Biogeography is defined as the study of life, in a spatial and temporal context, with respect to the analysis and explanation of patterns for a given area. The tendency for species richness and diversity to increases towards the equator, where both peak, is a much debated and tested pattern. Underlying mechanisms thought to cause this pattern are: gradients in temperature, stress, productivity, competition, predation, stability, effective evolutionary time, niche breadth, range size and area of occupancy. Evidence exists that both supports and negates most of these mechanisms. In addition to the richness gradient, a latitudinal gradient in geographical range size exists, whereby species range sizes decrease with latitude, referred to as Rapoport's Rule. This has been linked to species ability to tolerate changes in climate. The latitudinal gradient in species richness is thought to be a by-product of Rapoport's Rule and the "Rescue Effect".