a role in causing latitudinal gradients in species richness as well. The hypothesis is supported by much recent evidence, in particular, the studies of Allen et al. and Wright et al. 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, some support for a slower rate among plant species where water availability is limited 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