Ecotone

An ecotone is a boundary or transition zone between adjacent ecological communities or systems. Ecotones are produced where a combination of environmental conditions and biotic interactions exceed the physiological limits of the species on either side. They may result from steep environmental gradients such as abrupt changes in temperature or soil moisture or from nonlinear responses of species to more gradual changes in environmental factors. They are a focus of current research in physical geography due to their scale-dependent nature, their effect on species population dynamics and the movement of matter and energy, their relevance to the study of ecological [Page 866]complexity and self-organized ecosystems, and their potential sensitivity to climate change. Recent advances in spatial technologies and techniques have allowed biogeographers and ecologists to understand better the important role that ecotones play in the pattern and functioning of the biosphere.

An alpine tree line (forest-tundra) ecotone in Glacier National Park, Montana

Source: Photo by D. Cairns. From Bekker, M. F. (2005). Positive feedback between tree establishment and patterns of subalpine forest advancement, Glacier National Park, Montana, U.S.A. Arctic, Antarctic and Alpine Research, 37(1), 97–107. University of Colorado, Boulder. Used with permission.

Endogenous feedback between trees, wind, and snowdrift produces this crenulated, advancing tree line (forest-tundra) ecotone in Glacier National Park, Montana.

Source: Photo by G. Malanson. From Bekker, M. F. (2005). Positive feedback between tree establishment and patterns of subalpine forest advancement, Glacier National Park, Montana, U.S.A. Arctic, Antarctic and Alpine Research, 37(1), 97–107. University of Colorado, Boulder. Used with permission.

A view of upper and lower tree line ecotones in Rocky Mountain National Park, Colorado

Source: G. Malanson.

The structural characteristics and dynamics of ecotones are scale dependent; they may appear sharp or gradual with varying spatial scales and static or ephemeral with varying temporal scales (see images). The ecotone concept, however, is scale independent; ecotones can be identified and have been studied at various scales, ranging from edge-effect investigations of wood lots in agricultural fields, to the boundary between trees and tundra on a mountain slope, to continental-scale biome transitions, such as those between deciduous forest and grassland in North America.

Frederic Clements coined the term ecotone in 1907 from the Greek words oikos (home) and tone (tension), accentuating the interaction between the adjacent communities. However, the work of Clements and other early ecologists emphasized the importance of and interaction among species within communities, which directed attention away from the boundaries between them. Additionally, analytical methods in ecology were designed to deal with homogeneous communities, which led ecologists to further ignore transitional areas. Research that did focus on boundaries emphasized an “edge effect,” which included both positive (e.g., increases in species diversity) and [Page 867]negative (e.g., increases in bird nest predation) impacts on wildlife and plant communities. The emphasis of these studies was on the effects of edges as static elements.

Within the past 25 years, four factors have shifted interest back to ecotones, emphasizing their role as a dynamic component of pattern in the biosphere. First, the emergence of landscape ecology led to the explicit consideration of interactions between vegetation patterns and environmental processes and to a focus on the effects of scale on those interactions. Patterns arise from heterogeneity, itself a consequence of boundaries between relatively homogeneous patches of vegetation, land, and water, and researchers have studied how the patterns created by ecotones affect the ability of species, matter, and energy to flow across a landscape. New conceptualizations of scale produced greater interest in the scale-dependent properties of ecotones, including the recognition that different factors constrain ecotone patterns and dynamics at different spatial scales and that the constraints are more numerous and complex at fine scales than at broad scales.

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