Predators can exert top-down control on lower trophic levels, such that their removal or addition may trigger trophic cascades. Despite coastal ecosystems containing well known trophic cascades, there remains uncertainty about the abiotic and biotic factors governing the occurrence and strength of these cascades. Here, we sought 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 predator. From these data (147 predator-herbivore-producer measurements), we show that predators decreased herbivore populations between 2.1-4.76 times and increased producer populations by 1.62-2.83 times their original biomass, abundance, or density. Contrary to past research, these values are comparable to other ecosystems. Biotic factors related to species body size were most influential in determining herbivore population responses to the presence of predators, while abiotic factors, including nutrient concentration, best determined producer population responses. Our results also show that producers responded more strongly to changes in herbivore populations in high-nutrient and low-temperature environments. We found that herbivore populations in marine reserves were 2.83 times lower on average compared to areas outside the reserve, while producer populations were on average 1.90 times higher. Overall, this work advances understanding of factors modulating trophic cascade strength, demonstrates that reserves can have ecosystem-wide impacts, and provides new information about the average strength of trophic cascades in benthic marine ecosystems.