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Causes and consequences of dynamic feedback in population dynamics of Dendroctonus frontalis (Coleoptera: Scolytidae).
Ayres, Matthew1, Martinson, Sharon1, Ylioja, Tiina1, Hofstetter, Richard1, Klepzig, Kier2, 1 Biology, Hanover, NH, USA2 Southern Research Station, Pineville, LA, USA
ABSTRACT- High impact herbivores are frequently those that display intermittent outbreaks. Outbreaks can result from dynamic feedback systems that include delays and/or nonlinearities in per capita growth rates as a function of density. Dendroctonus bark beetles have large impacts on forests during outbreaks that can kill vast areas of pine. Dendroctonus typically employ pheromones to organize mass attacks of individual trees - apparently as a means of overwhelming tree defenses. This could generate destabilizing positive feedback (Allee effect) in which per capita reproduction tends to increase with increasing abundance of beetles to participate in attacks. We tested for an Allee effect in D. frontalis by comparing growth rates of local infestations that began at a range of population sizes. As predicted, the relative growth of infestations was positively related to initial size. To evaluate whether this was due to density-dependent interactions with tree defenses, we (1) developed a model that predicts dynamics of the tree resin system during attacks and eventual reproductive success of the beetles, and (2) tested model predictions by measuring attack rates, resin dynamics, and reproduction within natural infestations. Results supported predictions that resin flow declines most rapidly in trees sustaining high attack rates, that per capita reproduction declines with resin flow as a negative exponential, and that reproductive success is maximized at intermediate attack rates (due to increasing negative feedback from competition at high densities). However, the surprising additional result was that average beetle attack rates were unrelated to population size except at very low local abundance. This was because even low to moderate populations of beetles were efficient at concentrating their attacks on just a few trees. Thus, although beetle attack rates influence reproductive success, and vary greatly among trees, this does not seem to produce density-dependence among infestations of different sizes because attack rates are uncoupled from local population size. D. frontalis displays properties of a positive dynamic feedback, but the presumed mechanism seems to be wrong. Bark beetle demographics appear to involve feedback systems that operate at multiple spatial scales.
Key words: allee effect, population outbreaks, density-dependence, plant defense