Community Interactions Chapter 29. Forest of New Guinea Community includes nine species of pigeons that partition the food supply Pigeons disperse seeds.

Community Interactions

Chapter 29

Forest of New Guinea

• Community includes nine species of

pigeons that partition the food supply

• Pigeons disperse seeds of the trees

that provide their food (fruit)

• These are just a few of the many

interactions that shape this community

Community

• All the populations that live together in a

habitat

• Habitat is the type of place where

individuals of a species typically live

• Type of habitat shapes a community’s

structure

Factors Shaping Community Structure

• Climate and topography

• Available foods and resources

• Adaptations of species in community

• Species interactions

• Arrival and disappearance of species

• Physical disturbances

Niche

Sum of activities and relationships in

which a species engages to secure and

use resources necessary for survival

and reproduction

Realized & Fundamental Niches

• Fundamental niche – Theoretical niche occupied in the absence

of any competing species

• Realized niche – Niche a species actually occupies

• Realized niche is some fraction of the fundamental niche

Species Interactions

• Most interactions are neutral; have no

effect on either species

• Commensalism helps one species and

has no effect on the other

• Mutualism helps both species

Species Interactions

• Interspecific competition has a negative

effect on both species

• Predation and parasitism both benefit

one species at a cost to another

Symbiosis

• Living together for at least some part of

the life cycle

• Commensalism, mutualism, and

parasitism are forms of symbiosis

Mutualism

• Both species benefit

• Many examples in nature

• Some mutualisms are obligatory;

partners depend upon each other

Yucca and Yucca Moth

• Example of an obligatory mutualism

• Each species of yucca is pollinated only

by one species of moth

• Moth larvae can grow only in that one

species of yucca

Mycorrhizae

• Obligatory mutualism between fungus

and plant root

• Fungus supplies mineral ions to root

• Root supplies sugars to fungus

Competition

• Interspecific - between species

• Intraspecific - between members of the

same species

• Intraspecific competition is most intense

Forms of Competition

• Competitors may have equal access to

a resource; compete to exploit resource

more effectively

• One competitor may be able to control

access to a resource, to exclude others

Competitive Exclusion Principle

When two species compete for identical

resources, one will be more successful

and will eventually eliminate the other

Competitive Exclusion Expt

Paramecium caudatum

Paramecium aurelia

Keystone Species

• A species that can dictate community

structure

• Removal of a keystone species can

cause drastic changes in a community;

can increase or decrease diversity

Lubchenco Experiment

Tidepools Rocks exposed at high tide

Periwinkles promote or limit diversity in different habitats

Resource Partitioning

• Apparent competitors

may actually have

slightly different niches

• Species may use

resources in a different

way or time

• Minimizes competition

and allows coexistence

Predation

• Predators are animals that feed on other

living organisms

• Predators are free-living; they do not

take up residence on their prey

Coevolution

• Natural selection promotes traits that

help prey escape predation

• It also promotes traits that make

predators more successful at capturing

prey

Predator-Prey Cycles

• Predator and prey populations may show an apparent correspondence

PREY POPULATION

PREDATOR POPULATION

Variation in Cycles

• An association in predator and prey

abundance does not always indicate a

cause and effect relationship

• Variations in food supply and additional

predators may also influence changes

in prey abundance

Prey Defenses

• Camouflage

• Warning coloration

• Mimicry

• Moment-of-truth defenses

Predator Responses

• Any adaptation that protects prey may

select for predators that can overcome

that adaptation

• Prey adaptations include stealth,

camouflage, and ways to avoid chemical

repellents

Parasitism

• Parasites drain nutrients from their

hosts and live on or in their bodies

• Natural selection favors parasites

that do not kill their host too quickly

Types of Parasites

• Microparasites

• Macroparasites

• Social parasites

• Parasitoids

Succession

Change in the composition of species

over time

Types of Succession

• Primary succession - new

environments

• Secondary succession -

communities were destroyed

or displaced

Pioneer Species

• Species that colonize barren habitats

• Lichens, small plants with brief life

cycles

• Improve conditions for other species

who then replace them

Climax Community

• Stable array of species that persists

relatively unchanged over time

• Succession does not always move

predictably toward a specific climax

community; other stable communities

may persist

Cyclic Changes

• Cyclic, nondirectional changes also

shape community structure

• Tree falls cause local patchiness in

tropical forests

• Fires periodically destroy underbrush in

sequoia forests

Restoration Ecology

• Natural restoration of a damaged

community can take a very long time

• Active restoration is an attempt to

reestablish biodiversity in an area

• Ecologists are actively working to

restore reefs, grasslands, and wetlands

Community Instability

• Disturbances can cause a

community to change in ways that

persist even if the change is

reversed

Species Introductions

• Introduction of a nonindigenous species

can decimate a community

• No natural enemies or controls

• Can outcompete native species

Exotic Species

• Species that has left its home range

and become established elsewhere

• Becomes part of its new community

• Can have beneficial, neutral, or

harmful effects on a community

Endangered Species

• A species that is extremely vulnerable

to extinction

• Close to 70 percent of endangered

species have been negatively affected

by exotic competitors

Nile Perch in East Africa

• Nile perch were introduced into Lake

Victoria as a food source

• This predator ate native cichlids; drove

many species to extinction

• Now Nile perch species is close to

crashing

Rabbits in Australia

• Rabbits were introduced for food and

hunting

• Without predators, their numbers

soared

• Attempts at control using fences or

viruses have thus far been unsuccessful

Kudzu in Georgia

• Imported for erosion control

• No natural herbivores, pathogens, or competitors

• Grows over landscapes and cannot be dug up or burned out

• May turn out to have some commercial use

Diversity by Latitude

• Diversity of most groups is greatest in tropics; declines toward poles

Ant diversity

Why Are Tropical Species Rich?

• Resources are plentiful and reliable

• Species diversity is self-reinforcing

• Rates of speciation are highest in the

tropics

Distance Effect

• The farther an island is from a

mainland, the fewer species

• Closer islands receive more immigrants

• Species that reach islands far from

mainland are adapted for long-distance

dispersal and can move on

Distance Effect