As invasive predators spread across landscapes, their sudden presence may have significant effects on the behavior patterns of their new found prey. Here, I examined how predator-naive foxes responded to colonization by non-native golden eagles on Santa Cruz Island, California. First, using radio-telemetry, I investigated the effects of this diurnal, aerial predator on fox activity patterns. In 1992, just prior to the arrival of golden eagles, foxes showed substantial diurnal activity, but diurnal activity was 37.0% lower in 2003-7, after golden eagle colonization; concurrently, overall activity declined and nocturnal activity increased. Moreover, on nearby Santa Catalina Island, where golden eagles were absent but where the fox population recently crashed due to a disease epidemic, remaining foxes were significantly more diurnally active than were those on Santa Cruz Island. The weight of evidence suggested that the change in activity pattern was a response to predation, not to low population density, and that this was probably a heritable, rather than a learned, behavioral trait. Second, I used radio-telemetry, camera traps, sightings to investigate spatial patterns and habitat use in the wake of eagle colonization. When comparing pre- and post-colonization conditions, foxes demonstrated an inverse home range-density relationship, as fox core areas and home ranges expanded significantly in the low-density conditions following eagle colonization. I found that diurnal ranges were 11% smaller than nocturnal home ranges in post-colonization conditions, perhaps reflecting restricted diurnal movement as a form of predator avoidance of the diurnally hunting eagles. In terms of habitat preference, foxes preferred shrubland—a habitat which provides cover from aerial predators—over other habitat types. This suggested a way in which foxes may have mitigated golden eagle predation risk. Finally, I examined changes in fox diet before and after colonization using scat analysis. Because this invasion reduced the endemic fox population by 95% in a decade, these dietary changes could have been be attributed to behavioral change (e.g., reduced diurnal activity and movement), demographic change (e.g., reduced intra-specific density), and/or community level change (e.g., increased intra-specific competition with island spotted skunks), all of which were shifts associated with eagle predation. Concurrently, there were marked changes in the island's vegetation community, with the removal of introduced grazers and the subsequent increase in recruitment of shrubland. I hypothesized that these effects would cause fox diets to differ from historic diets, as indicated by scat analysis. I also evaluated seasonal differences and dietary breadth in the post-eagle colonization period. Although I did not observe an overall large scale modification of fox dietary patterns, statistically significant dietary changes were observed before and after eagle colonization. Results suggested that all four factors may have had an effect on fox diet, although evidence suggested that grazer removal may have had a more pronounced effect. Seasonal patterns and dietary breadth reflected the seasonal availability of fruiting shrubs in the dry season, and an emphasis on other food items, primarily mice and insects, in the wet season, as expected. While this fox population has subsequently rebounded successfully, understanding how animals respond to the sudden arrival of an invasive predator is crucial to improving approaches to conserving endangered species in the future.
|Commitee:||Lawler, Sharon P., Sih, Andrew|
|School:||University of California, Davis|
|School Location:||United States -- California|
|Source:||DAI-B 74/10(E), Dissertation Abstracts International|
|Subjects:||Ecology, Conservation biology, Behavioral Sciences|
|Keywords:||Anti-predators, Behavior, Golden eagle, Invasive species, Island fox, Santa cruz island|
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