University of Pennsylvania ecologist Peter Petraitis, California State Northridge biologist Steve Dudgeon, and their team have been returning to Maine’s rocky intertidal zone every spring and summer for nearly two decades. With support from the National Science Foundation (NSF), they survey a network of 60 experimental plots. The big question that brings them back year after year: Is an ecosystem like this a stable and permanent fixture, or, under harsh conditions, could it reach a tipping point? The idea is that changes in conditions could cause a switch from one community to another, such as from mussel beds to rockweed, and then back again. Part of the National Science Foundation Series “Science Nation.”