Predation pressure on sentinel prey does not necessarily diminish with advancing urbanization

Urbanization, one of the most significant global environmental issues of our time, causes significant environmental and structural changes in natural or seminatural habitat patches. These urbanization-related changes trigger significant impact on ecological interactions and functioning. Predation is one of the most important ecological interactions, and urbanization-related changes on predation pressure may have substantial ecological consequences. We studied predation pressure over a full season (from April to October) in rural versus urban forests using the sentinel approach in and around a large city (Debrecen) in the eastern part of the Great Hungarian Lowland. Model caterpillars made of nondrying green plasticine were readily attacked by arthropods, birds and mammals. From attack marks left by potential predators, a relatively high predation pressure was documented: up to 36% of the caterpillars exposed for 24 h showed attack marks. Seasonal differences were also obvious, with predation pressure during summer being significantly higher than in spring or autumn. This trend held for overall attack rates, also for attacks by arthropods and mammals but not birds. Surprisingly, attack rates were often higher in urban than rural habitats, contradicting the general hypothesis that predation pressure is lower in urbanized areas. As attack rates depend on both predator abundance and activity, and general data indicate lower predator abundances in urban habitats, this phenomenon may have been caused by hungrier predators in urban forest fragments or by the predator relaxation/safe habitat hypothesis that argues that a reduced need for vigilance allows more time to search for prey.