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.

Fig. 1

Representation of benchmark sites and experimental plots as constrained MDS (multidimensional scaling) ordination space. Panel a: plot of benchmark sites based on CAP analysis. Panel b: assignment of control plots and small experimental clearings (1 m in diameter) as mussel beds or rockweed stands based on PREDICTION analysis. Panel c: assignment of large experimental clearings (≥ 2 m in diameter) as a mussel beds or rockweed stands. Colors show assignment; symbol shapes define size of experimental clearing or control plots.

Fig. 2

Average percentage cover and 95% confidence limits by the three most common sessile organisms grouped by assignment and for Fucus, by bay. Arrows show declines associated with severe winter ice damage in winter of 2002−2003.

Fig. 3

Development of rockweed stands and mussel beds over time and space. On open coast shores there is a gradient in wave surge from protected to exposed shores with predators of mussels showing an increase in activity as wave surge diminishes. Work by Lubchenco and Menge (1978) suggested that rockweeds are confined to protected shores where predators are active enough to eliminate mussels. In sheltered bays, both mussels and rockweeds occur as a mosaic of patches and our work suggests development depends on which species becomes established first. The horizontal dotted line shows where a developing patch is “committed” to becoming either a mussel bed or rockweed stand. The vertical dotted line shows the hypothetical boundary between the two kinds of development, but the transition could also gradual or not linked to wave exposure alone.