Landscape transformation and global commerce are eroding barriers between historically allopatric species. Recently diverged species in novel sympatry may have high niche overlap and thus may compete intensely. Yet if competition is density-dependent, it could obscure looming conservation challenges until they become crises that are difficult to manage. We tested whether niche overlap between two such species, the spotted owl and barred owl, differed when the density of the colonizing species was low compared to when it was high. Barred owls have expanded from their historical range in eastern North America to fully colonize the range of the spotted owl in the Pacific Northwest (PNW). At the invasion front in the Sierra Nevada, where barred owl densities are low, we assessed both species’ niches at three hierarchies. First, we implemented multi-scale dynamic occupancy models using data from landscape-scale passive acoustic surveys; second, we developed resource selection functions with GPS tag data; third, we compared dietary overlap using stable isotopes. Finally, we reviewed the literature on spotted owl and barred owl dynamics in the PNW, where barred owl densities are high, and compared niche overlap between the two cases.
Results/Conclusions
Barred owl occupancy and colonization were related to more habitat attributes than were spotted owls, indicating that barred owls had a narrower niche at the landscape scale. Specifically, barred owls were more likely to be detected at locations with shallower slopes and more old forest, while spotted owls were less likely to be detected at locations with medium-aged forest (w=0.42, 0.96, respectively); barred owl site colonization was negatively related to open forest (w=0.46). While foraging, both species selected for medium-aged stands, and the two species were indistinguishable when compared directly (ΔAICcNull=0.0, w=0.28). Thus, their niches are equivalent in terms of foraging habitat selection. Preliminary stable isotope analyses indicate that barred owls consume a wider range of prey than spotted owls, and thus have a broader dietary niche. Our literature review indicated that when the two species’ densities are high (in the PNW), their niches are equivalent in terms of landscape-scale and foraging-scale habitat selection and that barred owls have a wider dietary niche. Our results suggest that competition between species in novel sympatry may be density dependent, with landscape-scale differences in habitat selection allowing for coexistence while the colonizing species has low population density. Diet and fine-scale habitat selection may be the best predictors of competition between species in novel sympatry.