Community Interactions Chapter 47

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 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 Some are obligatory; partners depend upon each other – Yucca plants and yucca moth – Mycorrhizal fungi and plants

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

Interference Competition Least chipmunk is excluded from piñon pine habitat by the competitive behavior of yellow pine chipmunks Yellow Pine Chipmunk Least Chipmunk Figure 47.3 Page 848

Competitive Exclusion Principle When two species compete for identical resources, one will be more successful and will eventually eliminate the other

Gause’s Experiment Paramecium caudatum Paramecium aurelia Figure 47.4 Page 848 Species grown together

Hairston’s Experiment Two salamanders species overlap in parts of their ranges Removed one species or the other in test plots Control plots unaltered 5 years later, salamander populations were growing in test plot

Resource Partitioning Apparent competitors may have slightly different niches May use resources in a different way or time Minimizes competition and allows coexistence Figure 47.6 Page 849

Predation Predators are animals that feed on other living organisms Predators are free-living; they do not take up residence on their prey

Coevolution Joint evolution of two or more species that exert selection pressure on each other as an outcome of close ecological interaction As snail shells have thickened, claws of snail-eating crabs have become more massive

Predator-Prey Models Type I model: Each individual predator will consume a constant number of prey individuals over time Type II model: Consumption of prey by each predator increases, but not as fast as increases in prey density Type III model: Predator response is lowest when prey density is lowest

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

Canadian Lynx and Snowshoe Hare Show cyclic oscillations Krebs studied populations for ten years Fencing plots delayed cyclic declines but didn’t eliminate them Aerial predators, plant abundance also involved Three-level model

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

Kinds of Parasites Microparasites Macroparasites Social parasites Parasitoids

Fungus and Frogs Amphibians are disappearing even in undisturbed tropical forests Infection by a parasitic chytrid is one of the causes of the recent mass deaths

Parasitic Plants Holoparasites – Nonphotosynthetic; withdraw nutrients and water from young roots Hemiparasites – Capable of photosynthesis, but withdraw nutrients and water from host

Parasitioids Insect larvae live inside and consume all of the soft tissues of the host Used as agents of biological control Can act as selective pressure on host

Price’s Sawfly Study Number emerging when wasp attacks were experimentally prevented Number emerging after wasp attacks Figure Page 855

Ecological Succession Change in the composition of species over time Classical model describes a predictable sequence with a stable climax community

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

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 TidepoolsRocks exposed at high tide Periwinkles promote or limit diversity in different habitats Figure Page 898

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 Figure Page 862

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 Figure Page 863

Area Effect Larger islands tend to support more species than smaller islands More habitats Bigger targets Larger populations decrease extinction risks