Cover: Models predicting interactions between the native Iris hexagona (blue lines) and invasive Iris pseudacorus (yellow lines) in different disturbance scenarios. In benign (freshwater) conditions, the invasive eventually drives the native species to extinction (top left). Hurricanes (lower right) reverse this result by increasing salinity, to which the native is better adapted. Depending on frequency, hurricanes can eliminate the invasive entirely or allow the species to coexist. Photo of I.hexagona by Mark Tobler, modified by Karl Hasenstein to represent the duality of species interactions. See pages 164–173.