PS 11-48 - Simulated effects of sex-ratio disparity on population dynamics of the endangered Houston toad

Tuesday, August 9, 2016
ESA Exhibit Hall, Ft Lauderdale Convention Center
Rebecca Aden1, Lela Culpepper2, Hsiao-Hsuan Wang1, Ivana Mali3, William E. Grant1 and Michael R.J. Forstner4, (1)Department of Wildlife and Fisheries Sciences, Texas A&M University, College Station, TX, (2)Department of Ecosystem Science and Management, Texas A&M University, College Station, TX, (3)Department of Biology, Eastern New Mexico University, Portales, NM, (4)Department of Biology, Texas State University-San Marcos, San Marcos, TX

Amphibian populations have been documented to be declining worldwide for the last three decades. Determining the risk of extinction is one of the major goals of amphibian conservation. However, sex-ratios are difficult to determine for amphibians with short, seasonal breeding cycles. The sex-ratio of reproductively mature individuals within a population is an important determinant of population dynamics, particularly for species such as the Houston toad, an endangered species endemic to east central Texas. The species demonstrates differences in the age at sexual maturity for males and females thus causing a large sex-ratio disparity. The objective of our research is to determine how this disparity affects the population dynamics and ultimately, the future of this critically endangered anuran. We conducted a thorough literature review to obtain the best basic demographic data available and developed a stage- and sex-structured population dynamics model for the Houston toad using STELLA. We then applied the model to estimate potential population growth rates under a variety of assumptions about the disparity in age at sexual maturity between males and females and the resulting sex-ratio of reproductively mature individuals within the population. Finally, we evaluated the performance of the model based on published literature.


Our results showed that the populations of Houston toad declined when the sex ratio (male:female) was around 4:1 and the populations exhibited stability when the sex ratio was around 1:1.  We then included uncertainty analyses to provide a range of potential sex ratios of Houston toad that supports a stable population.