Performance tradeoffs across resource gradients in anuran larvae

Studies investigating the factors that control the distribution of species across environmental gradients have often revealed fundamental tradeoffs limiting species distributions in nature. A tradeoff in performance has been suggested to influence species distributions across an important environmental gradient: the gradient in productivity or resource availability. Fast growing species are sensitive to conditions of resource scarcity, and are excluded from unproductive environments. Conversely, slow growing species thrive in conditions of resource scarcity but are less effective in preempting resources in productive environments. This tradeoff is taxonomically widespread, but its underlying mechanisms and demographic consequences are not well understood.;I investigated the occurrence, mechanisms, and a demographic consequence of a performance tradeoff using a system of anuran larvae distributed across a gradient in pond canopy cover. Leopard frogs are restricted to the productive open-canopy ponds; wood frogs occur in open-canopy ponds but also in the relatively unproductive closed-canopy ponds. I first demonstrate that resources are the primary factor influencing species performance across this gradient. I then document a performance tradeoff, whereby leopard frogs grow faster than wood frogs when fed resources of high quality, or in high quantities; but slower than wood frogs when fed resources of low quality, or in low quantities. This reversal in growth performance occurs because, when compared to wood frogs, leopard frogs apparently allocate a larger fraction of acquired resources into growth, but exhibit lower assimilation efficiency and higher minimum resource requirements. To understand how this tradeoff in performance arising at the individual level translates into species distributions, I further investigated the relationship between growth and mortality. Whereas in leopard frogs mortality is a negative function of growth, in wood frogs mortality is largely independent of growth. Most importantly, mortality in leopard frogs is higher than that of wood frogs even when resources are abundant. This suggests that, across species, maximization of growth capacity comes at the expense of increased probability of mortality. This tradeoff between growth and mortality appears to be general in nature, and may be influencing distributions of organisms as diverse as anuran larvae, crustaceans and plants.