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 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 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, 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 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 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 likely to survive long enough to adapt to local conditions and ultimately become new species.
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