Responses of American Alligators (Alligator mississippiensis) to Environmental Conditions: Implications for Population and Ecosystem Monitoring CORD B. EVERSOLE 1,3 ,SCOTT E. HENKE 1 ,DAVID B. WESTER 1 ,BART M. BALLARD 1 , AND RANDY L. POWELL 2 1 Caesar Kleberg Wildlife Research Institute, Texas A&M University–Kingsville, 1150 Engineering Avenue, MSC 218, Kingsville, TX 78363, USA 2 Department of Biological and Health Sciences, Texas A&M University–Kingsville, 700 University Avenue, MSC 158, Kingsville, TX 78363, USA ABSTRACT: Accurate monitoring of wildlife populations is critical for species management and conservation. If management practices are based on inaccurate data, managers might not be implementing management strategies appropriately, which can have severe ecological consequences. We used a generalized linear mixed-model analysis to predict and explain the relative abundance of American Alligators (Alligator mississippiensis) within inland freshwater wetlands. Models were developed for three alligator age classes: hatchlings (#30.5 cm total length [TL]), subadults (30.6–182.9 cm TL), and adults ($183 cm TL). Independent variables included environmental parameters that we measured during nighttime surveys. We conducted 135 nighttime alligator surveys, and recorded 7689 observations of alligators in three study lakes. We found that the relative abundance of alligators is variable among age classes and lakes. Final models for each age class differed when lakes were analyzed separately compared with when lakes were combined into one analysis; models differed across age classes as well. These results indicate that alligator occurrence and relative abundance is multifaceted and complex. Survey techniques should accurately quantify age- and population- specific data. Managers and scientists should target particular age classes during surveys on the basis of prevailing environmental conditions. Key words: Crocodilia; Habitat; Life history; Local environment; Texas; Wildlife management POPULATION surveys are commonly used to collect data on activity, relative abundance, and population dynamics of reptiles and amphibians (Sun et al. 2001; Brown and Shine 2002; Pellet and Schmidt 2005). The rates at which active animals are encountered during surveys can exhibit large variation across an array of environmental scales (Brown and Shine 2002). Survey results are dependent upon seasonal events such as overwintering, migration, and the production and care of offspring (Brown and Shine 2002). Animal activity and encounter rates also might vary from day to day as the result of changes in environmental conditions (Walls 1983; Sun et al. 2001; Brown and Shine 2002; Altwegg et al. 2005). American Alligators (Alligator mississippiensis) occur in the southeastern United States, and were declared an endangered species in 1967 (Conant and Collins 1998). The subsequent restoration of American alligator popula- tions and their habitats throughout the southeastern United States has been attributed to strict harvest regulations, intensive management strategies, and wetland conservation (Thompson et al. 1984; Saalfeld et al. 2008). To maintain sustainable populations, alligator populations are regularly monitored using nighttime surveys (Chabreck 1966; Mag- nusson 1982; Thompson et al. 1984; Fujisaki et al. 2011). Harvest regulations and management strategies are based upon these survey results; therefore, their accuracy is essential to the sound management of the species and their ecosystems (Subalusky et al. 2009). American Alligators are frequently used as an indicator of ecosystem health and habitat restoration success (Rice et al. 2005; Mazzotti et al. 2009; Fujisaki et al. 2011; Ugarte et al. 2013). It is because of the integral role that alligators play as apex predators and habitat modifiers that they are highly useful as ecological indicators (Mazzotti and Brandt 1994; Mazzotti et al. 2009). Thus, alligator surveys are a valuable ecological management and monitoring tool because resulting data can reflect the status of not only alligator populations but also the quality of the ecosystems in which they occur. Little is known, however, about how these surveys are affected by variability in environmental factors such as weather, water quality, and lunar parameters. This relationship is especially important for surveys because activity of the target species can affect the detection of alligators (Chabreck 1966; Subalusky et al. 2009). We evaluated the influence of environmental variables on the nighttime relative abundance of American Alligators. The objectives of our study were to determine: (1) which environmental factors are most influential on the relative abundance of alligators; (2) how these factors affect the detection of different age classes of alligators, and (3) methods to improve reliability of alligator surveys based on the first two objectives. MATERIALS AND METHODS Study Area This study was conducted at Brazos Bend State Park (BBSP; 29u22.9629N, 95u36.3439W, datum 5 WGS84), a 1982-ha park in Fort Bend County, Texas, USA. We surveyed seven lakes in the park (Fig. 1). Because of the small sample sizes of alligators observed in some lakes, we only used data from three lakes (Elm, Forty-Acre, and Pilant lakes) for data analysis. Although the three lakes are freshwater with similar aquatic plant communities, they differ in size, vegetation coverage, and alligator density (Table 1). Elm Lake is a heavily vegetated shallow-water lake (mean depth 5 0.6 m) occupied by mostly juvenile and nesting female alligators (Table 1). Forty-Acre Lake has sparser vegetation, more deep-water areas (mean depth 5 2.0 m), and a larger proportion of adult alligators (Table 1). Pilant Lake is similar to Elm Lake in terms of water depth but is the largest lake at the site (115.3 ha). Only 25% (29.1 ha) of the basin of Pilant Lake is likely usable by alligators on account of water levels and vegetation density (personal observation). These characteristics also severely limit the area that can be accurately surveyed using boats (Fig. 1). The area of Pilant Lake that was included in our study had little submerged aquatic vegetation, presumably 3 CORRESPONDENCE: e-mail, cord.eversole@students.tamuk.edu Herpetologica, 71(1), 2015, 37–45 E 2015 by The Herpetologists’ League, Inc. 37