Variation in the relative abundance of species:
Biodiversity refers to the variation among organisms and ecological systems at all levels, including genetic variation within populations, functional differences between species, and variation in biome structure. One of the most general indicators of biodiversity is the number of species within an area, or species richness.
In general, diversity increases the closer one goes to the equator, but variation also occurs within latitudinal belts, particularly between areas of distinct climate and topography. A variety of processes influence the number of species found in a particular place.
A general rule is that more species are found within large areas than within small areas. Species richness initially doubles as sample size doubles. Three processes determine the overall shape of the species-area relationship: local sampling, the formation of a variety of habitat types within an environmentally heterogeneous region, and the evolution of distinct lineages on isolated continents.
Extension is an issue for species, especially on islands where numbers are generally smaller and dispersal from other communities is more limited than for populations on the mainland.
Large-scale patterns of diversity:
In the Northern Hemisphere, the number of most plant and animal species increases from north to south. Numbers of mammal species increase from east to west in North America due to the greater heterogeneity of environments in the West. Trees and amphibians are more diverse in the moister eastern half of North America, while reptile species richness decreases fairly uniformly as temperature decreases towards the north.
More productive habitats and more complex habitats tend to harbor more species than less productive or simpler habitats. In regions north of 15°N, temperature provides the best prediction of animal species richness, while water availability has more influence throughout the tropics and into the Southern Hemisphere.
Local and regional components of diversity:
Local (or alpha) diversity is the number of species in a small area of homogeneous habitat. Regional (or gamma) diversity is the total number of species observed in all habitats within a geographic area that includes no significant barriers to the dispersal of organisms. The difference in species from one habitat to another is beta diversity. For example, eastern Asia has greater beta diversity than eastern North America because of its greater topographic complexity.
The species that occur within a region are referred to as its species pool, and the species within the regional species pool are sorted into different communities based on their adaptations and interactions, called species sorting. Competitive interactions between species play a major role in species sorting.
Higher species richness is associated with weaker competition between species. Also, high species richness in the tropics results in part from the presence of a greater variety of ecological resources.
Equilibrium theories of diversity:
The ultimate source of diversity is speciation (the production of new species by the splitting of evolving lineages). Differences in diversity between regions could arise due to differences in rates of speciation, extinction, or both. On islands, the equilibrium theory of island biogeography states that the number of species on an island balances regional processes governing immigration against local processes governing extinction. A steady state occurs when the immigration and extinction rates are equal. Smaller islands have higher extinction rates than larger islands, and far islands receive fewer immigrants than islands close to the mainland. New species can form on large islands, meaning large islands have higher species richness than would be expected from colonization rates alone.
High plant species diversity in the tropics could be explained because high levels of precipitation, sunlight, and low levels of soil organic matter create large differences between forest gaps caused by treefall and the surrounding environment, which should provide more opportunities for habitat specialization.