Introduction: Why Assembly Rules Are Important to the Field of Restoration Ecology

VICKY M. TEMPERTON, RICHARD J. HOBBS, TIM NUTTLE, MARZIO FATTORINI, AND STEFAN HALLE

How are ecosystems assembled? How do the species that make up a particular biological community arrive in an area, survive, and interact with other species? Why do only some species succeed in particular places? Why are similar assemblages of species seen in different parts of the landscape? These questions are fundamental elements of the science of ecology, and ecologists have been asking questions like these for centuries. This set of questions has been formulated into a search for what have been termed assembly rules: if only certain species can establish and survive in any given area, and if species tend to occur in recognizable and repeatable combinations or temporal sequences, then maybe we can identify a set of rules governing the assembly of ecosystems and communities.

Another group of people have been asking a different set of questions. How do we repair the damage caused to natural and managed ecosystems by overexploitation, misuse, pollution, mining, and so forth? How can we return a biological assemblage to mine-spoil heaps? How can we return a degraded area to a functioning ecosystem that can serve as habitat or perform useful ecosystem services? These are the questions asked by restoration ecologists, who aim to tackle the problems arising from the increasing human use—and misuse—of the planet's ecosystems.

In this book, we work from the premise that these two sets of questions are not really very different; indeed, there is considerable overlap between them. In fact, ecological restoration involves putting lost parts back into a system, and obviously any assembly rules that may exist must be considered to ensure success. Nevertheless, there have been few attempts so far to explore and develop that overlap.

Recent ideas about assembly rules have come primarily from community ecology (for example, Belyea and Lancaster 1999, Weiher and Keddy 1999), but this work has not yet been translated into practical outcomes. This fact is not really surprising, however. If one looks at the history of ecologists' work on constraints to community development, one finds intense debates running from the beginning of the twentieth century up to the present (see Chapter 3), none of which are adequately resolved as yet (Booth and Larson 1999). Two central questions have been at the root of this whole century of debate: How do communities of organisms come to be the way they are, and what are the constraints on membership in a community? Today's ecologists, who are having another shot at these two biggest questions in ecology, can be considered either the perseverant, ever-curious descendants of Darwin, Clements, Gleason, MacArthur, and Wilson, or quixotic adventurers trying to tame a windmill, as the complexity of nature has notoriously evaded simple description. Whatever your opinion, the questions are still there, and we need the answers. These answers are becoming more and more important in a world dominated and transformed by humanity, in which the understanding and repair of damaged ecosystems will be essential to our future survival (Hobbs and Harris 2001). Given that ecological restoration, as defined by the Society for Ecological Restoration International (SERI), is an intentional activity that initiates or accelerates the recovery of an ecosystem, it seems almost essential to consider possible rules or principles that can guide how components should be added to an ecosystem.

The concepts of succession and assembly rules are related, and yet there are important distinctions between the two (Young et al. 2001). One of the main distinctions is focus. Succession follows the dynamics of changes in a community's development, often with particular reference to an endpoint or deviations from this endpoint (climax). Assembly theory and the search for assembly rules focuses more on interactions among organisms within a community and the actual pathways a community can take in response to such interactions. Ecologists seek the mechanisms behind organism assemblages in different situations. This approach is directly relevant to ecological restoration, in which one seeks to guide an ecosystem toward a specific stable state after a disturbance. Lockwood (1997) and Young (2000) suggested that assembly and succession are the core concepts in restoration ecology. Yet very little...