Threshold facilitations of interacting species

The dynamics of species interactions are of central importance for the understanding of ecological coexistence, community structure and the effects of biological invasions. Using bark beetles that colonize the same habitat as an example, we explore species interactions in a resource-based model system with positive feedback between insect abundance and resource availability. The net interspecies interaction was found to be highly dynamic and may alternate in time between competition and mutualism. When both bark beetle species were able to kill trees (aggressive), our simulations showed strong facilitations between beetle species. This may lead to escape from control by competition, and increase the frequency of outbreaks of tree-killing. The frequency of net positive interactions varied with interaction strengths and the relative aggressiveness of the species and was highest when both species were strongly aggressive; which predicts disastrous outbreaks if, e.g., the European spruce bark beetle Ips typographus and the North American spruce beetle Dendroctonus rufipennis should become interacting species due to introductions. In imbalanced pairs, the relatively less aggressive species was facilitated more often than the aggressive species. Net positive interactions did not occur for strongly inferior species, but their survival was an increasing function of interaction strength with aggressive species and availability of resources. The benefits for the inferior species in the model are consistent with the structure of one aggressive and several less aggressive or non-aggressive species, which is common in bark beetle communities in many parts of the world.