Gradual regime shifts in Namibian fairy circles
The response of ecosystems to climate variability and anthropogenic disturbances is a fundamental aspect of ecology. Much attention has been devoted recently to the behavior of ecosystems near tipping points, where small environmental changes or disturbances can induce large responses. These "catastrophic regime shifts" are conceived as abrupt transitions between two alternative stable states occurring uniformly across the ecosystem. However, more complex responses have been suggested, stemming from the spatial nature of the system. Spatially extended ecosystems can self organize in a multitude of spatial patterns. In fluctuating environments, this can result in gradual rather than catastrophic regime shifts.
This prediction of pattern formation theory has never been tested in a real ecosystem, nor in a model describing the dynamics of a specific ecosystem. One candidate for such an investigation is the Namibian fairy-circle ecosystem, which is fairly homogeneous and undisturbed, and the subject of intense research. This ecosystem consists of a uniform matrix of perennial grass, punctured by circular gaps of sandy bare soil, the fairy circles, that on landscape scales form nearly periodic patterns. Moreover, dynamics of "birth" and "death" of these gaps have been reported, and are to date left unexplained.
Using empirical data and mathematical modeling we investigated the dynamics of the Namibian fairy-circle ecosystem as a case study of gradual regime shifts. Using our model we can reproduce the dynamics of birth and death of single fairy circles, as a result of variability in precipitation. Furthermore, using statistical analysis we correlate the change in both number and size of fairy circles to the precipitation, and show that the system exhibits different time scales in its response.
Our study provides new support for the view of fairy circles as a self-organization phenomenon driven by water-vegetation interactions, and interprets fairy-circle birth and death processes as transitions between multiple stable states. The study further demonstrates the feasibility of gradual regime shifts as unidirectional cascades of fairy-circle birth or death processes, and suggests that gradual regime shifts may occur in other pattern-forming ecosystems as well.