Determinants of spatial pattern at different scales in two populations of the marine alga Rissoella verruculosa

Understanding linkages between patterns and processes across scales is an important issue in ecology. We investigated the causes of spatial variation at different scales in 2 populations of the red alga Rissoella verruculosa (Bertoloni) J. Agardh, located on shores about 70 km apart in the northwest Mediterranean. We used transplantation and removal experiments in combination with measures of recruitment to test whether the similarities of patterns between populations, as documented in previous studies, were produced by the same processes. Results pointed to the importance of pre-emption of the substratum in setting both upper and lower margins of distribution of R. verruculosa, Low recruitment contributed to setting lower margins, but did not explain along-shore variation in cover. Intensity of recruitment and post-recruitment mortality were much greater at the northern location in high-shore habitats, whilst fronds attained a larger size at the southern location in low-shore habitats. These data suggested that the processes maintaining the upper and lower margins of distribution of R. verruculosa differed between populations, but produced similar patterns at the 2 locations. Thus, similarity of pattern did not imply that the same underlying processes were operating in different places. Identifying the processes that do not leave signatures in existing conditions, but that may do so if conditions change, can increase our ability to understand and predict spatially and temporally variable patterns in assemblages.