Salt-marsh invasions by the grass Elymus athericus (Poaceae) recently transformed usual areas dominated by Atriplex portulacoides (Chenopodiaceae) into homogeneous meadows. Two wolf spider species, Pardosa purbeckensis and Arctosa fulvolineata, show contrasting densities and habitat preferences in salt marshes (respectively dominant and co-dominant ground-living spiders) and oppositely respond to the grass invasion. This allowed us to test whether invasive species that alter habitat structure affect reproduction in addition to previously recorded changes in density. Reproductive traits (female mass, cocoon mass, number and volume of eggs, hatched cocoon as a proxy of reproduction date) were studied in both invaded and natural salt marshes during 2007 and 2008 in the Mont St-Michel Bay (France). In both species, reproductive outputs (cocoon mass) were higher in optimal habitats and volume of eggs was found to be independent from female mass, whereas the latter significantly influenced the number of eggs. In A. fulvolineata, lower reproductive outputs due to less numerous although larger eggs were found in suboptimal habitats whereas the opposite pattern was found in optimal habitats, showing the existence of plastic phenotypic trade-offs in habitats of different qualities. In P. purbeckensis, despite differences in population size among habitats, no reproductive trade-off was found. This study thus shows that two sympatric species belonging to the same family can differ in reproductive strategies and phenotypic plasticity under changes in habitat suitability.