Density-dependent regulation in an invasive seaweed: responses at plant and modular levels

1 The effects of density on the vital rates, growth form and population size structure of the invasive seaweed Sargassum muticum (Yendo) Fensholt were evaluated experimentally. 2 The initial difference in plant number between the highest and the lowest density (200 and 6400 plants m − 2 ) was considerably reduced by the end of the experiment. Surprisingly, this was mostly due to numbers increasing at lower densities, probably because microscopic forms were not removed during the experimental thinning. 3 The allometric length–dry mass relationship fitted a simple linear model on log-log scale for both the highest and the lowest densities, but had different slopes. At higher densities plants became taller and thinner as a consequence of variations in the production and growth of modules. 4 Mean size (dry mass) and the development of size hierarchies of plants were affected by both the addition of further (microscopic) recruits and asymmetrical competition among plants. Plant length distributions were also influenced by changes in the growth form of plants. The length hierarchies of main branches also suggested the presence of asymmetrical competition at this modular level. 5 Density influenced both mean size and morphology of the plants and thus induced changes in reproduction. The negative effect of density on individual plant size reduced the percentage of fertile plants and possibly their annual reproductive allocation, but these effects may be attenuated by morphological responses. 6 The responses of S. muticum to crowding are closely linked to its ability to colonize bare space. The massive reproductive output and very limited dispersal range account for local and dense recruitment patterns. Our results suggest that the responses of S. muticum to crowding allow the establishment of dense populations with high persistence and resistance to colonization by other species.