Plasticity in Metamorphic Traits of Natterjack Tadpoles: The Interactive Effects of Density and Pond Duration

Abstract

Plasticity in developmental time and size at metamorphosis were studied in natural and experimental populations of natterjack toad tadpoles (Bufo calamita), a species which breeds in temporary ponds of variable duration. Amount of rainfall directly affected pond hydroperiod and larval density through an increase in the number of spawning females. As both factors directly affected larval period and thus metamorphic success, we performed a factorial field experiment to examine the response of developmental rates to drought regime and its interaction with population density, by controlling pond duration, larval density and food level. Timing of metamorphosis was affected by larval density and food level. However, the response among larvae due to pond duration was not additive. At low larval density, tadpoles raised in short duration ponds exhibited shorter larval periods than in long duration ponds. In constrast, plasticity in developmental rates due to pond duration was less marked at high larval densities and no difference in length of larval period was found. This may be the result of higher competition levels in short duration ponds through an increase in actual larval density because of decreasing water level and lower metamorphic rates. This higher competitive stress was also evident by smaller metamorphic size in short ponds. At low densities tadpoles in longer duration ponds exhibited no additional growth, indicating that tadpoles that remained for a longer period did not achieve larger metamorphic size. This outcome does not support an adaptive explanation of developmental plasticity that merely may be the consequence of expressing a single program of development in different habitats.