Aaron Matthius Eger, Julia Kathleen Baum

Predators often exert top-down control on lower trophic levels, such that their removal or addition can trigger trophic cascades. Despite coastal ecosystems containing well known trophic cascades, the abiotic and biotic factors governing the occurrence and strength of these cascades are still unclear. We worked to explain the variability of trophic cascades in benthic marine ecosystems by conducting a meta-analysis of experimental (N = 17) and observational (N = 22) studies that recorded herbivore and producer populations in the presence and absence of a first level predator. From these data (147 predator-herbivore-producer measurements), we show that, although not as strong as previously estimated, the presence of predators decreased herbivore populations between 2.1 to 4.76 times and increased producer populations by 1.62 to 2.83 times. Biotic factors related to species’ body size were most influential in determining herbivore population response to predator presence, while abiotic factors, including nutrient concentration, best determined the producer population response. Our results also show producers responded more intensely to changes in herbivore populations in high nutrient and low temperature environments. Looking at populations in marine reserves we found that herbivore populations in reserves were 3.00 times lower on average, compared to areas outside the reserve, while producer populations were on average 1.84 times higher. Overall, this work advances our understanding of the factors modulating trophic cascade strength, demonstrates that reserves can have ecosystem wide impacts, and establishes a new baseline of trophic cascades in benthic marine systems.