Annual and Lifetime Home Ranges Reveal Movement Patterns Within and Among Local Populations of Gopher Tortoises (Gopherus polyphemus)

Abstract

Abstract. – To explore patterns of emigration of gopher tortoises (Gopherus polyphemus), we examined movements at a study site in south-central Alabama that consisted of 2 distinct aggregations of burrows located on adjacent north and south sandhills separated by a mesic depression wash. We collected telemetry data from 41 individuals over a complete season of activity (2000–2001) and capture-mark-recapture data of those same individuals during 14 trapping sessions from 1992 to 2020. We observed a bimodal distribution of telemetry fixes, with 1 mode created by individuals with few fixes (n = 9) who also were recaptured infrequently, which we interpreted to represent emigrants. Thus, our data suggest that the proportion of a local population that emigrates each year is on the order of 20%. To characterize the degree to which resident tortoises (those recaptured ≥ 5 times over the 29-yr period) used space across the entire study site, we used 100% minimum convex polygons to examine the proportion of annual home ranges (based on telemetry data) and lifetime home ranges (based on capture-mark-recapture and telemetry data) that involved both sandhills. For annual home ranges, only a single individual (of 14 residents) occupied burrows on both hills over 1 yr. For lifetime home ranges, 9 individual residents used burrows on both hills. Lifetime home ranges were 6 times larger than annual home ranges, with longest distances between vertices of lifetime home ranges approaching distances ascribed to emigration in previous studies. A model generated from the lifetime recapture data indicated a tendency for greater apparent survival of adults compared with juveniles on the study site and for juveniles and adult females to be more likely to move from the south hill to the north hill than the opposite direction. When added to 5 additional sites for which telemetry data were gathered, we argue that emigration rate is strongly and negatively correlated with local tortoise density. We conclude that current individual-based population models correctly separate movements of animals within a local population (residents) from emigration of animals among local populations, such models require addition of density dependence when addressing emigration rate, and dispersal distances are longer than those based on studies of movements within a single season of activity.