Regional Patterns

Within biogeographic realms, a variety of biomes can be distinguished on the basis of their characteristic vegetation or aquatic characteristics (see Chapter 2). Much of the variation in environmental conditions that produce biomes at the regional scale is the result of global circulation patterns and topography. Moun tain ranges and large rivers may be impassible barriers that limit the distribution of many species. Furthermore, mountains show relatively distinct elevational zonation of biomes (life zones). The area available as habitat becomes more limited at higher elevations. Mountaintops resemble oceanic islands in their degree of isolation within a matrix of lower elevation environments and are most vulnerable to climate changes that shift temperature and moisture combinations upward (see Fig. 5.2).

Geographic ranges for many, perhaps most, species are restricted by geographic barriers or by environmental conditions beyond their tolerance limits. Some insect species have broad geographic ranges that span multiple host ranges (e.g., forest tent caterpillar, Malacosoma disstria; Parry and Goyer 2004), whereas others have ranges restricted to small areas (e.g., species endemic to cave ecosystems; Boecklen 1991). Species with large geographic ranges often show considerable genetic variation among subpopulations, reflecting adaptations to regional environmental factors. For example, Istock (1981) reported that northern and southern populations of a transcontinental North American pitcher-plant mosquito, Wyeomyia smithii, showed distinct genetically based life history patterns. The proportion of third instars entering diapause increased with latitude, reflecting adaptation to seasonal changes in habitat or food availability. Controlled crosses between northern and southern populations yielded high proportions of diapausing progeny from northern x northern crosses, intermediate proportions from northern x southern crosses, and low proportions from southern x southern crosses for larvae subjected to conditions simulating either northern or southern photoperiod and temperature.

C. Island Biogeography

Ecologists have been intrigued at least since the time of Hooker (1847, 1853, 1860) by the presence of related organisms on widely separated oceanic islands. Darwin (1859) and A. Wallace (1911) later interpreted this phenomenon as evidence of natural selection and speciation of isolated populations following separation or colonization from distant population sources. Simberloff (1969), Simberloff and Wilson (1969), and E. Wilson and Simberloff (1969) found that many arthropod species were capable of rapid colonization of experimentally defaunated islands.

Although the theory of island biogeography originally was developed to explain patterns of equilibrium species richness among oceanic islands (MacArthur and Wilson 1967), the same factors and processes that govern colonization of oceanic islands explain rates of species colonization and metapopulation dynamics (see the following section) among isolated landscape patches (Cronin 2003, Hanski and Simberloff 1997, Leisnham and Jamieson 2002, Simberloff 1974, Soule and Simberloff 1986). Critics of this approach have argued that oceanic islands clearly are surrounded by habitat unsuitable for terrestrial species, whereas terrestrial patches may be surrounded by relatively more suitable patches. Some terrestrial habitat patches may be more similar to oceanic islands than others (e.g., alpine tundra on mountaintops may represent substantially isolated habitats) (Leisnham and Jamieson 2002), as are isolated wetlands in a terrestrial matrix (Batzer and Wissinger 1996), whereas disturbed patches in grassland may be less distinct (but see Cronin 2003). A second issue concerns the extent to which the isolated populations constitute distinct species or metapopulations of a single species (Hanski and Simberloff 1997). The resolution of this issue depends on the degree of heterogeneity and isolation among landscape patches and genetic drift among isolated populations over time.

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