We studied how substrate availability and dispersal ability influence the distribution pattern of a saproxylic beetle, Spasalus crenatus (MacLeay) (Coleoptera: Passalidae), in a subtropical forest. Members of this species consume deadwood and contribute to material cycling in forests. We tested two hypotheses to explain abundance and distribution patterns of S. crenatus in Puerto Rico: 1) substrate availability of fallen trunks limits the establishment of colonies (resource hypothesis) and 2) dispersal ability of the beetles limits their distribution (dispersal hypothesis). To test these hypotheses, we performed one field and one experimental study. In the field study, we examined the species composition of fallen trunks, bark density, and presence of bess beetle colonies in 16 transects (60 by 10 m) in the Luquillo Experimental Forest. The field results suggest that beetles have limited mobility and that they have preference for colonizing some species and avoiding other species. In the experimental study, we examined mobility and substrate association of S. crenatus. We placed three trunk species at various distances to observe colonization by experimental individuals. Beetles colonized trunks near the release point, with preference for harder woods. Both hypotheses are supported; availability of resources and dispersal ability contribute to the distribution pattern of this beetle in Puerto Rico. We discuss the results in light of evolutionary pressures underlying island ecology. In addition, understanding population dynamics of S. crenatus on an island will enable comparisons with mainland populations.
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Vol. 100 • No. 5