Ecological systems can be altered by many factors including habitat modification, eutrophication, species introductions and fishing. Thresholds may exist, allowing a natural system to withstand disturbance or switch to a distinct community. Alternate stable states are distinct communities in similar environments. Alternate stable state theory may be important for understanding dynamics and managing ecological systems, but experimental evidence is limited and controversial. To persist, alternate stable states must have positive feedback mechanisms that perpetuate the community at the expense of outsiders and allow it to recover from disturbance (resilience). Boccardia proboscidea, a native polychaete worm, has recently formed dense patches in a central California estuary adjacent to habitat dominated by phoronid worms. My data show that Boccardia and phoronid communities have discrete biological assemblages and physical characteristics and are alternate states. I examined multiple aspects of stability in the Boccardia and phoronid community states, including persistence, resilience and presence of positive feedback mechanisms. I experimentally manipulated Boccardia and phoronid communities using three sizes of reciprocal transplants, defaunated plots with coarse or fine sediment, and simulated natural and anthropogenic disturbances. These treatments were designed to measure persistence of transplanted communities and resilience in defaunated and disturbed plots.
Resilience in defaunated plots differed significantly between community type and physical habitat. The rate of change in all plots was related to the identity of the surrounding community, the type of physical habitat and the community structure of the plot. Larger defaunation plots had longer recovery times. Plots with animals (transplants) were better able to prevent invasion by the surrounding community than defaunated plots and thus had a smaller rate of change. The initial impacts following disturbance as well as resilience over time differed depending on community identity as well as type and spatial extent of disturbance. Results provide evidence for stability in two alternate soft-sediment communities. As a model system, these results demonstrate the potential importance of resilience and positive feedbacks in understanding and managing community structure and dynamics.