Ecological communities can undergo sudden and dramatic changes between alternative states. Understanding the mechanisms that trigger such shifts and those that maintain them is crucial for ecological prediction as well as natural resource management. Differentiating between potential mechanisms is difficult however, because shifts are often recognized only in hindsight, and many occur on such large spatial scales that field experiments to test their cause are not possible. To address this challenge, we focused on whether a single driver can both trigger the initial shift in state and maintain the new regime. We used this approach to evaluate a regime shift in which a filter feeding sea cucumber suddenly came to dominate reefs that had previously supported high abundances of macroalgae. We found that on these reefs a sustained period of low waves coincided with the shift to sea cucumber dominance, which persisted following the return to larger wave conditions, indicating that the new state was maintained by a mechanism other than low wave energy. Additionally, the period of sea-cucumber dominance occurred during a time when predators of the sea cucumber were rare, and the return of predators coincided with the demise of sea cucumber dominance and shift back to macroalgae. These results underscore the complex nature of regime shifts and demonstrate the utility of our proposed approach, suggesting that it is a productive first step for analyzing regime shifts in a range of systems.