General background, examples, exceptions
A central question in evolutionary ecology is: what are the reasons for differences in the abundance and diversity of organisms in different habitats and regions? Such differences are universal, i.e., it is highly unlikely that any two habitats will have the same number of species and organisms, and on a larger scale they are apparent between ecosystems and between latitudes, altitudes and different depths (in aquatic systems), as well as between different longitudes. By far the best-documented gradients are latitudinal ones, i.e., a very marked increase in diversity from high to low latitudes. An analysis of these gradients presents us with the opportunity to find a general explanation of the causes that determine diversity.
Latitudinal gradients in species diversity or better biodiversity were first described by Alexander von Humboldt in 1799 during his expedition in South America (Humboldt, 1808), and they have attracted the attention of numerous biologists since (for recent reviews and important papers see Rohde, 1992, 1999; Gaston, 2000; Willig 2001; Willig et al., 2003; Hillebrand, 2004; Mittelbach et al., 2007). They are known for most groups of plants and animals, although there are exceptions. A few examples are given in the following. According to Krebs (1985), the number of ant species at various localities is: Alaska 7 species, Utah 63, Cuba 101, and Brazil 222. Also according to Krebs, there are 22 species of snakes in Canada, 126 in the USA, and 293 in Mexico, and whereas a deciduous forest in Michigan, USA, contains 10–15 species on two hectares, a Malaysian tropical rainforest contains 227 in the same area. Wright (2002, references therein) gives even higher numbers for tropical rainforests: a 0.52 km2 plot in Borneo had 1175 species (with a diameter at breast height of at least 1 cm), and one hectare of Amazonian rainforests can have more than 280 species of trees in one hectare (with a diameter at breast height of at least 10 cm). In the oceans, latitudinal gradients are well documented for surface waters, but there is some evidence that they also occur in the deep sea (e.g., Lambshead et al., 2002).