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

Periwinkles promote or limit diversity in different habitats Tidepools Rocks exposed at high tide

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