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Different publications' theories regarding the latitudinal diversity gradient were examined and analysed to attempt to show how theories explaining the latitudinal diversity gradient have changed in the period 1980-2010.
In total, six main theories were identified from the literature and resources studied by searching abstracts and articles for relevant key words.
The main conclusion drawn was that no single theory can adequately explain the latitudinal diversity gradient. Species diversity is ultimately controlled by a combination of factors and until there is conclusive proof or agreement on this subject biogeographers and biologists will continue to hypothesize on the matter of the driving factors behind the latitudinal diversity gradient.
Latitudinal Diversity Gradient, Geographical, Species, Species Richness, Taxa, Hypothesis,
Biotic, Historical Perturbation, Climate Stability, Climate Harshness, Mid-Domain, Evolutionary Rate, Evolution.
The latitudinal diversity gradient is the term used to describe the decrease in species richness as one moves away from the equator.  Discovered by Alexander von Humboldt in 1799, it has remained one of the key questions in Evolutionary Ecology. Understanding the latitudinal diversity gradient is essential in our understanding of the spread of invasive species, disease and more pertinently, the effects of global climate change (Bradford et al 2006) Hillebrand (2004) identified that this subject has received great attention but the majority of studies have concentrated on only one or a small number of organisms. An example of this being Krebs (1985) who examined snake and ant species throughout America. In the past 30 years there have been many different theories that attempt to explain the latitudinal diversity gradient, from further reading into each individual theory it became apparent that no individual theory is entirely self supporting, rather a combination of each is perhaps the best way to try and answer the question of what is the driving factor behind the latitudinal diversity gradient.
The species-energy theorem centres on the hypothesis that it is the amount of available energy that governs the potential species richness of the system. Fraser and Currie examined this hypothesis in their 1995 paper 'The Species Richness-Energy Hypothesis in a System Where Historical Factors are Thought to Prevail: Coral Reefs.' They found that the best environmental predictors of diversity were temperature and biomass and also that there was little supporting evidence for other hypotheses such as environmental stability. Turner et al (1987) found that the diversity of butterflies, measured as the number of species is highly correlated with sunshine and temperature during May to September, with these two variables accounting for nearly 80% of the variance in diversity.
Originally proposed by Colwell and Hurtt, (1994) this hypothesis works on the basis that it is geographical constraints that contribute to species richness. Colwell et al (2004) state that if species' ranges are shuffled randomly within a bounded geographical domain free of environmental gradients, ranges overlap increasingly toward the centre of the domain, creating a mid domain peak of species richness. There is still debate over whether or not the Mid Domain Effect is responsible for the latitudinal diversity gradient as empirical support for it is often weak. Zapata et al (2005)
Effective Evolutionary Time
Effective evolutionary time hypothesizes that it is evolutionary time and the factors associated with it, such as environmental energy, mutation, generations and selection that are responsible for the diversity gradient. Rhode (1992)
Climate Harshness and Climate Stability
The Climate Harshness theory speculates that fewer species are found at high latitudes due to their inability to cope with the pressures that the environment places upon them.
Climate Stability proposes that the reason for the diversity gradient is due to species specializing into narrower and narrower niches due to stable climate conditions, resulting in increased speciation and therefore resulting in temperate areas at high latitude having a low diversity as they experience more changeable weather events throughout the year. Climate stability is even observed in marine environments with strong latitudinal gradients being observed. Kaustuv et al (1999) as well as in terrestrial environments. Lima-Ribeiro et al (2009).
Cardillo et al (2005) argue that the increased diversity observed at lower latitudes is due to high evolutionary rates resulting in increased speciation. Mettelbach et al (2007) found that there was evidence for higher rates of diversification in the tropics, with studies of latitudinal variation suggesting greater speciation at lower latitudes. The results from these studies show that there is indeed a link between low latitudes and evolutionary rate resulting in increased speciation.
Since the 1980's there have been suggestions of new theories and re examinations of others, for example Zapata et al (2005) re examine the mid domain effect and address its criticisms. Each of the individual theories discussed earlier proposes its own explanation for the latitudinal diversity gradient. Some hypotheses are circular and some are founded on insufficient evidence, Rhode (1992). In the past 30 years theories purporting to explain the latitudinal diversity gradient have changed from supporting one viewpoint as the sole cause of the latitudinal diversity gradient to being more inclusive of other hypotheses, the Evolutionary time theory is the most inclusive of other theories as it recognises that no single factor can be the cause of such a complex system.
References and Literature Cited
1. On-Line Biology Book, L, Latitudinal diversity gradient
Accessed on 17-08-2010
Hawkins, B.A, Diniz-Filho, J.A.F, Jaramillo, C.A., Soeller, S.A., 2006. Post-Eocene climate change, niche conservatism, and the latitudinal diversity gradient of New World birds. Journal of Biogeography.
Cardillo, M. Orme, C. D. L., Owens, I.P.F. 2005. Testing for Latitudinal Bias in Diversification Rates: An Example Using New World Birds. Ecology.
Colwell, R.K. Hurtt, G.C 1994. Nonbiological Gradients in Species Richness and a Spurious Rapoport Effect. The American Naturalist, Vol. 144, No. 4 October 1994
Colwell, R,K. Rahbeck,C. Gotelli, N,J. 2004 The Mid-Domain Effect and Species Richness Patters: What have we learned so far? Vol. 163. No.3 March 2004
Fraser, R.H, Currie, D.J. 1995. The Species Richness-Energy Hypothesis in a System Where Historical Factors are Thought to Prevail: Coral Reefs. The American Naturalist Vol.148, No. 1 July 1996
Hillebrand, H. 2004. On the Generality of the Latitudinal Diversity Gradient. The American Naturalist, Vol 163, No. 2. 2004.
Kaustuv, R., Jablonski, D., Valentine, J. W. 1999. Dissecting Latitudinal Diversity Gradients: functional groups and clades of marine bivalves. The Royal Society
Krebs, C.J.1985. Ecology. The Experimental Analysis of Distribution and Abundance. Third edition. Harper&Row, New York.
Lima-Ribeiro, M., Diniz-Filho, J. A. F., Barberi, M. 2010. Climate Stability and the Current Patterns of Terrestrial Vertebrate Species Richness on the Brazilian Cerrado. Quaternary International, Volume 222, Issue 1-2. August 2010
Mittelbach, G. G., Schemske, D. W., Cornell, H. V., Allen, A. P., Brown, J. M., Bush, M. B., Harrison, S. P., Hurlbert, A. H., Knowlton, N., Lessios, H. A., McCain, C. M., McCune, A. R., McDade, L. A., McPeek, M. A., Near, T. J., Price, T. D., Ricklefs, R. E., Roy, K., Sax, D. F., Schluter, D., Sobel, J. M. and Turelli, M. 2007. Evolution and the latitudinal diversity gradient: speciation, extinction and biogeography. Ecology Letters
Rhode, K. 1992. Latitudinal Gradients in Species Diversity: The search for the primary cause. Oikos. Vol. 65, No. 3. December 1992
Turner, J.R.G. Gatehouse, C.M and Corey, C.A. 1987. Does Solar Energy Control Organic Diversity? Butterflies, Moths and the British Climate. Oikos, Vol. 48. No.2
Zapata, F.A., Gaston, K. J., Chown, S. L. 2005 The Mid-Domain Effect Revisited. The American Naturalist, Vol. 166, No. 5 November 2005.