Direct observation of increasing recovery length before collapse of a marine benthic ecosystem.

Ecosystems can experience catastrophic transitions to alternative states, yet recent results have suggested that slowing down in rates of recovery after a perturbation may provide advance warning that a critical transition is approaching. Perturbation experiments with microbial populations have supported this hypothesis under controlled laboratory conditions, but evidence from natural ecosystems remains rare. Here, we manipulated rocky intertidal canopy algae to test the hypothesis that the spatial scale at which the system recovers from a perturbation in space should increase as the system approaches the tipping point, marking the transition from a canopy-dominated to a turf-dominated state. Empirical estimates of recovery length, a recently proposed spatial indicator of an approaching tipping point, were obtained by comparing the spatial scale at which algal turfs propagated into canopy-degraded regions with decreasing canopy cover. We show that recovery length increased along the gradient in canopy degradation, providing field-based evidence of spatial signatures of critical slowing down in natural conditions.