Chapter 46 Community Structure and Biodiversity

Impacts, Issues: Fire Ants in the Pants Argentine fire ants first entered the US in the 1930s, probably on cargo ships Fire ants are more dangerous and competitive than native ants Their invasion disturbs ground-nesting birds and animals that feed on native ants

Impacts, Issues: Fire Ants in the Pants Ecologists enlist the aid of two fly species that attack fire ants in their native habitat Another option is fungi or protists that infect fire ants but not native ants Knowledge of community structure is vital to understanding the effects of invading species

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

Fig. 46-3a, p.823

Fig. 46-2b, p.822

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

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

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

Resource Partitioning Apparent competitors may have slightly different niches May use resources in a different way or time Minimizes competition and allows coexistence Figure 47.8 Page 825

Fig. 46-8b, p.825 bristly foxtail Indian mallow smartweed

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

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

Fig b, p.827

Fig a, p.827

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

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

Fig d, p.832

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

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

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

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

Fig , p.839