SURVEY
Ecological thresholds: a survey

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Abstract

The existence of ecological discontinuities and thresholds has been recognised by ecological economics as a key feature to take into account in the study of environment–economy interactions. This paper reviews some theoretical developments and empirical studies dealing with ecological phenomena involving non-linear dynamics. The literature about this issue reveals that there is abundant evidence of discontinuities and threshold effects as the consequence of human perturbations on ecological systems. However, due to the complexities involved, the predictive capacity of ecology is limited and large uncertainties still remain.

Introduction

Vis-a-vis neo-classical economics, ecological economics has paid more attention to feedback effects, self-organisation, uncertainties and non-linear dynamics. From a co-evolutionary and adaptive perspective (Norgaard, 1992), ecological discontinuities, being an important source of uncertainty, are key to understanding the complex interactions between the economy and the biophysical environment where it is embedded. The ecological economics literature about this topic has put emphasis on the formulation of environmental policy or management models as well as the proposition of adequate institutional responses when ecological thresholds are likely. However, there have not been many attempts to integrate the ecological literature into this discussion. This paper tries to review the theoretical developments and empirical studies dealing with discontinuities in the ecological science. It pretends to systematise useful ecological information for economists and policy-makers interested in non-linear economy–environment interactions. How often do ecological discontinuities occur in nature? How uncertain are they? Are the underlying mechanisms well understood? Around these kinds of questions some fruitful dealings between economists and ecologists may arise. The present paper pretends to collaborate with this dialogue.

Ecological discontinuities can be defined as a sudden change in any property of an ecological system as a consequence of smooth and continuous change in an independent variable. Ecological discontinuities imply critical values of the independent variable around which the system flips from one stable state to another, that is, ecological thresholds. The definition of thresholds can be arbitrary in the sense that it depends on the temporal and spatial scale adopted. In general, no description in ecology makes sense without reference to particular temporal and spatial scales (Levin, 1992). Ecological discontinuities are phenomena of special interest for economics when they comprise abrupt changes in the ecological services provided by the involved ecological system (Myers, 1996; Daily, 1997; Pimentel et al., 1997). These services can be supplied by different levels of the hierarchical organisation of ecological systems. Therefore, the most suitable temporal and spatial scales to address ecological discontinuities can vary broadly depending not only on the ecological system involved but also on the dynamics of the environmental services provided by it.

For the current theory of environmental externalities, to take into account ecological discontinuities is not a serious problem. Although present models usually assume a continuous relationship between production and environmental externalities (Turner, 1999), it would be not complicated to relax this assumption and to assume the possibility of discontinuities in the external cost curve. Rather, ecological discontinuities are a challenge for the economic theory because the threshold values and the magnitude of the change are generally uncertain. Perrings and Pearce (1994) deal with this problem by proposing a penalty to be paid when an environmental standard is surpassed. This standard would be a value of the independent variable that imposes a limit to the economic activity in order to maintain the risk of surpassing the ecological threshold at acceptable levels. The aim of this measure is to assure the resilience of ecosystems, which allows statistically predictable environmental externalities. This notion is based on the belief that once the ecological threshold is surpassed, the general equilibrium effects cannot be estimated ex ante. This kind of approach, based on safe-minimum standards, seems more suitable than market mechanisms to deal with complex economy–environment interactions because prices are unable to detect when a system is approaching a threshold (Bishop, 1993, Folke, 1999). However, if the critical values which may trigger an ecological discontinuity are basically uncertain, the safe-minimum standards can be very elusive and the process of standard-definition very conflictive. Hence, ecological research on the occurrence and predictability of ecological discontinuities can be a crucial input to the environmental decision-making process. The ‘quality’ of this input will determine the relevance of scientific advisory in the decisional process (Functowicz and Ravetz, 1994). Because of that, it is helpful to check the ‘state of the art’ of the ecological science on this subject.

The existence of ecological discontinuities is also a key feature to take into consideration for designing environmental management regimes. Perrings and Walker (1997) have shown, at a theoretical level, how different management strategies may influence non-linear and long-term dynamics of ecosystems. They point out that optimal management should be sensitive to key variables conditioning long-term resilience of the system, like fire frequency in grasslands (see below). Gunderson et al. (1997) introduce a set of propositions about the adequate scales of analysis, institutional strategies and variable characterisation and monitoring to deal with uncertainty linked to non-linear behaviour in natural systems. The aim of the present paper is not to go further on these issues, but instead to visualise the current state of knowledge about ecological discontinuities.

The second and third sections of this paper link the concept of ecological thresholds to the debate on the relationship between biodiversity and (a) resilience and stability of ecosystems and (b) ecosystem functions, respectively. Then, a fourth section reviews some examples of multiple states and ecological discontinuities in each of the current most important human impacts on ecosystems, population harvesting, pollution, habitat fragmentation, ecosystem management and biological invasions. The paper finishes with some concluding remarks.

Section snippets

Biodiversity, resilience and stability

The reference to the maintenance of ecosystems’ resilience as a key aspect of sustainability and as a crucial aim for environmental policy has been very present in the ecological economics literature (Common and Perrings, 1992, Arrow et al., 1995, Folke et al., 1996, Ludwig et al., 1996, Berkes et al., 1998, Levin et al., 1998). According to this position, ecological discontinuities may arise when human intervention has reduced enough of the ecosystem's resilience. When the system loses

Theoretical hypotheses

Environmental services may rely upon the functioning of ecosystems, that is, on aggregated ecosystem processes involved in the flux of mass or energy. The relationship between biodiversity and ecosystem processes is a topic receiving increasing interest in the ecological literature. Vitousek and Hooper (1993) recognise three possible relationships between biological diversity and ecosystem-level biogeochemical functions, (1) linear; (2) asymptotic and (3) non-existent. Case 1 is also called the

Examples of multiple states and thresholds

The existence of multiple stable states or alternative equilibria in ecological systems has been recognised since the 1960s in the ecological literature. Multiple stable points may occur at different levels of the ecological organisation, but usually they are defined as alternative species assemblage in a community (Scheffer et al., 1993) or multiple possible stable densities of a population (Loenardsson, 1994). Alternative assemblage of species may arise due to differences in the historical

Concluding remarks

There are multiple theoretical developments in ecology predicting the existence of ecological discontinuities triggered by threshold values of internal or external variables to the ecological systems. Empirical studies reveal that ecological discontinuities as the consequence of human impacts are not uncommon in nature. Nevertheless, these studies face some difficulties, mainly because the definition of alternative stable states is highly dependent on the chosen temporal and spatial scales, as

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