Implications of Spatial Heterogeneity for Catastrophic Regime Shifts in Ecosystems

Abstract

Although alternative stable states are commonly found in simple models, it seems reasonable to assume that the response of real ecosystems to environmental change should often be smoothed by spatial heterogeneity and other stabilizing mechanisms. Here, we systematically explore the effect of spatial heterogeneity on regime shifts for three different models, which we run on a one-dimensional lattice with different spatial distributions of an environmental factor (e.g., soil fertility, water level). If dispersion between patches is negligible, the response to gradual change in some overall stressor (e.g., precipitation, nutrient load) is straightforward. Because of the environmental heterogeneity, each patch shifts to the other stable state at different values of the overall control variable. Therefore, the response of the ecosystem as a whole is gradual (i.e., the average of many asynchronous small shifts) instead of catastrophic. However, in response to a reverse change in the global stressor, the system always shows hysteresis, as each individual patch shifts back to the original state at a different value of the control parameter than the critical threshold for the forward shift. If dispersion between patches occurs, the response to change in the overall control parameter becomes dependent on the spatial pattern of environmental heterogeneity. If the environmental parameter is randomly distributed in space, the overall response tends to remain surprisingly catastrophic, and hysteresis is hardly reduced as compared to the homogeneous case. By contrast, in a smooth environmental gradient, the response of the overall system is gradual, and hysteresis is much smaller. In fact, hysteresis is largely reduced to the initial phases, in which none of the patches have shifted to the alternative state yet. As soon as the first patch shifts, a domino effect occurs, pushing over the neighboring patches. In conclusion, our results suggest that spatial heterogeneity may weaken the tendency for large-scale catastrophic regime shifts if dispersion is unimportant or if local environmental characteristics vary along a smooth gradient.

Shallow lakes theory revisited: various alternative regimes driven by climate, nutrients, depth and lake size

“…Consequently hysteresis may be less pronounced (and can even disappear) if the depth profile of a lake is less flat (Van Nes et al, 2002). More in general, it has been shown for a range of ecological models that spatial heterogeneity of the environment tends to reduce the chance that large scale shifts between alternative stable states are found (Van Nes & Scheffer, 2005). This seems intuitively straightforward as the shifts in the local states do not happen at the same moment, simply because different sites have different local properties.…”

“…For instance, if water circulates through a lake rapidly, one expects the clearing effects of submerged plants to contribute to the overall change of turbidity in the lake, but not to result in a locally higher clarity. Analysis of different models shows that such mixing can counteract the smoothing effect of environmental heterogeneity, and cause the system to show large scale synchronous sharp shifts again (Van Nes & Scheffer, 2005).…”

Toxicity of reduced nitrogen in eelgrass (Zostera marina) is highly dependent on shoot density and pH

“…However, in our case, the feedback mechanism may be more complex: a high eelgrass density can also imply a higher photosynthetic activity and thus a higher pH, which in turn may lead to a higher toxicity. Therefore, the outcome in a particular Weld situation will strongly depend on the NH x loads, the photosynthetic activity, and the exchange rates between the seagrass meadow and the surrounding water layer (van Nes and ScheVer 2005). The occurrence of alternative stable states may at least in part explain the worldwide observed seemingly high resilience to change, sudden ecosystem collapses, and mass mortalities in seagrasses (van der Heide et al 2007).…”

The importance of within-system spatial variation in drivers of marine ecosystem regime shifts

“…One rarely addressed challenge is to account for the role of within-system spatial heterogeneity for regime shifts [1]. However, theoretical examinations suggest that such heterogeneity may thwart regime shifts if, for example, environmental characteristics vary along a smooth gradient [10]. Also, the spatial scales at which key drivers operate are critical [11,12] as spatial averaging has a blurring effect on the evaluation of the roles of potential mechanisms [13].…”