Long-term ecological dynamics: reciprocal insights from natural and anthropogenic gradients

Abstract

Many ecological dynamics occur over time-scales that are well beyond the duration of conventional experiments or observations. One useful approach to overcome this problem is extrapolation of temporal dynamics from spatial variation. We review two complementary variants of this approach that have been of late increasingly employed: the use of natural gradients to infer anthropogenic effects and the use of anthropogenic gradients to infer natural dynamics. Recent studies have considered a variety of naturally occurring gradients associated with climate, CO2, disturbance and biodiversity gradients, as well as anthropogenic gradients such as those created by biological invasions, habitat fragmentation and land abandonment. These studies show that natural gradients are useful in predicting long-term consequences of human-induced environmental changes, whereas anthropogenic gradients are helpful in inferring the mechanisms behind natural dynamics because covarying factors are often more clearly understood in anthropogenic gradients than in natural gradients. We classify these studies into several categories, each with different strengths and weaknesses, and outline how the limitations can be overcome by combining the gradient-based approach with other approaches. Overall, studies reviewed here demonstrate that the development of basic ecological concepts and the application of these concepts to environmental problems can be more effective when conducted complementarily than when pursued separately.

Figure 1

Examples of gradient analyses involving (a) direct, (b) indirect or (c) no space-for-time substitution. In (a), a gradient of site age across the Hawaiian Islands (left panel) was used to infer long-term changes in the diameter and height of the dominant tree species (right panel) and other community and ecosystem properties (adapted from Vitousek 2004). In (b), a gradient of atmospheric CO₂ concentrations created by a natural CO₂ spring in New Zealand (left panel) was used to infer long-term changes in microbial C (right panel) and other community and ecosystem properties (adapted from Ross et al. 2000). In (c), a gradient of the density (presence or absence) of introduced ungulates in Argentina (left panel) was used to infer long-term effects on plant densities (right panel), pollination and reproduction (adapted from Vázquez & Simberloff 2004). L-C, S-A, M and Q in the right panel are the initial letters of the site names shown in the left panel.