Keystone predators in invaded communities

The importance of keystone predators in structuring biological communities and ecosystems has been one of the cornerstones of ecology. Surprisingly though, almost nothing is known about the effect of keystone predators in systems that are dominated by non-natives. Morro Bay, California, USA is a highly invaded estuary that is also home to two well-known keystone predators, sea otters and sea stars in the genus Pisaster.

Between 2000 and 2002, the non-native bryozoan, Watersipora subtorquata became the dominant space occupier on pier pilings in Morro Bay. This increase coincided with two other changes; 1) an increase in the presence of sea otters foraging in Morro Bay and 2) the almost complete loss of mussels (the previous dominant space occupiers) from Morro Bay pier pilings and floating docks. Additionally, Pisaster spp. seemed to have also increased during this time. Could these events be related? 

In our study, “Trophic cascades in an invaded ecosystem: Native keystone predators facilitate a dominant invader in an estuarine community” we experimentally tested whether sea otters and Pisaster spp. could facilitate the invasion of Watersipora. We found that both keystone predators caused significant increases in the abundance of the bryozoan Watersipora subtorquata by preying on mussels and opening up space. We also found that sea otters increased the abundance of Watersipora even when there were no mussels initially in the system, indicating that the cascading effect of otters on Watersipora subtorquata is not limited to removing space competitors.

These findings have important management implications in that predators are high conservation priorities and management to increase predators could, in some cases, also increase unwanted invaders. In addition, Pisaster spp. have recently been decimated by wasting disease along the Pacific Coast of North America, generating concern about the negative impact that the loss of sea stars will have on biological communities.  However, in invaded communities this loss may instead have ecosystem benefits and recovery of sea stars in the future could facilitate invasion of Watersipora.  This study provides a novel twist on how native keystone species may have unanticipated negative impacts once environments have been altered by invasion.

The authors through Lisa Needles