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.

1. The Tea Bag Index (Keuskamp et al. 2013) has been developed to allow worldwide comparable data on litter decomposition and has become an important part of several citizen science projects. This commentary shows that it is unsuitable for assessing total decomposition in most environments as it does not account for the role of macrofauna in the decomposition process. 2. Tea bags were placed following the standardised protocol in old growth forest in Borneo (Sabah, East Malaysia). 3. The results shows that more than half of the bags were attacked by termites and they tended to remove the majority of the tea, rendering the Tea Bag Index incalculable. 4. The Tea Bag Index can measure the microbial component of decay, but will significantly underestimate total decay rates in most environments. This method will not give an accurate estimate of decomposition in most biomes.