| Summary: | Author's post-print is released with a Creative Commons Attribution Non-Commercial No Derivatives License (CC BY-NC-ND 4.0) as required by journal policy. Marine and terrestrial communities are often hierarchically structured by one or more foundation species, which provide habitats for many other taxa. Interactions between coexisting habitat modifiers may have strong effects on patterns and processes in the dependent assemblage. Yet they are rarely studied, especially at a small scale. Small epibenthic patches co-dominated by barnacles Balanus crenatus Brugiere and several species of solitary ascidians in the White Sea soft bottoms support many dependent species. Barnacles occupy bivalve shells, small stones and conspecifics. Ascidian clumps develop on barnacles and their empty shells. Previous observations suggest that at the patch scale ascidians may replace barnacles over several years likely because of the negative interactions between them. Barnacles have distinct annual growth rings on their shells, which we used to trace their growth and survival in the field. No difference between the patches with different dominants would evidence no pronounced negative effect of ascidians. In the patches dominated by ascidians (A) or barnacles (B) collected at the same subtidal site in 2004 and 2005 we compared lengths of recent annual vertical growth increments and dead:live ratios of barnacles of the same age class according to the growth rings. Barnacles grew slower in A than in B, regardless of the biomass of conspecific neighbors. Dead:live ratios were higher in A for age classes 1+…2+ and 4+…9+. Estimated mortality risk between A and B increased with age of barnacles, from around 1:1 to 5.6 times greater in the 9+ age group. Because of the observed difference in growth and survival, the negative effect of adult ascidians on barnacles could not be excluded; alternatively, ascidians may prefer the patches with declining barnacles, or there could be an unknown external process that negatively affects barnacles and favors ascidians. Russian Foundation for Basic Research (grants no. 05-04-48927A, 05-04-63041K, 06-04-63077K, 08-04-01373A, 08-04-10109K)
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