Communities can be distinguished on a taxonomic basis at a global scale because of the distinct faunas among biogeographic realms (A.Wallace 1876). However, similar community types on different continents often are dominated by unrelated species with similar attributes, termed ecological equivalence. For example, grassland communities on every continent should show similar food web structure and functional group organization, reflecting similar environmental conditions, regardless of taxonomic representation. A number of studies have indicated global patterns in community structure related to latitudinal gradients in temperature and moisture and to the ecological history of adaptive radiation of particular taxa.
Latitudinal gradients in temperature and precipitation establish a global template of habitat suitability, as discussed in Chapters 2 and 7. Equatorial areas, characterized by high sun angle and generally high precipitation, provide favorable conditions of light, temperature, and moisture, although seasonal patterns of precipitation in some tropical areas create periods of adverse conditions for many organisms. The strongly seasonal climate of temperate zones requires specific adaptations for survival during seasonally unfavorable conditions, thereby limiting species diversity. The harsh conditions of temperate deserts and high-latitude zones generally restrict the number of species that can be supported or that can adapt to these conditions.
Species richness generally decreases with latitude for a wide variety of taxa (Price 1997, J. Stout and Vandermeer 1975, Willig and Lyons 1998). This gradient may be particularly steep for insects, which would be expected to show increasing species richness toward warmer latitudes, but may not be reflected by all taxa (e.g., aphids, Dixon 1985) or component communities (Vinson and Hawkins 1998). Vinson and Hawkins (1998) reviewed literature for stream communities and concluded that species richness is highly variable and no strong latitudinal trends are apparent.
Some studies suggest that the tropics may support several million new arthropod species (Erwin 1995,May 1988,E.Wilson 1992). Global arthropod diversity currently is estimated at 4-6 million species, with most species in the tropics (Novotny et al. 2002).Although diversity may be high in the tropics, densities may be low and make detection of many species difficult.
Although increasingly favorable climate toward the equator is an attractive explanation for the observed trend in diversity, several alternative hypotheses have been proposed. Terborgh (1973) showed that the apparent trend in species richness with latitude may reflect increasing land area toward the equator. He noted that climate is relatively constant across a wide belt between 20°N and S latitudes but shows a distinct latitudinal gradient above 20°N and S latitudes. Combining climate and surface area gradients yielded a latitudinal gradient in habitat area available within each climate class, with a preponderance of global surface area in tropical habitat. These data suggest that gradients in species richness reflect habitat area available for within-habitat speciation (see discussion later in this chapter).
Latitudinal gradients in species richness also may reflect greater primary productivity in the tropics (Rosenzweig and Abramsky 1993,Tilman and Pacala 1993, Waide et al. 1999; see later in this chapter). Furthermore,Willig and Lyons (1998) showed that latitudinal gradients can result from chance.
Superimposed on the latitudinal gradients are the relatively distinct biogeo-graphic realms identified by A. Wallace (1876). These biogeographic realms reflect the history of continental breakup, with southern floras and faunas largely distinct from northern floras and faunas (see Chapter 7). However, the southern continents show a varied history of reconnection with the northern continents that has resulted in invasion primarily by northern species. The proximity of North America and Eurasia has facilitated movement of species between these land masses, leading to development of a Holarctic species component, especially within the arctic and boreal biomes. Whereas many genera, and even some species, occur throughout the Holarctic realm, the flora and fauna of Australia have remained relatively distinct as a result of continued isolation.
Species richness also may be related to geologic time. E. Wilson (1969) suggested that co-evolution should improve the efficiency of total resource exploitation and lead to further increase in co-existing species over time. In other words, a habitat or resource that has persisted for a longer period of time would acquire more species than a more recently derived habitat or resource. Birks (1980) found that the residence time of tree species in Britain was strongly correlated with the diversity of associated insect species. Tree species that had a longer history of occurrence in Britain hosted a larger number of species than did tree species with shorter residence times. Again, because residence time is correlated with area of occurrence (habitat area), the effects of these two factors cannot be distinguished easily (Price 1997; see later in this chapter).
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