Evolution of dispersal traits in a biogeographical context: a study using the heterocarpic Rumex bucephalophorus as a model

1. Seed dispersal is crucial in determining the spatial patterns of plant populations, and selection pressure for dispersal varies over different scales. However, few empirical studies have assessed differences in dispersal traits in a biogeographical context. 2. We studied dispersal traits in 46 populations of Rumex bucephalophorus from mainland and island locations, and from different habitats to test predictions of how insularity and habitat characteristics affect both dispersability (dispersal capacity and number of dispersible diaspores) and dispersal-distance strategy. We also conducted a greenhouse experiment to compare dispersability of plants in nature and a controlled environment. 3. Dispersability drastically changed when plants were cultivated in the greenhouse, revealing a high phenotypic plasticity for dispersal and showing that the maternal environment directly influences dispersal. However, dispersal-distance strategy, estimated by the production of either long diaspores (LD) or short diaspores (SD), seems to be genetically determined. Dispersability was markedly affected by the habitat in which the plants develop, but the patterns found are not consistent with theoretical predictions. We found two contrasting patterns of dispersal on islands: Atlantic island populations showed a trend to seed monomorphism and mid-dispersal strategy, whilst Mediterranean island populations showed seed heteromorphism and long-dispersal strategy. These contrasting patterns support theoretical predictions that long-distance dispersal evolves in heteromorphic plants, whilst in monomorphic plants only local dispersal evolves. Lastly, we have found a clear mid-dispersal-distance strategy in coastal sands and Atlantic islands; we suggest that the low environmental variation shared by these two zones selects for shorter distances of dispersal. 4. Synthesis. Most models describing the evolution of dispersal strategies assume that forces selecting for decreased dispersability also select for decreased dispersal distances. However, in R. bucephalophorus dispersal distance and dispersability showed contrasting patterns of variation. The fact that these two traits are differently determined could suggest that they can respond in a different manner to selective pressures.