Predatory impacts and variables determining the spatiotemporal occupancy of the invasive American mink (Neovison vison) on a pristine island ecosystem in Southern Chile
Invasive species are the second most significant driver of biodiversity loss. Islands are the most vulnerable regions to invasions because the evolutionary isolation generally results in a lack of behavioral responses to predation by the local biota. The American mink (Neovison vison) is a mustelid native to North America that was recently introduced in Navarino Island, southern Chile. American mink have no competitors or natural enemies in Navarino, establishing them as a new top predator in this fragile ecosystem, threatening local biodiversity. In this project I aimed to determine how the invasive American mink adapts to a pristine-island ecosystem and to assess how mink impact native biodiversity. During 2014 I used camera traps and occupancy models to investigate mink occupancy and habitat selection. Also, I analyzed 113 scats to study mink diet, and investigated the predatory-risk perception of a native rodent species to the novel predator. Finally, I proposed new management strategies to be implemented by the Chilean Agriculture and Livestock Bureau (SAG) to control the mink population on the island.
American mink presented a high occupancy (over 60%) during the summer. However, occupancy dropped significantly in winter (36%) and spring (49%). During summer, occupancy was not associated with fresh-water systems. Mink were almost completely occupying the coastal areas, where resources are more abundant, but also occupied the interior forests and wetlands. The understory vegetation height and density explained occupancy, probably because it is in dense vegetation where mink may have better hunting opportunities. Winter and spring occupancy were associated with fresh-water systems and the coast, respectively. The drop in occupancy may be explained by high mortality, as other studies show that mink are not well adapted to winter fasting.
Mammals represented the main prey item of the American mink, followed by fish. Muskrats represented more than 55% of the bulk of mink diet in inland waters. Most (77%) scat collected in inland wetlands were found at beaver dams. This shows the importance of the community dynamics between these two invasive species.
Native rodents did not perceive direct cues of mink as a predator. However, they avoided open areas, suggesting that they did perceive indirect clues of raptor predation. These results are in accordance with other studies that showed similar rodent responses to novel terrestrial predators, supporting the thesis that short periods of time are not enough to allow prey to develop anti-predatory behaviors.
On Navarino Island, SAG is the agency in charge of managing invasive species. Unfortunately, the lack of human resources and deep understanding of American mink ecology undermines successful control of this invasive species. Given that most trapping activity occurs during the summer when mink are more active with juveniles dispersing, this study suggests that most of the animals that are being removed likely will not survive the winter famine anyway. Therefore, trapping during the summer is unlikely to be effective in controlling the population because animals living inland will simply occupy empty territories. To improve protection of local biodiversity and better control the mink population, I recommend allocating trapping efforts at the end of the winter and beginning of spring when mink are occupying mostly shrub coastal areas, and reproductive adults are more likely to be eliminated from the population. ($8,730)