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 specifically 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 to 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 ten 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 2,575 artificial prey and 3,825 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,
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 rate. Our results suggest that artificial and
live prey produce strongly divergent estimates of predation intensity.
Thus, while artificial prey might be used as a rapid-screening tool, live
prey should be favored in comprehensive studies to assess this fundamental
ecosystem service.