Terrestrial Ecosystems Can Be So Nonlinear:
Implications for Science and Policy

James Reynolds
Department of Biology &
Division of Environmental Science and Policy
Duke University

Ecological Thresholds Meeting
November 4-5, 2002

Abstract

Terrestrial ecosystems often exhibit nonlinear and discontinuous responses to environmental forcings, e.g., climate and disturbance, even when the forcings themselves are continuous and gradual. Hence, it is reasonable to expect that future changes in regional and global environments will not necessarily result in smooth, linear changes in local ecosystems but, instead, we should be prepared for surprises and unanticipated outcomes. There is ample evidence to support this conclusion: Paleorecords document that community shifts to past climatic change have been highly nonlinear; experimentalists often find nonlinear responses of ecosystems to disturbance and rising levels of atmospheric carbon dioxide concentrations (CO2); landscapes have known to have critical thresholds of fragmentation that affect key ecological processes, such as the ability of natural enemies to control pest outbreaks in agricultural systems and the balance between native and exotic species; and there are increasing data to suggest minimal threshold values of species biodiversity exist that, when surpassed, negatively impact the ability of ecosystems to provide the essential goods and services of importance to human populations. Notwithstanding this evidence, I maintain that a great deal of ecological research continues to employ 'linear thinking' and, not surprisingly, linear thinking is also entrenched in the way policy-makers perceive environmental change and ways to manage it, as illustrated by the common (and usually false) assumption of the linear relationship between action and outcome. Given that the socio-economic consequences of abrupt nonlinear changes in terrestrial ecosystems are potentially more damaging if societies are not prepared, it is obvious that linear thinking is both risky and unacceptable. In this talk, I will first define what is meant by nonlinearity, describe its origins in ecological systems, and provide several examples to illustrate its unpredictable nature, drawn from studies of the effects of elevated CO2 on plants and ecosystems, regime shifts in terrestrial ecosystems, and how climate change and disturbance may affect biodiversity. Second, I consider whether current experimental and modeling approaches are adequate to reduce uncertainty brought about by nonlinear behavior. Third, I discuss the implications and importance for policy-makers to adopt nonlinear thinking in their dealings with global change and terrestrial ecosystems. Lastly, I describe a new initiative under the Global Change and Terrestrial Ecosystems (GCTE) project as a contribution to efforts of the International Biosphere-Geosphere Programme (IGBP) to emphasize research on ecosystem nonlinearity as a vital and challenging component of global change science and which impacts on how we design experiments, build models, and perceive ecosystem dynamics in a rapidly changing-and unpredictable-world.