Latitudinal gradient in species richness

Abstract

What is the gradient? Amongst the earliest remarked, and best documented, of broad-scale spatial patterns of life on Earth is a trend for more species to be found towards lower latitudes. The latitudinal gradient in species richness is such that most extant eukaryote, perhaps also prokaryote, species are found in the tropics. Even when counted over quite small areas in which it is present, the species richness of a taxonomic group might commonly vary by orders of magnitude between high and low latitudes.How general is the gradient? The latitudinal gradient in species richness occurs in terrestrial, freshwater and marine realms, is repeated for local and regional assemblages, and has been documented for a wide range of taxonomic groups, including viruses, bacteria, plants, arthropods and vertebrates. There are some exceptions, but these tend foremost to concern relatively species-poor taxonomic groups. This said, for any group typically only at rather coarse spatial resolutions is the gradient one of relatively continuous increase in richness towards lower latitudes. More usually, it is quite variable, with substantial regions in which richness can be notably high or low in comparison with the broad trend, and the peak often being displaced from the equator. In particular, the gradient can be influenced by the shape of continents or ocean basins, marked discontinuities in environmental conditions, and major historical events (such as periods of glaciation). It is helpful to consider the gradient in the context of the wider patterns of spatial variation in richness of which it is a part, rather than in isolation.How long has the gradient persisted? It is strongly suspected that the latitudinal gradient in species richness has been a persistent feature of life on Earth for much of its history, although obviously the positions of land and water masses have shifted, and the geographic pattern of environmental conditions has altered dramatically. There is evidence, however, that for some particular groups of organisms that remain quite speciose the latitudinal gradient has become steeper through evolutionary time.Why does the gradient occur? Ultimately, the latitudinal gradient in species richness must be a consequence of a greater period of net diversification in the tropics, likely following the origins of life there, or higher speciation rates and/or lower extinction rates at low latitudes compared with other regions. There is evidence that many temperate clades had their origins in the tropics. The prevailing view is that the tropics seem also to have been a ‘cradle of diversity’ (with high speciation rates), and that they have served as a ‘museum of diversity’ (with low extinction rates). The balance between the two rates is not well understood, and is a topic of much research interest. Some empirical evidence suggests that rates of speciation may have been lower at low latitudes but, accompanied by sufficiently low extinction rates, have nonetheless led to higher richness in the tropics.The proximate reasons for any latitudinal differences in speciation and extinction rates continue to be much debated. It is difficult to disentangle the potential influences of the many abiotic and biotic variables that show systematic patterns of variation with latitude. In the absence of substantial constraints on water supply, high speciation rates in the tropics may have resulted from higher and more stable temperatures leading to high biological rates, and hence shorter generation times and higher mutation rates. Extinction rates may be lower in the tropics because of environmental conditions that support a greater and more stable biomass, enabling more species to persist at low abundances. Speciation rates may also have been heightened and extinction rates reduced by the greater extent of the tropics in the past, and the accumulation of species at higher latitudes may have been reduced by the historical adaptation of most clades to tropical environments.What are the consequences of the gradient for conservation? Perhaps the most obvious consequence of the latitudinal gradient in species richness is that maintenance of the absolute numbers of species on Earth, or at least reduction of their current rates of loss, requires disproportionate attention to be paid to conservation activities in the tropics. Indeed, global schemes for identifying priority areas for conservation almost invariably place the majority of these in the tropics. A high proportion of tropical species tend also to have rather limited geographic distributions, which means that conservation efforts need to be dispersed throughout much of the tropics.Are there other significant implications of the gradient? The vast majority of studies in biology have been conducted on organisms from mid- to high northern latitudes. This means that they have usually concerned species that exist under conditions, and perhaps face challenges, that are very different from those relevant to most extant species. The consequences for understanding of many facets of biology could be profound.