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Chapter 7

Community Ecology

Chapter Overview Questions

Ø What determines the number of species in a community?

Ø How can we classify species according to their roles in a community?

Ø How do species interact with one another? Ø How do communities respond to changes in

environmental conditions? Ø Does high species biodiversity increase the

stability and sustainability of a community?

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Core Case Study: Why Should We Care about the

American Alligator? Ø Hunters wiped out

population to the point of near extinction.

Ø Alligators have important ecological role.

Figure 7-1

Core Case Study: Why Should We Care about the

American Alligator? Ø Dig deep depressions (gator holes).

l  Hold water during dry spells, serve as refuges for aquatic life.

Ø Build nesting mounds. l  provide nesting and feeding sites for birds. l  Keeps areas of open water free of vegetation.

Ø Alligators are a keystone species: l  Help maintain the structure and function of the

communities where it is found.

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COMMUNITY STRUCTURE AND SPECIES DIVERSITY

Ø Biological communities differ in their structure and physical appearance.

Figure 7-2

Species Diversity and Niche Structure: Different Species Playing

Different Roles

Ø Biological communities differ in the types and numbers of species they contain and the ecological roles those species play. l  Species diversity: the number of different

species it contains (species richness) combined with the abundance of individuals within each of those species (species evenness).

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Species Diversity and Niche Structure

Ø Niche structure: how many potential ecological niches occur, how they resemble or differ, and how the species occupying different niches interact.

Ø Geographic location: species diversity is highest in the tropics and declines as we move from the equator toward the poles.

TYPES OF SPECIES

Ø Native, nonnative, indicator, keystone, and foundation species play different ecological roles in communities. l  Native: those that normally live and thrive in a

particular community. l  Nonnative species: those that migrate,

deliberately or accidentally introduced into a community.

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Case Study: Species Diversity on Islands

Ø MacArthur and Wilson proposed the species equilibrium model or theory of island biogeography in the 1960’s.

Ø Model projects that at some point the rates of immigration and extinction should reach an equilibrium based on: l  Island size l  Distance to nearest mainland

Indicator Species: Biological Smoke Alarms

Ø Species that serve as early warnings of damage to a community or an ecosystem. l  Presence or absence of trout species because

they are sensitive to temperature and oxygen levels.

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Keystone Species: Major Players

Ø Keystone species help determine the types and numbers of other species in a community thereby helping to sustain it.

Figures 7-4 and 7-5

Foundation Species: Other Major Players

Ø Expansion of keystone species category. Ø Foundation species can create and enhance

habitats that can benefit other species in a community. l  Elephants push over, break, or uproot trees,

creating forest openings promoting grass growth for other species to utilize.

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Case Study: Why are Amphibians Vanishing?

Ø Frogs serve as indicator species because different parts of their life cycles can be easily disturbed. Figure 7-3

Case Study: Why are Amphibians Vanishing?

Ø Habitat loss and fragmentation. Ø Prolonged drought. Ø Pollution. Ø  Increases in ultraviolet radiation. Ø Parasites. Ø Viral and Fungal diseases. Ø Overhunting. Ø Natural immigration or deliberate introduction

of nonnative predators and competitors.

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How Would You Vote?

To conduct an instant in-class survey using a classroom response system, access “JoinIn Clicker Content” from the PowerLecture main menu for Living in the Environment.

Ø  Do we have an ethical obligation to protect shark species from premature extinction and treat them humanely? l  a. No. It's impractical to force international laws on

individual fishermen that are simply trying to feed their families with the fishing techniques that they have.

l  b. Yes. Sharks are an important part of marine ecosystems. They must be protected and, like all animals, they should be humanely treated.

SPECIES INTERACTIONS: COMPETITION AND PREDATION

Ø Species can interact through competition, predation, parasitism, mutualism, and commensalism.

Ø Some species evolve adaptations that allow them to reduce or avoid competition for resources with other species (resource partitioning).

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Resource Partitioning

Ø Each species minimizes competition with the others for food by spending at least half its feeding time in a distinct portion of the spruce tree and by consuming somewhat different insect species.

Figure 7-7

Niche Specialization

Ø Niches become separated to avoid competition for resources.

Figure 7-6

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SPECIES INTERACTIONS: COMPETITION AND PREDATION

Ø Species called predators feed on other species called prey.

Ø Organisms use their senses their senses to locate objects and prey and to attract pollinators and mates.

Ø Some predators are fast enough to catch their prey, some hide and lie in wait, and some inject chemicals to paralyze their prey.

PREDATION

Ø Some prey escape their predators or have outer protection, some are camouflaged, and some use chemicals to repel predators.

Figure 7-8

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SPECIES INTERACTIONS: PARASITISM, MUTUALISM, AND

COMMENSALIM Ø Parasitism occurs when one species feeds

on part of another organism. Ø  In mutualism, two species interact in a way

that benefits both. Ø Commensalism is an interaction that benefits

one species but has little, if any, effect on the other species.

Parasites: Sponging Off of Others

Ø Although parasites can harm their hosts, they can promote community biodiversity. l  Some parasites live in host (micororganisms,

tapeworms). l  Some parasites live outside host (fleas, ticks,

mistletoe plants, sea lampreys). l  Some have little contact with host (dump-nesting

birds like cowbirds, some duck species)

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Mutualism: Win-Win Relationship

Ø Two species can interact in ways that benefit both of them.

Figure 7-9

Commensalism: Using without Harming

Ø Some species interact in a way that helps one species but has little or no effect on the other.

Figure 7-10

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ECOLOGICAL SUCCESSION: COMMUNITIES IN TRANSITION

Ø New environmental conditions allow one group of species in a community to replace other groups.

Ø Ecological succession: the gradual change in species composition of a given area l  Primary succession: the gradual establishment

of biotic communities in lifeless areas where there is no soil or sediment.

l  Secondary succession: series of communities develop in places containing soil or sediment.

Primary Succession: Starting from Scratch

Ø Primary succession begins with an essentially lifeless are where there is no soil in a terrestrial ecosystem

Figure 7-11

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Secondary Succession: Starting Over with Some Help

Ø Secondary succession begins in an area where the natural community has been disturbed.

Figure 7-12

Can We Predict the Path of Succession, and is Nature in

Balance? Ø The course of succession cannot be

precisely predicted. Ø Previously thought that a stable climax

community will always be achieved. Ø Succession involves species competing for

enough light, nutrients and space which will influence it’s trajectory.

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ECOLOGICAL STABILITY AND SUSTAINABILITY

Ø Living systems maintain some degree of stability through constant change in response to environmental conditions through: l  Inertia (persistence): the ability of a living system

to resist being disturbed or altered. l  Constancy: the ability of a living system to keep

its numbers within the limits imposed by available resources.

l  Resilience: the ability of a living system to bounce back and repair damage after (a not too drastic) disturbance.

ECOLOGICAL STABILITY AND SUSTAINABILITY

Ø Having many different species appears to increase the sustainability of many communities.

Ø Human activities are disrupting ecosystem services that support and sustain all life and all economies.