Many populations are thought to be regulated, in part, by their natural enemies. If so, disruption of this regulation should allow rapid population growth. Such “enemy escape” may occur in a variety of circumstances, including invasion, natural range expansion, range edges, suppression of enemy populations, host shifting, phenological changes, and defensive innovation. Periods of relaxed enemy pressure also occur in, and may drive, population oscillations and outbreaks. We draw attention to similarities among circumstances of enemy escape and build a general conceptual framework for the phenomenon. Although these circumstances share common mechanisms and depend on common assumptions, enemy escape can involve dynamics operating on very different temporal and spatial scales. In particular, the duration of enemy escape is rarely considered but will likely vary among circumstances. Enemy escape can have important evolutionary consequences including increasing competitive ability, spurring diversification, or triggering enemy counteradaptation. These evolutionary consequences have been considered for plant–herbivore interactions and invasions but largely neglected for other circumstances of enemy escape. We aim to unite the fragmented literature, which we argue has impeded progress in building a broader understanding of the eco-evolutionary dynamics of enemy escape.

Understanding the combined effects of stressors on plants is important for understanding how they will tolerate herbivory and other damage under unfavorable conditions. When two stresses have synergistic effects, plants may experience particularly strong impacts. We examined individual and combined effects of drought stress and clipping (simulated herbivory) on two species of goldenrods (Solidago altissima L. and S. gigantea Ait.). Each stress reduced production of most plant structures, with drought stress having stronger impacts than clipping. The effects of the two stresses were additive for S. gigantea but synergistic for S. altissima, at least for aboveground biomass and inflorescence biomass. Both species, when under stress, reallocated resources toward asexual reproduction (rhizomes) and away from sexual reproduction (inflorescences). Our results suggest that even closely related plants may tolerate damage differently when under abiotic stresses, and that predicting the additive vs. synergistic action of combined stresses will be difficult.