A Comparison of Vertebrate Associates of Gopher Tortoise and Nine-Banded Armadillo Burrows in South Georgia

Abstract

Burrowing organisms augment the availability of important resources for other species. The gopher tortoise (Gopherus polyphemus) is a keystone excavator in open canopy pine-forest ecosystems in the southeastern United States because its burrows are utilized by over 360 species. Across its range, the gopher tortoise is declining, which is thought to negatively affect burrow-associated species and ecosystem functionality. The nine-banded armadillo (Dasypus novemcinctus) is another burrower of similar size that has become syntopically distributed with the gopher tortoise as a result of range expansion. Recent studies have documented vertebrates utilizing armadillo burrows, linking armadillo burrowing to support of local biodiversity similar to the gopher tortoise. We sought to determine the potential for ecological redundancy between gopher tortoises and armadillos and test quantitatively for differences in associate events at their burrows in a mixed-pine–hardwood forest where they co-occur. Using motion activated game cameras to monitor burrows, we compared metrics of vertebrate occurrence between armadillo and tortoise burrows and examined the effects of environmental variables using a series of regression models. A total of 40 vertebrate taxa were observed visiting burrows between October 2019 and December 2020. Richness, diversity, and community composition were not significantly different between the two burrow types. However, associate event counts were significantly greater at tortoise burrows. Burrow and microhabitat variables had varying effects on associate event counts, with consistently positive effects for tortoise burrows, active burrows, and increased richness of tree species, while negative effects were detected for increased canopy cover as well as increased proportions of hardwood trees. Our study provides a framework for testing redundancy of function between syntopic ecosystem engineers, adds to the growing body of work on the ecological significance of armadillo range expansion, and identifies aspects of the habitat that cause fluctuations in the importance of burrows for associate species.