Adapting to beach and dune environments: A study of fitness and ecological factors in the coastal plant Cakile edentula

Species’ geographical and ecological ranges are limited by the interaction of genotypes with the biotic and abiotic environment, and the quality of the habitat relative to the species’ niche requirements. A fundamental question in ecology and evolution is how species will respond to changes in their environment; whether species will shift their distribution to track the niche environment, locally adapt, or become extirpated. Along an ecological gradient, the factors defining a species’ limits and abundance are expected to vary. Species are defined by the interactions of genotypes with the environment, and these interactions may be constant in space with a strong genetic basis or vary plastically with the environment. This study assessed the phenotypic differences of Cakile edentula (sea rocket; Brassicaceae) plants along the beach to dune gradient within the species’ Atlantic Canadian range, and attempted to disentangle whether plants growing along this gradient plastically respond or are locally adapted to their home microenvironment (beach vs. dune) within the coastal habitat. Empirical plant surveys in wild populations, and a greenhouse and a reciprocal transplant experiment all revealed significant phenotypic differentiation between beach and dune plants. The patterns of variation for most traits observed in the wild were not reflected in the controlled environment, suggesting plasticity rather than genetically based trait variation accounts for most variation observed in the wild. However, there was evidence that some traits (stem elongation and seed set) had a genetic basis as they varied consistently between beach and dune plants in all environments. A greenhouse experiment manipulating growing density and salinity exposure provided evidence that increased growth and fitness associated with wild beach plants compared to dune plants may be attributed, at least in part, to the combined effects of plant density and sea spray. Moreover, this work supports the expectation for temporally and spatially variable environments that phenotypic variation in plants is primarily plastic, with some influence of maternal plant environment to offspring growth and fitness in the next generation. Overall, this study suggests C. edentula plants are highly plastic, and fitness variation between beach and dune plants likely indicates variation in habitat quality relative to the species’ niche