Survey techniques for determining distribution, abundance, and occupancy of the carnivore guild in the Apostle Islands National Lakeshore (2014-2017)
Carnivores are important components of ecosystems with wide-ranging effects on ecological communities. These wide-ranging effects are complex and vary with carnivore size, natural history, and hunting tactics, and researchers and managers must understand the ecological roles of carnivores and their interactions with their local environment. We studied the carnivore guild in the Apostle Islands National Lakeshore (APIS), where the distribution, abundance, and occupancy of carnivores was largely unknown. This knowledge was needed to understand island-level variation in carnivore communities and how this variation affects the community-level ecology of APIS. We developed a systematic method to deploy a grid of camera traps while targeting fine-scale features to maximize carnivore detection (Appendix 1) and for organizing and tagging the resulting photograph data (Appendix 2).
In this report, we document our findings from deploying 160 camera traps on 19 islands and mainland Wisconsin from 2014-2017. We collected 203,385 photographs across 49,280 trap nights, with 7,291 total wildlife events and 1,970 carnivore events. We had a mean 7.68 functioning camera traps per island (range 1-30), and our camera trap density averaged 1.89 (range 0.75-12.50) camera traps/ km2. We detected 10 terrestrial carnivores among 21 unique species detected, including unanticipated detections of American martens (Martes americana) and gray wolves (Canis lupus). The mean richness of carnivores on an island was 3.10 (range 0-10) species/island.
The most supported single variable to explain carnivore richness on the Apostle Islands was island size, while the most supported model was island biogeography, which included island size (positive correlation), distance to mainland (negative correlation), and distance to nearest island (negative correlation). The relative abundance of a species was significantly correlated with the number of islands on which they were found. Mean carnivore occupancy across islands ranged from 0.24 for gray wolves to a high of 0.93 for black bears (Ursus americanus). Detection rates for species were generally higher in summer than winter, with the exception of coyotes (Canis latrans) and red foxes (Vulpes vulpes).
Low levels of human activity and development in APIS may play a role in supporting carnivore species that tend to avoid human disturbance. However, none of the islands in the archipelago are likely large enough to sustain populations of mammalian carnivores in the face of demographic stochasticity or the genetic effects of small population size. Therefore, one important area for future study is determining how carnivores colonize and move between islands, as well as how the carnivore guild interacts and affects each other. Fuller understanding of APIS ecology will require on-going monitoring of carnivores to evaluate temporal dynamics as well as related ecological evaluations (e.g. small mammal dynamics, plant community dynamics) to understand trophic effects.