Population Viability of Reddish Egrets (Egretta rufescens) in Texas: A System Dynamics Approach to Conservation and Management of North America’s Rarest Heron
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Reddish egrets (Egretta rufescens) are a threatened waterbird species that inhabit coastal areas of North, Central, and South America, including Cuba and the Bahamas. An estimated one-third to one-half of the global E. rufescens population occurs in the United States, with Texas having approximately 75% of the breeding pairs. The plume trade of the late 1800’s drastically reduced global population numbers so that by the 20th century the species was decimated and possibly extirpated in many parts of its range. While population numbers may be increasing throughout portions of the range, many factors continue to threaten the persistence of the species in Texas. Population viability analyses (PVAs) are a common method of predicting a species’ persistence into some future time. The purpose of developing a population viability analysis for E. rufescens is to identify possible factors impeding growth of the Texas population. By assessing the relative threat of each contributing factor and identifying vulnerable life stages, a robust PVA can estimate how different management actions may affect population demographics. I created a dynamic population demographic model of Texas reddish egrets based on difference equations, with stochastic variables drawn from normal distributions. I simulated the Texas E. rufescens population to 50 years and evaluated my model by comparing my results with current population trend and parameter estimates reported in the literature. Using a quasi-extinction criterion of ≤50 individuals, probability of persistence to 50 years was 98.2% (766 of 780 simulations) for the breeding population. I found four-year-old female survivorship to be the most influential model parameter, which is consistent with similar studies of long-lived avian species that mature late and lay relatively few eggs. Additionally, I found that while the breeding population is projected to remain stable (λ ~ 1.0) over the next 50 years there is only around a 5% probability of achieving the breeding population goal. These findings suggest that management actions focusing on increasing adult survivorship, such as habitat protection, would be most beneficial to population growth and persistence of reddish egrets in Texas.