Crumb bums? Context dependence in ecosystem services supplied by common urban animals

Bolstering the supply of animal-mediated ecosystem services is an emerging priority in human-altered landscapes. Such services are driven not only by environmental factors that shape communities of species that provide the service but also by the ecological context that affects the behavior of these species. In this study, we used a field experiment to investigate an ecosystem service that depends on resource use behavior—the removal of littered food waste by birds and squirrels in urban green spaces. We first explore how landscape-scale urbanization affects the composition of the litter-removing species community. We then examine two facets of waste removal provisioning—the amount of food removed and the speed of removal—and how they vary across ecological contexts represented by green space type (picnic areas, urban parks, and forest preserves), bird and squirrel abundance, number of people, amount of existing litter, and weather conditions. We found that although landscape-scale urbanization affected the composition of species within green spaces, service provisioning was context-dependent. Littered food removal services were provided at higher rates in park and picnic sites than in forest preserves and the abundance of eastern gray squirrels (Sciurus carolinensis) was a main driver of littered food removal services. Where squirrels were abundant, more food was removed, and food removal began and was completed more quickly. When squirrel abundance is accounted for, removal from picnic areas is higher than park sites, indicating context dependence in this service is likely driven by squirrel behavioral responses to ambient food waste levels in these habitats. This study highlights the role of common urban species in providing a valuable ecosystem service and the importance of ecological context in its supply. Efforts to account for animal-mediated ecosystem services in human-altered landscapes should address the potential for services to be driven by a single species and context-dependent factors that influence behavior.