Understanding how species interactions shape communities is a central goal of ecology and is a prerequisite for effective conservation on our rapidly changing planet. Predator-prey interactions have profoundly influenced both life history evolution and breeding ecology of birds, as nest predation is the major source of avian reproductive failure in most systems. Human activities can change the nature of predator-prey relationships by manipulating populations and by providing resource subsidies. However, the extent to which resource subsidies affect species interactions in anthropogenic landscapes is poorly understood. We hypothesized that resource subsidies in urban landscapes would decouple predator-prey relationships between breeding birds and nest predators, as predators switched from natural to anthropogenic foods. From 2004-2009, we monitored 2942 nests of five species of forest songbirds and surveyed nest predators at 19 riparian forests distributed along a rural-to-urban landscape gradient in Ohio, USA. Our previous research demonstrated that urban landscape matrices provided more anthropogenic foods (e.g., birdseed, exotic fruits, refuse) to generalist and opportunistic predators than rural landscapes. We used video cameras at a subset of nests to identify the suite of nest predators active at sites.
Amount of urban development surrounding riparian forests was positively associated with numbers of nearly all confirmed nest predators, including free-ranging domestic cats (Felis catus), corvids, common grackles (Quiscalus quiscala), native mesopredators, raptors, and squirrels. Brown-headed cowbird (Molothrus ater) was the only species of nest predator not significantly associated with urbanization. Avian nest survival strongly declined with increasing predator activity, but only in the most rural landscapes. The relationship between birds and nest predators diminished as landscapes surrounding forests urbanized and was entirely disconnected in the most urban landscapes. Our work demonstrates that decoupling of predator-prey relationships can arise when synanthropic predators are heavily subsidized by anthropogenic resources. Thus, we show that human drivers can alter, and completely disarticulate, relationships among species that are well established in less developed systems.