On a global scale, species diversity typically declines with increasing latitude toward the poles (Rosenzweig, 1995; Stevens and Willig, 2002). Although the latitude diversity gradient is the most striking biodiversity pattern, the dynamics that generate and maintain this trend remain poorly understood. The latitudinal diversity gradient is commonly viewed as the net product of in situ origination and extinction, with the tropics serving as either a generator of biodiversity (the tropics-as-a-cradle hypothesis), or an accumulator of biodiversity (the tropics-as-museum hypothesis), eventually both.
The causes for latitudinal gradients in biodiversity (http://www.ecology.info/gradients-biodiversity.htm).
The determinant of biological diversity is not latitude per se, but the environmental variables correlated with latitude. More than 25 different mechanisms have been suggested for generating latitudinal diversity gradients, but no consensus has been reached yet (Gaston, 2000).
One factor proposed as a cause of latitudinal diversity gradients is the area of the climatic zones. Tropical landmasses have a larger climatically similar total surface area than landmasses at higher latitudes with similarly small temperature fluctuations (Rosenzweig, 1992). This may be related to higher levels of speciation and lower levels of extinction in the tropics (Rosenzweig, 1992; Gaston, 2000; Buzas et al., 2002). Moreover, most of the land surface of the Earth was tropical or subtropical during the Tertiary, which could in part explain the greater diversity in the tropics today as an outcome of historical evolutionary processes (Ricklefs, 2004).
The higher solar radiation in the tropics increases productivity, which in turn is thought to increase biological diversity. However, productivity can only explain why there is more total biomass in the tropics, not why this biomass should be allocated into more individuals, and these individuals into more species (Blackburn and Gaston, 1996). Body sizes and population densities are typically lower in the tropics, implying a higher number of species, but the causes and the interactions among these three variables are complex and still uncertain (Blackburn and Gaston, 1996).
Higher temperatures in the tropics may imply shorter generation times and greater mutation rates, thus accelerating speciation in the tropics (Rohde, 1992). Speciation may also be accelerated by a higher habitat complexity in the tropics, although this does not apply to freshwater ecosystems. The most likely explanation is a combination of various factors, and it is expected that different factors affect differently different groups of organisms, regions (e.g. northern vs. southern hemisphere) and ecosystems, yielding the variety of patterns that we observe.
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