Testing the sentinel method: live and artificial prey display
contrasting patterns of predation across an urban gradient
Abstract
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, 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 plasticine prey to determine
differences in predation intensity between these prey types. We released
2,575 plasticine prey and 3,825 live prey, either separately (plasticine
or live prey alone) or combined, in a randomized sequence. We found
there was a positive our index of predation and level of urbanization.
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.