Estimating abundance of the endangered Houston toad on a primary recovery site

Abstract

The Griffith League Ranch is one of the primary recovery sites for the endangered Houston toad Bufo (Anaxyrus) houstonensis. New recovery initiatives have recently been implemented to increase Houston toad abundance; however, no robust estimate of population size has been conducted in the last decade of study, nor from this recovery site. To assist with inferences regarding efficacy of current and future management actions, we estimated adult Houston toad abundance on the Griffith League Ranch. Houston toads were sampled at breeding ponds during the 2010 breeding season using a mark-recapture methodology. We analyzed the data using a modified Jolly-Seber open population model in Program MARK. Models were built whereby the probability of capture remained constant, the apparent survival varied with time or was constant, and the probability of entry varied with time. Model averaging was used to account for uncertainty and the derived adult male Houston toad abundance ranged from 201 to 307 individuals. Using a previously determined Griffith League Ranch Houston toad functional sex ratio of 5:1, we estimated the abundance of the total adult Houston toad population on this primary recovery site to be from 241 to 368 individuals. This study is the first to report a robust abundance estimate of a Houston toad population and provides a foundation for further research to quantify the impact of current and future management actions.