Why are there so many species in the tropics? Effective evolutionary time

Tropical habitats, generally, have many more species than temperate and cold habitats. Such gradients are referred to as latitudinal gradients in species diversity. No agreement has been reached on the causes of the gradients. The hypothesis of effective evolutionary time (K. Rohde: Latitudinal gradients in species diversity: the search for the primary cause, Oikos, 65, 514-527,1992) attempts to explain diversity gradients, and in particular latitudinal gradients. It was originally named “time hypothesis” (K. Rohde: Latitudinal gradients in species diversity and their causes. I. A review of the hypotheses explaining the gradients. Biologisches Zentralblatt 97, 393-403, 1978, K. Rohde: Latitudinal gradients in species diversity and their causes. II. Marine parasitological evidence for a time hypothesis. Biologisches Zentralblatt 97, 405-418, 1978).


Low (warm) latitudes contain significantly more species than high (cold) latitudes. This has been shown for many animal and plant groups, although exceptions exist. An example of an exception is helminths (parasitic worms) of marine mammals, which have the greatest diversity in northern temperate seas, possibly because of small population densities of hosts in tropical seas that prevented the evolution of a rich helminth fauna, or because they originated in temperate seas and had more time for speciation there. It has become more and more apparent that species diversity is best correlated with environmental temperature and more generally environmental energy. These findings are the basis of the hypothesis of effective evolutionary time. Species have accumulated fastest in areas where temperatures are highest. Mutation rates and speed of selection due to faster physiological rates are highest, and generation times which also determine speed of selection, are smallest at high temperatures. This leads to a faster accumulation of species, which are absorbed into the abundantly available vacant niches, in the tropics. Vacant niches are available at all latitudes, and differences in the number of such niches can therefore not be the limiting factor for species richness.

The hypothesis of effective evolutionary time offers a causal explanation of diversity gradients, although it is recognized that many other factors can also contribute to and modulate them.

Historical aspects

Some aspects of the hypothesis are based on earlier studies. (B. Rensch: Neuere Probleme der Abstammungslehre. Die transspezifische Evolution. Encke, Stuttgart, 1954), for example, stated that evolutionary rates also depend on temperature: numbers of generation in poikilotherms, but sometimes also in homoiotherms, are greater at higher temperatures and the effectiveness of selection is therefore greater. Ricklefs refers to this hypothesis as “hypothesis of evolutionary speed“ oder “higher speciation rates“ (E. Ricklefs: Ecology. Nelson and Sons, London, 1973). Genera of Foraminifera in the Cretacious and families of Brachiopoda in the Permian have greater evolutionary rates at low than at high latitudes (F.G. Stehli, E.G. Douglas and N.D. Newell: Generation and maintenance of gradients in taxonomic diversity. Science 164, 947-949, 1969). That mutation rates are greater at high temperatures has been known since the classical investigations of N.W. Timofeeff-Ressovsky, K.G. Zimmer und M. Delbrück: Über die Natur der Genmutation und der Genstruktur. Nachrichten aus der Biologie der Gesellschaft der Wissenschaften Göttingen I, 189-245, 1935), although few later studies have been conducted. Also, these findings were not applied to evolutionary problems.

The hypothesis of effective evolutionary time differs from these earlier approaches as follows. It proposes that species diversity is a direct consequence of temperature-dependent processes and the time ecosystems have existed under more or less equal conditions. Since vacant niches into which new species can be absorbed are available at all latitudes, the consequence is accumulation of more species at low latitudes. All earlier approaches remained without basis without the assumption of vacant niches, as there is no evidence that niches are generally narrower in the tropics, i.e., an accumulation of species cannot be explained by subdivision of previously utilized niches. The hypothesis, in contrast to most other hypotheses attempting to explain latitudinal or other gradients in diversity, does not rely on the assumption that different latitudes or habitats generally have different “ceilings” for species numbers, which are higher in the tropics than in cold environments. Such different ceilings are thought to be, for example, determined by heterogeneity or area of the habitat. But such factors, although not setting ceilings, may well modulate the gradients.

A considerable number of recent studies, discussed in K. Rohde: Nonequilibrium Ecology, Cambridge University Press, Cambridge, 2005b, 223 pp., http://www.cambridge.org/9780521674553 support the hypothesis. Thus, diversity of marine benthos, interrupted by some collapses and plateaus, has risen from the Cambrian to the Recent, and there is no evidence that saturation has been reached (D.Jablonski: The future of the fossil record, Science 284, 2114-2116, 1999). Rates of diversification per time unit for birds and butterflies increase towards the tropics (M. Cardillo: Latitude and rates of diversification in birds and butterflies. Proceedings of the Royal Society London 266, 1221-1225,1999). Allen et al. found a general correlation between environmental temperature and species richness for North and Central American trees, for amphibians, fish, Prosobranchia and fish parasites. They showed that species richness can be predicted from the biochemical kinetics of metabolism, and concluded that evolutionary rates are determined by generation times and mutation rates both correlated with metabolic rates which have the same Boltzmann relation with temperature. They further concluded that these findings support the mechanisms for latitudinal gradients proposed by Rohde (A.P. Allen, J.H. Brown, and J.F. Gillooly: Global biodiversity, biochemical kinetics, and the energetic-equivalence rule. Science, 297, 1545-1548, 2002). Gillooly et al. (2002) described a general model also based on first principles of allometry and biochemical kinetics which makes predictions about generation times as a function of body size and temperature (J.F. Gillooly, E.L. Charnov, G.B. West, M.Van Savage, and J.H. Brown: Effects of size and temperature on developmental time. Nature 417, 70–73, 2002). Empirical findings support the predictions: in all cases that were investigated (birds, fish, amphibians, aquatic insects, zooplankton) generation times are negatively correlated with temperature. Brown et al.(2004) further developed these findings to a general metabolic theory of ecology (J.H. Brown, J.F. Gillooly, A.P. Allen, M. Van Savage, and G.. West,. (2004). Toward a metabolic theory of ecology. Ecology 85, 1771-1789). Indirect evidence points to increased mutation rates at higher temperatures (C. Bazin, P. Capy, D. Higuet, and T. Langin, T.: Séquences d’AND mobiles et évolution du génome. Pour Sci., Hors. Sér. Janvier 97, 106-109., 1997), and the energy-speciation hypothesis is the best predictor for species richness of ants (M. Kaspari, P.S. Ward and M.Yuan: Energy gradients and the geographical distribution of local ant diversity. Oecologia 140, 407-413, 2004). Finally, computer simulations using the Chowdhury eosystem model have shown that results correspond most closely to empirical data when the number of vacant niches is kept large (K. Rohde and D. Stauffer: “Simulation of geographical trends in Chowdhury ecosystem model”, Advances in Complex Systems 8, 451-464, 2005). For a detailed discussion of these and other examples see K. Rohde: Nonequilibrium Ecology, Cambridge University Press, Cambridge, 2005b, 223 pp., http://www.cambridge.org/9780521674553 and K. Rohde: Eine neue Ökologie. Aktuelle Probleme der evolutionären Ökologie”. Naturwissenschaftliche Rundschau, 58, 420-426, 2005.

Depth gradients

Species diversity in the deepsea has been largely underestimated until recently (e.g., Briggs 1994: total marine diversity less than 200,000 species) (J. C.Briggs. Species diversity: land and sea compared. Systematic Biology 43, 130-135, 1994). Although our knowledge is still very fragmentary, some recent studies appear to suggest much greater species numbers (e.g., Grassle and Maciolek 1992: 10 million macroinvertebrates in soft bottom sediments of the deepsea) (J. F. Grassle and N. J. Maciolek: Deepsea species richness: regional and local diversity estimates from, quantitative bottom samples. American Naturalist 139, 313-341, 1992). Further studies must show whether this can be verified (K. Rohde: Ecology and biogeography of marine parasites. Advances in marine biology 43,1-86, 2002). A rich diversity in the deepsea can be explained by the hypothesis of effective evolutionary time: although temperatures are low, conditions have been more or less equal over large time spans, certainly much larger than in most or all surface waters.

This post is based on an earlier article of mine in Wikipedia. See also post on vacant niches.

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