Boyero L (2012) Latitudinal Gradients in Biodiversity. ECOLOGY.INFO 32

Latitudinal Gradients in Biodiversity

Luz Boyero
School of Tropical Biology
James Cook University
Australia

Note: This online review is updated and revised continuously, as soon as results of new scientific research become available.  It therefore presents state-of-the-art information on the topic it covers.

The exuberant tropics: truth or myth?

The word 'tropical' usually evokes images of a dense jungle packed with all sorts of different creatures, or a coral reef full of fishes of many different colours. However, 'the tropics', i.e. the zone extending approximately 30º north and south of the Equator (Pringle 2000), includes also huge extents of desert and savannah which appear, on the contrary, to contain low numbers and diversities of living organisms.

Are then, the tropics, more biologically diverse than other climatic zones? In fact, this is the most universally accepted and oldest recognized pattern in ecology (Hawkins 2001). The tropics have extraordinarily high species richness or, viewed from a different perspective, areas outside the tropics have extraordinarily low species richness (Blackburn and Gaston 1996).

Generality of latitudinal diversity gradients

Despite this recognition of the generality of latitudinal diversity gradients, our knowledge is biased towards some taxonomic groups, regions and ecosystems. First, studies have been biased towards vertebrates, which make up less than 5% of species on Earth. For example, many mammals peak in species richness in the tropics (Kaufman 1995), while some insect groups show reversed latitudinal gradients (Kouki et al. 1994).

Second, diversity gradients described for the northern hemisphere seem to be invalid for the southern hemisphere (Platnick 1991, Boyero 2002). For example, terrestrial vertebrates are more species rich in Central America than North America, but that is not the case in tropical versus temperate Australia (Schall and Pianka 1978). Species richness of Odonata per unit area is similar in tropical and temperate South America, while it is about 29 times greater in Central America than in North America (Boyero 2002). The so-called 'boreal bias' (Platnick 1991) exists because the vast majority of studies have been performed by ecologists from North America and Europe.

Third, most of the available information comes from terrestrial or marine ecosystems, while fresh waters have received little attention, even though they contain 20% of the Earth's vertebrate species (Rohde 1998). Nevertheless, available data show that freshwater fish and macroinvertebrates are more diverse in the tropics. For example, the number of fish species in tropical lakes far exceeds that of temperate lakes (e.g. 1450 species in the lakes Victoria, Tanganyika and Malawi versus 212 species in the North American Great Lakes and Lake Baikal; Rohde 1998).

A similar pattern is found in rivers (e.g. 2000 species in the Amazon and 700 in the Congo, versus 250 species in the Mississippi and 70 in the Danube, Pringle 2000). Some stream macroinvertebrates are more diverse in the tropics in Australia (Boulton et al. 2005) and America (e.g. 25 species of Odonata and 32 of Ephemeroptera per unit area (10⁶ km²) in North America, versus 717 Odonata and 206 Ephemeroptera in Central America; Boyero 2002).

Regional vs. local diversity

When considering any ecological pattern (Levin 1992) and, specifically, latitudinal trends in species richness, spatial scale is a fundamental factor to take into account (Lyons and Willig 1999, Kaspari et al. 2003, Rahbeck 2005). The two main approaches to diversity patterns are the study of regional diversity –the number of species within a region – and that of local diversity – the number of species within a site or locality.

The number of species within a biome, a continent, or a climatic zone are examples of regional diversity, which is determined by regional factors such as geology, climate, migration, or extinction. On the other hand, local diversity refers to the number of species within a patch of, for example, coral or forest, or in a stream reach. Local diversity is usually (but not always) strongly related to regional diversity (Caley and Schluter 1997), which determines the pool of available species. However, members of the species pool may or may not be present at a site depending on local factors such as habitat structure, productivity, disturbance, or biotic interactions (Rohde 1992, Rosenzweig and Abramsky 1993).

For example, in streams, regional diversity tends to be higher in the tropics (Boyero 2002, Boulton et al. 2005), while local diversity is fairly constant across latitudes, or does not show a clear latitudinal gradient (Vinson & Hawkins 2003). This may be explained by a higher turnover or replacement of species across sites in the tropics, as has been demonstrated for macroinvertebrates (Lake et al. 1994) and suggested for stream frogs (Boulton et al. 2005). While biogeographic processes may generate higher species richness in the tropics for many taxonomic groups, the highly variable and unpredictable stream habitat imposes an upper limit to local diversity and homogenizes the number of local species around the world (Boulton et al. 2005). This habitat, although highly heterogeneous at relatively small scales (Boyero 2003, Boyero and Bosch 2004), is extraordinarily similar around the world (Hynes 1970), which makes streams ideal to test hypotheses about broad-scale diversity and ecological gradients (Vinson and Hawkins 2003). Nevertheless, local diversity of stream fishes seems to have stronger regional influences (Angermeier and Winston 1998), and the same occurs with many groups of organisms in terrestrial and marine ecosystems (Caley and Schluter 1997).

The causes for latitudinal gradients in biodiversity

The determinant of biological diversity is, clearly, not latitude per se, but the environmental variables correlated with latitude. More than 25 different mechanisms have been suggested for generating latitudinal diversity gradients, but no consensus has been reached yet (Gaston 2000).

One of the factors proposed as a cause of latitudinal diversity gradients is the area of the climatic zones. Tropical land masses have a larger climatically similar total surface area than land masses at higher latitudes with similarly small temperature fluctuations (Rosenzweig 1992). This may be related to higher levels of speciation and lower levels of extinction in the tropics (Rosenzweig 1992, Gaston 2000, Buzas et al. 2002). Moreover, most of the land surface of the Earth was tropical or subtropical during the Tertiary, which could in part explain the greater diversity in the tropics today as an outcome of historical evolutionary processes (Ricklefs 2004).

The higher solar radiation in the tropics increases productivity, which in turn is thought to increase biological diversity. However, productivity can only explain why there is more total biomass in the tropics, not why this biomass should be allocated into more individuals, and these individuals into more species (Blackburn and Gaston 1996). Body sizes and population densities are typically lower in the tropics, implying a higher number of species, but the causes and the interactions among these three variables are complex and still uncertain (Blackburn and Gaston 1996).

Higher temperatures in the tropics may imply shorter generation times and greater mutation rates, thus accelerating speciation in the tropics (Rohde 1992). Speciation may also be accelerated by a higher habitat complexity in the tropics, although this does not apply to freshwater ecosystems. The most likely explanation is a combination of various factors, and it is expected that different factors affect differently different groups of organisms, regions (e.g. northern versus southern hemisphere) and ecosystems, yielding the variety of patterns that we observe.

The importance of understanding latitudinal gradients in biodiversity

Understanding the global distribution of biodiversity is one of the most significant objectives for ecologists and biogeographers (Gaston 2000). But, beyond purely scientific goals, this understanding is essential for applied issues of major concern to humankind, such us the spread of alien invasive species, the control of diseases and their vectors, and the likely effects of global environmental change on the maintenance of biodiversity (Gaston 2000).

Tropical areas, usually located in developing countries, play a prominent role in this picture, as their rates of habitat degradation and biodiversity loss are exceptionally high. Just as very little information existed on ‘natural’ conditions of temperate ecosystems before they were dramatically altered, this information is very scarce today for the tropics. The difference is that, today, it is not too late to collect this information (Pringle 2000).

References

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Boulton AJ, Boyero L, Covich AP, Dobson MK, Lake PS, Pearson RG (2005) Are tropical streams ecologically different from temperate streams? In Tropical Stream Ecology. Dudgeon D, Cressa C (eds.). Academic Press, San Diego (Aquatic Ecology Series) (in press)

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The author is:  Dr. Luz Boyero (PhD in Biology)