Community Ecology

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Community Ecology

• Chapter 47

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Outline

• Diversity and Composition Models
• Island Biogeography
• Habitat and Ecological Niche
• Competition Between Populations
• Predator-Prey Interactions
• Symbiotic Relationships
• Community Development
• Community Diversity

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Community Concept

• A community is an assemblage of populations
  interacting with one another within the same
  environment.
• Composition is a listing of various species in
  the community.
• Diversity includes both species richness and
  species diversity.

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Diversity and Composition Models

• Gleason - Individualistic Model
• Each population in community is there because its
  own particular abiotic requirements are met by a
  particular habitat.
• Clements - Interactive Model
• Community is the highest level of organization.
• Dependent on biotic interactions.

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Island Biogeography

• MacArthur and Wilson developed a general model of
  island biogeography to explain and predict the
  effects of distance from the mainland and size of
  an island on community diversity.
• Island size and distance from population source
  affects species diversity.

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Model of Island Biogeography
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Community Structure

• Competition for limited resources between two
  species has a negative effect on the abundance of
  both species.
• Predation and parasitism are expected to increase
  the abundance of the predator and parasite at the
  expense of the abundance of the prey and its host.

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Habitat and Ecological Niche

• Habitat is the area an organism lives and
  reproduces in.
• Ecological niche is the role an organism plays in
  its community, including its habitat and its
  interactions with other organisms.
• Fundamental niche - All conditions under which
  the organism can survive.
• Realized niche - Set of conditions under which it
  exists in nature.

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Feeding Niches for Wading Birds
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Competition Between Populations

• Interspecific competition occurs when members of
  different species try to utilize a resource in
  limited supply.
• Competitive Exclusion Principle - No two species
  can occupy the same niche at the same time.
• Resource Partitioning decreases competition.
• Can lead to character displacement.

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Competition Between Barnacle Species
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Character Displacement in Galápagos Finches
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Predator-Prey Interactions

• Predation occurs when one living organism, the
  predator, feeds on another, the prey.
• Presence of predators can decrease prey
  densities, and vice-versa.

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Lynx-Snowshoe Hare Interactions
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Prey Defenses

• Prey defenses are mechanisms that thwart the
  possibility of being eaten by a predator.
• Spines
• Tough Epidermis
• Poisonous Chemicals
• Camouflage
• Bright Coloration
• Flocking Behavior

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Camouflage in the Anglerfish
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Mimicry

• Mimicry occurs when one species resembles another
  that possesses an overt antipredator defense.
• Batesian - Mimic lacks defense of the organism it
  resembles.
• Müllerian - Mimic shares same protective defense.

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Symbiotic Relationships

• Symbiosis refers to interactions in which there
  is a close relationship between members of two
  populations.
• Parasitism
• Parasite derives nourishment from a host, and may
  use host as habitat and mode of transmission.
• Endoparasites
• Ectoparasites

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Commensalism

• Commensalism is a symbiotic relationship in which
  one species is benefited and the other is neither
  benefited nor harmed.
• Remoras and Sharks
• Many examples may turn out to be mutualism or
  parasitism.
• Amount of harm or benefit two species do to one
  another is partially determined by the
  investigator.

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Mutualism

• Mutualism is a symbiotic relationship in which
  both members of the association benefit.
• Often help organisms obtain food or avoid
  predation.
• Bacteria in human intestinal tact.
• Need not be equally beneficial to both species.
• Cleaning Symbiosis

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Mutualism Between Bullhorn Acacia and Ants
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Cleaning Symbiosis
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Community Development

• Ecological Succession
• A change involving a series of species
  replacements in a community following a
  disturbance.
• Primary Succession occurs in areas where there is
  no soil formation.
• Secondary Succession begins in areas where soil
  is present.
• Pioneer Species

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Secondary Succession in a Forest
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Succession Models

• Facilitation Model
• Succession in a particular area will always lead
  to the same type of community.
• Climax Community
• Each stage facilitates invasion and replacement
  by organisms of the next stage.

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Succession Models

• Inhibition Model
• Colonists hold onto their space and inhibit
  growth of other plants until the colonists are
  damaged or die.
• Tolerance Model
• Different types of plants can colonize an area
  at the same time.
• Chance determine which seeds arrive first.

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Community Diversity

• Community stability can be recognized in three
  ways.
• Persistence through time.
• Resistance to change.
• Recovery once a disturbance has occurred.

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Intermediate Disturbance Hypothesis

• Moderate amounts of disturbances at moderate
  frequency are required for a high degree of
  community diversity.
• If widespread disturbances occur frequently,
  diversity will be limited.

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Intermediate Disturbance Hypothesis
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Predation, Competition, and Biodiversity

• In certain communities, predation by a particular
  species reduces competition and increases
  diversity.
• Predators that regulate competition and maintain
  community diversity are referred to as keystone
  predators.
• Introduction of exotic species into a new area
  may lead to unbridled competition and resultant
  reduction in biodiversity.

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Review

• Diversity and Composition Models
• Island Biogeography
• Habitat and Ecological Niche
• Competition Between Populations
• Predator-Prey Interactions
• Symbiotic Relationships
• Community Development
• Community Diversity

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