Species richness, or biodiversity, increases from the poles to the tropics for a wide variety of terrestrial and marine organisms, often referred to as the latitudinal diversity gradient. The latitudinal diversity gradient is one of the most widely recognized patterns in ecology. It has been observed to varying degrees in Earth's past. A parallel trend has been found with elevation (elevational diversity gradient), though this is less well-studied. Explaining the latitudinal diversity gradient has been called one of the great contemporary challenges of biogeography and macroecology (Willig et al. 2003, Pimm and Brown 2004, Cardillo et al. 2005). The question "What determines patterns of species diversity?" was among the 25 key research themes for the future identified in 125th Anniversary issue of Science (July 2005). There is a lack of consensus among ecologists about the mechanisms underlying the pattern, and many hypotheses have been proposed and debated. A recent review noted that among the many conundrums associated with the latitudinal diversity gradient (or latitudinal biodiversity gradient) the causal relationship between rates of molecular evolution and speciation has yet to be demonstrated. Understanding the global distribution of biodiversity is one of the most significant objectives for ecologists and biogeographers. Beyond purely scientific goals and satisfying curiosity, this understanding is essential for applied issues of major concern to humankind, such as the spread of invasive species, the control of diseases and their vectors, and the likely effects of global climate change on the maintenance of biodiversity (Gaston 2000). Tropical areas play prominent roles in the understanding of the distribution of biodiversity, as their rates of habitat degradation and biodiversity loss are exceptionally high. Patterns in the past The latitudinal diversity gradient is a noticeable pattern among modern organisms that has been described qualitatively and quantitatively. It has been studied at various taxonomic levels, through different time periods and across many geographic regions (Crame 2001). The latitudinal diversity gradient has been observed to varying degrees in Earth's past, possibly due to differences in climate during various phases of Earth's history. Some studies indicate that the gradient was strong, particularly among marine taxa, while other studies of terrestrial taxa indicate it had little effect on the distribution of animals. Hypotheses for pattern Although many of the hypotheses exploring the latitudinal diversity gradient are closely related and interdependent, most of the major hypotheses can be split into three general hypotheses. Spatial/Area hypotheses There are five