The growth-mortality tradeoff: evidence from anuran larvae and consequences for species distributions.

A tradeoff affecting the ability to grow under high versus low resource levels has been commonly hypothesized to influence species distributions across resource gradients in a wide variety of taxa. This influence is dependent on individual growth being proportional to traits that affect demographic processes such as mortality. However, data on how individual growth scales with demographic performance are rare. We conducted a mesocosm experiment, and re-analyzed data from a similarly designed field experiment, to examine the relationship between growth and mortality in two tadpole species that segregate across a resource gradient. Overall, environmental conditions leading to faster growth also lead to lower mortality rates. However, species differed in this relationship. Leopard frogs achieved faster growth than wood frogs, but their absolute mortality was greater and increased steeply as growth decreased. Conversely, absolute mortality of wood frogs was lower and less strongly dependent on growth. These interspecific differences suggest a second tradeoff, that between maximizing growth rates or minimizing mortality, with potentially important demographic consequences. Leopard frogs grow faster than wood frogs in productive ponds, but are excluded from unproductive ponds dominated by wood frogs due to accelerating mortality rates with declining realized growth. A review of the literature suggests that in diverse taxa, including plants, microcrustaceans and drosophilids, patterns in mortality are consistent with this tradeoff indicating that the mechanism we demonstrate could be a link between individual performance and demographic rates influencing species distributions in other systems.