A POPULATION IS A GROUP OF INTERBREEDING MEMBERS of a species. A number of more or less discrete subpopulations may be distributed over the geographic range of a species population. Movement of individuals among these "demes" (composing a "metapopulation") and newly available resources compensate for local extinctions resulting from disturbances or biotic interactions (Hanski and Gilpin 1997). Populations are characterized by structural attributes, such as density; dispersion pattern; and age, sex, and genetic composition (Chapter 5) that change through time (Chapter 6) and space (Chapter 7) as a result of responses to changing environmental conditions.
Population structure and dynamics of insects have been the subject of much ecological research. This is the level of ecological organization that is the focus of evolutionary ecology, ecological genetics, biogeography, development of sampling methods, pest management, and recovery of endangered species. These disciplines all have contributed enormously to our understanding of populationlevel phenomena.
Abundance of many insects can change orders of magnitude on very short time scales because of their small size and rapid reproductive rates. Such rapid and dramatic change in abundance in response to often-subtle environmental changes facilitates statistical evaluation of population response to environmental factors and makes insects useful indicators of environmental change. The reproductive capacity of many insects enables them to colonize new habitats and exploit favorable conditions or new resources quickly. However, their small size, short life span, and dependence on chemical communication to find mates at low densities limit persistence of small or local populations during periods of adverse conditions, frequently leading to local extinction.
Population dynamics reflect the net effects of differences among individuals in their physiological and behavioral interactions with the environment. Changes in individual success in finding and exploiting resources, mating and reproducing, and avoiding mortality agents determine numbers of individuals, their spatial distribution, and genetic composition at any point in time. Population structure is a component of the environment for the members of the population and provides information that affects individual physiology and behavior, and hence fitness (see Section I). For example, population density affects competition for food and oviposition sites (as well as other resources), propensity of individuals to disperse, and the proximity of potential mates.
Population structure and dynamics also affect community structure and ecosystem processes (Sections III and IV). Each population constitutes a part of the environment for other populations in the community. Changes in abundance of any one species population affect the population(s) on which it feeds and population(s) that prey on, or compete with, it. Changes in size of any population also affect the importance of its ecological functions. A decline in pollinator abundance will reduce fertilization and seed production of host plants, thereby affecting aspects of nutrient uptake and primary productivity. An increase in phytophage abundance can increase canopy "porosity," increasing light penetration and increasing fluxes of energy, water, and nutrients to the soil. A decline in predator abundance will release prey populations from regulation and contribute to increased exploitation of the prey's resources. A decline in detritivore abundance can reduce decomposition rate and lead to bottlenecks in biogeochemical cycling that affect nutrient availability.
Population structure across landscapes also influences source-sink relationships that determine population viability and ability to recolonize patches following disturbances. For example, the size and distribution of demes determine their ability to maintain gene flow or to diverge into separate species. Distribution of demes also determines the source(s) and initial genetic composition of colonists arriving at a new habitat patch. These population attributes are critical to protection or restoration of rare or endangered species. Isolation of demes as a result of habitat fragmentation can reduce their ability to reestablish local demes and lead to permanent changes in community structure and ecosystem processes across landscapes.
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