Habitat Influence on American Alligator Densities in Coastal South Carolina Derived from Nightlight Survey Data (1971-2012)

<p>Presented at the Annual Conference of the Wildlife Society in Milwaukee, WI, October 2013. Describes influence of habitat on raw alligator densities (adults, juveniles, and total) in South Carolina rivers and lakes from 1971-2012.</p> <p> </p> <p>ABSTRACT:</p> <p>The American alligator (Alligator mississippiensis; hereafter alligator) is an iconic species of the southeastern United States of significant cultural, ecological, and economic importance. In South Carolina, habitat loss and degradation pose conservation threats to alligators due to rapid population growth and land conversion rates in the last thee decades. Our objective was to identify habitat features that influence alligator abundance and distribution on South Carolina’s coastal plain. Such knowledge is critical for effective landscape-level conservation planning. Alligator nightlight surveys were conducted by boat on major rivers and lakes between 1971-1977 and 2009-2012. We digitized survey routes in ArcGIS and examined land cover patterns within 1, 3, and 5 km of each route. We constructed linear regression models to determine which land cover categories best-described juvenile, adult, and total alligator densities for each survey period. Average alligator density increased from 0.48 ± 0.54 (SE) alligators per km during the 1970s to 2.84 ± 2.03 for current surveys. Congruently, total wetland area within South Carolina also increased by 23% between 1980 and 2006, though the proportion of water cover (rivers, lakes) decreased by 10% and urban cover increased by 87%. The proportion of urban land cover within each route area during the 1970s positively influenced total alligator densities (β: 0.21, 95% CI: 0.16-0.26), though current total alligator densities were negatively influenced by agriculture and rangeland (β: -0.60, 95% CI: -0.29 - -0.97). Interestingly, the explanatory variables for juvenile density differed among survey periods, as well as from variables that best described adult and total densities. These preliminary results indicate that alligator densities are influenced by different habitat features among age classes, and that said features have changed over time. An improved knowledge of fine-scale habitat requirements for juveniles and adults is warranted for successful conservation of alligators in a changing landscape.