Loss of fitness correlates in overwintering tadpoles that use artificial poolls in a lowland agroecosystem

Background/Question/Methods

The conversion of natural habitats to agricultural land is one of the most significant impacts affecting amphibians throughout the world.  Altered landscapes affect the survival, movement, and life history traits of amphibians.  However, some agricultural activities create new habitats might attract amphibians.  These habitats differ in size, distribution, and physical features in ways that could influence the behavior, life history, morphology, and physiology of frogs. Polypedates braueri typically breeds in the spring or summer, and tadpoles metamorphose before winter; however, we have found P. braueri tadpoles in water tanks on lowland orchads throughout the winter. The purpose of our study was to describe the breeding ecology of P. braueri, to elucidate the cause of overwintering in tadpoles occupying man-made water containers, and to evaluate how overwintering might affect individual fitness. Specifically, we 1) documented breeding phenology, prevalence of overwintering tadpoles, and tadpole development, 2) assessed the influence of temperature and food on both growth and metamorphosis in the laboratory, 3) conducted a field experiment to evaluate the factors that might induce overwintering, and 4) measured the lipid content and jumping performance of metamorphs.

Results/Conclusions

Polypedates braueri bred from March to August, but tadpoles were present year round.  Laboratory experiments demonstrated that tadpole overwintering was facultative; low temperatures and limited food retarded both growth and development, resulting in overwintering in the tadpole stage.  Tadpoles at the lowest experimental temperature (15^oC) never reached metamorphosis.  A field experiment demonstrated that 78, 28, and 4% of tadpoles raised in high, medium, and low food regimes, respectively, metamorphosed before the onset of winter.  Tadpoles that did not metamorphose by fall continued to grow slowly and either metamorphosed during the winter or the following spring.  These findings indicate that food availability plays a key role in inducing overwintering in tadpoles.  Jumping performance of metamorphs was positively correlated with food regimes, but body lipid content was significantly higher in metamorphs raised with either low or high food regimes than in those with medium levels of food.  Overwintering by P. braueri tadpoles has not been previously reported; however, agricultural activities have created new breeding habitats (i.e. man-made bodies of water), some of which are sufficiently food-limited that tadpoles overwinter in order to complete development and metamorphosis.  An understanding of the survivorship, life history traits, and physiology of these frogs is needed to shed light on how man-made breeding sites affect the population dynamics of native frog populations.