Evidence of compensatory density dependence in a larval amphibian

Background/Question/Methods

Conditions experienced in early developmental stages can have long term consequences for individual fitness. High intraspecific density during the natal period can affect juvenile and eventually adult growth rate, metabolism, immune function, survival, and fecundity. While these effects have been broadly documented, the mechanistic relationship between intraspecific density and resulting phenotypes is poorly understood. To identify a functional relationship between natal density and juvenile traits, we experimentally manipulated the initial larval density of a pond-breeding amphibian, the ringed salamander (Ambystoma annulatum). We tested 11 initial densities. As individuals metamorphosed, we measured size, growth, and length of larval period. Using an information theoretic approach, we compared seven a priori mechanistic models with parameters estimated using nonlinear regression.

Results/Conclusions

We found strong support for a compensatory density dependent relationship between initial density and length of larval period, body size at metamorphosis, and energy stores at metamorphosis. Specifically, our data indicate that compensatory density dependence is occurring, and that there is a threshold in the response of individual parameters to initial density. As initial density increased, there was a diminishing change in response variables (increase for length of larval period, decrease for mass, length and growth). Our study suggests that at low densities, a small change in the number of larvae could have large fitness consequences. However, at high densities, a change in the number of larvae would have minimal fitness implications for any resulting juveniles. The compensatory response to initial intraspecific density presents an alternate hypothesis to previously proposed self-thinning or Allee effects in larval amphibians, and suggests that ringed salamanders may be under selective pressure for tolerance to high density and increased efficiency in resource utilization.