Trophic cascades and connectivity in coastal benthic marine ecosystems: a meta-analysis of experimental and observational research

Abstract

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.

Global kelp forest restoration: past lessons, present status, and future directions

“…A limited number of organisations are currently exploring these solutions in Norway, California, Australia, and Japan (Larby, 2020; Urchinomics, 2020). Restoration of natural sea urchin predators could be achieved either through marine reserves which may allow them to recover without further intervention (Eger & Baum, 2020), or through planned reintroductions/range expansions where key predators are missing (Eger et al ., 2020a). Managers could combine reserves and reintroductions with active restoration efforts to maximise chances of success.…”

“…Transplant experiments demonstrated that while the environment was now suitable for P. comosa , propagule supply and/or post‐settlement survival was likely insufficient for the species to re‐establish populations naturally (Campbell et al ., 2014). While other passive approaches like MPAs can succeed in restoring predator species and kelp forests (Eger & Baum, 2020), they can also fail to facilitate the re‐establishment of a kelp forest (Leung, Yeung & Ang, 2014). As a result, managers are increasingly considering active restoration approaches in combination with removing or mitigating the causes of decline (Morris et al ., 2020; Layton et al ., 2020b).…”

Meta-analysis reveals controls on oyster predation

“…We measured oyster recruitment as the number or density of newly settled oyster larvae ('spat') that naturally recruited to a surface and survived at least two weeks (Newell et al, 2000). If data were reported as a time series, we used data from the final sampling period to maximize the probability of detecting a predator effect (Englund et al, 1999;Eger and Baum, 2020).…”

A Roadmap for the Restoration of Mediterranean Macroalgal Forests

“…In this line, while examples of macroalgae restoration can be found in both MPAs and nonprotected areas (e.g., Verdura et al, 2018;Medrano et al, 2020), studies commonly point to herbivory as the major threat to restoration processes (Carney et al, 2005;Campbell et al, 2014;Tamburello et al, 2019;Savonitto et al, 2021). Some MPAs have worked to restore kelp populations, especially where cascading effects have facilitated grazer control and where no other stressors are present (e.g., increases in the populations of urchin predators such as sea otters, fish, or lobsters) (e.g., Shears and Babcock, 2003;Watson and Estes, 2011;Caselle et al, 2018;Eger and Baum, 2020). However, to successfully revegetate barren grounds, a synergy between passive (MPAs) and active restoration strategies (intentional activity to help macroalgae recovery) might be key, indicating that restoration actions should be encouraged in already protected areas to enhance and speed up the recovery of degraded macroalgal forest (Gianni et al, 2013;Filbee-Dexter and Scheibling, 2014;Medrano et al, 2019).…”