C. D. Bjurlin

Effects of Habitat on Competition Between Kit Foxes and Coyotes

Abstract San Joaquin kit foxes (Vulpes macrotis mutica) are an endangered species with a narrow geographic range whose natural populations are limited by predation by coyotes (Canis latrans). In the warm, arid grassland and shrubland habitats where kit foxes occur, coyotes are more cover dependent than kit foxes, creating the possibility of habitat segregation. Effects of habitat variation on coyote and kit fox competition are unknown. We assessed exploitation and interference competition between coyotes and kit foxes in grassland and shrubland habitats to determine if such competition varies among habitats. With respect to exploitation competition, we evaluated habitat and spatial partitioning, diet, prey abundance, and survival for kit foxes and coyotes at the Lokern Natural Area in central California, USA, from January 2003 through June 2004. Kit foxes partitioned habitat, space, and diet with coyotes. Coyotes primarily used shrubland habitats whereas kit foxes selectively used burned grasslands. Kit foxes and coyotes had high dietary overlap with regards to items used, but proportional use of items differed between the 2 species. Kit foxes selected for Heermanns kangaroo rats (Dipodomys heermanni), which were closely tied to shrub habitats. With respect to interference competition, predation was the primary source of mortality for kit foxes, and survival of individual kit foxes was inversely related to proportion of shrub habitat within their home ranges. Our results suggest that a heterogeneous landscape may benefit kit foxes by providing habitat patches where predation risk may be lower.

Survival during Early Life Stages of the Desert Tortoise (Gopherus agassizii) in the South-Central Mojave Desert

Abstract Early age classes of the Desert Tortoise (Gopherus agassizii) are particularly vulnerable to predation by several mammal and bird species. We studied tortoise survival at the Sand Hill Training Area of the Marine Corps Air Ground Combat Center, Twentynine Palms, California, from 1998 to 1999. We radiographed and thread-spooled 25 females to determine reproductive and nesting ecology. Predators consumed 11 of 42 monitored nests during the first 70 days of incubation. Ninety-one of 132 eggs in nests fenced after 70 days developed into healthy neonates. We recorded 0.84 and 0.91 neonate survival probability during dispersal in 1998 and 1999, respectively. Applying neonatal survival to egg success, we predict that 40% of individuals survived from oviposition to hibernation. Our findings support other chelonian life-history models indicating relatively high mortality early in life. Our data also suggest that neonatal Desert Tortoises are less susceptible to predation than was previously thought, perhaps because of their cryptic coloration and secretive habits. The Common Raven (Corvus corax) was not found to be a source of neonate mortality during this study.

Effects of Roads on Endangered San Joaquin Kit Foxes

Abstract San Joaquin kit foxes (Vulpes macrotis mutica) occur in central California, USA, and are endangered due to habitat loss and degradation. As the human population of California grows, more roads are being constructed in remaining kit fox habitat. We examined effects of 2-lane roads on demographic and ecological patterns of kit foxes on the Lokern Natural Area (LNA) from August 2001 to June 2004. Of 60 radiocollared kit foxes, only one was struck by a vehicle. Foxes were assigned to 1 of 3 risk categories (high, medium, or low) based on proportion of time spent in road-effect zones, which were defined by the probability of a fox encountering a road during nocturnal movements. Fox survival probabilities, reproductive success, litter size, nocturnal movements, and den placement all were similar among risk categories. Nocturnal locations of foxes were closer to roads than were den locations, and den fidelity was lowest for medium-risk foxes and highest for low-risk foxes but intermediate for high-risk foxes. Food availability and use were not affected by proximity to roads. We were unable to detect any significant detrimental effects from 2-lane roads on kit fox demography and ecology. Our results suggest that standard mitigation strategies, such as crossing structures and exclusionary fencing, would not benefit kit foxes on the LNA.