Contrasting activity times and habitat use of Virginia opossums (Didelphis virginiana) and raccoons (Procyon lotor) in urban natural areas

Background/Question/Methods

We have a relatively limited understanding of how natural processes like habitat selection and species interactions function in urban landscapes that have been highly modified by humans. Urban environments feature reduced competition from top predators and increased availability of anthropogenic resources, both of which may alter the activity of co-occurring, generalist mesopredators. Given the lack of top predators, we may expect increased competition of ecologically released mesopredators. However, resource subsidies may buffer these competitive interactions. To address these knowledge gaps, we used camera traps to determine activity times and the patch and matrix landscape features that affect occupancy in two human commensal species, Virginia opossums (Didelphis virginiana) and raccoons (Procyon lotor) in natural areas in Seattle. We set camera traps at 41 samples units for 3-week sessions during 2 field seasons. We tested the hypothesis that greater niche partitioning between these human commensal mesopredator species would occur in an urban environment.

Results/Conclusions

Our results showed a significantly different distribution of activity times for the two species (U²_0.05,280,258 = 1.45, P < 0.001). Opossums were predominately nocturnal (𝜒²₁ = 36.0, P < 0.001),while raccoons distributed their activity between nocturnal and crepuscular periods (𝜒²₁ = 3.72, P = 0.054). While the two species co-occurred at many locations, we did observe differences in site occupancy, which were driven largely by the average percentage of impervious surfaces within a 250 m radius of the camera trap. Other covariates such as patch size, distance to nearest edge, human population density, and road density, were poorly-supported (by AIC) or had low effect sizes. Predicted opossum occupancy was highest at low levels of impervious surfaces and dropped considerably as impervious surfaces increased. Raccoons showed an opposite pattern, with higher overall occupancy that increased as impervious surfaces increased. Our results suggest opossums have a relatively narrow niche in urban environments. In comparison to previous studies of raccoon activity times, we have documented an apparent niche expansion. Because the two species’ activity times and presence do overlap, we did not document niche partitioning. Our results instead reflect raccoons’ greater behavioral plasticity and adaptability to urbanization.