Experimental confirmation of multiple community states in a marine ecosystem

Abstract

Small changes in environmental conditions can unexpectedly tip an ecosystem from one community type to another, and these often irreversible shifts have been observed in semi-arid grasslands, freshwater lakes and ponds, coral reefs, and kelp forests. A commonly accepted explanation is that these ecosystems contain multiple stable points, but experimental tests confirming multiple stable states have proven elusive. Here we present a novel approach and show that mussel beds and rockweed stands are multiple stable states on intertidal shores in the Gulf of Maine, USA. Using broad-scale observational data and long-term data from experimental clearings, we show that the removal of rockweed by winter ice scour can tip persistent rockweed stands to mussel beds. The observational data were analyzed with Anderson's discriminant analysis of principal coordinates, which provided an objective function to separate mussel beds from rockweed stands. The function was then applied to 55 experimental plots, which had been established in rockweed stands in 1996. Based on 2005 data, all uncleared controls and all but one of the small clearings were classified as rockweed stands; 37% of the large clearings were classified as mussel beds. Our results address the establishment of mussels versus rockweeds and complement rather than refute the current paradigm that mussel beds and rockweed stands, once established, are maintained by site-specific differences in strong consumer control.

A quantitative framework for assessing ecological resilience

“…When an ecosystem crosses a threshold or tipping point, its capacity to adapt to and cope with disturbances has been exhausted, and it abruptly reorganizes into a new regime with new structures, functions, and processes. Thresholds have been assessed in, for instance, intertidal marine ecosystems switching from rock weed beds to mussel stands (Petraitis et al 2009) or the encroachment of woody plants into grasslands as a function of fire intensity (Twidwell et al 2013). Scale Ecosystem structure is compartmentalized by spatial and temporal scales (Levin 1998), which can be assessed objectively using statistical tools ).…”

Analysis of abrupt transitions in ecological systems

“…Similarly, the timing of disturbance events with respect to seasonal period can determine their effects on biological responses (Nystrom et al 2000). Understanding spatiotemporal context can help to reconcile differences among case studies illustrating general types of transitions and state changes (e.g., Petraitis et al 2009). Contextual information also can help translate scientific analyses into meaningful policy recommendations and management interventions (Carpenter et al 2011).…”

Multiple stable states and relationship between thresholds in processes and states

“…They demonstrated that these effects translated into changes in adult densities and divergent successional patterns (Petraitis & Dudgeon 2005, Methratta & Petraitis 2008) and that some areas that were seaweed stands are now becoming mussel beds (Petraitis et al 2009). There has been some discussion whether the thresholds in the mussel-seaweed system are an example of multiple states or 2 states under different levels of top-down control (Bertness et al 2002, Petraitis & Dudgeon 2004a), although Petraitis et al (2009) suggested how these 2 seemingly different views can be reconciled.…”

“…abrupt regime shifts from corals to macroalgae in tropical systems, and from macroalgae to barrens or mussel beds in temperate systems, which result in changes in abundance of exploited fish stocks (Done 1992, Hare & Mantua 2000, Konar & Estes 2003, deYoung et al 2004, Paine & Trimble 2004, Mumby 2009, Petraitis et al 2009). Ecologists are especially interested in these sudden shifts because they are common in a variety of situations, often occur unexpectedly, and are difficult to reverse.…”