Efficient implementation of management programs for invasive species depends on accurate surveillance for guiding prioritization of surveillance and control resources in space and time. Occupancy probabilities can be used to determine where surveillance should occur. Conversely, knowledge of the certainty of site-level absence is of special interest in situations where the objective is to completely remove populations despite substantial risk of reinvasion. Indeed, the decision to shift from emphasizing control activities over the full range to emphasizing reinvasion prevention, surveillance, and response near the borders, depends on accurate knowledge of absence across space. We used a dynamic occupancy model to monitor changes in the distribution of an invasive species, feral swine (Sus scrofa), based on camera-trap data collected as part of a management program from June 2014 to January 2016 in San Diego County, California. Site usage of feral swine declined overall. The most informative predictors of site usage were spatial (latitude and longitude). Site-level nonusage rates increased over time and in response to management removal efforts; and site-level usage rates were heavily impacted by having neighboring sites that were used. Combining the detection probability estimated from the occupancy model and Bayes Theorem, we demonstrated how certainty of local (site-level) absence can be estimated iteratively in time in areas with negative surveillance (no detections) data. Our framework provides a means for using management-based surveillance data to quantify certainty of site-level absence of an invasive species, allowing for adaptive prioritization of surveillance and control resources. Our approach is flexible for application to other species and types of surveillance (e.g., track-plates, eDNA).
Until 2006, San Diego County remained one of two counties in California that did not have a resident population of non-native wild pigs. Since that time, three or more introductions of pigs resulted in the establishment of several populations of wild pigs that grew and were believed to span the backcountry of San Diego County. Feral pigs have the potential to harm sensitive habitats, compete with native species, negatively impact drinking water quality, damage agriculture and rangelands, destroy archeological sites, and transmit diseases. They also pose a significant threat to the network of protected areas in San Diego County. In 2009, affected public land management agencies began working together to address San Diego's pig problem in an alllands approach by forming an Intergovernmental Pig Group. The Group determined that eradicating pigs was feasible, especially given recent drought conditions, and should be the ultimate goal of the project. In the summer of 2014, USDA APHIS Wildlife Services began a large-scale effort to remove pigs across San Diego County. We provide an update on the status of the eradication effort and share information we have gathered on the San Diego pig population from remote cameras and samples collected from pigs taken during this effort. We also outline our strategy for the future to achieve eradication, including how an independent monitoring study will be used to certify that eradication has been, and remains, successful.
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