Proactively Manage Pests Based on Predator-Prey Ecology

Ecological theory predicts that raptors exert top-down effects on their prey by direct consumption, indirectly by imposing risk of predation, or a combination of both. However, empirical evidence for these effects is mixed and inconsistent. Understanding relationships between raptors and their prey is especially important where raptors may contribute to the control of agricultural pests. Earlier work shows promise that barn owls may control rodent pests, but empirical evidence is limited and the distinction between direct and indirect effects remains unresolved. Here, we investigate the top-down effects of barn owls on rodent pests in winegrape vineyards in Napa Valley, California. We monitored barn owl occupancy, reproductive status, and fledgling success in over 300 nest boxes in 2023. We used this to create a spatially and temporally explicit barn owl hunting pressure model from hunting locations and from prey delivery rates that vary as nestlings develop. The hunting pressure layers were used to examine the direct effect of barn owls on rodent abundance (measured with chew blocks), and the indirect effect of barn owls on rodent perceived predation risk and activity (measured with giving-up density trays and game cameras respectively). We found that barn owls exert both direct and indirect effects on rodent pests and that dense vegetation favors the acoustically attuned barn owls over the visually-oriented rodent pests. We contribute to growing evidence that barn owls can be used as biological control in agricultural systems, and we show that growers can manipulate vegetation to enhance the effect of barn owls and to induce more fear in rodents.