Trophic redundancy and predator size class structure drive differences in kelp forest ecosystem dynamics

Abstract

Ecosystems are changing at alarming rates because of climate change and a wide variety of other anthropogenic stressors. These stressors have the potential to cause phase shifts to less productive ecosystems. A major challenge for ecologists is to identify ecosystem attributes that enhance resilience and can buffer systems from shifts to less desirable alternative states. In this study, we used the Northern Channel Islands, California, as a model kelp forest ecosystem that had been perturbed from the loss of an important sea star predator due to a sea star wasting disease. To determine the mechanisms that prevent phase shifts from productive kelp forests to less productive urchin barrens, we compared pre-and postdisease predator assemblages as predictors of purple urchin densities. We found that prior to the onset of the disease outbreak, the sunflower sea star exerted strong predation pressures and was able to suppress purple urchin populations effectively. After the disease outbreak, which functionally extirpated the sunflower star, we found that the ecosystem response-urchin and algal abundances-depended on the abundance and/or size of remaining predator species. Inside Marine Protected Areas (MPAs), the large numbers and sizes of other urchin predators suppressed purple urchin populations resulting in kelp and understory algal growth. Outside of the MPAs, where these alternative urchin predators are fished, less abundant, and smaller, urchin populations grew dramatically in the absence of sunflower stars resulting in less kelp at these locations. Our results demonstrate that protected trophic redundancy inside MPAs creates a net of stability that could limit kelp forest ecosystem phase shifts to less desirable, alternative states when perturbed. This highlights the importance of harboring diversity and managing predator guilds.

Geographic variation in responses of kelp forest communities of the California Current to recent climatic changes

“…Likewise, southern California also has functional redundancy in urchin predators with California sheephead and spiny lobster Selden, Gaines, Hamilton, & Warner, 2017;Tegner & Levin, 1983), especially in several MPAs, which are included in our surveys there. Thus, the persistence of kelps in our southern California survey sites may reflect the protection of key sea urchin predators and an enhanced resistance and resilience of kelp forest communities to the loss of the sunflower star (Eisaguirre et al, 2020). These are only two examples of strong environmental and ecological differences among the ecoregions of the California Current large marine ecosystem.…”

“…As a foundation species, changes in the abundance of kelp can have cascading effects on community structure and ecosystem function Byrnes et al, 2011;Carr & Reed, 2016;Edwards et al, 2020;Lamy et al, 2020;Miller et al, 2018;Schiel & Foster, 2015;Vergés et al, 2016;Wernberg et al, 2012). In contrast, variability in community structure and species interactions may influence the resilience of kelps to projected warming (Eisaguirre et al, 2020;Ling et al, 2015). Building on global analyses of kelp dynamics (Krumhansl et al, 2016), a critical next step for understanding potential geographic variability in response to warming is to assess responses in the biological communities associated with these important foundation species.…”

Trophic downgrading reduces spatial variability on rocky reefs

“…In the Aleutian Archipelago, we have demonstrated that the trophic downgrading that has greatly eliminated foundational kelp species has also reduced variability in overall community structure and in the structure of trophic guilds. While functional redundancies among predators and herbivores can make diverse systems more stable 7 , 9 , simple food webs, such as those found in the Aleutian Archipelago, do not have the functional redundancies at higher trophic levels needed to maintain stability. Trophic rewilding (i.e., the restoration of apex predators) has been suggested for systems that have been downgraded 82 and may be one way that Aleutian kelp forests and their spatial variability can be restored.…”

“…Trophic downgrading occurs when apex predators have been extirpated over large geographic regions, which can lead to important consequences for ecosystem functioning due to both direct and indirect cascading effects 1 . This has been observed globally across a variety of terrestrial and marine ecosystems 2 – 7 . Often, trophic downgrading triggers increases in herbivore populations, thereby changing overall community structure 6 , 8 , 9 and altering patterns of ecosystem productivity 10 – 12 .…”

“…Further, when a community has been downgraded, its resilience (i.e., recovery and stability after a disturbance) can decrease in comparison to intact communities 3 . This has been an important consideration in the design of marine protected areas (MPAs) and terrestrial parks and reserves, which typically protect apex predators to maintain diversity and normal ecosystem functioning 7 , 18 – 20 .…”