pol2e_ch44

Summary

A community is a group of species that coexist and interact with one another within a defined geographic area.
A community is characterized by the particular mix of species it contains (species composition), by the number of species it contains, and by their abundances. Together these attributes of communities constitute community structure.
A community contains those species that are able to colonize an area and persist there.

Spatial variation in physical conditions and in habitat structure is associated with predictable spatial variation in species composition. Review Figure 44.2
Even in a constant environment, ongoing extinction and colonization result in ongoing change, or turnover, in the species composition of communities over time.
After a sudden disturbance, species sometimes replace one another in a more or less predictable sequence called succession. Succession following a disturbance that removes the original community often results in a community that resembles the original one. Review Figure 44.4 and ANIMATED TUTORIAL 44.1
Some disturbances cause an ecological transition to a different type of community, and removal of the disturbance factor does not restore the original type of community.
Climate change can cause the species composition of a community to change over time. Review Figure 44.5

An ecological community is a dynamic system with inputs, internal workings, and outputs. A fundamental aspect of this community function is the flux of energy into and out of the community.
The total amount of energy that primary producers capture and convert into chemical energy per unit of time is called gross primary productivity (GPP). The amount of GPP that becomes available to consumers per unit of time is called net primary productivity (NPP). Review Figure 44.6 and ACTIVITY 44.1
The ecological efficiency of energy transfer from one trophic level to the next is only about 10 percent.
The term niche refers to the physical and biological environments in which a species has a positive per capita growth rate, as well as to the species’ functional role in the community.
Species diversity has two components: the number of species in the community, called species richness, and the distribution of those species’ abundances, called species evenness. Review Figure 44.7 and ACTIVITY 44.2
The outputs of a community are measures of its function. Community outputs increase with the species diversity of the community and the number of functional roles those species fill. Review Figure 44.8

Species richness is greatest in the tropics and decreases with increasing latitude. Possible causes of this pattern include greater climate stability, greater energy input, and greater habitat complexity at lower latitudes. Review Figure 44.9
Species richness also varies among oceanic islands. Large islands support more species than small islands, and islands close to a mainland support more species than distant islands. Review Figure 44.10
The theory of island biogeography explains these island patterns as a balance between the rate at which new species colonize and the rate at which resident species go extinct. Review Figures 44.11 and 44.12 and ANIMATED TUTORIAL 44.2

Ecosystems provide humans with a variety goods and services. These ecosystem services have economic value. Review Table 44.1
Production of ecosystem services is threatened by human activities, including the fragmentation of natural habitats, that reduce species diversity. The detrimental effects of habitat fragmentation can be mitigated by land-use management that minimizes extinction in each habitat “island” and maximizes dispersal among them. Review Figures 44.13 and 44.14 and ANIMATED TUTORIAL 44.3
Ecosystem function often can be preserved or restored by focusing on particular species that play especially important roles in the community, and by maintaining or restoring overall species diversity. Review Figure 44.15

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