3: Spatial Gradients in Biodiversity

Generally speaking, warm tropical ecosystems are richer in species than cold temperate ecosystems at high latitudes (see Gaston and Williams, 1996, for general discussion). A similar pattern is seen for higher taxonomic groups (genera, families). Various hypotheses (e.g., environmental patchiness, solar energy, productivity; see Blackburn and Gaston, 1996) have been raised to explain these patterns. For example, it is assumed that warm, moist, tropical environments, with long day-lengths provide organisms with more resources for growth and reproduction than harsh environments with low energy resources (Hunter, 2002). When environmental conditions favor the growth and reproduction of primary producers (e.g., aquatic algae, corals, terrestrial flora) then these may support large numbers of secondary consumers, such as small herbivores, which also support a more numerous and diverse fauna of predators. In contrast, the development of primary producers in colder temperate ecosystems is constrained by seasonal changes in sunlight and temperature. Consequently, these ecosystems may support a less diverse biota of secondary consumers and predators.

Recently, (Allen et al. 2002) developed a model for the effect of ambient temperature on metabolism, and hence generation time and speciation rates, and used this model to explain the latitudinal gradient in biodiversity. However, these authors also noted that the principles that underlie these spatial pattern of biodiversity are still not well understood.

Species and ecosystem diversity is also known to vary with altitude Walter (1985) and Gaston and Williams (1996: 214-215). Mountainous environments, also called orobiomes, are subdivided vertically into altitudinal belts, such as montane, alpine and nival, that have quite different ecosystems. Climatic conditions at higher elevations (e.g., low temperatures, high aridity) can create environments where relatively few species can survive. Similarly, in oceans and freshwaters there are usually fewer species as one moves to increasing depths below the surface. However, in the oceans there may be a rise in species richness close to the seabed, which is associated with an increase in ecosystem heterogeneity.

By mapping spatial gradients in biodiversity we can also identify areas of special conservation interest. Conservation biologists are interested in areas that have a high proportion of endemic species, i.e., species whose distributions are naturally restricted to a limited area. It is obviously important to conserve these areas because much of their flora and fauna, and therefore the ecosystems so-formed, are found nowhere else. Areas of high endemism are also often associated with high species richness (see Gaston and Spicer, 1998 for references).

Some conservation biologists have focused their attention on areas that have high levels of endemism (and hence diversity) that are also experiencing a high rate of loss of ecosystems; these regions are biodiversity hotspots. Because biodiversity hotspots are characterized by localized concentrations of biodiversity under threat, they represent priorities for conservation action (Sechrest et al., 2002). A terrestrial biodiversity hotspot is defined quantitatively as an area that has at least 0.5%, or 1,500 of the world's ca. 300,000 species of green plants (Viridiplantae), and that has lost at least 70% of its primary vegetation (Myers et al., 2000; Conservation International, 2002). Marine biodiversity hotspots are quantitatively defined based on measurements of relative endemism of multiple taxa (species of corals, snails, lobsters, fishes) within a region and the relative level of threat to that region (Roberts et al., 2002). According to this approach, the Philippine archipelago and the islands of Bioko, Sao Tome, Principe and Annobon in the eastern Atlantic Gulf of Guinea are ranked as two of the most threatened marine biodiversity hotspot regions.

Conservation biologists may also be interested in biodiversity coldspots; these are areas that have relatively low biological diversity but also include threatened ecosystems (Kareiva and Marvier, 2003). Although a biodiversity coldspot is low in species richness, it can also be important to conserve, as it may be the only location where a rare species is found. Extreme physical environments (low or high temperatures or pressures, or unusual chemical composition) inhabited by just one or two specially adapted species are coldspots that warrant conservation because they represent unique environments that are biologically and physically interesting. For further discussion on spatial gradients in biodiversity and associated conservation practices see the related modules on "Where is the world's biodiversity?" and "Conservation Planning at a Regional Scale."