Although many of the
hypotheses exploring the latitudinal diversity gradient are closely related and interdependent, most of the major hypotheses can be split into three general hypotheses.
Spatial/Area hypotheses There are five major hypotheses that depend solely on the spatial and areal characteristics of the
tropics.
Mid-domain effect Using
computer simulations, Colwell and Hurtt (1994) and Willing and Lyons (1998) first pointed out that if species' latitudinal ranges were randomly shuffled within the
geometric constraints of a bounded biogeographical
domain (e.g. the continents of the
New World, for
terrestrial species), species' ranges would tend to overlap more toward the center of the domain than towards its limits, forcing a
mid-domain peak in species richness. Colwell and Lees (2000) called this
stochastic phenomenon the
mid-domain effect (MDE), presented several alternative analytical formulations for one-dimensional MDE (expanded by Connolly 2005), and suggested the hypothesis that MDE might contribute to the latitudinal gradient in species richness, together with other explanatory factors considered here, including
climatic and historical ones. Because "pure" mid-domain models attempt to exclude any direct environmental or evolutionary influences on species richness, they have been claimed to be
null models (Colwell et al. 2004, 2005). On this view, if latitudinal gradients of species richness were determined solely by MDE, observed richness patterns at the biogeographic level would not be distinguishable from patterns produced by random placement of observed ranges. Others object that MDE models so far fail to exclude the role of the
environment at the population level and in setting domain boundaries, and therefore cannot be considered null models (Hawkins and Diniz-Filho 2002; Hawkins et al. 2005; Zapata et al. 2003, 2005). Mid-domain effects have proven controversial (e.g. Jetz and Rahbek 2001, Koleff and Gaston 2001, Lees and Colwell, 2007, Romdal et al. 2005, Rahbek et al. 2007, Storch et al. 2006; Bokma and Monkkonen 2001, Diniz-Filho et al. 2002, Hawkins and Diniz-Filho 2002, Kerr et al. 2006, Currie and Kerr, 2007). While some studies have found evidence of a potential role for MDE in latitudinal gradients of species richness, particularly for
wide-ranging species (e.g. Jetz and Rahbek 2001, Koleff and Gaston 2001, Lees and Colwell, 2007, Romdal et al. 2005, Rahbek et al. 2007, Storch et al. 2006; Dunn et al. 2007) others report little correspondence between predicted and observed latitudinal diversity patterns (Bokma and Monkkonen 2001, Currie and Kerr, 2007, Diniz-Filho et al. 2002, Hawkins and Diniz-Filho 2002, Kerr et al. 2006).
Geographical area hypothesis Another spatial hypothesis is the geographical area hypothesis (Terborgh 1973). It asserts that the tropics are the largest
biome and that
large tropical areas can support more species. More area in the tropics allows species to have
larger ranges and consequently larger
population sizes. Thus, species with larger ranges are likely to have lower
extinction rates (Rosenzweig 2003). Additionally, species with larger ranges may be more likely to undergo
allopatric speciation, which would increase rates of speciation (Rosenzweig 2003). The combination of lower extinction rates and high rates of speciation leads to the high levels of
species richness in the tropics. A critique of the geographical area hypothesis is that even if the tropics is the most extensive of the biomes, successive biomes north of the tropics all have about the same area. Thus, if the geographical area hypothesis is correct, these regions should all have approximately the same species richness, which is not true, as is referenced by the fact that
polar regions contain fewer species than
temperate regions (Gaston and Blackburn 2000). To explain this, Rosenzweig (1992) suggested that if species with partly tropical distributions were excluded, the richness gradient north of the tropics should disappear. Blackburn and Gaston 1997 tested the effect of removing tropical species on latitudinal patterns in
avian species richness in the
New World and found there is indeed a relationship between the land area and the species richness of a biome once predominantly tropical species are excluded. Perhaps a more serious flaw in this hypothesis is some biogeographers suggest that the terrestrial tropics are not, in fact, the largest biome, and thus this hypothesis is not a valid explanation for the latitudinal species diversity gradient (Rohde 1997, Hawkins and Porter 2001). In any event, it would be difficult to defend the tropics as a "biome" rather than the geographically diverse and disjunct regions that they truly include. The effect of area on biodiversity patterns has been shown to be scale-dependent, having the strongest effect among
species with small geographical ranges compared to those species with large ranges who are affected more so by other factors such as the
mid-domain and/or
temperature. argues higher
evolutionary rates due to shorter generation times in the tropics have caused higher
speciation rates and thus increased
diversity at low latitudes. and increased selection pressure from other species that are themselves evolving. Faster rates of
microevolution in
warm climates (i.e. low latitudes and altitudes) have been shown for
plants,
mammals,
birds,
fish and
amphibians.
Bumblebee species inhabiting lower, warmer elevations have faster rates of both
nuclear and
mitochondrial genome-wide
evolution. Based on the expectation that faster rates of microevolution result in faster rates of speciation, these results suggest that faster evolutionary rates in warm climates almost certainly have a strong influence on the latitudinal diversity gradient. However, recent evidence from marine fish and
flowering plants have shown that rates of speciation actually decrease from the
poles towards the
equator at a global scale. Understanding whether
extinction rate varies with
latitude will also be important to whether or not this hypothesis is supported.
The hypothesis of effective evolutionary time The hypothesis of
effective evolutionary time assumes that diversity is determined by the evolutionary time under which
ecosystems have existed under relatively unchanged conditions, and by
evolutionary speed directly determined by the effect of
temperature on
mutation rates,
generation times, and
speed of selection.
The integrated evolutionary speed hypothesis The integrated evolutionary speed hypothesis argues that species diversity increases due to faster rates of
genetic evolution and speciation at lower latitudes where ecosystem productivity is generally greater. It differs from the effective evolutionary time hypothesis by recognizing that species richness generally increases with increasing ecosystem productivity and declines where high environmental energy (temperature) causes water deficits. It also proposes that evolutionary rate increases with population size,
abiotic environmental heterogeneity, environmental change and via
positive feedback with biotic heterogeneity. There is considerable support for faster rates of genetic evolution in warmer environments, and for a slower rate among bird species with small population sizes. Many aspects of the hypothesis, however, remain untested.
Biotic hypotheses Biotic hypotheses claim ecological species interactions such as
competition,
predation,
mutualism, and
parasitism are stronger in the tropics and these interactions promote species coexistence and specialization of species, leading to greater speciation in the tropics. These hypotheses are problematic because they cannot be the ultimate cause of the latitudinal diversity gradient as they fail to explain why species interactions might be stronger in the tropics. An example of one such hypothesis is the greater intensity of predation and more specialized predators in the tropics has contributed to the increase of diversity in the tropics (Pianka 1966). This intense predation could reduce the importance of competition (see competitive exclusion) and permit greater niche overlap and promote higher richness of prey. Some recent large-scale experiments suggest predation may indeed be more intense in the tropics, although this cannot be the ultimate cause of high tropical diversity because it fails to explain what gives rise to the richness of the predators in the tropics. Interestingly, the largest test of whether biotic interactions are strongest in the tropics, which focused on predation exerted by large fish predators in the world's open oceans, found predation to peak at mid-latitudes. Moreover, this test further revealed a negative association of predation intensity and species richness, thus contrasting the idea that strong predation near the equator drives or maintains high diversity. Other studies have failed to observe consistent changes in ecological interactions with latitude altogether (Lambers et al. 2002), suggesting that the intensity of species interactions is not correlated with the change in species richness with latitude. Overall, these results highlight the need for more studies on the importance of species interactions in driving global patterns of diversity. ==Synthesis and conclusions==