Testing the Sentinel Method: Live and Artificial Prey Display Contrasting Patterns of Predation Across an Urban Gradient

Assessing changes in the intensity of biotic interactions across environmental gradients is a central issue in ecology. The sentinel method has been widely adopted to study predator-prey interactions by establishing patches of prey under different conditions that predators can attack. Sentinels, proxies for prey, are frequently worm-shaped prey resembling caterpillars and are commonly used to assess predation by arthropod-feeding predators, with predation measured as the rate of disappearance or evidence of predation after a certain period of exposure. While it has been suggested that artificial sentinel prey might produce divergent results from live prey, previous studies showed mixed results in the difference between these two prey types. Results are likely to vary with context, and the assessment of different prey types along urban gradients is still lacking. Here, we performed an experiment at 10 sites across a natural-to-urban gradient in Suzhou (East China) combining live prey and artificial prey to determine differences in predation intensity between these prey types. We released 2575 artificial prey and 3825 live prey, either separately (artificial or live prey alone) or combined, in a randomized sequence. We found a positive relationship between our index of predation and the level of urbanization using both types of prey. However, the predation rate using artificial prey was lower than with live prey and showed a different pattern with urbanization. The predation rate using live prey was higher for avian predators and lower for insect predators with increasing urbanization. Our results show that artificial and live prey can produce divergent estimates of predation intensity. Thus, while artificial prey may be used as a rapid-screening tool, live prey could be favored in comprehensive studies to assess this fundamental ecosystem service.