Simulated effects of sex ratio disparity on population dynamics of the endangered Houston toad (Bufo houstonensis)

Abstract

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 (Bufo houstonensis), 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®7.0.1. 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 also determined that sex ratio disparity predominantly affects Houston toad populations that are already at low numbers, as sex ratio disparity intrinsically increases as juvenile survivorship increases.