Multiyear demographic study of three co-occurring pond-breeding amphibian species

Background/Question/Methods

Long-term studies of undisturbed amphibian populations are critical to understanding natural population fluctuations and distinguishing them from human induced fluctuations.   Such studies are challenging because they demand sufficient time and energy to make large scale replication by a single researcher difficult.  To add to a small, but growing, collection of such long-term studies, in 2003 I initiated a long-term demographic study of three amphibian species, Spotted Salamanders (Ambystoma maculatum), Wood Frogs (Rana sylvatica), and American Toads (Bufo americanus), at a relatively undisturbed breeding site in central Ohio.  The data presented here represent a broad look at the demographics at this pond, but I anticipate continuing to follow these populations for at least another six years.  Thus far, using a drift fence and pitfall trap system, we monitored the annual breeding population sizes and recruitment rates for all three species.  Additionally, for Spotted Salamanders, individual sexes, weights, and SVLs were recorded, and a subset of the adult population was PIT tagged each year (except 2009) to allow for estimations of annual survival and recapture probabilities.

Results/Conclusions

Between 2003 and 2010, breeding population sizes ranged from 487 to 1,061 for Spotted Salamanders, from 324 to 3,166 for Wood Frogs, and from 38 to 516 for American Toads.  Numbers of emerging juveniles were highly variable, with a range of 0 to 304 for Spotted Salamanders, 5 to 76,809 for Wood Frogs, and 1 to 3,224 for American Toads.  For Spotted Salamanders, we found that sex ratios (male to female) ranged from 1.5 to 3.8, and recruitment rate (juveniles/female) ranged from 0 to 0.98. Finally, we estimate that annual survival rate for mature Spotted Salamanders (77.0%) is not dependent on sex or year, but that the probability of recapture is time dependent, with the lowest recapture probability (36.2%) occurring between 2009 and 2010, and the highest recapture probability (60.0%) occurring between 2006 and 2007.  Throughout the last six years, we have seen dramatic demographic fluctuations in every metric except annual survival, often with no evidence of a temporal pattern.  The nature of these results at our undisturbed breeding site reinforces the need for long-term studies on amphibian populations if we intend to use demographic metrics to evaluate population health and/or stability.