Developmental dynamics of Ambystoma tigrinum in a changing landscape

Loss of pond habitat is catastrophic to aquatic larval amphibians, but even reduction in the amount of time a breeding site holds water (hydroperiod) can influence amphibian development and limit reproductive success. Using the landscape variation of a glacial valley in the Greater Yellowstone Ecosystem as the context for a natural experiment, we examined variation in growth pattern and life history of the salamander Ambystoma tigrinum melanostictum and determined how these developmental characteristics varied with hydroperiod over several summers.

In ponds that dried early in the season, maximum larval size was reduced relative to the sizes achieved in permanent ponds. Ephemeral ponds were associated with early metamorphosis at small body sizes, while permanent ponds facilitated longer larval periods and later metamorphosis. Paedomorphosis resulted from indefinite metamorphic postponement, and was identified only in the most permanent environments. Patterns of growth and allometry were similar between ponds with different hydroperiods, but considerable life history variation was derived from modulating the timing of and size at metamorphosis. Considering maximum rates of growth and inferring the minimum size at metamorphosis across 25 ponds over the course of three years, we calculated that hydroperiods longer than three months are necessary to support these populations through metamorphosis and/or reproductive maturity.

Landscape heterogeneity fosters life history variation in this natural population. Modulation of the complex ambystomatid life cycle allows this species to survive in unpredictable environments, but current trends towards rapid pond drying will promote metamorphosis at smaller sizes and could eliminate the paedomorphic phenotype from this region. Metamorphosis at small size is has been linked to altered fitness traits, including reduced survival and fecundity. Thus, widespread environmental truncation of larval periods may lead to decreased population persistence. We found that the hydroperiods of many ponds in this region are now shorter than the developmental period required for larvae to reach the minimum size for metamorphosis; these locations serve as reproductive sinks that may be detrimental for persistence of the species in the region.