When the same habitat types (forests, oceans, grasslands, etc.) in regions of different latitudes are compared, it becomes apparent that the overall number of species increases from pole to equator. This latitudinal gradient is probably even more pronounced than current records indicate, since researchers believe that most undiscovered species live in the tropics.

One hypothesis to explain this phenomenon, the "time theory," holds that diverse species adapted to today's climatic conditions have had more time to emerge in the tropical regions, which, unlike the temperate and arctic zones, have been unaffected by a succession of ice ages. However, ice ages have caused less disruption in some temperate regions than in others and have not interrupted arctic conditions.

Alternatively, the species-energy hypothesis proposes the following positive correlations: incoming energy from the Sun correlated with rates of growth and reproduction; rates of growth and reproduction with the amount of living matter (biomass) at a given moment; and the amount of biomass with number of species. However, since organisms may die rapidly, high production rates can exist with low biomass. And high biomass can exist with few species. Moreover, the mechanism proposed—greater energy influx leading to bigger populations, thereby lowering the probability of local extinction—remains untested.

A third hypothesis centers on the tropics' climatic stability, which provides a more reliable supply of resources. Species can thus survive even with few types of food, and competing species can tolerate greater overlap between their respective niches. Both capabilities enable more species to exist on the same resources. However, the ecology of local communities cannot account for the origin of the latitudinal gradient. Localized ecological processes such as competition do not generate regional pools of species, and it is the total number of species available regionally for colonizing any particular area that makes the difference between, for example, a forest at the equator and one at a higher latitude.

A fourth and most plausible hypothesis focuses on regional speciation, and in particular on rates of speciation and extinction. According to this hypothesis, if speciation rates become higher toward the tropics, and are not negated by extinction rates, then the latitudinal gradient would result—and become increasingly steep.

The mechanism for this rate-of-speciation hypothesis is that most new animal species, and perhaps plant species, arise because a population subgroup becomes isolated. This subgroup evolves differently and eventually cannot interbreed with members of the original population. The uneven spread of a species over a large geographic area promotes this mechanism: at the edges, small populations spread out and form isolated groups. Since subgroups in an arctic environment are more likely to face extinction than those in the tropics, the latter are more likely to survive long enough to adapt to local conditions and ultimately become new species.