Thresholds and multiple community states in marine fouling communities: integrating natural history with management strategies

The epifaunal communities characteristic of the southern New England, USA, region between eastern Long Island Sound, CT, and Cape Cod, MA, comprise a complex system in which different mechanisms at certain threshold levels can cause the establishment of 4 distinct community states: (1) a diverse native community dominated by bryozoans and sponges most commonly found in open coast areas, (2) an invasive ascidian community characteristic of marinas and areas of coastal development, (3) a mussel-dominated community occurring after massive recruitment and (4) an ascidian community dominated by Diplosoma listerianum that occurs only in years following warm winters. Each of these states is fairly resilient, but the spatial extent and duration of each state can be highly variable. Transitions among the states occur if some set of threshold conditions are surpassed and reasonable predictions can be made based on knowledge of the natural history of the species within the system. Two sets of processes seem to control the resilience of each state and the thresholds beyond which a transition to a new state occurs: (1) fast and local processes such as within-population recruitment, predation or bioengineering by mussels and (2) slow regional processes such as climate change, coastal development or habitat restoration. Of these, coastal development and restoration efforts are under management control and could have large effects on these community states, in particular the native community. However, we may not be able to influence large-scale regional processes such as climate change that may favor non-native communities. Nevertheless, the different community states can be used as indicators of both local and regional management success and allow local management efforts to be put in the context of larger scale shifts in threshold conditions that affect regional community patterns.