorns. Will be required not to eat all acorns to helpspread seeds”.

_______________ 13. “Must break down dead organic matter to help recycle it. Most needed for thebreakdown of dead trees. Position urgently needed for health of the entirecommunity.”

Name:


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4.4 Food for Energy

All organisms, both producers andconsumers, use the chemical energy storedin glucose and other organic compounds tofuel their life processes. In most cells, thisenergy is obtained by aerobic cellularrespiration. Aerobic respiration usesoxygen to convert organic nutrients backinto carbon dioxide and water. Just likephotosynthesis, the hundreds of chemicalsteps can be represented by a simpleequation:

glucose + oxygen carbon dioxide + water +energy

or

C6H12O6 + 6O2 6CO2 + 6H2O +energy

Note that the overall chemical reaction foraerobic respiration is the opposite of thereaction for photosynthesis. The reactantsneeded for photosynthesis are thoseproduced by aerobic respiration.

All organisms store energy in the chemicalbonds making up the molecules of theirbodies. The molecules composing anorganism are thus potential sources of foodfor other organisms. A mayfly eats algae.A bleeding shiner eats mayflies. A snappingturtle eats the bleeding shiner. After thealgae, bleeding shiners, and snapping turtlesall die decomposers obtain their energy fromthem. As a result, there is little matterwasted in natural ecosystems.

4.5 Models of Energy Flow

Organisms are linked together in feedingrelationships called food chains. Manyfood chains follow the pattern describedbelow. They begin with producers that areeaten by herbivores that, in turn, are eatenby carnivores. The food chain illustrates

how energy and nutrients move from oneorganism to another through an ecosystem.

Kelp Sea Urchins Sea Otters

Feeding interactions can affect ecosystemprocesses by influencing species’abundances. The removal of sea otters byRussian fur traders caused an explosion inthe population of sea urchins that thenovergrazed the kelp.

Ecologists assign each organism to atrophic level or feeding level depending onwhether it is a producer or a consumer andon what it eats or decomposes. Producersbelong to the first trophic level, primaryproducers to the second trophic level,secondary consumers to the third, and so on.Detritus feeders and decomposers feed at alllevels along the chain and thus, are notassigned to a specific trophic level.

Most organisms are in more than one foodchain. A certain species of plant can beeaten by several species of animal.Likewise, one animal species can eat morethan one type of food. Because most speciesparticipate in more than one food chain, theorganisms in most ecosystems form acomplex network of interconnected foodchains called a food web. The figure belowdemonstrates a sample food web. Thearrows can be followed to find out what eatswhat. Even though a food web is morerealistic than a food chain, it is still highlysimplified. An actual web would includemany more organisms at each trophic level,and all the animals would have a more

Mountain Lion

Snakes Rabbits

DeerShrews

GrassesMice

Insects

Bacteria andToadstools


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diverse diet than that illustrated.Food chains usually proceed from verysmall organisms to larger and larger ones.Therefore, the number of organisms at eachtrophic level tends to decrease as you movealong the food chain. In other words, manysmall producers feed fewer larger herbivoresthat, in turn, feed still fewer and largercarnivores; such relationships are oftenrepresented by a pyramid of numbers as inthe figure below.

Not all pyramids of numbers have such aregular shape. In fact, some aren’t pyramidsat all. Think about the hundreds of wormsand insects that feed on the acorns producedby one oak tree. This pyramid would lookupside down!

Although the pyramid of numbers is simple,it is not used much by ecologists. This isbecause it treats all organisms only in termsof numbers. It ignores differences in size.Yet size to a hungry trout is important. Onestonefly with a mass of 1.2 g makes a bettermeal than one mayfly with a mass of only0.4 g. To avoid this fault in the pyramid ofnumbers, ecologists often use a pyramid ofbiomass. Each trophic level in this pyramidshows the biomass (total dry weight of allthe organisms at that level) at certain trophiclevel. It makes much more sense to talkabout the masses of organisms at eachtrophic level than about the numbers oforganisms. After all, the important thing fora trout is the total mass not the number ofstoneflies it eats. Likewise, you probably

don’t care how many potatoes you eat fordinner. Rather, you care about the totalamount, or mass, of potatoes. One bigpotato may feed you better than three smallones.

In order to construct a pyramid of biomass,ecologists harvest all organisms, plants,animals and microorganisms, from randompatches or narrow strips in an ecosystem.The samples of organisms are then sorted

according to trophic levels, dried andweighed. The data are used to plot apyramid of biomass.

For most land ecosystems, the totalbiomass at each successive trophic leveldecreases. In the open waters of aquaticecosystems, however, the biomass ofprimary consumers (zooplankton) can

exceed that of producers. The reason is thatthe producers are microscopicphytoplankton that grow and reproducerapidly, not large plants that grow andreproduce slowly. The pyramid is not anupright pyramid because the zooplankton eatthe phytoplankton almost as fast as they areproduced so the producer population isnever very large.

There is one problem with pyramids ofbiomass. They imply that equal masses ofall organisms have equal energy contents.Gram for gram, not all organisms have equalenergy contents. Different types of tissueshave different energy contents. The mostnoticeable difference occurs between plantand animal tissue.The most useful pyramid then is one whichshows energy instead of masses or numbers.All animals need energy to live. Some eatplants to get this energy. Other animals eatanimals to get their energy. The more easilyanimals can get the energy they need, thebetter. Therefore the efficiency with whichenergy is passed along the food chain is


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more important than either the numbers oforganisms or their biomasses. Ecologistsdirect most of their attention towardpyramids of energy that represent thestorage of biomass at various trophic levelsin the ecosystem.

All living things need matter and energy.The matter is needed to make new cells andto repair worn-out parts. The energy isneeded to “power” life processes such asmovement and growth. Sitting quietly bythe edge of a lake, you will notice theconstant activity of the organisms aroundyou. Fish cause ripples in the water as theybreak the surface to capture food. Birds dartback and forth over the water and land.Insects crawl, hop and fly just abouteverywhere. The air is filled with thesounds of animals you cannot see. Activityis the essence of life. In order to haveactivity, energy is needed.

All the energy used by living things comes,in the first place, from the sun. Each dayplanet Earth receives far more solar energythan is necessary to support the biosphere.Most of the sun’s energy is absorbed,scattered or reflected by the atmosphere orby the Earth’s surface. Of the visible lightthat reaches plants, algae and bacteria, onlyabout 1% is converted to chemical energyby photosynthesis. These photosynthesizingorganisms use some of this energy for theirown life processes (like active transport,synthesis of molecules, cytoplasmicstreaming, etc.) and the rest is stored. On aglobal scale, autotrophs produce about 170billion tons of organic material per year inthe biosphere.

In a food chain or web, chemical energystored in biomass is transferred from onetrophic level to another. However, witheach transfer of energy some usable energyis degraded and lost to the environment as

low-quality heat. This is partly aconsequence of the principle called thesecond law of thermodynamics. Energytransfer along a food chain is not veryefficient. Because of this energy loss only asmall portion of what is eaten and digestedis actually converted into an organism’sbodily material (or biomass) and the amountof usable energy available to eachsuccessive trophic level declines.

Use this food chain as an example:

Diatoms Mayflies Fish

The diatoms, through photosynthesis, storesome of the sun’s energy in carbohydratemolecules. Much of this energy is lost asheat when diatoms burn these carbohydratesby cellular respiration. Mayflies eat thediatoms and get the stored energy. Themayflies, in turn, lose much of the energythey took in by burning the food moleculesduring respiration to support their lifeactivities. In addition, some moleculesmaking up the cells of consumed diatoms,such as cellulose in the cell walls, are notdigestible by the mayflies and they, alongwith their stored energy, will be lost aswaste. Like the mayflies, when the fish eatsthe mayflies it obtains the mayflies’ storedenergy. However, the fish does not get allthe mayflies’ energy. Parts of theexoskeletons of the mayflies are indigestibleby the fish. The fish, just like the diatomsand mayflies, will then lose much of itsacquired energy through life activities.

The percentage of usable energy transferredas biomass from one trophic level to the nextis called ecological efficiency. It rangesfrom 2% - 20% (that is, a loss of 80 - 98%)depending on the types of species and theecosystem involved, but 10% is consideredvery typical, so common in fact that the10% rule was coined to represent the


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phenomenon that only 10% of the energystored in the molecules of one trophic levelis transferred to the next trophic level.

Regardless of the source of its food, anorganism uses the energy from that food tomaintain itself, to fuel its activities, and togrow and reproduce. Many components offood are not easily digested: hair, feathers,insect exoskeletons, cartilage, and bone inanimal foods, and cellulose and in plantfoods. These substances may be defecatedor regurgitated, and there is very littleenergy obtained from them. The moredigestible portions of the food are used tomeet metabolic and respiratory needs, mostof which escapes the organism as heat. Theenergy retained by the organism becomesavailable for the synthesis of new biomassthrough growth and reproduction, whichanimals feeding at the next higher trophiclevel may then consume.

Assuming 10% ecological efficiency (90%loss) at each trophic transfer, if green plantsin an area manage to capture 10,000 units ofenergy from the sun, then only about 1,000units of energy will be available to supportherbivores and only about 100 units tosupport first order carnivores. Only 10units of energy must support all of thesecond order carnivores.

The more trophic levels or steps in a foodchain or web, the greater the cumulative lossof usable energy as energy flows through thevarious trophic levels. The pyramid ofenergy figure shown in the next columnillustrates this energy loss for a simple foodchain, assuming a 90% energy loss witheach transfer. Unlike a pyramid of numbersor biomass, a pyramid of energy always hasthe upright pyramidal shape. Some energyis always degraded to heat each time energy

is transferred and no organism in the foodchain can recapture this heat energy - it islost forever to that ecosystem. Thus energyflow is one-way along a food chain. For anecosystem to keep operating, energy mustalways enter it from the sun.

The earth could support more people if morewere to eat at low trophic levels byconsuming grains, vegetables and fruitsdirectly (for example, grain humans)rather than passing such crops throughanother trophic level and eating grain eaters(grain cow human).

The large loss in energy between successivetrophic levels also explains why food chainsand webs rarely have more than four or fivetrophic levels. In most cases, too littleenergy is left after four or five transfers tosupport organisms feeding at these trophiclevels. This explains why there are so fewtop carnivores such as eagles, hawks,mountain lions, and white sharks and whythese top carnivores usually are the first tosuffer when the ecosystems which supportthem are disrupted and why they are sovulnerable to extinction.


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The flowchart below illustrates the movement of energy through an ecosystem. The boxesrepresent the total mass of organisms at each trophic level. The arrows show the amount ofenergy passing through each trophic level. Energy enters the producer level as sunlight. Someof this energy is stored in molecules produced in photosynthesis. Energy enters each of theconsumer trophic levels when the consumers feed on the level below. Much of the energy in thefood entering any level is used to power life processes; the food is used as fuel in cellularrespiration, and its energy ends up as heat. Some energy is wasted; it is lost to the detritus foodweb in the form of dead leaves or droppings. A small portion of the energy is stored up in tissuewhen organisms grow or reproduce; this production – roughly 5-20% of energy intake at anytrophic level – is the only energy available to the next level. Label the trophic levels on thediagram: producers, primary consumers, secondary consumers, and tertiaryconsumers. Label the pattern of energy flow: sunlight, production energy, energyused in cellular respiration, and energy of wastes.

Name:


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Multiple Choice

9. When you eat an apple, you are aa. primary consumerb. secondary producerc. producerd. secondary consumere. tertiary consumer

10. An organism’s “trophic level” refers toa. the rate at which it uses energy.b. where it lives.c. what it eats.d. the intensity of its competitionwith other species.

11. The energy for nearly every organism innearly every ecosystem ultimately comesfrom

a. minerals in the soil.b. the sun.c. heat from the earth.d. respiration.e. decomposition.

12. Why is a diagram of energy flow fromtrophic level to trophic level shaped like apyramid?

a. Organisms at each level store mostthe energy and pass little on.

b. There are more producers thanprimary consumers, and so on.

c. Organisms eventually die as theyget older.

d. Most energy at each level is lost,leaving little for the next.

e. There are always fewer secondaryconsumers than primary consumers, and soon.

13. Which group of organisms in the foodchain has the largest biomass?

a. plantsb. grasshoppersc. miced. owlse. snakes

14. In an ecosystem the ______ isalways greater than the ______.a. number of producers . . . number of

primary consumers.b. biomass of secondary consumers . . .

biomass of producers.c. energy used by primary consumers . .

. energy used by secondaryconsumers.

d. biomass of producers . . . biomass ofprimary consumers.

e. energy used by primary consumers . .. energy used by producers.


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4.6 Primary Productivity

Gross Primary Productivity (GPP) is therate at which plants or other producers usephotosynthesis to make more plant material(biomass). The gross primary productivityfor the entire biosphere is about 170 billiontons of biomass per year. Becauseconsumers in an ecosystem acquire theirorganic fuels from producers, either directlyor via other consumers, primary productivitysets the biosphere’s spending limits forenergy.

Look at the figure below to see how grossprimary productivity varies across differentecosystems on earth. GPP is generallygreatest in the shallow waters nearcontinents, along coral reefs where abundantlight, heat and nutrients stimulate algalgrowth and where upwelling currents bringnitrogen and phosphorus from the oceanbottom to the surface. The lowest GPP is indeserts because of their low precipitationand high temperatures and the open oceanbecause of a lack of nutrients and sunlightexcept near the surface.

To stay alive, grow and reproduce, anecosystem’s producers must use some of thetotal biomass they produce for their ownrespiration. Only what is left, called NetPrimary Productivity (NPP), is availablefor use as food by other organisms(consumers) in an ecosystem.

NPP = GPP - Respiration

NPP is the rate at which energy for use byconsumers is stored in new biomass (cells,leaves, roots, and stems). It is measured inunits of energy or biomass available toconsumers in a specified area over a giventime. Various ecosystems and life zonesdiffer in their NPP. The figure aboveactually depicts the NPP not the GPP ofdifferent ecosystems but the trend is thesame for both. The most productive areestuaries, swamps and marshes, and tropicalrain forests. The least productive are theopen ocean, tundra, and desert. Even thoughthe open ocean has a very low net primaryproductivity per meter square per year, thereis so much open ocean that it produces moreof the earth’s NPP per year than any of the

other ecosystemsand life zones.

Agricultural land ishighly modifiedand managed. Thegoal of agricultureis to increase theNPP and biomassof selected cropplants by addingwater (irrigation)and nutrients.Nitrogen andphosphorus are themost commonnutrients infertilizers becausethey are most often


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the nutrients limiting crop growth. Despitesuch inputs, the NPP of agricultural land isnot particularly high compared with that ofother ecosystems.

Producers are the source of all food in anecosystem. The planet’s NPP ultimatelylimits the number of consumers (andremember, humans are consumers,) that cansurvive on earth. At first look, it mightappear that a good way to feed the world’shungry millions would be to harvest plantsin estuaries, swamps and marshes.However, people cannot eat most plants inthese areas and the plants are also vital foodsources for fish, shrimp and other aquaticlife forms that provide humans and otherconsumers with protein.

It might also seem logical that we shouldgrow more food for human consumption byclearing tropical forests and planting foodcrops. According to most ecologists, this isalso a bad idea. In tropical forests most ofthe nutrients needed to grow food crops arestored in the vegetation rather than in thesoil. When the trees are removed, cropproduction and frequent rains rapidlydeplete the nutrient poor soils. Crops can begrown only for a short time without massiveand expensive commercial fertilizers.

Why don’t we harvest the open ocean’sprimary productivity, those floating anddrifting phytoplankton, to feed the world’shungry people? The open ocean, after all,provides the largest percentage of the earth’snet primary productivity. The problem isthat harvesting the widely disperse, tinyfloating producers would take much morefossil fuel and other types of energy than thefood energy we would get and would disruptthe food webs of the open ocean that provideus with other important consumers from fishto shellfish.

Peter Vitousek and other ecologists haveestimated that humans now use, waste ordestroy about 27% of the earth’s totalpotential NPP and 40% of the NPP of theplanet's terrestrial ecosystems. This is themain reason why we are crowding out oreliminating the habitats and food supplies ofa growing number of other species. Thinkof what will happen to our use of the earth’sNPP if the human population doubles in thenext 40-50 years.

4.7 Disturbances & Succession

When a large, mature tree falls in a forest,this local disturbance increases sunlight andnutrients for growth of plants in theunderstory. When a log hits a rock in anintertidal zone, it dislodges or kills many ofthe organisms that are growing on the rockbut also provides space for colonization ofnew intertidal organisms.

Many people think of all environmentaldisturbances as only harmful. Largecatastrophic disturbances can definitelydevastate communities, however, manyecologists contend that in the long run, sometypes of disturbances such as fires can bebeneficial for the species diversity of somecommunities. Such moderate disturbancescreate new conditions that can discourage oreliminate some species but encourage othersby releasing nutrients and creating unfilledniches.

According to the intermediate disturbancehypothesis, communities that experiencefairly frequent but moderate disturbanceshave the greatest species diversity. Themoderate disturbances are large enough tocreate openings for colonizing species indisturbed areas but mild and infrequentenough to allow the survival of some maturespecies in undisturbed areas.


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The tallgrass prairie ecosystem was shapedand is maintained with fire. At KonzaPrairie Biological Station fire is a researchtool to help scientists understand andmanage the ecosystem. Long-term data setshave been collected on the effect of fire onthe tallgrass prairie. Some parts of KonzaPrairie are burned yearly and others areburned every 2, 4, 10 or 20 years. Twentyyears of data on the effect of fire show thatannual burning promotes the dominance ofgrasses while no burning allows woodyshrubs and some trees to take over largeareas of prairie. Frequent fires favor grassesbecause their roots, where nutrients arestored, and their rhizomes, where newshoots emerge, are below ground and notaffected by fire. New growth takes placeshortly after the above ground parts havebeen burned. The above ground growingpoints (buds) of woody species are damagedby fire. The woody stems are burned but theroot system may remain alive.

The change in an ecosystem that happenswhen one community replaces another as aresult of changing abiotic and biotic factors

is ecological succession. There are twotypes of ecological succession – primarysuccession and secondary succession.

Primary succession begins when acommunity becomes established in an areawhere there is no topsoil (usually exposedrock). These communities are usuallystarted by heat and sun tolerant organismslike bacteria, fungi, and lichen (lichen isactually a symbiotic relationship between afungus and an algae). The acids producedby these organisms penetrate and breakdown the rock to help form soil. As theseplants become established they also begin totrap wind blown soil. With a little soilpresent mosses can join the limitedcommunity. The mosses will trap sand andalso secrete acids to further break the rockdown and create more soil.

The first organisms to live in a new habitatare called pioneer species. Pioneer speciesare usually small, fast growing plants andlichen. As the pioneer organisms die anddecompose they make up the first stages ofsoil development (silt). Small weedy plants


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including ferns, fungi, and insects begin tomove in and when there is enough soil builtup shrubs begin to appear. After enoughtime shade-intolerant trees like pines andcedars begin to move in which areeventually replaced by shade-tolerant trees.The final stable, mature ecologicalcommunity with little changes in the numberof species present represents a climaxcommunity.

Secondary succession is a process ofchange that takes place after a community oforganisms has been removed but the soilremain intact. The pioneer species duringsecondary succession are usually grassesand other herbaceous plants. Secondarysuccession follows some disturbance eventlike a fire, flood, drought, hurricane,tornado, disease, plowing, deforestation, etc.Secondary succession rarely restores theexact system that experienced thedisturbance because of inherent randomnessof ecological processes.

It is important to note that successiondoes not just affect the plant life of thecommunity but also the animal life.Different animals prefer differenthabitats and the habitats of successionare undergoing substantial change.Succession can also occur in aquaticecosystems like ponds and lakes. Ayoung pond will mature to old age andfinally to death. The rate of successionis different for different aquatic ecosystems(just like for terrestrial ecosystems). Anoligotrophic lake is a very young lake thatcontains low levels of nutrients.Oligotrophic lakes contain few organisms,very little organic material, and aregenerally very clear. Examples ofoligotrophic lakes include Lake Tahoe andCrater Lake. A mesotrophic lake is moremature and contains larger levels ofnutrients and a diverse community.

Examples include Lake Erie and LakeOntario. A eutrophic lake is an old anddying lake with very high levels of nutrientsand organic matter. The water of aeutrophic lake can deteriorate quicklyresulting in fish-kills and weed-chokedshores.

Aging of aquatic ecosystems can be sped upby artificially increasing nutrient levels.This is usually the result of runoff fromagricultural fields and feedlots rich innitrates and phosphates. This process iscalled cultural eutrophication and isdifferent than natural eutrophicationdescribed in the previous paragraph.

A pond or lake is gradually filled in by builtup organic matter and by soil runoff. Thisresults in a pond changing from a pond to amarsh to a meadow and finally to a forest.


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Autotrophs like plants, algae, and some bacteria convert light energy and transform it into theenergy of chemical bonds in carbon-based compounds. This process is called primaryproduction, and its rate is quantified as primary productivity. Photosynthesis unites carbondioxide and water to form the sugar glucose with the release of oxygen. The overall chemicalbalance of the photosynthetic reaction is 6CO2 + 6H2O C6H12O6 + 6O2. Ecologists areinterested in the primary productivity of an ecosystem because this determines the total energyavailable to the ecosystem. Label the diagram below with the following labels gross primaryproduction, respiration, and net primary production.

Rough estimates of food chain data from various field studies

CommunityNPP

(kcal/m2/yr)

Consumeringestion

(kcal/m2/yr)

EcologicalEfficiency (%)

# of TrophicLevels

Open ocean 500 0.1 25 7.1Coastal marine 8000 10.0 20 5.1Temperate grassland 2000 1.0 10 4.3Tropical forest 8000 10.0 5 3.2

4. From the estimates presented in the table above, what factor contributes most to variations infood chain length among ecosystems?

5. What biological factors account for variation in the factor you chose as your answer toquestion 4? (You may have to refer to section 4.5)

Name:


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Sunlight, moisture, and nutrient availability determine how much food the producers of anecosystem can make – primary production. After studying the bar graph in section 4.6, rank thefollowing ecosystems in terms of primary production per square meter per year (highest pp = #1,lowest pp = #5).

_____ A. Desert _____ B. Coral Reef _____ C. Deciduous Forest _____ D. Open Ocean

_____ E. Tropical Rain Forest

Gradual transition in the species composition of a community that occurs after a disturbance iscalled ecological succession. Fire is a common occurrence in some communities and has animportant role in shaping and perpetuating those communities. State whether each of thefollowing represents a relatively early (E) stage in succession, an intermediate (I) stage, or arelatively late (L) stage. (Hint: Ask yourself if the community were left untouched, whether itwould look the same or different in fifty years. If it would look nearly the same but capable ofchanging without further disturbance, it is an early stage; if very different, it is in an intermediatestage, if pretty much the same, it is a late stage.) Next, for those communities in an early stage,state whether those examples represent primary (P) or secondary (S) succession.

Early, Intermediate,or Late?

Primary orSecondary

Lichen-covered rocks near a melting glacier in Alaska

The forest in Drey Land

An abandoned cornfield in Missouri

A lava flow on the island of Hawaii

Redwood forest in California

The JBS pond (mesotrophic)


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4.8 Community diversity

Just as a population has certaincharacteristics, a community has its own setof properties. As you have read its definingcharacteristics are its diversity, its prevalentform of vegetation, its stability, and itstrophic structure.

The biodiversity of a community measuresthe variety of different kinds of organismsthat make it up. Community diversityencompasses both species richness, whichis the total number of different species in thecommunity, and relative abundance, whichis how common each species is in the area.Ecologists can only get a true picture ofdiversity by examining both species richnessand relative abundance.

The second property, a community’sprevalent form of vegetation, applies toterrestrial communities. For example,deciduous trees are the prevalentcomponents of the community in temperatedeciduous forest. When looking at thedominant vegetation, one can also start tounderstand its structure or arrangement. Forinstance, a deciduous forest has apronounced vertical structure. The treetopsform a top layer, or canopy, under whichthere is a subcanopy of lower branches andthe crowns of shorter, understory trees, thensmall shrubs and herbs carpet the forestfloor. The types and structural features ofthe plants mostly determine the kinds ofanimals that live in the community.

The third property of a community,stability, refers to the community’s abilityto resist change and return to its originalcomposition after being disturbed. Stabilitydepends on both the type of community andthe nature of disturbances. For example, aforest dominated by oak and hickory trees(as is found in parts of Missouri) is a highly

stable community that may last forthousands of years with little change inspecies composition. Large oaks andhickories can withstand most lightening-caused fires, which kill small trees andshrubs growing in forest openings or in theshade of dominant trees. However, when afire is big enough to kill the dominant trees,an oak/hickory forest might seem less stablethan a grassland, because it will take muchlonger for the forest to return to its originalspecies composition.

The fourth property of a community is itstrophic structure, the feeding relationshipsamong the various species making up thecommunity. As we have learned, trophicstructure determines the passage of energyand nutrients from plants and otherphotosynthetic organisms to herbivores andthen to carnivores.

Three main forces -- or interactions amongthe species themselves -- tie populationstogether into communities. Competition,predation and symbiosis, the three maintypes of interactions, are influenced byevolution through natural selection.

If predators were to eat all of their prey,predators would reduce the diversity ofspecies in communities. However, predatorsrarely drive their prey to extinction, forseveral reasons. For one thing, naturalcommunities are complex, with manyspecies; predators themselves are oftenpreyed upon, limiting their numbers. Also, apredator may be able to switch to analternative food source when the populationof one prey species dwindles. The defensemechanisms of the prey themselves play amajor role in keeping prey populations fromextinction.

Several studies have shown that predatory-prey relationships can actually help maintain


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community diversity rather than reduce it.A classic set of experiments by Americanecologist Robert Paine in the 1960s wereamong the first to provide this evidence.Paine removed the dominant predator, a seastar of the genus Pisaster, fromexperimental areas within the intertidal zoneof the Washington coast.

The result was that Psaster’s main prey, amussel of the genus Mytilus, out-competedmany of the other shoreline organisms (suchas barnacles and snails) for space on therocks. The number of species dropped fromfifteen to eight.

These experiments have generated theconcept of keystone predators. A keystonepredator is a species, such as Pisaster,which reduces the density of the strongestcompetitors in a community. In so doing,the predator helps maintain species diversityby preventing competitive exclusion ofweaker competitors. In Paine’s study,predation by Pisaster was a key factor inmaintaining populations of at least sevenother species.

Pisaster is just one example of a toppredator keystone species that exerts astabilizing effect on their ecosystems byfeeding on and helping regulate thepopulations of certain species. Otherexamples of top predator keystone speciesinclude the wolf, leopard, lion, alligator, seaotter, and great white shark.

Other species are keystone species becausethey are habitat modifiers. Elephants pushover, break and uproot trees, creatingopenings in the savanna grasslands andwoodlands of Africa. This activity promotesthe growth of grasses and other forage plantsthat benefit smaller grazing species such asantelope and accelerates nutrient cyclingrates.

Bats and birds regenerate deforested areasby depositing plant seeds in their droppings.Beaver’s dams can change a fast-movingstream into a pond or lake. This attractsfish, muskrats, herons, and ducks that preferdeeper, slower-moving water, as well aswoodpeckers that feed on dead treesemerging from the pond. Beavers do,however, destroy large expanses ofterrestrial forests by destroying trees to builddams.

Even a dung beetle can be a keystonespecies. Dung beetles rapidly remove, bury,and recycle animal wastes. Without themwe would be up to our eyeballs in suchwaste, and many plants would be starved fornutrients. They establish new plantsbecause the dung they bury contains seedsthat have passed through the digestive tractsof fruit-eating birds. They churn and aeratethe soil, making it more suitable for plantlife. They also reduce populations ofmicroorganisms that spread disease to wildand domesticated animals because the beetlelarvae feed on parasitic worms and maggotsthat live in the dung.

The designation of some species as keystonespecies is somewhat controversial becauseall species play some role in theirecosystems and are thus inherentlyimportant. Some ecologists consider somespecies to be more important because of therole they play in maintaining the structureand function of their ecosystems.


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Section 4.8 reiterates the four major characteristics of biological communities.

Each of the italicized phrases in the essay below relates to one of these four characteristics. Statewhether each phrase relates to biodiversity (B), prevalent form of vegetation (V), response todisturbance (D), or trophic structure (T), by writing the corresponding letter in the parentheses.

The tropical rain forest may hold a greater variety of species (1. __________), but no

place on Earth can rival the great conifer forests (2. ________) of America’s Pacific Northwest

for the mass of living things they contain. The major tree species of these forests are Douglas

fir, Sitka spruce, western hemlock, western red cedar, and coastal redwood (3. ________). The

trees may be 2 or 3 meters in diameter and more than 60 meters tall. There are groves of

redwoods whose tops are 100 meters above the forest floor. A ten-story building would fit

comfortably beneath their lowest branches. Along streams and where the tall trees have fallen, a

tangle of bushes and smaller trees (4. ________) reach upward for sunlight, but in the shade

under the dense canopy the ground is covered mostly by a thick blanket of needles. Sunlight

slants through the canopy and illuminates scattered shrubs and ferns.

The forest may seem quite silent on a warm summer day because much of its activity

takes place high in the canopy or in the thick layer of litter on the ground. In fact, many animals

spend their entire lives on the trunks and branches of tall Douglas firs. As many as 1500 species

of invertebrates (5. ________) - insects, spiders, mites, and their relatives – have been counted

on a single tree. A small rodent called the red tree vole never leaves the canopy, where it feeds

on the needles of Douglas fir (6. ________). The flying squirrel nests in hollow trees but glides

to the forest floor to eat lichens and fungi (7. ________). Some of the fungi combine with tree

roots and help trees absorb water and nutrients; the flying squirrel and other small mammals help

to spread the spores of these fungi to new areas during their nocturnal feeding forays. Also in

the soil and litter under the trees lives a variety of insects (8. ________) that may rival the insect

life of the tropical rain forest canopy.

Periodic fires scar the trunks of the biggest trees. Their thick bark and the large amount

of water stored in their trunks usually protect the trees, but perhaps once every few centuries a

stand is destroyed by fire (9. ________). Plants from the surrounding forest gradually invade

the clearing (10. ________), and over a period of hundreds of years a stand of big trees may

become reestablished. Natural disasters have given the natural forest a patchy appearance, with

groves of ancient trees adjoining meadows and stands of immature trees.

Name:


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Essay Questions

11. A researcher noted that in many small ponds, fish species A preyed on several smallerspecies, B, C, D, E, and F. He suspects that A may be a keystone predator in the pondcommunities. What kind of experiment could you suggest to test whether this hypothesis isvalid? If A is a keystone predator, how would you expect the experiment to turn out?

12. A forested area of a new subdivision in western St. Louis county has been set aside for apark. The developer cuts down most of the trees and plants a lawn. How would you expect thepark to compare with the original forest, in terms of prevalent form of vegetation, diversity,trophic structure, and stability?

advanced biology: bahe & deken - john burroughs...

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