The Importance of Scale in Testing the Origins of Alternative Community States

Abstract

The possibility that different species assemblages may represent persistent alternative community states remains largely unexplored by experimental ecologists because of a variety of conceptual and experimental problems. We discuss some of the conceptual roadblocks to experimentation and propose several avenues for attacking the problem experimentally. We address the conceptual issues involved in (1) the blurring of the distinction between the processes that initiate the switch among alternative states and the positive-feedback processes that maintain those states, and (2) the role of spatial scale in initiating the switch. We suggest that the switch between alternative states requires, first, a disturbance that removes species involved in the positive feedbacks needed for maintenance and, second, the arrival of other individuals that initiate the switch to the alternative assemblage. The removal of the species that maintain the system must be large enough and over a long enough time to allow the arrival and establishment of members of the alternative assemblage, and so we hypothesize that the switch among alternative states is scale dependent. This scenario suggests that the switch among alternative states can be investigated experimentally through the manipulation of the scale of the disturbance and of the arrival of members of the alternative state. Small-scale disturbances should consistently fail to initiate a switch, while larger-scale events should initiate a switch at least part of the time. We also note that in some cases the scale of disturbance and/or the arrival of recruits cannot be manipulated or controlled and suggest that several approaches other than factorial experiments with ANOVA, such as spatial autocorrelation methods, may be useful. We illustrate the potential and the difficulties of various approaches by discussing two systems in eastern North America that may contain alternative states. Mosaics of mussel beds and algal beds occupy rocky coasts from New England northward, and patchworks of forests and heathlands occur in eastern Canada and in the Appalachian highlands. While the study of alternative states in the marine system can be approached experimentally, the scale of disturbance required to switch forests to heathlands is too large for experimentation and must rely on the use of other approaches.