Fig. 1. Transient invaders can induce shifts between alternative stable states in a laboratory ecosystem.

(A) We exposed cocultures of Lp and Ca to a serial dilution protocol that includes daily migration of fresh cells from both species. (B) Average fraction (three replicates, SE smaller than linewidth) of Lp cells in the community at the end of each dilution cycle, as described in (A). Depending on the initial species fraction, cocultures reach a different outcome in which either species grows to dominate the system. The inset cartoon shows a mechanical analog of the ecosystem: Each of the two basins of attraction can keep the marble (the community) in an alternative stable state. (C) We explored the effects of invasions into this bistable ecosystem. The cartoon shows an unsuccessful invasion by Pc that nevertheless induced a shift toward an alternative stable state. (D) Time series for the cell densities during an unsuccessful invasion by Pc (bars show the SE of three replicates). The inset cartoon depicts this invasion event as a perturbation that drives the system toward an alternative basin of stability, where it remains after the perturbation is gone.