One major geographic pattern in biodiversity is the latitudinal gradient in species richness. As one travels further away from the equator, for most taxa, the number of species declines. For example, Figure 3 demonstrates the latitudinal richness gradient for richness in ants of the Northern Hemisphere. This general pattern holds true for most taxa and ecosystem types in both marine and terrestrial environments. There is broad agreement that this pattern is caused by differences in the abiotic, climatic environment, but the specific mechanism or mechanisms causing this pattern are a continued topic of discussion and investigation. One set of theories, broadly grouped together as “species-energy theory” is based on the fact that the amount of radiant energy from the sun captured by ecosystems is negatively associated with latitude. As energy is distributed throughout ecosystems through trophic processes, it is thought that species richness will track the energy following one or more mechanisms. Models of species-energy theory incorporate variables such as temperature, net primary productivity, speciation, and extinction. Other ideas that have been proposed to account for the latitudinal gradient are related to the physiological responses of animals to climatic conditions and the effects of the abiotic environment on historical processes. Most of these theories are not mutually exclusive.
Another recurring pattern in biogeographic theory is the elevational gradient in species richness. As one travels to higher elevations, the number of species declines, or, in many cases, peaks at mid-elevations. Aside from the environmental mechanisms driving this diversity gradient, there is a phenomenon that is based on the geography of species range distributions called the mid-domain effect. The mid-domain effect predicts a peak of diversity at the midpoint along any domain simply by the fact that the ranges of more species overlap in the middle of a domain (like a mountain or an island) than on the edges, and this effect works together with environmental determinants to affect the net distribution of species along many elevational gradients.
A third recurring pattern in the distribution of species is the area effect on species richness. The larger an place is, the more species it can support. This applies to actual islands in bodies of water, as well as habitat islands such as those surrounded by human development. The species-area relationship is presented in more detail in an article about the theory of island biogeography.