Vertebrate scavenging is an important ecosystem process that increases nutrient cycling and stabilizes ecosystems. The responses of vertebrate scavengers to human activity also varies according to species and community level traits. Understanding these responses is pivotal as urbanization and population levels are projected to rise. Therefore, we assessed both the vertebrate scavenger community composition and the functional efficacy along a 400-mile urbanization gradient.
We conducted 196 successful camera trap trials in 7 localities spanning from New York City to the Adirondack Mountains between 2016 and 2018. We used generalized linear mixed models to test the impact of urbanization and seasonality on: (1) scavenger community composition (e.g. species richness and percent occurrence), (2) scavenger efficiency (e.g. time to detection and carcass fate), and (3) carcass persistence. We characterized urbanization levels with a principal components analysis of land use data, NDVI values, and impervious surfaces ratio and extracted the values for the first two components for each locality with a 10 km buffer. Seasonality was characterized by means of a binary variable (e.g. cold or warm).
Results/Conclusions
Our models show effects of both urbanization and seasonality on the scavenger community. Moderately urbanized areas have higher species richness than either urban or natural habitats (range=3-15). Further, larger species dominate more natural habitats, while smaller animals occurred more frequently in urban habitats (meannatural=16.4 kg; meanurban=10.4 kg). The community was more diverse during warmer months in every habitat except for the most urban locality. Scavenger efficiency and carcass persistence were stable across urbanization levels. However, seasonality effected scavenger efficacy. Scavengers were less likely and slower to find carcasses during colder months. However, summer and winter carcass persistence were comparable when time to discovery and non-scavenged carcass variables were controlled.
The high consumption rates support the assumption that carrion is a relatively high value food source. Further, competitive exclusion of smaller carnivores suggests that top down mediation is a primary driver in natural areas. However, high urbanization levels appear to act as a filter for larger scavengers. Smaller species dominate carcass removal due to competitive release in more urban areas. Therefore, we propose that the vertebrate scavenger community’s response to urbanization supports the intermediate disturbance hypothesis, insofar as moderate urbanization levels allow for both urban exploiters and avoiders to co-exist.