Revisiting Connell: Competition but not as we know it.

Abstract

<jats:p>Space is one of the primary limiting resources for organisms on the intertidal rocky shore. This paper examined the effect of reduced density on key traits (mortality and growth) on the intertidal barnacles, <jats:italic>Chthamalus montagui</jats:italic> and <jats:italic>Semibalanus balanoides</jats:italic>, on the mid-shore in Plymouth, UK. Intra- and interspecific treatments comprising of <jats:italic>C. montagui</jats:italic> and <jats:italic>S. balanoides</jats:italic> were manipulated to reduce densities at two similar sites. Changes in mortality and operculum growth were assessed over an 8-week period using digital photography. Covariates of growth included nearest neighbour distance, competition between closest pairs and initial size. Conflicting patterns were observed when comparing growth rates between treatments and sites. At Site 1, interspecific treatments had a lower growth rate than intraspecific treatments, whereas at Site 2, interspecific growth rates were higher. ANCOVA showed that nearest neighbour distance had no significant effect on growth, but when comparing differences in growth of closest neighbouring pairs, <jats:italic>C. montagui</jats:italic> treatment showed evidence of competition whereas <jats:italic>S. balanoides</jats:italic> did not. ANCOVA analysis indicated no difference in growth between each outcome of pair competition, suggesting winners are initially bigger than losers. Comparisons of mortality between treatments indicated mortality over time with no significant differences observed between treatments, but response surface methodology (RSM) revealed no effects of competition on mortality of <jats:italic>S. balanoides</jats:italic>, but negative effects of both intra- and interspecific competition on <jats:italic>C. montagui</jats:italic> survivorship. Examination of natural populations of barnacles in the mid-shore indicated there was strong spatial variation in growth rates, perhaps driven by small-scale differences within sites.</jats:p>