Resource partitioning and overlap in three sympatric species of Ips bark beetles (Coleoptera : Scolytidae)

The bark beetles Ips pini, I. perroti, and L grandicollis are sympatric in pine forests of the northcentral United States. They share the same limited phloem resource and often coexist within the same host trees. We tested wh ether phloem resources are partitioned in time and space by measuring spati al and seasonal colonization of logs. Differences among species in flight p henology, development time, voltinism, and spatial colonization patterns wi thin logs reduce, but do not eliminate, species overlap. The bark beetle sp ecies share predation by Thanasimus dubius (Cleridae) and Platysoma cylindr ica (Histeridae), which exploit pheromone signals for prey location. We emp loyed pheromone traps to test for chemical communication among, bark beetle species. Heterospecific signals tend to be deterrents when they are added to conspecific signals but attractants when they are alone, indicating that the communication system can both reduce and increase species overlap in r esource use depending upon relative abundance of the species. Deterrence by heterospecific signals is probably a result of selection for minimizing in terspecific competition. However, individuals may sometimes benefit from jo ining aggregations of other species because of (1) predator swamping, (2) i mproved success in attacking live trees, and (3) location of suitable, rece ntly dead, trees. These benefits should be greatest for males (which locate and colonize host trees before signalling females) and indeed males tended to be more attracted than females by heterospecific signals. Shared resour ces, shared predators, and heterospecific pheromone communication all contr ibute to species interactions in this guild of bark beetles, but predicting whether the removal of one species will tend to increase or decrease the a bundance of remaining species remains difficult. Species interactions are l ikely conditional and coexistence is probably promoted by benefits to rare species of multispecies associations.