Community Structure & Biodiversity

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 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 Theoretical niche occupied in the absence of any competing species  Realized niche Niche a species actually occupies 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 Yucca plants and yucca moth Mycorrhizal fungi and plants 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

Fig. 46-4, p.823 Sea Anemone and Fish

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

Fig. 46-5a, p.824

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 47.6 Page 825 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

Fig. 46-7, p.825 P. glutinosis P. jordani

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

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

Fig. 46-9a, p.826

Fig. 46-9c, p.826

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

Fig a, p.827

Fig b, p.827

Fig c, p.827

Prey Defenses  Camouflage  Warning coloration  Mimicry  Moment-of-truth defenses

Fig a, p.828 Camouflage

Fig b, p.828 Camouflage

Fig c, p.828 Camouflage

Fig a, p.829 Mimicry

Fig b, p.829 Mimicry

Fig c, p.829 Mimicry

Fig d, p.829 Mimicry

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

Fig a, p.829

Fig b, p.829

Fig d, p.829

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

Fig a, p.830

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 Nonphotosynthetic; withdraw nutrients and water from young roots  Hemiparasites Capable of photosynthesis, but withdraw nutrients and water from host Capable of photosynthesis, but withdraw nutrients and water from host

Fig a, p.830 Devil’s Hair

Fig b, p.830 Devil’s Hair

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

Fig , p.831

The Cowbird  Brown-headed cowbirds lay their eggs in nests constructed by other “host” bird species. These hosts are unable to differentiate between cowbird eggs and their own  Cowbird hatchlings shove the other eggs out of the owner’s nest and demand to be fed.

The Cowbird  Parasitic behavior has perpetuated cowbird genes for thousands of years

Fig a, p.831

Fig b, p.831

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 a, p.832

Fig b, p.832

Cyclic Changes  Cyclic, nondirectional changes also shape community structure  Tree falls cause local patchiness in tropical forests  Fires periodically destroy underbrush in sequoia forests

Fig a, p.833

Fig b, p.833

Fig c, p.833

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

Fig a, p.834

Fig b, p.834

Lubchenco Experiment TidepoolsRocks exposed at high tide Periwinkles promote or limit diversity in different habitats Figure Page 834

Species Introductions  Introduction of a nonindigenous species can decimate a community  No natural enemies or controls  Can outcompete native species