The relationship between habitat permanence and larval development in California spadefoot toads: field and laboratory comparisons of developmental plasticity

We evaluated differences in larval habitats and life history of three species of spadefoot toads, then compared their life histories in a common garden study. Our field work defined the selective regime encountered by each species. Our Great Basin spadefoot (Spea intermontana) bred asynchronously in permanent streams and springs where there was no risk of larval mortality due to drying. The water chemistry remained fairly stable throughout the larval period. The western spadefoot toad, Sp. hammondii, bred fairly synchronously following heavy spring rains in temporary pools that remained filled an average of 81 d. Fifteen % of the breeding pools dried completely on or before the day the first larvae metamorphosed. The desert spadefoot toad, Scaphiopus couchii, bred synchronously after heavy summer showers in very short duration pools; 62% of the breeding pools dried completely on or before the day the first larvae metamorphosed. The concentration of ammonium nitrogen and CaCO3 increased markedly as the Sp. hammondii and S. couchii pools dried. S. couchii attained metamorphosis at a much earlier age and smaller size than the other two species. S. couchii also showed little variation in the age at metamorphosis but considerable variation in the size at metamorphosis, while the other two species varied in both age and size. The results identify some variables that could serve as cues of pool drying and demonstrate an association between breeding pool duration, breeding synchrony, development rate, and larval development. Our laboratory study yields information about the genetic basis of the differences in development and controlled comparisons of phenotypic plasticity. We manipulated food supply to study the plastic response of age and size at metamorphosis and hence construct the reaction norm for these variables as a function of growth rate. The growth rates ranged from below to above those observed in natural populations. As in the field, in the lab S. couchii attained metamorphosis at an earlier age and smaller size than the other two species. All three species had a similarly shaped reaction norm for size(y-axis) and age (x-axis) at metamorphosis, which was a concave upward curve. A consequence of this shape is that age at metamorphosis changes more readily at low levels of food availability and size at metamorphosis changes more readily at high levels of food availability. If we restrict our observations to just those growth rates that are seen in nature, then S. couchii has almost no variation in the age at metamorphosis but considerable variation in size at metamorphosis, while the other two species vary in both age and size at metamorphosis. All three species increased in size at metamorphosis with increased food levels. Our comparative reaction norm approach thus demonstrates that S. couchii has adapted to ephemeral environments by shifting its growth rate reaction norm so that age at metamorphosis is uniformly fast and is not associated with growth rate. The realized variation is concentrated in size rather than age at metamorphosis.