Nonadditive interactions between trophic levels bias the appraisal of the strength of mortality factors

Abstract

AbstractThe role of bottom‐up and top‐down factors in determining the abundance of organisms is still often evaluated in terms of the nominal (i.e., apparent) cause of death. To determine whether estimates of mortality can be influenced by nonadditive multitrophic interactions, we generated both marginal (i.e., the entirety of mortality attributable to a given factor, including its obscured fraction) and nominal estimates of the trophic forces acting on an herbivore. This was accomplished by comparing mortality rates of the willow leaf beetle Phratora vulgatissima on willows affected or relatively unaffected by local growing conditions, in the presence or absence of natural enemies. Marginal estimates indicated that bottom‐up and top‐down mortality factors interacted in a synergistic or compensatory way, with the nature of the interaction varying in response to the time elapsed since the last harvest of willow plantations. In contrast, nominal estimates could not discriminate synergistic and compensatory interactions from top‐down influence. Combined with recent evidence of compensatory mortality between trophic forces in another system, this study suggests that nonadditive effects between bottom‐up and top‐down mortality factors may be common, offering an explanation through which the contrasting evidences previously presented by proponents of the bottom‐up and top‐down views can be understood.