Allee effects and conspecific cueing jointly lead to conspecific attraction.

Conspecific attraction is the preferential settlement into habitat patches with conspecifics. To be a good proximate strategy, fitness gains from settling with conspecifics must outweigh the costs of higher conspecific densities, such as intraspecific competition. Two types of benefits have been proposed to explain conspecific attraction: Allee effects (i.e., positive density dependence) and conspecific cueing (using conspecifics as an indicator of habitat quality). I present empirical evidence for conspecific attraction in the settlement of the porcelain crab, Petrolisthes cinctipes Randall (Anomura: Porcellanidae). Previous work demonstrated that P. cinctipes experiences strong intraspecific competition and that both Allee effects and conspecific cueing are present in P. cinctipes life-history. I developed an empirically-based fitness model of the costs and benefits of settling with conspecifics. Based on this model, I simulated optimal settlement to habitat patches that varied in conspecific density and habitat quality, where the correlation between density and habitat quality determined the level of conspecific cueing. I tested whether Allee effects alone, conspecific cueing alone, or Allee effects and conspecific cueing together could provide an ultimate explanation for the proximate settlement behavior of P. cinctipes. The settlement simulation was consistent with empirical settlement only when Allee effects and conspecific cueing were both included. Three life-history features are critical to this conclusion: (1) fitness is maximized at intermediate density, (2) fitness depends on the decisions of previous settlers, and (3) conspecific density provides good information about habitat quality. The quality of information garnered from conspecifics determines whether conspecific attraction is a good proximate strategy for settlement. I present a graphical illustration demonstrating how Allee effects and conspecific cueing work together to influence fitness, providing a conceptual framework for other systems.