Document: EVA-3-8-5

Managing trophic cascades: too many, too little - too late?

COOCH, E.* ¹, R.JEFFERIES ² and R.ROCKWELL ³

Department of Natural Resources, Cornell University ¹
Department of Botany, University of Toronto ²
Department of Ornithology, American Museum of Natural History ³

Abstract:
Significant theoretical work has shown that the dynamics of trophic interactions can lead to rapid changes in the structure of ecological communities, and the development of new and potentially stable equilibrium states. Such 'trophic cascades' have been described as chain-like responses of one or more trophic layers following perturbation of another trophic level, and are commonly characterized as situations where strong interaction effects of species lead to exponentially increasing dominance of one or more trophic levels through the food chain. In terrestrial systems, trophic cascades are typically the result of anthropogenic enhancement of top-down or bottom-up dominance, generally by increasing the abundance of one or more trophic levels beyond the stability of the original equilibrium state(s). This might be achieved, for example, by an increasing energy or nutrient subsidy that promotes the population growth of (typically) the top-level organism. One example of the significant consequences of external and nutrient subsidies involves the impact of increasing numbers of several species of Arctic-breeding geese (which are obligate herbivores) on the trophic stability of the ecosystems in which they typically breed. While many factors are believed to have contributed to these increases, it is generally believed that changes in agricultural practices resulting in increased over-winter forage availability has been a significant cause of the increase. While intense herbivory does not result necessarily in the initiation of trophic cascades, there is an "a priori" expectation that the foraging activities of large colonial populations of geese will exert strong top-down effects on the ecosystem. Data from a long-term study of a colony of Snow Geese has shown that the pattern and extent of various grazing behaviors exhibited by the geese over the course of the breeding season significantly mediate the temporal and spatial dynamics of their primary food plants. Increasingly heavy feeding pressure by growing numbers of geese has resulted in a classical positive-feedback trophic loop, resulting in a rapid trophic cascade characterized by extreme degradation of large geographic areas of traditional goose foraging habitat. All of these factors are in turn influenced by underlying large-scale geological processes, and climatic change. Since the rates of change in the dynamics of all three primary trophic levels (soil, plants, and herbivores) are not synchronous, the pattern and process of changes in this system are complex. Further, because geese are mobile, the pattern of dispersal (spatially and temporally) can add to the complexity of the dynamics of this system. The extreme impacts of the increasingly abundant geese on Arctic ecosystems is cause for considerable concern, and has lead to several different management initiatives. However, adaptive management of goose populations is complicated by the complexity of the trophic interactions of this system, both spatially and temporally. In particular, the extreme non-linear and rapid response of the system to perturbation makes is difficult to specify the management objective function in any meaningful way if standard approaches to the problem are used. Several potential options for dealing with this potentially common problem are discussed.

Keywords: herbivory, trophic cascades, dispersal, adaptive management

This abstract is being presented at: 3:35 PM in session:
Symposium # 18: The Role of Theoretical Ecology in Biodiversity Conservation and Management.