POSITIVE INTRASPECIFIC EFFECTS TRUMP NEGATIVE EFFECTS IN HIGH-DENSITY BARNACLE AGGREGATIONS

Abstract

High-density aggregations of conspecifics have long been presumed to have negative effects on individual fitness, yet this has not been adequately tested for multiple life history traits in the field. Here I report a series of investigations that comprehensively evaluate the influence of conspecific density on key traits (survival, growth, and repro- duction) in the intertidal barnacle Balanus glandula. Density was manipulated in mid- intertidal barnacle populations living on both natural rock surfaces and on settlement plates, and monitored in natural populations. Survival was positively related to density: populations at the lowest density level were 19 times more likely to experience $95% mortality than those at the highest level. In contrast, growth and individual reproductive output were negatively related to density: animals at lower experimental densities (16-50 adult bar- nacles/100 cm 2 ) grew to 1.6 times the diameter of individuals living at higher densities (200-460 adult barnacles/100 cm 2 ), and larger barnacles produced larger brood masses than smaller individuals. Overall, the frequency of brooding barnacles in experimental and natural populations was greatest at intermediate densities. Estimation of the basic repro- ductive rate for the experimental animals on the settlement plates indicated that the primary effect of increased conspecific density was enhanced survival, and consequently, a larger mean number of larvae produced per original recruit. Thus in this case, the positive intra- specific effects of high-density aggregations outweighed the negative effects. My results demonstrate that key life history traits may not have similar responses to changes in density, and provide a strategy for evaluating the relative importance of positive vs. negative in- traspecific interactions.