Latitudinal gradients

An increase in numbers of species as one proceeds from high latitudes toward the tropics is one of the most general of geographic patterns of species richness, and it unquestionably is true for the fishes of running waters. Despite the inadequate state of taxonomic knowledge, well over 3,000 species of freshwater fish are estimated to occur in tropical South America, primarily in riverine habitats (Moyle and Cech 2006). This greatly exceeds the roughly 700 species found in the lakes and rivers of temperate North America and 250 species of Europe. Spe cies richness is highly correlated with basin area for rivers worldwide, and stronger statistical relationships are observed when analyzed separately by continent (Amarasinghe and Wel-comme 2002). The exponent of the species-area relationship always is greater in tropical than temperate regions (0.25 for Europe, 0.26 for Asia, 0.49 for Africa, and 0.51 for South America), demonstrating a trend toward a more rapid increase in species richness with increasing river size at low latitudes.

Surprisingly, no one has yet established convincingly that a similar latitudinal trend exists in numbers of invertebrate species, and it has been suggested that the biota of streams might be an exception to the general trend toward more species in the tropics (Patrick 1966). Whether stream insects contradict (Flowers 1991) or support (Stout and Vandermeer 1975, Jacobson et al. 1997) this pattern remains controversial. For a very large data set of mayflies, stoneflies, and caddisflies, Vinson and Hawkins (2003) found no simple latitudinal gradient other than a decline in diversity at very high latitudes, and instead noted taxon-specific peaks and troughs at particular latitudes. Some of this variation likely is due to incomplete sampling, and some to areas of radiation and spread. Because tropical studies are few and taxonomic knowledge is limited, the absence of a latitudinal trend in the invertebrate diversity of streams is far from established. It is interesting to speculate that the spectacular diversity of tropical fishes, many of which are insectivores, could represent a constraint on insect diversity in tropical streams.

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