Community stability: effects of limpet removal and reintroduction in a rocky intertidal community

Limpet disturbance effects on barnacle recruitment are related to recruitment intensity but not recruit size

Intertidal limpets are important grazers along rocky coastlines worldwide that not only control algae but also influence invertebrates such as common barnacles. For instance, grazing limpets ingest settling barnacle cyprid larvae (hereafter cyprids) and push cyprids and barnacle recruits off the substrate. Such limpet disturbance effects (LDEs) can limit barnacle recruitment, a key demographic variable affecting barnacle population establishment and persistence. In this study, we examined limpet (Lottia cassis) disturbance to barnacle (Chthamalus dalli, Balanus glandula) recruitment on the Pacific coast of Hokkaido, Japan, as information on limpet-barnacle interactions from this region is missing. We investigated, for the first time, whether barnacle size and recruitment intensity influence LDEs on barnacle recruitment. Small barnacles may be less susceptible to LDEs than larger barnacles, because small size may reduce the propbability of limpet disturbance. Moreover, recruitment intensity can influence LDEs, as high recruitment can compensate for LDEs on barnacle recruitment density. In Hokkaido, C. dalli cyprids are smaller than B. glandula cyprids, and C. dalli recruitment is higher than B. glandula recruitment. Thus, we hypothesized that LDEs on C. dalli recruitment would be weaker than those on B. glandula recruitment. To test our hypothesis, we conducted a field experiment during which we manipulated limpet presence/absence on the interior surfaces of ring-shaped cages. After four weeks, we measured barnacle recruitment and recruit size on the interior surfaces of the cages and found negative LDEs on C. dalli and B. glandula recruitment and recruit size. As hypothesized, the LDEs on C. dalli recruitment were weaker than the LDEs on B. glandula recruitment. Additionally, C. dalli recruits were smaller than B. glandula recruits. However, the LDEs on C. dalli recruit size were as strong as the LDEs on B. glandula recruit size, indicating that the smaller C. dalli recruits are not less susceptible to LDEs than B. glandula recruits. Since C. dalli recruitment was higher than B. glandula recruitment, we propose that the higher C. dalli recruitment compensated for the LDEs on C. dalli recruitment. Our findings indicate that the detected differences in LDEs on barnacle recruitment are related to barnacle recruitment intensity but not recruit size.

Effects of herbivorous snails and macroalgal canopy on recruitment and early survivorship of the barnacle Semibalanus balanoides (L.)

This study investigated the effects of grazing by the herbivorous snail Littorina littorea (L.) and the presence of a macroalgal canopy on recruitment and early survivorship of the barnacle Semibalanus balanoides (L.) at four sites of various distances from the mouth of Toothacher Cove on Swans Island in the Gulf of Maine in 1998. Recruitment plates were attached to the substrate at each site in all possible treatment combinations, including either presence of absence of macroalgal canopy, presence or absence of the herbivore, and presence or absence of a cage. Significant differences in recruitment were observed among sites in April and May, but not in June. The average number of recruits also varied significantly among herbivore treatments in May and June, and in the May macroalgaexherbivore interaction. Survivorship did not differ among sites, but did vary significantly among herbivore treatments. These differences were due almost entirely to consistently low recruitment and survival in the uncaged or open controls. Comparisons of the caged and open treatments explained between 88 and 99% of the variation in the respective main effects tests. The presence or absence of L. littorea or of macroalgal canopy had little effect on S. balanoides recruitment or survivorship. Results support previous conclusions that recruitment is greater on more exposed shores and suggest that exclusion cages may protect barnacle recruits from factors such as desiccation and the whiplash effect of macroalgal fronds.