Effects of Plasticity on Elevational Range Size and Species Richness

Elevational ranges within many taxa are greater in the north temperate region than the tropics. Two hypotheses to explain the pattern are, first, that large elevational ranges in the temperate region arise because species have evolved broad tolerance curves in response to seasonality and, second, that a low diversification rate in the temperate region (speciation minus extinction) has led to relatively few species, each of which occupies a large elevational range in the absence of competitors (character release). We build a quantitative genetic model of selection on a phenotypic trait, whereby increased tolerance is modeled as arising from plasticity in the trait. We show that broad tolerances result in evolution of large elevational ranges because they induce shallower genotypic clines and hence reduced maladaptive gene flow. The evolution of large elevational ranges results in relatively few competing species arranged along the elevational gradient at a species carrying capacity. In such saturated communities, species have much elevational overlap. In contrast, in similar-sized communities that could accommodate many more species, the resulting character release is associated with smaller elevational overlaps. Empirical assessment of these predictions should contribute to assessing any role for ecological limits in driving the latitudinal diversity gradient in species richness.