Regime shifts by front dynamics

Ecosystems are highly nonlinear dissipative systems that often admit multiple alternative stable states, but seldom have the time span to converge to these states as they are repeatedly interfered by environmental changes and disturbances. Multiplicity of stable states in variable and disturbed environments is of high concern because of the possible occurrence of regime shifts, i.e. state transitions that involve large, abrupt decline in ecosystem function. State transitions induced by disturbances, however, need not be abrupt as the disturbances are often limited in their spatial extent. In that case they induce local state transitions that result in spatially confined alternative-state domains. The subsequent dynamics crucially depend on the dynamics of the transition zones or fronts that bound these domains. In this paper we study three aspects of front dynamics that affect the nature of regime shifts and their outcomes: single-front motion, front interactions and front instabilities. Using models of dryland vegetation as a case study we show that fronts can change their speed and invert their direction of motion as parameters change across a threshold value - the Maxwell point, that fronts may slow down or stop entirely when interacting with other fronts, and that disturbances may change one front type to another, each with its distinct speed. We further study the implications of these three aspects of front dynamics to gradual regime shifts. We show that such shifts can occur due to the propagation of a perturbation beyond its original extent, that such a transition does not necessarily culminate in the alternative state, i.e. it may remain incomplete, and that the time it takes for a regime shift to occur can be changed by front manipulations. Since gradual regime shifts can take place far from the typical tipping point considered, early warning signals of an approaching tipping point would fail in predicting gradual regime shifts. A more relevant approach for these regime shifts may be the development of detection and prevention methods, which can be applied during an ongoing gradual regime shift due to its slow dynamics. We conclude with a discussion of the need to develop such methods and of other ecological contexts that may benefit from them.