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Species diversity decreases from eqator towards poles.Ecologists proposed various hypoyesis.Suggest two hypothyisis for this phenonmenon?
Most groups of organisms are more species-rich in the tropics. This well-known increase in species richness toward the equator became apparent by the middle of the eighteenth century as taxonomists, led by Carolus Linnaeus, described tropical species sent back to Europe by explorers. These explorers and later naturalists, such as Humboldt, Darwin, and Wallace, described overwhelming biological diversity in the tropics. Today, two and a half centuries later, we are stilt trying to catalog this diversity and do not even know within an order of magnitude its full extent.

  Bird species richness (Dobzhansky 1950) decrease toward the poles. Despite some exceptions to the equatorial peak in species diversity,  the pattern of increased numbers of species in the tropics is pervasive and dramatic.The time since perturbation hypothesis proposes that there are more species in the tropics because the tropics are older and they are disturbed less frequently. That is, more species occur in the tropics because (1) there has been more time for speciation and (2) less frequent perturbation reduces extinction rates. The proponents of this hypothesis assume that the tropics have remained relatively stable while middle and high latitudes have been repeatedly disrupted by the advance and retreat of glaciers. However, in chapter 16 we saw that intermediate levels of disturbance may increase local diversity. In counterpoint to this hypothesis, Joseph Connell (1978) proposed that the extraordinary diversity of tropical rain forests and coral reefs is maintained by frequent disturbance.

Productivity

The authors of the productivity hypothesis observe that two of the most diverse environments on earth, coral reefs and tropical rain forests, are also extraordinarily productive. This hypothesis proposes that high productivity contributes to high species richness. It assumes that with more energy to divide among organisms, specialized consumers will have larger populations. Since larger populations generally have lower probabilities of extinction than smaller populations, extinction rates should be lower in more productive environments.The environmental heterogeneity hypothesis proposes that the tropics contain more species because they are more heterogeneous than temperate regions. Daniel Janzen (1967) and GeorgeStevens (1989) pointed out that, compared to high-latitude species, most tropical species occur in far fewer environments along altitudinal and latitudinal gradients. (This is case study done by scientists).


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