Trophic cascades in terrestrial systems: A review of the effects of carnivore removals on plants

We present a quantitative synthesis of trophic cascades in terrestrial syst ems using data from 41 studies, reporting 60 independent tests. The studies covered a wide range of taxa in various terrestrial systems with varying d egrees of species diversity. We quantified the average magnitude of direct effects of carnivores on herbivore prey and indirect effects of carnivores on plants. We examined how the effect magnitudes varied with type of carniv ores in the study system, food web diversity, and experimental protocol. A meta-analysis of the data revealed that trophic cascades were common among the studies. Exceptions to this general trend did arise. In some cases, tro phic cascades were expected not to occur, and they did not. In other cases, the direct effects of carnivores on herbivores were stronger than the indi rect effects of carnivores on plants, indicating that top-down effects atte nuated. Top-down effects usually attenuated whenever plants contained antih erbivore defenses or when herbivore species diversity was high. Conclusions about the strength of top-down effects of carnivores varied with the type of carnivore and with the plant-response variable measured. Vertebrate carn ivores generally had stronger effects than invertebrate carnivores. Carnivo res, in general, had stronger effects when the response was measured as pla nt damage rather than as plant biomass or plant reproductive output. We cau tion, therefore, that conclusions about the strength of top-down effects co uld be an artifact of the plant-response variable measured. We also found t hat mesocosm experiments generally had weaker effect magnitudes than open-p lot field experiments or observational experiments. Trophic cascades in ter restrial systems, although not a universal phenomenon, are a consistent res ponse throughout the published studies reviewed here. Our analysis thus sug gests that they occur more frequently in terrestrial systems than currently believed. Moreover, the mechanisms and strengths of top-down effects of ca rnivores are equivalent to those found in other types of systems (e.g., aqu atic environments).