Our planet is inhabited by millions of species of plants, animals and microbes. The distribution of these species is uneven and their groups exhibit various patterns.
The most widely recognised pattern of biodiversity is the latitudinal gradient, in which there is an increase in the species richness from the poles towards the tropics. Except for a few areas such as the deserts, tropical regions are richer in species diversity as compared to the temperate and polar regions. Moreover, tropical forests have greater species diversity than the temperate forests.
According to one of the hypotheses, evolution of species is a function of time. In the past, the temperate regions have often faced glaciations. In contrast, the tropical regions have been relatively undisturbed for millions of years. As a result, the tropical regions have had a long evolutionary time for species diversification.
Another hypothesis stated that unlike temperate regions, the tropical environments are relatively more constant and predictable. Such constant environments encouraged niche specialisation and led to greater species diversity.
According to yet another hypothesis, the availability of abundant solar energy in the tropics contributes to higher productivity, which indirectly leads to greater diversity. Apart from latitude, the area of a place too determines the species diversity or richness. A relationship between species and area was proposed by a German naturalist and geographer Alexander Von Humboldt while he was exploring the wild in the South American jungles. According to him, within a region, species richness increased with the increasing explored area, but only up to a limit. To analyse the relation between the two entities – species richness and area, he plotted a graph which turned out to be a rectangular hyperbola with the equation S = CAZ . However, on a logarithmic scale, this relationship is linear and is given by the equation log S = log C + Z log A. A higher value of Z suggests that the nature of habitat changes and becomes more accommodating causing a proportionate number of species to spread across a wider area. In contrast, as the value of Z decreases, there is a decrease in the biodiversity of that habitat. Therefore, in nature, species diversity of a region depends on its latitudinal location and its area.