Search

A persistent lack of juvenile recruitment in populations of the imperiled Mojave desert tortoise (Gopherus agassizii) has been attributed to artificially inflated populations of coyotes (Canis latrans) and common ravens (Corvus corax) in some areas. Subsidized by resources in the form of road-killed prey, movement corridors, or nesting perches provided by ever-expanding road and powerline networks, these “subsidized predators” threaten tortoises and other species thanks to subsidies that help support their populations in areas where they might otherwise have been less abundant. Here, we quantified the risk of predation from these subsidized predators on juvenile Mojave desert tortoises by quantifying attack rates on 3D-printed replicas. We evaluated predation risk as a function of replica size, distance from powerlines, and season—variables that can inform release strategies of head-started and translocated tortoises. We found significantly greater risk of predation from ravens in the spring than in the fall. Replica size did not significantly influence predation risk from ravens or coyotes. Replica distance from a powerline or road also did not significantly influence predation risk, supporting previous studies that have found the risk of predation can persist for up to 1.6 km from raven nesting structures. Our results support recommendations for releasing juvenile head-started and translocated tortoises in the fall and away from areas close to raven nesting structures. Our study provides insight into the interactions of prey and subsidized predators and helps guide management on potential mitigation strategies for minimizing the impacts of subsidized predators on species like the imperiled Mojave desert tortoise.

A major challenge in the recovery of long-lived at-risk taxa like turtles is low juvenile recruitment. Head-starting—the raising of juveniles to larger sizes to improve survival—is one tool that can be used in circumstances where juvenile recruitment is limited. Due to declining populations and difficulty detecting juveniles, however, lack of knowledge of the ecology of juveniles can hinder efforts to develop and evaluate head-starting programs for many turtle species. We sought to inform recovery efforts of Mojave desert tortoises by quantifying multi-year space use and survival of head-started juveniles after release. We radio-tracked tortoises head-started under three different husbandry treatments that varied in rearing duration (from two to over six years) and whether head-starting included an indoor rearing component the first year. We compared post-release space use and survival as a function of treatment, release size, and time since release. We found that space use, including home range size and site fidelity, varied by husbandry treatment, with smaller and younger tortoises having smaller home ranges and higher site fidelity. Additionally, home range size decreased and site fidelity increased with time since release across treatments. Tortoises with an indoor-rearing component experiencing increased risk of mortality as movement increased compared to tortoises reared solely outdoors. Nevertheless, survival did not differ among treatments or with tortoise age or size. Regardless of husbandry treatment, head-started tortoises exhibited similar space-use and survival overall. Our study provides insight into juvenile tortoise behavior and head-starting as a tool for tortoise conservation.

Conservation translocations are often used to connect disjunct populations or reintroduce them to otherwise suitable areas from which they have become locally extinct. Translocations, however, can alter wildlife movements and increase their exposure to hazards such as predation, thus increasing their risk of mortality. The giant gartersnake, Thamnophis gigas, is a wetland-dependent threatened species that has lost > 90% of its historical habitat. Certain species recovery may depend on the success of translocations in restoring populations. We translocated snakes from two donor sites—one managed wetland and one rice agriculture site—to a third restored wetland in Sacramento County, California, USA, to evaluate the impact of translocations on home ranges and movements. Translocated snakes had smaller home ranges after translocation for the 95% and 100% MCP estimation, had smaller net displacements, but had similar measures of sinuosity (index that measures the curvature of a movement path), total distance moved, and start-to-end distance compared to their individual measurements before translocation. Landscape type, however, had the greatest effect on home range estimates and movement measures, with resident snakes from the rice agriculture site having greater home ranges and net displacements, and greater, less sinuous seasonal movement paths than snakes from both the donor and recipient wetland sites. Results varied depending on the specific metric, but overall, translocated snakes tended to have intermediate values for home range size, movement distances, and sinuosity compared to the two resident groups. Our results suggest that although translocation did affect movement and space use relative to resident snakes, its effect depended on the donor habitat from which translocated snakes were removed. Because irregular movements are often linked to low survival after translocation, it is encouraging that translocated snakes did not move more than snakes from a donor rice agriculture site, a landscape that represents much habitat for remaining populations.