In 2013-14, sea star wasting decimated populations of Pisaster ochraceus along the west coast of North America. Loss of this keystone predator created a unique perturbation that enabled testing of resilience mechanisms across a wide geographic area. We examine the role of compensatory predation by the small six-armed sea star Leptasterias spp. and the predatory dogwhelk Nucella ostrina in controlling mussel population growth at the lower edge of the mussel bed under reduced Pisaster predation. We ask: do persisting small predators compensate for keystone predation loss by exerting greater control on mussel bed dynamics (Q1), and does the strength of compensatory predation vary spatially (Q2)?
To test the effects of predation by these two focal species on community structure in the absence of the keystone, we conducted a cage exclusion experiment at 6 sites spanning 3 Oregon coastal capes: Foulweather (CF), Perpetua (CP), and Blanco (CB). Treatments were: (1) both predators included, (2) Nucella only, (3) Leptasterias only, and (4) no predators. Plots were monitored monthly and recleared at the end of the experiment 18 months later, after which samples were sorted to functional group, dried at 80oC, and analyzed by site, functional group, and experimental treatment.
Results/Conclusions
Analysis of preliminary biomass data revealed variable effects of predation by Nucella and Leptasterias on community composition and biomass of prey functional groups (mussels and barnacles; Q1). However effects of each predator were highly variable at the site level (Q2). CF sites had lower total prey biomass compared to CB and CP sites and predator effects were weak. CP had high total biomass but again, weak predator effects. Since predators are more abundant at CP sites, we attribute this to high recruitment of barnacles and mussels that overwhelmed predation. CB had high total biomass and strong predator effects. Leptasterias + whelk and Leptasterias-only treatments had fewer barnacles and no mussels, but whelks only and predator exclusion treatments contained small, intact mussel beds at the end of the experiment.
These results provide insight into the relative importance of bottom-up (especially recruitment limitation) vs. top down control in meditating resilience mechanisms at a relatively fine spatial scale. Understanding the variability of resilience mechanisms in rapidly changing ecosystems is important to improve our predictive capacity in the face of catastrophic events (e.g. disease outbreak, marine heat waves) which are predicted to increase in frequency and severity due to climate change.