Plant and Animal Ecology
Albert J. Parker
Kathleen C. Parker
Thomas R. Vale
Ecology is the study of interactions of organisms with the physical and biological elements of their environments. The breadth of the North American continent and the diversity of the biota that inhabits it frustrate efforts to summarize these patterns of interaction in simple or tidy ways. In this chapter we attempt to synthesize the ecological factors that structure the biota of North America while acknowledging at the outset that an even and comprehensive coverage of the vast array of interesting and important interactions that shape this structure must remain elusive. Our purpose here is to address the principal processes that influence the geographic heterogeneity of plant cover and animal life in North America. The scope is generally restricted to patterns evident from the landscape scale to the continental scale. Overall, then, the intent is to explore recent ecological themes and trends that are deemed important in understanding spatial patterns of biotic response to the physical environment as it varies across the face of the continent.
Four broad categories of influence act over time to shape geographic variation in the North American biota: genetic constraints and evolutionary history, physical environmental gradients, biotic interactions, and environmental history (which includes disturbance regimes, dispersal dynamics, unique events, and chance). The role of human action in shaping the North American biota is well documented (Marsh,  1965, Goudie, 1981). Whether human influence should be treated as separate from these four— or subsumed within one or more of them—is a debate best left to environmental philosophers; some elements of human influence will be considered in the discussion of environmental history.
Genes provide an inherited set of encoded instructions (collectively, a genotype) that impose limits on the range of expression of form and function (or phenotype) of individuals. To the extent that genetic material dictates the manner in which organisms acquire the energy and material necessary to survive, it imposes limits on the range of physical environments in which these organisms might be found.
For a population, the gene pool will vary over time as natural selection, genetic drift, gene flow, and other evolutionary forces modify the frequency of different genes. Such changes in gene frequency in a population generally accrue through the operation of demographic processes: