55Community EcologyCommunity Ecology

Intro to ChapterIntro to Chapter

MutualismMutualism: worker : worker Acacia ant & Acacia Acacia ant & Acacia tree (Costa Rica)tree (Costa Rica)

CommensalismCommensalism: plant : plant feeds larvae of antfeeds larvae of ant

55 Community Ecology

• 55.1 What Are Ecological Communities?55.1 What Are Ecological Communities?

• 55.2 What Processes Influence Community 55.2 What Processes Influence Community Structure? Structure?

• 55.3 How Do Species Interactions Cause 55.3 How Do Species Interactions Cause Trophic Cascades?Trophic Cascades?

• 55.4 How Do Disturbances Affect Ecological Communities?

• 55.5 What Determines Species Richness 55.5 What Determines Species Richness in Ecological Communities?in Ecological Communities?

55.1 What Are Ecological Communities?

• all the species that live and interact in all the species that live and interact in a given areaa given area

Gleason & Clements, 1920s - 1930sGleason & Clements, 1920s - 1930s

• debated the nature of communitiesdebated the nature of communities

Henry Gleason Henry Gleason

• argued that plant communities were argued that plant communities were loose associations of speciesloose associations of species

• each species was distributed based each species was distributed based on its environmental requirements.on its environmental requirements.

Frederick ClementsFrederick Clements

• argued that plant communities were argued that plant communities were tightly integrated “superorganisms.” tightly integrated “superorganisms.” Communities in similar areas would Communities in similar areas would have the same species.have the same species.

Who was right?Who was right?

Figure 55.1 Plant Distributions along an Environmental Gradient, Whitaker’s Study—Oregon Mts.

Different combinations Different combinations of plants occurred in of plants occurred in different locationsdifferent locations

(Gleason)(Gleason)

Ecological Communities don’t move together as a unit--

Each species has unique interactions Each species has unique interactions with its environment.with its environment.

So why do we care?So why do we care?

understand how these “loose understand how these “loose assemblages of species” function... assemblages of species” function...

interactions with us among other interactions with us among other things!things!

Trophic Levels of Community Members

based on their source of based on their source of energyenergy

autotrophsautotrophs

Photosynthesizers or primary producers Photosynthesizers or primary producers that get their energy directly from that get their energy directly from sunlight.sunlight.

heterotrophsheterotrophs

consume, directly or indirectly, the energy-consume, directly or indirectly, the energy-rich molecules made by the primary rich molecules made by the primary producers.producers.

Trophic Levels: ConsumersTrophic Levels: Consumers

Herbivores eat plants, and constitute Herbivores eat plants, and constitute the ____________ level.the ____________ level.

Organisms that eat herbivores are Organisms that eat herbivores are ________________.________________.

Organisms that eat secondary Organisms that eat secondary consumers are consumers are ________________________..

________________________________ eat dead bodies eat dead bodies and waste products. and waste products. (specific name)(specific name)

Table 55.1 The Major Trophic Levels

Trophic Levels, cont.

Organisms that get their food from Organisms that get their food from more than one trophic level are more than one trophic level are __________________..

Trophic levels are not clearly distinctTrophic levels are not clearly distinct—so why do we use them?—so why do we use them?

useful way of thinking about energy useful way of thinking about energy flow in a communityflow in a community

Food Chain vs Food Web

Linear sequence in which a plant is Linear sequence in which a plant is eaten by an herbivore, which is eaten eaten by an herbivore, which is eaten by a secondary consumer, etc. by a secondary consumer, etc.

Food chains are interconnected to Food chains are interconnected to make food webs.make food webs.

Figure 55.2 Food Webs Show Trophic Interactions in a Community

Most comm have only 4-5 trophic levels: WHY?

Energy is lost between trophic levels. Energy is lost between trophic levels.

What kind of diagram shows this?What kind of diagram shows this?

Pyramids of energy –or- Pyramids of energy –or-

Pyramids of biomass (weight of living Pyramids of biomass (weight of living matter)matter)

Figure 55.3 Diagrams of Biomass and Energy Distributions

Pattern?Pattern?

Energy Energy decreases from decreases from lower to higher lower to higher levelslevels

Photo 55.1 Cloud forest in Costa Rica.

Energy diagrams are variable--

A forest has high biomass in the A forest has high biomass in the producer levelproducer level

Much energy is stored as wood Much energy is stored as wood (difficult to digest) that is unavailable (difficult to digest) that is unavailable to most herbivores.to most herbivores.

What about in aquatic ecosystems?

Primary producers: bacteria & protistsPrimary producers: bacteria & protists

High rates of cell division support a high High rates of cell division support a high biomass of herbivores, and result in an biomass of herbivores, and result in an inverted biomass distribution.inverted biomass distribution.

Examples of Detritivores?

• bacteria, fungi, worms, mites, insectsbacteria, fungi, worms, mites, insects

What do they eat/how?What do they eat/how?

• transform transform detritusdetritus (dead remains and (dead remains and waste products) into free mineral waste products) into free mineral nutrients that can be taken up and used nutrients that can be taken up and used again by plants.again by plants.

How does this help nutrient cycles?How does this help nutrient cycles?

• Continued ecosystem productivity Continued ecosystem productivity depends on the decomposition of depends on the decomposition of detritus.detritus.

Plant-animal mutualism: Pollen transfer by honeybees

55.2 What Processes Influence Community Structure?55.2 What Processes Influence Community Structure?

Categories of species interactions:

• Predation or parasitism: one participant is harmed, the

other benefits.

• Competition: two organisms using same

resource that is insufficient to supply needs of both.

• Mutualism: both species benefit.

Plant-animal mutualism: Pollination of a night-blooming cactus by a bat

Categories of species interactions:

• Commensalism: one participant benefits and the

other is unaffected.

• Amensalism: one participant is harmed, and the

other is unaffected.

These interactions may increase or decrease the range of conditions over which a species can exist…how?

Table 55.2 Types of Ecological Interactions

Animal-animal mutualism: rattle ants & small oak blue caterpillar

• Online Activity--relationships

Parasites

Parasites are usually _____________ Parasites are usually _____________ than their hosts, and live _________ than their hosts, and live _________ or ___________ the host. or ___________ the host.

They often feed on the host without They often feed on the host without killing it.killing it.

Microparasites are much smaller: Microparasites are much smaller: bacteria, viruses, protists.bacteria, viruses, protists.

Predators

typically larger, live outside the bodies of their prey.

Predators of animals typically kill their prey

Herbivores are predators of plants, and often do not kill the plants.

Predators can reduce the size of prey populations, but predator-prey relationships are usually more complex.

Predator and prey population densities Predator and prey population densities can can oscillateoscillate together. together.

• Growth of Growth of predator populationpredator population nearly nearly always always lagslags growth of prey growth of prey population.population.

• WHY?WHY?

• As predator population grows, it As predator population grows, it reduces size of prey population, then reduces size of prey population, then predators run out of food and predators run out of food and population crashes.population crashes.

Figure 55.4 Hare and Lynx Populations Cycle in Nature (Part 1)

Figure 55.4 Hare and Lynx Populations Cycle in Nature (Part 2)

Testing the Model

To test whether oscillations of To test whether oscillations of snowshoe hare and Canadian lynx snowshoe hare and Canadian lynx populations were due populations were due only to the only to the interactions between the speciesinteractions between the species, , enclosures were built to exclude enclosures were built to exclude lynx, but not hares.lynx, but not hares.

Population cycles of the hares were Population cycles of the hares were influenced by food supply as well as influenced by food supply as well as predators.predators.

Figure 55.5 Prey Population Cycles May Have Multiple Causes (Part 1)

Figure 55.5 Prey Population Cycles May Have Multiple Causes (Part 2)

Predators may restrict range of prey species.

The Megapode Bird Story (nests)The Megapode Bird Story (nests)

Megapodes are birds that do not Megapodes are birds that do not incubate their eggs but instead lay incubate their eggs but instead lay them in a large mound of them in a large mound of decomposing plants; heat from the decomposing plants; heat from the decomposition warms the eggs.decomposition warms the eggs.

Megapodes have colonized many Megapodes have colonized many islands but are islands but are absentabsent wherever there wherever there are Asian mammalian predators that are Asian mammalian predators that eat eggs.eat eggs.

Figure 55.6 Megapode Distributions are Limited by Mainland Predators

Prey have evolved lots of adaptations!

WHY?WHY?

Make them more difficult to capture, Make them more difficult to capture, subdue, or eatsubdue, or eat

Like What?Like What?

Includes toxic hairs, tough spines, noxious Includes toxic hairs, tough spines, noxious chemicals, camouflage, and mimicrychemicals, camouflage, and mimicry

So, have predators evolved?So, have predators evolved?

YES!!! Predators, in turn, have evolved YES!!! Predators, in turn, have evolved more effective ways to capture preymore effective ways to capture prey

Mimicry is well studied—2 types?Mimicry is well studied—2 types?

Batesian mimicryBatesian mimicry: :

a palatable species mimics an a palatable species mimics an unpalatable or noxious species.unpalatable or noxious species.

MMüllerian mimicryüllerian mimicry: :

two or more unpalatable or noxious two or more unpalatable or noxious species converge to look alike.species converge to look alike.

How do they persist in the ecosystem?How do they persist in the ecosystem?

Batesian mimicryBatesian mimicry can be maintained if can be maintained if the the mimicmimic is is less commonless common in the in the environment than the unpalatable environment than the unpalatable species.species.

In In MMüllerian mimicryüllerian mimicry all species in the all species in the system benefitsystem benefit when an when an inexperienced predator eats one inexperienced predator eats one individual, and individual, and learns to avoidlearns to avoid individuals of all the species.individuals of all the species.

Figure 55.7 Batesian and Müllerian Mimicry Systems

Development of mimicry in a swallowtail species

How do microparasite populations persist???How do microparasite populations persist???

New hosts must become infected New hosts must become infected before the current host dies.before the current host dies.

Microparasites can invade a host Microparasites can invade a host population with many susceptible population with many susceptible individualsindividuals

Microparasite population decreases Microparasite population decreases as more hosts become immuneas more hosts become immune

Examples of microparasite modes of infection?

breath, body fluids, water, animal vectorsbreath, body fluids, water, animal vectors

Infected hosts can sometimes continue to Infected hosts can sometimes continue to infect others, even after death.infect others, even after death.

Cholera?Cholera?

caused by the bacterium caused by the bacterium Vibrio choleraeVibrio cholerae

bacterium is ingested from water suppliesbacterium is ingested from water supplies

thousands are released when the victim thousands are released when the victim defecates.defecates.

Figure 55.8 Filtering Water Can Help Combat Cholera

How can competition for resources influence How can competition for resources influence abundance & distribution of species?abundance & distribution of species?

Interference competitionInterference competition: one species : one species interferes with the activities of another.interferes with the activities of another.

Exploitation competitionExploitation competition: one species : one species reduces the availability of a resource.reduces the availability of a resource.

Intraspecific vs Interspecific CompetitionIntraspecific vs Interspecific Competition

Competition can occur between individuals Competition can occur between individuals of the of the same speciessame species: : intraspecificintraspecific competitioncompetition. A primary cause of density-. A primary cause of density-dependent birth and death rates.dependent birth and death rates.

InterspecificInterspecific competition competition occurs between occurs between individuals of individuals of different speciesdifferent species. .

Competitive exclusion?Competitive exclusion?

occurs when a superior competitor occurs when a superior competitor prevents another species from using a prevents another species from using a habitathabitat

How can competition restrict a species’ habitat?

Plants compete for space. The shoots need sunlight and the roots compete for water and minerals.

Sessile animals also compete for space.

Two Barnacle Species….this is in the OPTIONS MATERIAL!

Two barnacle species compete in the intertidal zone, ending up in two distinct bands. Chthamalus lives in the higher zone because it is more tolerant of desiccation.

In the lower zone it is outcompeted by Balanus.

If only one species of barnacle is present, it occupies a larger zone than when both are present.

Figure 55.9 Competition Restricts the Intertidal Ranges of Barnacles

55.2 What Processes Influence Community Structure?One species may restrict another species’ One species may restrict another species’

range by reducing populations of a shared range by reducing populations of a shared prey species…prey species…

Two parasitoid waspsTwo parasitoid wasps

prey on scale insects; both were introduced to control scale.

When the second species was introduced

it reduced the scale population so much that it displaced the first species.

Figure 55.10 A Parasitoid Wasp Outcompetes Its Close Relative

Ammensal interactions are widespread…EXAMPLES?Ammensal interactions are widespread…EXAMPLES?

herds of mammals trampling plants herds of mammals trampling plants around a water holearound a water hole

tree branches falling on smaller plants tree branches falling on smaller plants or animals.or animals.

A rhinoceros grazing on an African A rhinoceros grazing on an African plain demonstrates several types of plain demonstrates several types of interactions.interactions.

Figure 55.11 A Single Small Community Demonstrates Many Interactions (Part 1)

Figure 55.11 A Single Small Community Demonstrates Many Interactions (Part 2)

Mutualism Examples?Mutualism Examples?

• Plants and mycorrhizaePlants and mycorrhizae

• plants and N-fixing plants and N-fixing bacteriabacteria

• corals and corals and photosynthetic protistsphotosynthetic protists

• termites and protists in termites and protists in their guts that digest their guts that digest cellulosecellulose

• plants and their plants and their pollinatorspollinators

Figure 55.12 Plant–Animal Mutualisms Are Important in Pollination (A)

How are they MUTUALISTIC?How are they MUTUALISTIC?

Many plants provide nectar on the Many plants provide nectar on the vegetative parts of the plant to vegetative parts of the plant to attract ants.attract ants.

Ants provide protection by attacking Ants provide protection by attacking herbivores. herbivores.

Many species of Many species of AcaciaAcacia trees have trees have associations with ants. The ants associations with ants. The ants receive food and a place to live.receive food and a place to live.

Plant & Pollinator/Seed Disperser-not always mutualistic-WHY?

Seed dispersers are also Seed dispersers are also seedseed predatorspredators that destroy some of the that destroy some of the seeds. seeds.

Some animals Some animals cut holes in petalscut holes in petals to to gain access to nectar gain access to nectar without without transferring any pollentransferring any pollen..

Some plants exploit pollinators:Some plants exploit pollinators:

Orchid flower example—Orchid flower example—

mimics female insects, mimics female insects, enticing male insects enticing male insects to copulate with them. to copulate with them.

The males transfer The males transfer pollen which benefits pollen which benefits the plant, but the male the plant, but the male insect has no nectar insect has no nectar reward, and no reward, and no offspring.offspring.

Figure 55.12 Plant–Animal Mutualisms Are Important in Pollination (B)

How can a change in predator population cascade to lower trophic levels?

Yellowstone National Park—Yellowstone National Park—wolves & elkwolves & elk

wolveswolves were extirpated by 1926 were extirpated by 1926

elk were culled each year to prevent elk were culled each year to prevent them from exceeding carrying them from exceeding carrying capacity, until 1968capacity, until 1968

elkelk population then rapidly population then rapidly increased…increased…

When wolves were absent, elk When wolves were absent, elk browsed aspen trees so heavily that browsed aspen trees so heavily that no young no young aspensaspens could get a start. could get a start.

Elk also browsed streamside willows Elk also browsed streamside willows to the point that to the point that beaversbeavers were nearly were nearly exterminated.exterminated.

Wolves were Wolves were reintroducedreintroduced in 1995, and in 1995, and preyed primarily on elk. Aspen and preyed primarily on elk. Aspen and willow regrew, and beaver population willow regrew, and beaver population increased.increased.

Figure 55.13 Wolves Initiated a Trophic Cascade (Part 1)

Figure 55.13 Wolves Initiated a Trophic Cascade (Part 2)

Figure 55.13 Wolves Initiated a Trophic Cascade (Part 3)

Figure 55.14 Trophic Cascades May Cross Habitats

Trophic cascades may affect multiple ecosystems-another ex.

dragonfly larvae are more abundant in ponds without fish.

In a study of ponds with and without fish, researchers found that adult dragonflies were more common around fish-free ponds, and the insect pollinators they preyed on were less common…

Flowers of St. John’s wort near fish-Flowers of St. John’s wort near fish-free ponds were visited by insect free ponds were visited by insect pollinators much less, and produced pollinators much less, and produced fewer seeds.fewer seeds.

Thus, fish predation on dragonfly Thus, fish predation on dragonfly larvae in one habitat influenced larvae in one habitat influenced another habitat where fish do not another habitat where fish do not live.live.

How can beavers cause a trophic cascade?

by by preferentially cuttingpreferentially cutting some speciessome species of trees they alter the species of trees they alter the species composition of the vegetation.composition of the vegetation.

They also create aquatic habitat for They also create aquatic habitat for many other species by building many other species by building dams.dams.

“ “ ecosystem engineersecosystem engineers””

What’s a KEYSTONE SPECIES?What’s a KEYSTONE SPECIES?

A species that exerts influence A species that exerts influence out of out of proportionproportion with its abundance with its abundance

They may influence species richness They may influence species richness and the flow of energy and materials.and the flow of energy and materials.

CLASSIC EXAMPLE…ROCKY INTERTIDAL ZONECLASSIC EXAMPLE…ROCKY INTERTIDAL ZONE

Sea stars in the rocky Sea stars in the rocky intertidal zones of the intertidal zones of the Pacific northwest prey Pacific northwest prey on mussels.on mussels.

If sea stars are absent…If sea stars are absent…

mussels crowd out all mussels crowd out all other competitors. other competitors.

By eating mussels, the By eating mussels, the sea stars create space sea stars create space that other species can that other species can colonize.colonize.

Figure 55.15 Some Sea Stars are Keystone Species

Keystone species may not be predators.Keystone species may not be predators.

Fig treesFig trees in tropical forests may act as in tropical forests may act as keystone species.keystone species.

The fruits ripen at a time of year when The fruits ripen at a time of year when fruit is otherwise scarce. fruit is otherwise scarce.

Dozens of fruit-eating species depend Dozens of fruit-eating species depend on the figs at this time of year.on the figs at this time of year.

55.5 55.5 What Determines What Determines Species Richness in Species Richness in Ecological Communities?Ecological Communities?

55.5 What Determines Species Richness in Ecological Communities?

Species richness is the number of species living in a community.

An observed biogeographic pattern is that more species are found in the low latitudes than high latitudes.

Example: gradient in mammal species richness in Central and North America

Figure 55.20 The Latitudinal Gradient of Species Richness of North American Mammals

55.5 What Determines Species Richness in Ecological Communities?

The mountainous regions also have greater species richness. More vegetation types and climates exist in these topographically diverse areas.

Islands and peninsulas generally have fewer species than a similar area on the mainland (theory of island biogeography).

55.5 What Determines Species Richness in Ecological Communities?

Species richness is correlated with ecosystem productivity, but the relationship is complex.

Richness often increases with productivity, but only to a point, then declines.

Interspecific competition may be more intense when productivity is high, resulting in competitive exclusion.

Figure 55.21 Species Richness Peaks at Intermediate Productivity

55.5 What Determines Species Richness in Ecological Communities?

Species richness might enhance productivity, because more species would be using all possible resources.

If environment changes, a species-rich ecosystem is more likely to have species already adapted to new conditions.

The species-rich ecosystem would be more stable, or change less over time.

55.5 What Determines Species Richness in Ecological Communities?

This hypothesis was tested by planting grasses in controlled plots. Species number ranged from a few to 25 species.

Over 11 years, plots with more species were more productive, and productivity varied less from year to year.

Population densities were not stable because different species performed better in drought years and wet years.

Figure 55.22 Species Richness Enhances Community Productivity (Part 1)

Figure 55.22 Species Richness Enhances Community Productivity (Part 2)

Figure 55.22 Species Richness Enhances Community Productivity (Part 3)