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Outline

• The Concept of the Community– Diversity and Composition Models

• The Structure of Communities– Island Biogeography– Habitat and Ecological Niche– Competition Between Populations– Predator-Prey Interactions– Symbiotic Relationships

• Community Development• Community Biodiversity

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Community Concept

• 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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Community Structure

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Diversity and Composition Models

• Gleason - Individualistic Model– Each population is there because its abiotic

requirements are met• Clements - Interactive Model– Community is the highest level of organization– Dependent on biotic interactions

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Species Richness of Communities

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Island Biogeography

• MacArthur and Wilson– Developed a general model of island biogeography– Explains and predicts how the community

diversity of an island is affected by• Distance from the mainland, and• Size of the island

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Model of Island Biogeography

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Habitat and Ecological Niche

• Habitat– The area an organism lives and reproduces in

• Ecological niche– The role a species plays in its community

• Includes 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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Community Structure Competition

• When two species compete, the abundance of both species is negatively impacted

Predation (or parasitism)• Expected to increase the abundance of the predator (or parasite)• And reduce the abundance of the prey (or host)

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Competition Between Populations• Interspecific competition

– Members of different species require the same resource

– The supply of the resource is limited

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Competition Between

Two Laboratory Populations of Paramecium

Competitive Exclusion Principle

No two species can occupy the same niche at the same time

Resource Partitioning decreases competition

Can lead to character displacement

Competition Between Populations

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Character Displacement in Finches

on the Galápagos Islands

When on the same island, the beak sizes are appropriate for eating small, medium, and large-sized seeds.

When on separate islands, beaks have the same intermediate size, allowing them to eat seeds that vary in size.

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Niche Specialization Among

Five Species of Coexisting Warblers

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Competition BetweenTwo Species of Barnacles

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Predator-Prey Interactions• Predation– One living organism, the predator, feeds on another, the

prey• Predator is larger• Predator has lower reproductive rate• Prey usually entirely consumed

– Presence of predators can decrease prey densities, and vice-versa

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Predator-prey Interaction Between

Paramecium caudatum and Didinium nasutum

Didinium ate all the Paramecium and then dies out

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Predator-prey InteractionBetween a Lynx and a Snowshoe Hare

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Prey Defenses• Prey defenses– Mechanisms that thwart the possibility of being

eaten by a predator• Spines• Tough Epidermis• Poisonous Chemicals• Camouflage

(cryptic coloration)• Bright Coloration

(warning coloration)• Flocking Behavior

http://www.youtube.com/watch?v=B2JdRPKYyTc

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Camouflage in the

Anglerfish

http://www.youtube.com/watch?v=fUDY9Y_3TNc

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Mimicry– One species resembles another species– Mimicked species possesses an overt

antipredator defense (warning coloration)

• Batesian Mimicry - Mimic lacks defense of the organism it resembles

• Müllerian Mimicry - Mimic shares same protective defense

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Anti-predator Defenses

Poison arrow frog used to make arrows lethal weapons.

Eastern tiger swallowtail butterfly caterpillar has false eyespots to confuse predator.

South American lantern fly has a large false head that resembles and alligator.

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Mimicry Among Insects with

Yellow and Black Stripes

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Symbiotic Relationships• Symbiosis– Interactions in which there is a close relationship

between members of two species– Parasitism• Parasite derives nourishment from a host, and may use

host as habitat and mode of transmission– Endoparasites -Inside host body– Ectoparasites – Outside host body

Beetle infected with wasp larvae Fleas on your dog/cat/you

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Commensalism

• Symbiosis, cont.– Commensalism• A symbiotic relationship in which one species benefits

and the other is indifferent– Remoras and Sharks

• Many supposed examples may turn out to be mutualism or parasitism• Inferred amount of harm or benefit two species do to

one another is subject to investigator bias

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Clownfish AmongSea Anemone’s Tentacles

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Mutualism

• Symbiosis, cont.

– Mutualism

• A symbiotic relationship in which both members of the association benefit

• Need not be equally beneficial to both species

– Cleaning Symbiosis

• Often help each other obtain food or avoid predation

– Bacteria in human intestinal tract

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Mutualism Betweenthe Bullhorn Acacia Tree

and Antsa. Tree provides nourishment for ants. Ants live inside hollow thorns.

b. Nectaries at base of leaves from which ants feed.

c. Ants harvest larval food at tips of leaves

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Cleaning Symbiosis

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Community Development

• Ecological Succession– A predictable pattern of change in species

replacements 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

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Secondary Successionin a Forest

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Succession Models

• Facilitation Model– Each stage facilitates invasion and replacement by

organisms of the next stage– Succession in a particular area will always lead to

the same type of community– Climax Community

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Succession Models

• Inhibition Model– Colonists remain 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

• If widespread disturbances occur frequently, diversity will be limited

• If diversity is high, only moderate disturbances have been occurring with moderate frequency

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The Intermediate Disturbance Hypothesis

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Predation, Competition, and Biodiversity

• Predation by a particular species may reduce competition and increases diversity– Such predators are referred to as keystone

predators• Exotic species– May lead to unbridled competition– Resultant reduction in biodiversity

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Effect of a Keystone Species

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Review

• The Concept of the Community– Diversity and Composition Models

• The Structure of Communities– Island Biogeography– Habitat and Ecological Niche– Competition Between Populations– Predator-Prey Interactions– Symbiotic Relationships

• Community Development• Community Biodiversity