Ecosystem services, like natural pest control, are essential for developing sustainable urban agricultural systems. However, the strength and functioning of natural pest control is not fully understood in urban environments because it is not known whether the species that can tolerate urban conditions are also good at controlling pests. Here, we assessed the biological control (biocontrol) of pests by ground-level arthropods on mixed-vegetable farms across an urbanization gradient extending outward from Philadelphia, PA. According to the percentage of impervious surface area in a 1.2km radius surrounding each farm, we classified farms as urban (50-85%), suburban (16-49%), or rural (1-15%), and sampled five farms in each category. We conducted biocontrol strength assays on farms by measuring predation rates of housefly (Musca domestica) pupae deployed as sentinel prey. On each farm, half of the biocontrol strength assays were covered in mesh that excluded larger bodied predators to determine the contributions of differently sized predators to overall biocontrol. In concert with these biocontrol strength assays, we sampled the composition of arthropod predators on each farm using pitfall traps.
Results/Conclusions
Although we found equal abundance (F2,25 = 2.00, P = 0.21) and richness (F2,25 = 2.16, P = 0.14) of predators on farms across the development gradient, the composition of species differed: suburban farms lacked the large bodied predators that contributed significantly to biocontrol on urban and rural farms (chi-squared = 61.44, P <0.001). This indicates that although the species present on urban farms differs from those on suburban farms, likely due to tolerance for urban conditions, the urban species significantly contribute to biocontrol. Our results support the ‘opportunistic species hypothesis’ stating that generalist species will dominate in highly disturbed areas, as seen on the urban farms. Under the ‘mean body size hypothesis’ claiming that mean body sizes decrease as disturbance intensity increases, suburban areas may be the most disturbed in our system. Our results showing strong biocontrol on urban farms provide positive encouragement for the development of sustainable urban agriculture in Philadelphia, PA.