Chapter 53

Community Ecology

A Community

A community is all of the species within a given area that have the ability to interact with one another and their environment.

Community structure is chiefly governed by the interactions of the organisms and their environments.

Interspecific Interactions

Interspecific interactions are the relationships in the life cycles of the organisms and their interactions with other species in the community.

Possible Linking Interactions:1. Competition

2. Predation

3. Herbivory

4. Symbiosis• Parasitism, • Mutualism, • Commensalism

Interspecific Competition

Occurs when species compete for a particular resource that is limited in some way.

When both organisms compete for it, it may be detrimental to one or both organisms and may lead to competitive exclusion.

Competitive Exclusion

Occurs when one organism has a means to use a resource better than another.

Thus, it is better able to reproduce and ultimately leads to the elimination of the the other organism.

G.F. Gause

Arrived at the Principle of Competitive Exclusion while studying 2 species of paramecium.

Each would grow well on their own--reaching a carrying capacity.

When grown together, one would drive the other to extinction.

Niche

This is a species role in the environment--where and how it fits into an ecosystem.

A species ecological niche is the sum total of all biotic and abiotic resources available to an organism within an environment.

Niche

In terms of the Competitive Exclusion Principle, two species cannot coexist in an ecosystem if their niches are identical.

Niche

Similar species can coexist if they are in a community where there are one or more significant differences in their niches

NicheAs a result of competition, a species may occupy a realized niche rather than a fundamental niche.– Fundamental niche is the entire geographic

range suitable to a particular organism.– Realized niche is the part of the

fundamental niche actually occupied.

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Resource Partitioning

As a direct result of competition, 2 organisms may evolve the capacity to use a different set of resources.

This enables 2 competing species to coexist.

Character Displacement

A comparison of 2 closely related species whose populations overlap.

They may be allopatric or sympatric species.

Character Displacement

In some cases, allopatric populations have similar morphology and use similar resources.

Character Displacement

In contrast, sympatric populations compete for resources and show differences in body structure and the resources they use.

Character DisplacementThus, character displacement is the tendency for characteristics to be more divergent in sympatric populations and convergent in allopatric populations as a result of competition.

Predation

Predators kill things.They have acute senses and many adaptations.– Claws, fangs, teeth, etc.

They have to have these adaptations because they are chasing prey that are often fast and agile, or bigger and stronger.

Prey

They have evolved many adaptations to avoid being caught.Hiding, fleeing, self-defense, alarm calls.They have morphological and physiological adaptations.Cryptic coloration, mechanical and chemical defenses.

Aposematic ColorationMany times animals with effective chemical defenses have bright warning coloration--aposematic coloration.It is likely adaptive.Evidence supports the adaptive idea.Predators avoid prey with bright coloration.

Mimicry

This occurs when one species mimics another for some benefit.

There are two types:

1. Batesian

2. Müllerian

1. Batesian MimicryThis is where a non-poisonous species tricks (baits) a potential predator into thinking that it is poisonous.They mimic the appearance of a poisonous species.

2. Müllerian Mimicry

Two or more poisonous species resemble one another.

When the prey mimic one another, it is beneficial to both species because predators will quickly learn to avoid certain coloration patterns.

Convergent Evolution

Müllerian mimicry is a good example of convergent evolution because many different species have similar patterns of coloration.

Example: bees

PredationPredation can take on many different forms.

Herbivory--eating of plants.

Parasitism--deriving nutrients from a host with no benefit to the host.– Endoparasites, ectoparasites, parasitoidism

Mutualism--symbiotic type of relationship.

Commensalism--two species interact, one benefits and the other is neither harmed not benefits.

Predation

The interspecific interactions of the species result in selective forces such as those seen in coevolution and convergent evolution.

Interspecific Interactions

Interspecific interactions and adaptations that result in coevolution must result in a genetic change between two interacting species.

One species changes which results in a change in another species, which results in a change in the first species, etc.

Convergent Evolution

In contrast, when more than two species are involved, convergent evolution occurs.

We see this with aposematic coloration.

Changes occur in multiple species as a result of a selective force of a predator.

Community Structure

Community structure is governed by a few species.

They control composition, relative abundance and diversity among species.

2 Fundamental Features of Community Structure

1. Species diversity

2. Feeding Relationships

1. Species Diversity

The variety of different kinds of organisms that comprise a community.

There are 2 components:– A. Species richness– B. Relative abundance

A. Species Richness

The number of different types of species in a community.

Correlates to rates of evapotranspiration--the measure of evaporation of water from soil plus the transpiration of water from plants.

B. Relative Abundance

The proportion of the total each species represents.

Consider 2 Communities:Community #1: 25A, 25B, 25C, 25DCommunity #2: 80A, 5B, 5C, 10DEach community has 4 species: richness is the same.Relative abundance is different.

2. Feeding Relationships

The structure and dynamics of a community depend on the feeding relationships between organisms for the most part.

This makes up the trophic structure of the community.

Food Webs

They are very complex and many species weave in and out at different levels.

They are linked to food chains.

Food Chains

They are relatively short.

1. The energetic hypothesis:– The length is limited by the inefficiency of

energy transfer.

2. The dynamic stability hypothesis:– Long food chains are less stable than short

ones.

1. The Energetic Hypothesis

Most data supports this. Only about 10% of the energy stored in each trophic level is converted into organic matter of the next level.

2. The Dynamic Stability Hypothesis

Wild fluctuations in smaller populations are magnified at higher levels.

In variable environments, top predators can have a difficult time adjusting with shocks to the food chain.

Species Impact

Certain species have a large impact on the structure of a community.– They are highly abundant.– They play a key role in community dynamics.

They can be classified as:– Dominant species– Keystone species– Foundation Species

Dominant Species

Most abundant--greatest biomass.

Control the distribution of other species.

There is no single explanation for why a species becomes dominant.– They outcompete other species for

resources.– They are successful at avoiding predation.

Keystone Species

Not the most abundant species.

Do exert a strong control--stems from niche.

Sea-Star--Mussel Example:

The mussel Mytilus californianus is a dominant species in the rocky intertidal community of western N. America.

They compete for space.

The sea star Pisaster ocharaceous preys on the mussel removing it and allows for other animals to move in.

Sea-Star--Mussel Example:

When the sea star is experimentally removed, the mussels dominate the area and diversity declines.

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Sea-Star--Mussel Example:

Thus, the sea star acts as a keystone species and exerts an influence over the entire community.

Models Describing Trophic Levels

Useful for describing biological communities.– Bottom-Up model– Top-Down model– Numerous intermediate models.– Nonequilibrium model

Bottom-Up Model

Hypothesis that there is a unidirectional influence from lower to higher trophic levels.

Vegetation→Herbivore linkage.

Top-Down Model

The hypothesis is that predators control organization because they reduce the herbivore population.Nutrients←Vegetation←Herbivore←Predator

Intermediate Models

Many models between bottom-up and top-down are proposed.

The direction of flow in these models is also hypothesized to fluctuate from bottom-up and top-down over time.

Nonequilibrium Model

Originally, scientists used to think that communities were stable.

Now, it is obvious that communities change much more than they are stable.

This gave rise to the nonequilibrium model.

Nonequilibrium Model

Communities are in a constant state of change as a result of this continued disturbances.

Disturbances: things that change a community by altering its resources and/or organisms.– Example: fires, floods, droughts

Intermediate Disturbance Hypothesis

Suggests that moderate levels of disturbance can create conditions that foster species diversity.

It is supported by a broad range of studies from terrestrial and aquatic communities.

Ecological Succession

The process by which a disturbed area gets colonized by a variety of species.

These are gradually replaced by still other species.

Primary Succession

Occurs when the process begins in a “lifeless” area where soil has not yet formed.– Example: moraine, volcanic island.

• Prokaryotes are initially present• Mosses and lichens are the 1st large enough to

see.

Primary Succession

As time passes, soil forms from weathering and the chemical breakdown and plants eventually become the main form of vegetation.

Secondary Succession

Occurs when existing communities become cleared by some disturbance and get repopulated with plants over time.

Succession

Mount St. Helens

Secondary Succession

There are three processes that link early and late arriving species:

1. Early arrivals make the environment more hospitable.– They facilitate the appearance of later

species by making the environment hospitable.

Secondary Succession

2. Early arrivals may inhibit the arrival of later species.– Colonization by later plants occurs in spite

of the plants rather than because of them.

Secondary Succession

3. Early and late arrivals are independent of one another.– Early arrivals tolerate later species but

neither help nor hinder them.

Biodiversity

Is controlled by biogeographical features.

The location and size of the island are correlated to species biodiversity.

As Darwin and Wallace pointed out, life is more varied and abundant than in other parts of the world.

Equatorial-Polar Gradients

There are two key factors observed in equatorial-polar gradients:– Evolutionary history and climate.

Tropical regions are “older” than polar regions because their growing season is longer.Equatorial regions have tended to avoid major disturbances such as glaciation compared to temperate regions.

The Island Equilibrium Model

Island biogeography provides a great way to study species.

The Island Equilibrium Model helps us study this.

Islands--terrestrial islands and islands in the water.

The Island Equilibrium Model

Consider a newly formed island:Species come from a mainland.2 factors determine the number of species on the island:The rate of immigration and the rate of extinction.

The Island Equilibrium Model

2 physical features of the island affect immigration and extinction rates:

1. Size.

2. Distance from mainland.

The Island Equilibrium Model

1. Size

Small islands generally have low immigration rates.

The Island Equilibrium Model2. Distance from the mainland:With 2 islands of the same size, the one closer to the mainland will have a higher immigration rate and a lower extinction rate.

The Island Equilibrium Model

It is called the island equilibrium model because eventually extinction rates will equal the immigration rates.It is somewhat of an oversimplification.It can only be applied over short time periods and on small islands.Large islands are subject to a number of changes.