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Community Ecology

Chapter 54

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Biological Communities

• A community consists of all the species that occur together at any particular locality.

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Fundamental and Realized Niches

• An organism’s niche is the total of all ways it utilizes the resources of its environment.

– fundamental - entire niche potentially available to an organism

– realized - actual niche utilized by an organism

• Habitat is the place where an organism lives.

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Realized Niche

• Interspecific competition - Occurs when different species attempt to utilize the same resource.

– interference - individuals fighting over the same resource

– exploitative - individuals utilizing shared resources

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Principle of Competitive Exclusion• The principle of competitive exclusion states

that no two species utilizing the same niche can coexist indefinitely.

– one will eventually eliminate the other Gause’s Paramecium experiments

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Detecting Interspecific Competition

• Negative effects of one species on another do not automatically indicate competition.

– Presence of one species may attract a predator that consumes both, causing one species to have a lower population size than the other.

must always look at underlying ecological mechanisms

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Predation and Prey Populations

• Predation occurs when one organism consumes another.

– provides strong selective pressure on prey populations

Any physiologic characteristic or behavior that would decrease the probability of capture should be strongly favored.

increased fitness

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Plant Defenses Against Herbivores

• Morphological – thorns, spines, etc.

• Chemical – secondary chemical compounds

• Evolutionary response of herbivores– Certain groups of herbivores are associated

with each group of plants protected by a particular secondary compound.

coevolution

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Animal Defenses Against Predators

• Some animals that feed on plants rich in secondary compounds receive an extra benefit.

– caterpillars that feed on members of the milkweed family

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Animal Defenses Against Predators

• Chemical defenses– poisons and stings

• Defensive coloration– aposematic coloration (warning coloration)

individuals advertise poisonous nature– cryptic coloration

camouflage (blending coloration)

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Mimicry

• Batesian mimicry– Palatable insects resemble brightly colored, distasteful species.

• Mullerian mimicry– Unrelated protected species resemble one another.

predators learn more quickly

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Coevolution and Symbiosis

• Coevolution involves long-term mutual evolution of two or more species.

– predator-prey interactions– symbiotic relationships

two or more kinds of organisms live together in permanent relationships

commensalism, mutualism, and parasitism

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Commensalism

• One species in symbiotic relationship benefits while the other is neither helped or hurt.

– tropical fish and sea anemones

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Mutualism

• Both species in relationship benefit.– mutual cooperation

ants and acacias

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Parasitism

• Parasite benefits while inflicting some form of harm to the prey.

– ectoparasites external parasites - lice

– endoparasites internal parasites

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Interactions Among Ecological Processes

• Keystone species– Species that have a particularly strong

effect on community composition. top predators

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Succession• Succession is the process of ecosystems

changing from a simple to a more complex structure.

– primary succession - occurs in bare areas– secondary succession - occurs following disturbance

• Succession occurs because species alter the habitat and available resources in ways that favor other species.

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An ecosystem includes all the organisms living in a particular place, and the abiotic environment in which they interact.

ProducersAutotrophs - capture light energy and manufacture own chemical energy

ConsumersHeterotrophs - must obtain organic molecules from autotrophs

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Trophic Levels - Food Chain

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Tertiaryconsumer

Primary consumer

Primary producer

Secondaryconsumer

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Trophic Levels

• Trophic level refers to the feeding level of an organism.• Organisms from each trophic level constitute a food chain.

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The Energy in Food Chains• Second Law of Thermodynamics

food chains are generally limited to 3 or 4 steps (trophic levels) ultimately determined by the amount of sunlight

• On average, 10% of the organic matter (energy) transfers from one trophic level to the next.

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Interactions Among Trophic Levels• Trophic cascading

effect of one trophic level flows down to lower levels. (top-down effects)

Bottom-up effectsproductivity of an ecosystem is low, herbivore populations will be too small to support any predators.

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Veg

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Productivity

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Interconnected food chains constitute a food web.