THE RELATIVE IMPORTANCE OF PROCESSES AFFECTING RECRUITMENT OF TWO TEMPERATE REEF FISHES

Abstract

Local population densities of most benthic marine organisms are determined by a variety of processes that act before, during, and after settlement of planktonic propagules to the benthos. Considerable disagreement exists over the relative contribution of each process to patterns of abundance. Using multifactorial experiments, I assessed the effects of resident conspecifics, potential interspecific competitors, predators, and reef location on patterns of recruitment of two temperate reef fishes. Recruitment of bluebanded gobies (Lythrypnus dalli) and blackeye gobies (Coryphopterus nicholsii) consistently increased with distance from a nearby large reef. Recruitment of Lythrypnus was enhanced by the presence of older resident conspecifics. I found no strong evidence of any competitive effects between the two species. Predators had a variety of important effects on the density of recruits. Recruitment of Lythrypnus was halved by predators; this was the result of direct and indirect effects of predators: in addition to directly reducing recruit density, predators also reduced recruitment indirectly by reducing the density of older residents to which recruits responded. I developed two related methods for quantifying the contributions of the direct and indirect components of the predator effect. The effect of predation was spatially variable for both species: the net effect of this variability was the elimination of recruitment patterns related to reef location. Hence, the effects of spatially variable input of recruits was dependent upon the presence of predators, and thereby, predation altered the relative importance of the other factors that affected recruitment of the gobies. I argue that a much broader understanding of the “relative importance” of multiple causal processes will be gained by striving to understand how the contribution of each process is modified by other processes than by attempting to quantify the “importance” of each process.