Ecological and Evolutionary Dynamics of Zostera japonica and Spartina alterniflora Invasions in the Eastern Pacific
Dwarf eelgrass (Zostera japonica) and smooth cordgrass (Spartina alterniflora) are ecologically important invaders of intertidal mudflats in the eastern Pacific. S. alterniflora and Z. japonica invasions alter estuarine nutrient dynamics, cause sediment and infaunal community changes, and modify waterfowl foraging habitats. Ecological and evolutionary mechanisms of the invasion success of Z. japonica and S. alterniflora were addressed with a combination of experimental and observational field methods and population genetic approaches. The following specific objectives were addressed. 1) Genetic correlates of S. alterniflora invasion success were assessed by comparing the multilocus microsatellite genotypes of individuals from multiple populations in the invasive (eastern Pacific) and native (western Atlantic and Gulf of Mexico) ranges. 2) Dispersal and colonization patterns of S. alterniflora in Willapa Bay and Grays Harbor (Washington, USA) were assessed using multilocus microsatellite genotype data. 3) The roles of competition and disturbance in the invasion success of Z. japonica and the concomitant decline of its native congener, Z. marina, in the Pacific Northwest (USA) were assessed using experimental and observational field data. 4) The interaction of landscape variation in hydrodynamic stress and competition with Z. japonica in the seedling recruitment of invasive S. alternif!ora was assessed using experimental and observational data. Invasive S. alterniflora populations contained high frequencies of novel genotypes and were comparable in their genetic diversity to populations in the native range. The genetic structure of invasive S. alterniflora in Willapa Bay was consistent with colonization by multiple founding foci and local dispersal. Native Z. marina and invasive Z. japonica both experienced substantial reductions in aboveground biomass in response to interspecific competition, relative to intraspecific competition. However, when both species were subjected to disturbance, Z. japonica productivity and fitness improved dramatically, while Z. marina performance correspondingly declined. Finally, Z. japonica had a consistently negative influence on S. alterniflora seedling recruitment, but the relative strength and importance of this interaction varied spatially with hydrodynamic stress. These results demonstrate that both ecological and evolutionary genetic processes can play important roles in the success of invasive species.