Chapter 54

Community Ecology

Reminder: Earth Day

April 22nd, 2010

Earth Day 2010 (40th anniversary of Earth day)

Iowa DNR 2009 self-assessment

The DNR also rated water quality at C-.

Numerically the water quality rated a D, but they gave themselves extra credit for effort!

Iowa’s water quality (DNR self-assessment)

A biological community– Is an assemblage of populations of various

species living close enough for potential interaction

The animals and plants surrounding this watering hole are all members of a savanna community in southern Africa

Community InteractionsA community’s interactions include competition, predation, herbivory, symbiosis, and disease

Populations are linked by interspecific interactions that affect the survival and reproduction of the species engaged in the interaction

Interspecific Interactions

Competition

Competition

Interspecific competition– Occurs when species compete for a particular

resource that is in short supply

Strong competition can lead to competitive exclusion– The local elimination of one of the two competing

species

The Competitive Exclusion Principle

The competitive exclusion principle– States that two species competing for the same

limiting resources cannot coexist in the same place

Ecological Niches

The ecological niche– Is the total of an organism’s use of the biotic

and abiotic resources in its environment

So if we take the niche concept into account– Then the competitive exclusion principle can be

reworded as:• Two species cannot coexist in a community if their

niches are identical

Ecologically similar species can coexist in a community. If there are one or more significant difference in their nichesFundamental niche (the niche potentially occupied) versus realized niche (the niche actually occupied)

When Connell removed Balanus from the lower strata, the Chthamalus population spread into that area.

The spread of Chthamalus when Balanus was removed indicates that competitive exclusion makes the realizedniche of Chthamalus much smaller than its fundamental niche.

RESULTS

CONCLUSION

Ocean

Ecologist Joseph Connell studied two barnacle speciesBalanus balanoides and Chthamalus stellatus that have a stratified distribution on rocks along the coast of Scotland.

EXPERIMENT

In nature, Balanus fails to survive high on the rocks because it isunable to resist desiccation (drying out) during low tides. Its realized niche is therefore similar to its fundamental niche. In contrast, Chthamalus is usually concentrated on the upper strata of rocks. To determine the fundamental of niche of Chthamalus, Connell removed Balanus from the lower strata.

Low tide

High tide

Chthamalusfundamental niche

Chthamalusrealized niche

Low tide

High tideChthamalus

Balanusrealized niche

Balanus

Ocean

As a result of competition– A species’ fundamental niche may be different

from its realized niche

A. insolitususually percheson shady branches.

A. distichus perches on fence posts and

other sunny surfaces.

A. distichus

A. ricordii

A. insolitus

A. christophei

A. cybotes

A. etheridgei

A. alinigar

Resource PartitioningResource partitioning is the differentiation of niches that enables similar species to coexist in a community

G. fortis

Beak depth (mm)

G. fuliginosa

Beak depth

Los Hermanos

Daphne

Santa María, San Cristóbal

Sympatric populations

G. fuliginosa, allopatric

G. fortis, allopatric

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Character DisplacementIn character displacement there is a tendency for characteristics to be more divergent in sympatric (geographically overlapping) populations of two species than in allopatric (geographically separate) populations of the same two speciesThe allopatric populations have similar beaks and use similar resources.

The sympatric populations would potentially compete for resources and their characteristics are more divergent

Predation

Predation

Predation refers to an interaction where one species, the predator, kills and eats the other, the prey

Adaptations– Feeding adaptations of predators include claws, teeth,

fangs, stingers, and poison

– animals also display a great variety of defensive adaptations to conteract predation

Predation (Hawk-snake)

Cryptic coloration, or camouflage makes prey difficult to spot

Aposematic coloration: warns predators to stay away from prey

Warning coloration in poisonous frogs (movie)

Mimicry

In some cases, one prey species– May gain significant protection by mimicking

the appearance of another

Batesian mimicry

Müllerian mimicry

In Batesian mimicry: A palatable or harmless species mimics an unpalatable or harmful model

(a) Hawkmoth larva

(b) Green parrot snake

In Müllerian mimicry– Two or more unpalatable species resemble each

other

(a) Cuckoo bee

(b) Yellow jacket

Fooling predators (movie)

HerbivoryParasitismDiseaseMutualismCommensalism

HerbivoryHerbivory– Has led to the evolution of plant mechanical

and chemical defenses and consequent adaptations by herbivores

ParasitismIn parasitism, one organism, the parasite– Derives its nourishment from another organism,

its host, which is harmed in the process

Parasitism can have a significant effect on survival, reproduction, density, and productivity of a populationEffects can be direct or indirect

Tomato Hornworm covered in cocoons of pupating parasitic wasp

Disease

Pathogens (disease-causing agents) have effects on populations and communities that are similar to that of parasites

Deer with Chronic Wasting Disease

Mutualism

Mutualistic symbiosis, or mutualism is an interspecific interaction that benefits both species

Mutualism

Mutualism

CommensalismIn commensalism: One species benefits and the other is not affectedDifficult to positively document in nature, because in any association between organisms both are probably affected

Cattle egrets and water buffalo

Interspecific Interactions and Adaptation

Evidence for coevolution– Which involves reciprocal genetic change by

interacting populations, is scarce

However, generalized adaptation of organisms to other organisms in their environment appears to be a fundamental feature of life

Species in the Structure of a Community

In general, a small number of species in a community exert strong control on that community’s structure.

Dominant and Keystone species (more on these soon)

Species Diversity

The species diversity of a community– Is the variety of different kinds of organisms that make

up the community– Has two components: species richness and relative

abundanceSpecies richness– Is the total number of different species in the

communityRelative abundance– Is the proportion each species represents of the total

individuals in the community

Two different communities can have the same species richness, but a different relative abundance

Community 1A: 25% B: 25% C: 25% D: 25%

Community 2A: 80% B: 5% C: 5% D: 10%

D

C

BA

A community with an even species abundance– Is more diverse than one in which one or two

species are abundant and the remainder rare

Trophic Structure

Trophic structure– Is the feeding relationships between organisms

in a community– Is a key factor in community dynamics

Food chains link the trophic levels from producers to top carnivores

Quaternary consumers

Tertiary consumers

Secondary consumers

Primary consumers

Primary producers

Carnivore

Carnivore

Carnivore

Herbivore

Plant

Carnivore

Carnivore

Carnivore

Zooplankton

Phytoplankton

A terrestrial food chain A marine food chain

Food Webs

A food web is a branching food chain with complex trophic interactions

Humans

Baleen whales

Crab-eater seals

Birds Fishes Squids

Leopardseals

Elephant seals

Smaller toothed whales

Sperm whales

Carnivorous plankton

Euphausids (krill)

Copepods

Phyto-plankton

Limits on Food Chain Length

Each food chain in a food web– Is usually only a few links long

There are two hypotheses that attempt to explain food chain length– The energetic hypothesis suggests that the

length of a food chain is limited by the inefficiency of energy transfer along the chain

– The dynamic stability hypothesis proposes that long food chains are less stable than short ones

Species with a Large Impact

Certain species have an especially large impact on the structure of entire communities– Either because they are highly abundant or

because they play a pivotal role in community dynamics

Dominant Species

Dominant species– Are those species in a community that are most

abundant or have the highest biomass– Exert powerful control over the occurrence and

distribution of other species

One hypothesis suggests that dominant species– Are most competitive in exploiting limited

resources

Another hypothesis for dominant species success– Is that they are most successful at avoiding

predators (human introduced invasive species as an example)

Keystone Species

Keystone species– Are not necessarily abundant in a community– Exert strong control on a community by their

ecological roles, or niches

Field studies of sea stars exhibit their role as a keystone species in intertidal communities

(a) The sea star Pisaster ochraceous feeds preferentially on mussels but will consume other invertebrates.

With Pisaster (control)

Without Pisaster (experimental)

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(b) When Pisaster was removed from an intertidal zone, mussels eventually took over the rock face and eliminated most other invertebrates and algae. In a control area from which Pisaster was not removed, there was little change in species diversity.

Observation of sea otter populations and their predation shows the effect the otters have on ocean communities

Food chain beforekiller whale involve-ment in chain

(a) Sea otter abundance

(b) Sea urchin biomass

(c) Total kelp density

Num

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1972 1985 1989 1993 19970

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Food chain after killerwhales started preyingon otters

Ecosystem “Engineers” (Foundation Species)

Some organisms exert their influence– By causing physical changes in the

environment that affect community structure

Beaver dams can transform landscapes on a very large scale

Some foundation species act as facilitators– That have positive effects on the survival and

reproduction of some of the other species in the community

Salt marsh with Juncus (black rush) (foreground)

With Juncus

Without Juncus

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Conditions

Juncus slows salt buildup in soils and thereby increases species richness

Bottom-Up and Top-Down Controls

The bottom-up model of community organization– Proposes a unidirectional influence from

lower to higher trophic levels

In this case, the presence or absence of abiotic nutrients– Determines community structure, including

the abundance of primary producers

The top-down model of community organization– Proposes that control comes from the trophic

level above

In this case, predators control herbivores– Which in turn control primary producers

Disturbances

DisturbanceDisturbance influences species diversity and compositionA disturbance– Is an event that changes a community– Removes organisms from a community– Alters resource availability

Decades ago, most ecologists favored the traditional view that communities are in a state of equilibriumThe current view is a nonequilibrium model– Which describes communities as constantly changing

after being buffeted by disturbances

Fire– Is a significant disturbance in most terrestrial

ecosystems

– Is often a necessity in some communities

(a) Before a controlled burn.A prairie that has not burned forseveral years has a high propor-tion of detritus (dead grass).

(b) During the burn. The detritus serves as fuel for fires.

(c) After the burn. Approximately one month after the controlled burn, virtually all of the biomass in this prairie is living.

The intermediate disturbance hypothesis– Suggests that moderate levels of disturbance

can foster higher species diversity than low levels of disturbance

The large-scale fire in Yellowstone National Park in 1988– Demonstrated that communities can often

respond very rapidly to a massive disturbance

(a) Soon after fire. As this photo taken soon after the fire shows, the burn left a patchy landscape. Note the unburned trees in the distance.

(b) One year after fire. This photo of the same general area taken the following year indicates how rapidly the community began to recover. A variety of herbaceous plants, different from those in the former forest, cover the ground.

Human Disturbance

Humans are the most widespread agents of disturbance…no surprise there!

Human disturbance to communities– Usually reduces species diversity

Humans also prevent some naturally occurring disturbances which can be important to community structure (fire suppression)

Ecological Succession

Ecological succession– Is the sequence of community and ecosystem changes

after a disturbance

Primary succession– Occurs where no soil exists when succession begins

( as in a glacier retreat)

Secondary succession– Begins in an area where soil remains after a disturbance

( as in a post-fire forest)

Possible effects of early-arrivers

Early-arriving species– May facilitate the appearance of later species

by making the environment more favorable– May inhibit establishment of later species– May tolerate later species but have no impact

on their establishment

McBride glacier retreating

0 5 10

Miles

GlacierBay

Pleasant Is.

Johns HopkinsGl.

Reid Gl.

GrandPacific Gl.

Canada

Alaska

1940 1912

1899

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Muir G

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Riggs G

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Retreating glaciers– Provide an opportunity to study succession

Glacier Bay, Alaska

Retreating glaciers leave behind rocky moraines

Succession on the moraines in Glacier Bay, Alaska follows a predictable pattern of change in vegetation and soil characteristics

The soil conditions change during succession as well

Species Diversity

Biogeographic factors affect community diversity

Two key factors correlated with a community’s species diversity– Are its geographic location and its size

Equatorial-Polar GradientsThe two key factors in equatorial-polar gradients of species richness

– evolutionary history – climate

Species richness generally declines along an equatorial-polar gradient (And is especially great in the tropics)

The greater age of tropical environments may account for the greater species richness.Polar and temperate areas have had to repeatedly “start-over” after big disturbances like glaciers.Climate is likely the primary cause of the latitudinal gradient in biodiversity due to differences in solar energy availability and water availability.

Area Effects

The species-area curve quantifies the idea that– All other factors being equal, the larger the

geographic area of a community, the greater the number of species

A species-area curve of North American breeding birds supports this idea

Area (acres)

1 10 100 103 104 105 106 107 108 109 1010

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Island Equilibrium Model

Species richness on islands– Depends on island size, distance from the

mainland, immigration, and extinction

Studies of species richness on the Galápagos Islands– Support the prediction that species richness

increases with island size

The results of the study showed that plant species richness increased with island size, supporting the species-area theory.

FIELD STUDY

RESULTS

Ecologists Robert MacArthur and E. O. Wilson studied the number of plant species on the Galápagos Islands, which vary greatly in size, in relation to the area of each island.

CONCLUSION

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Area of island (mi2) (log scale)

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Community Structure

Two different views on community structure

The integrated hypothesis of community structure– Describes a community as an assemblage of closely

linked species, locked into association by mandatory biotic interactions

The individualistic hypothesis of community structure– Proposes that communities are loosely organized

associations of independently distributed species with the same abiotic requirements

Rivet and Redundancy Models

The rivet model of communities– Suggests that all species in a community are linked

together in a tight web of interactions– Also states that the loss of even a single species has

strong repercussions for the community

The redundancy model of communities– Proposes that if a species is lost from a community,

other species will fill the gap

Community hypotheses and models represent extremes, and that most communities probably lie somewhere in the middle