Back to the future: Using fecundity estimates to understand population dynamics in the past

Background/Question/Methods   Understanding the mechanisms that produce population dynamics is an important goal of ecologists, particularly in species that are of conservation concern. Amphibian populations often naturally fluctuate by several orders of magnitude, yet much focus has been on the dramatic population declines linked to anthropogenic effects. Over the past 20 years, we have studied a subalpine population of tiger salamanders in order to better understand natural amphibian population fluctuations in the absence of anthropogenic disturbance. We have observed two “boom and bust” cycles in this population and have experimentally shown the potential influence of cannibalism and resource depression by paedomorphs (aquatic adults) on the decline phase of these cycles. Understanding the population dynamics of this system also depends on knowing the number of embryos produced each year, which provides the initial baseline for tracking boom cohorts and for quantifying mortality effects via cannibalism and resource depression. Our previous data suggest two non-exclusive hypotheses for the production of boom cohorts. First, such cohorts may be due to a demographic convergence between metamorphic and paedomorphic females that reproduce at the same time because of variation in frequency of breeding. Second, the potential for boom cohorts may occur frequently, but is only realized when the senescence of paedomorphs reduces cannibalism and creates a window for recruitment. Using a non-invasive clutch-bag technique, we determined fecundity/body size relationships for both metamorphic and paedomorphic salamanders over multiple years. We utilized these relationships and our population estimates of the number and sizes of females breeding in each pond during each year to back-calculate the number of offspring produced in each pond during past years.

Results/Conclusions   Estimates varied across years and ponds depending on the relative numbers and sizes of metamorphic versus paedomorphic females. Our results are consistent with the hypothesis that boom cohorts are both limited by cannibalism by paedomorphic salamanders, and stimulated by the demographic convergence of egg-laying by females of each morph. This insight allows us to better understand the natural population fluctuations that we have documented, and provides further evidence for the underlying mechanisms producing such fluctuations. Because there is little to no evidence of anthropogenic disturbance in this system, our findings may provide a baseline for understanding the ecological interactions that affect other amphibian populations, including those that are declining because of human impacts.