Do Saproxylic Curculionids Affect the Fitness of Co-occurring Cerambycids?

Saproxylic insects sometimes coexist in incredibly high numbers under bark and share common resources. Thus, interactions between species are possible and could even explain their coexistence. This study investigates evidence of negative or positive effects of curculionid beetles (Coleoptera: Curculionidae) on cerambycid beetles (Coleoptera: Cerambycidae) that coexisted in dead tree branches in Costa Rica. Co-occurrence analysis and generalized regressions were used to test associations between cerambycid and curculionid species. Three cerambycid species that each co-occurred with a curculionid species were selected to measure fitness. Fitness measures of the cerambycid were compared with abundance of the co-occurring curculionid to assess the possible impact of curculionids on cerambycids. Elytral length and emergence week were measured and compared as proxies for fitness. One-hundred and nineteen species pairs showed significantly positive co-occurrence, and none showed significant negative co-occurrence. The three focal species showed no strong evidence for facilitation or competition with co-occurring curculionids, but there were significant differences in fitness measures and they deserve further study. In a canopy branch of the tree species Eschweilera biflava, many species and individuals emerged, and, despite the high density, the cerambycid N. mutilatus emerged in high numbers, early and quickly, and with similar elytral lengths as those in other branches. This could be evidence for facilitation by co-occurring ambrosia beetles, or for cryptic variation among conspecific trees