Possible influences of plasticity and genetic/maternal effects on species coexistence: native Gammarus fasciatus facing exotic amphipods

1. Native species might be able to persist in the face of a detrimental exotic invader by occupying refuge habitats where the exotic is not successful. These refuges might then provide demographic subsidies that enhance persistence of the native species in areas of coexistence with the exotic species. Phenotypic plasticity of the native species could play a critical role in this process by allowing individuals dispersing from refuge to invaded habitats to adaptively modify their phenotypes, thereby enhancing the subsidy. By contrast, locally adapted genetic or maternal differences between native populations could result in fitness disadvantages for dispersers at invaded habitats, thereby reducing the subsidy. 2. We studied phenotypic variation in a native amphipod (Gammarus fasciatus) across ion gradients in Lac St. Louis, Qu ebec, Canada. Ion gradients determine refuge vs. invaded habitats because native amphipods are found in both ion-poor and ion-rich habitats, whereas exotic (Echinogammarus ischnus) amphipods are found only in the latter. 3. We tested for plastic vs. genetic/maternal contributions to spatial variation (ion-rich vs. ionpoor habitats) in native amphipod fitness components: postmoult calcification, body size, larval survival, time to first reproduction and fecundity. We did so by comparing the following: (i) F0 (collected from the wild) individuals between the two habitats – some of which were reared for a time under varying conditions in the laboratory; and (ii) F1 (reared in the laboratory for their entire lives) individuals from the two habitats under a variety of water conditions. 4. We found strong plastic effects on postmoult calcification – it was more rapid in ion-rich water. We found genetic/maternal differences in time to reproduction (almost twice as long in ion-poor amphipods), fecundity and larval survival (higher in ion-poor amphipods) in early summer. These plastic effects and genetic/maternal differences are likely to be adaptive. 5. Plasticity and genetic/maternal effects both could influence the efficacy of ion-poor populations in providing demographic subsidies that aid native persistence at ion-rich habitats where exotic amphipods are present. These effects could thus have important consequences for native species persistence in the presence of non-native species.