A model for range expansion of coastal algal species with different dispersal strategies:: the case of Fucus serratus in northern Spain

A model was used to investigate the nature of the distributional boundary of the brown alga Fucus serratus in northern Spain. The model explored the colonisation of a shore by invaders with contrasting dispersal modes: an exponentially bounded mode, as an example of short-range dispersal; an extreme, long-range dispersal mode; and a mixed mode with variable proportions of short- and long-distance dispersers. The organisms were dioecious, had a limited life-span and reproduced only when plants of different sexes were within a short distance. For species with exponentially bounded dispersal and interannual variability in the environment, the model reproduced the basic features of the distribution of F serratus: survival and reproduction beyond the distributional boundary, sharp boundary and displacements of the boundary through time. Species with exclusively long-range dispersal exhibited continually accelerating rates of spread under favourable conditions. However, long-range dispersers might fail to invade or become extinct when exposed to a short series of unfavourable years. Regional abundance is critical for the persistence of local populations. For long-range dispersers, no distinct distributional boundary was recognised. At the opposite extreme, invaders with exponentially bounded dispersal had small and decelerating rates of spread, high resistance to disturbances and distinct distributional boundaries; the persistence of populations mainly relies upon local abundance. Mixed strategies, with most of the propagules having exponentially bounded dispersal and a small fraction having long-range dispersal, appear to be the superior strategy; they combine rapid colonisation rates and high resistance to disturbances. The combination of Allee effects and propagule pressure (i.e. number of propagules arriving at a given locality) explains the differences among dispersal strategies.