Harnessing trophic cascades to improve foundation species restoration: A meta‐analysis

Abstract

AbstractGlobal declines in foundation species and associated losses of biodiversity and ecosystem services have made restoration of foundation species and the habitats they create a top priority. Because restoration success has been highly variable, the field is increasingly using both experimentation and ecological theory to augment success. We conducted a meta‐analysis of studies (18) in estuarine and marine systems, which increased the abundance of top predators as a restoration approach, thus improving biogenic habitat‐forming foundation species recovery by reducing consumers. Beyond testing the general idea of whether top predator addition enhances foundation species restoration, we examined whether the degree of restoration success could be explained by classic predictors of top‐down strength, such as taxonomic identity of the foundation species and top predators, direct and indirect effects of top predators, and the latitude and duration of the study. Top predators successfully elicited a trophic cascade by decreasing consumer abundances (direct effect) and increasing foundation species recovery (indirect effect) when compared with control restoration projects without top predators. Direct effects are expected to be stronger than indirect effects, in accordance with classic ecological theory. We found no evidence of magnitude differences between direct and indirect effects of top predator addition. Increasing invertebrate top predators produced a 3.5× greater increase in the basal trophic level than adding vertebrate top predators. Despite the strong effect of predator type, the log response ratio of all foundation species types (algae, animal, and plant) increased 10‐fold in top predator addition treatments. We found diminishing effects of top predator addition as study duration increased, which supports recent developments that the positive effects of marine protected area designation decline over time. Our results highlight the promise of harnessing top‐down control via top predator addition as a mechanism to aid foundation species recovery.