Habitat linkages do not predict occurrence for common California wildlife species

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Autumn R. Iverson, Department of Animal Science, University of California Davis, Davis, CA, David Waetjen, Environmental Science & Policy, University of California, Davis, CA and Fraser Shilling, Department of Environmental Science and Policy, University of California, Davis, Davis, CA

Background/Question/Methods
Habitat linkages are hypothetical objects developed in geographic information systems (GIS) proposed to connect areas of habitats in fragmented landscapes. Assuming they are used by organisms in nature, linkages could be an important tool for the maintenance of viable wildlife populations and biodiversity conservation. Connectivity models have been developed using common, least-cost path techniques for California, including the California Essential Habitat Connectivity Project (CEHCP;), the Areas of Conservation Emphasis (ACE; California Department of Fish and Wildlife) and a linkage plan for the Desert Region Ecosystem Conservation Plan area (DRECP, Bureau of Land Management). However, these linkages have not been evaluated using wildlife detection data.
We used twenty years of wildlife detection data (n > 180,000 total observations) collected from six biodiversity databases. Using these detections and grid cells over the DRECP area and California, we employed occupancy modeling to assess the effect of variables such as major roads and proposed linkages on the occupancy probability of 33 native reptiles, amphibians, and mammals species and/or groups.
Results/Conclusions
In the California desert, the distance to a major road was a significant detection predictor for two species, and a significant occupancy predictor for five species. For the state-wide analysis, mean distance to road was a significant detection predictor for seven of 12 species, and road density was a significant occupancy predictor for 10 species. Relationships varied by species and road variable with no consistently positive or negative relationship for any road variable.
Proposed linkages within the DRECP area were not important occupancy predictors for most species, with two showing positive (glossy snake, desert tortoise) and two showing negative (zebratail lizard, desert spiny lizard) relationships to the linkages. For the state-wide linkage models, results were similar. For the ACE model, two of 12 species had a positive relationship between linkages and occupancy (western rattlesnake and CA newt) and one had a negative relationship (CA ground squirrel). For the CEHCP model, two species of 12 had a positive relationship between linkages and occupancy (mule deer during winter, CA slender salamander) and three had a negative relationship (CA ground squirrel, coyote, gopher snake).
These results show that hypothetical habitat linkages are not a one-size fits all strategy, and caution should be exercised when using them in land and transportation-based conservation planning. Next steps will include connectivity modeling based on the opportunistic wildlife detection data.