Proposal summary

Community interactions and the population ecology Dendroctonus frontalis
USDA Competitive Grant: Two years, 1998-2000. 
Understanding why some species regularly attain very high abundance is a central problem in theoretical population ecology and applied entomology. Time series analyses of many important insect pests reveal evidence of population cycles, implicating some biological source of complex endogenous feedback. However, the forces producing this feedback remain generally unknown. The southern pine beetle, Dendroctonus frontalis (Coleoptera: Scolytidae) is one such species. We hypothesize that outbreak population dynamics of southern pine beetle are the result of endogenous feedback from community interactions involving the host tree, mutualistic mycangial fungi, a bluestain fungus (Ophiostoma minus) that competes with the mycangial fungi, and Tarsonemus mites (phoretic on the southern pine beetles) that transport and feed upon the bluestain fungus. Destabilizing positive density-dependence is hypothesized to arise from the effects of beetle attack rate on the tree resin system; this would have the effect of accelerating population growth and is thought to play a critical role in the increase phase of population outbreaks. Delayed inverse density dependence is hypothesized to result from web of interactions involving mites, Ophiostoma minus, mycangial fungi, and D. frontalis; this would have the effect of limiting the duration of the outbreak phase and driving populations back to endemic levels. We propose to test these hypotheses by experimentally manipulating tree resin flow, beetle attack rates, the abundance of Ophiostoma minus, and the abundance of Tarsonemus mites. Results could falsify our model either by rejecting one or more of the necessary qualitative predictions (e.g., measurable effects of Tarsonemus on the transmission of O. minus) or by yielding quantitative estimates for the species interactions that do not permit the observed endogenous dynamics (e.g., cycle durations of 3-5 years instead of 8 years). This research will clarify the role of community interactions in the population ecology of an important forest pest and provide a template for exploring the mechanisms by which community interactions can influence population dynamics in other foodwebs.
Investigators: Matthew P. Ayres, Kier Klepzig, John Moser, Peter L. Lorio, Fina Lombardero, Richard Hofstetter
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