The latitude-niche width hypothesis claims that niches of species are narrower in high diversity tropical than in low diversity cold-temperate regions.
A priori reasons for the hypothesis
The hypothesis is implicit in equilibrium ecology, i.e., it is a consequence of the view that habitats are generally saturated with individuals and species and that a greater number of species can be accommodated in a habitat only if their niches are narrower (e.g., Rosenzweig and Ziv 1999 ). Since tropical habitats are generally much more species-rich than cold-temperate ones (“latitudinal gradients in species diversity”), niches in the former therefore must be generally narrower.
Assumptions on which the hypothesis are based
The hypothesis is based on two widely accepted assumptions (e.g., May 1973 ), i.e., that there is a latitudinal gradient in population variability, and that there is a relationship between population variability and niche breadth. In other words, the tropics have greater stability and less seasonality than temperate regions, which makes populations there more stable and thereby permits narrower niches.
History of the concept
The hypothesis can be traced back to MacArthur (MacArthur 1965 , 1969 , 1972 , MacArthur and Wilson 1967 ), but is possibly even older.
Empirical evidence for and against the hypothesis
MacArthur (1965 , 1969 ) and Moore (1972 ), among others, have provided some evidence for the hypothesis; others (e.g., Rohde 1980 , Novotny and Basset 2005 ) have given evidence against it. A niche has many dimensions which can vary with latitude. We discuss some aspects of the niche that have been examined in this regard.
Latitudinal ranges (Rapoport’s rule).
Stevens (1989 ) provided evidence that latitudinal ranges of some groups of plants and animals are narrower in the tropics. Rohde et al. (1993 ) have shown that for marine teleosts at least, niches are wider and not narrower in the tropics than at higher latitudes; however, freshwater teleosts show an increase in latitudinal ranges with latitude, but only above latitudes of about 40oN . Numerous subsequent studies examining the rule have been made, some providing support for it, but an even greater number failing to find support or even finding support for an opposite trend. Rohde (1998 ) therefore suggested two opposing trends: newly evolved species with little vagility may have narrower ranges in the tropics, species with greater vagility and of sufficient age to have spread into adjacent areas may have larger ranges in the tropics. In those species that have larger latitudinal ranges in cold regions, the increase is often restricted to high latitudes above approximately 40-50° N and S (review in Rohde 1999 ); Rohde (1996 ) concluded that the rule describes a local phenomenon, the result of the extinction of species with narrow ranges at very high latitudes during the ice ages. Stauffer and Rohde (2006 ), using the Chowdhury Ecosystem Model, also failed to find support for the rule. For a review see Rohde (1999 ) (for further details on Rapoport’s rule see the knol Rapoport’s rule).
According to Moore (1972 ), the average tropical species occupies about half as much of the intertidal zone as the average temperate species. According to MacArthur (1965 , 1969 ), tropical species often have a spottier distribution than high- latitude ones. Several authors (e.g. Beaver 1979 , review in Novotny and Basset 2005 ) have studied possible differences in host specificity of herbivorous insects in tropical and high latitude habitats. No major differences were found. – Rohde (1978 ) has shown that host ranges (the number of host species infected) of monogenean flatworms infecting the gills of marine fish are more or less the same at all latitudes, whereas host ranges of another group of (endoparasitic) flatworms, the Digenea, are markedly greater at high latitudes. After correction for intensity and prevalence of infection, however, host specificity was the same and very high at all latitudes for both groups (Rohde 1980 ). Other niche dimensions of these parasites, such as geographical range and microhabitat width, were also examined and found not to be correlated with diversity, although the data sets were small and more studies are needed (Rohde 1989 ). Lappalainen and Soininen (2006 ) found that the regional occupancy of fish species in freshwater was more strongly governed by the habitat position than the habitat breadth. The cool water species (percids and cyprinids) showed significant decrease in habitat breadth towards higher latitudes (and not towards lower latitudes, expected by the latitude-niche breadth hypothesis). Stauffer at al. (2007 ), using the Chowdhury ecosystem model, have show that tropical species with sufficient vagility and time to spread into adjacent habitats, tend to have wider habitats than high latitude ones, contradicting the latitude-niche breadth hypothesis.
Metanalysis (evaluation of all previous studies considered to be relevant)
Vázquez and Stevens (2004 ) made a thorough meta-analytical study of the evidence for the latitude-niche breadth hypothesis, concluding that the null hypothesis (i.e, that there is no correlation between latitude and niche width) cannot be rejected. They also critically examined the two assumptions on which MacArthur’s hypothesis is based, i.e., that there is a latitudinal gradient in population variability, and that there is a relationship between population variability and niche breadth, and found no convincing evidence for either of these assumptions. Indeed, as shown by Rohde (1992 ), there may be extreme variations (occurring over very short time spans of a few hours) in temperature, salinity and currents in tropical shallow waters, such as high diversity coral reefs.
Flawed a priori assumptions of the hypothesis
As pointed out above, the latitude-niche breadth hypothesis makes equilibrium assumptions, implicitly and explicitly assuming that niche space is more or less saturated with species. However, there is much evidence for an overabundance of vacant niches and that most ecological including tropical systems are far from saturation (for a discussion and examples see Rohde 2005  and the knol vacant niches).
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Competitive exclusion, vacant niches, niche restriction and segregation, effective evolutionary time, latitudinal gradients in species diversity