Alternate attractors in the population dynamics of a tree-killing bark beetle
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ORIGINAL ARTICLE
Alternate attractors in the population dynamics of a tree-killing bark beetle Sharon J. Martinson • Tiina Ylioja • Brian T. Sullivan • Ronald F. Billings Matthew P. Ayres
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Received: 31 July 2011 / Accepted: 18 November 2012 / Published online: 8 December 2012 The Society of Population Ecology and Springer Japan 2012
Abstract Among the most striking changes in ecosystems are those that happen abruptly and resist return to the original condition (i.e., regime shifts). This frequently involves conspicuous changes in the abundance of one species (e.g., an oubreaking pest or keystone species). Alternate attractors in population dynamics could explain switches between low and high levels of abundance, and could underlie some cases of regime shifts in ecosystems; this longstanding theoretical possibility has been difficult to test in nature. We compared the ability of an alternate attractors model versus two competing models to explain population fluctuations in the tree-killing bark beetle, Dendroctonus frontalis. Frequency distributions of abundance were distinctly bimodal, a prediction of the alternate
Electronic supplementary material The online version of this article (doi:10.1007/s10144-012-0357-y) contains supplementary material, which is available to authorized users. S. J. Martinson (&) 184 Running Iron Rd, #5, Bishop, CA 93514, USA e-mail: [email protected] T. Ylioja Finnish Forest Research Institute, P.O. Box 18, FI-01301 Vantaa, Finland B. T. Sullivan Southern Research Station, USDA Forest Service, 2500 Shreveport Highway, Pineville, LA 71360, USA R. F. Billings Texas Forest Service, J.B. Connally Building Suite 364, 301 Tarrow St., College Station, TX 77840, USA M. P. Ayres Department of Biological Sciences, Dartmouth College, Hanover, NH 03755, USA
attractors model, strongly indicating the lack of a single, noisy equilibrium. Time series abundance data refuted the existence of strong delayed density-dependence or nonlinearities, as required by the endogenous cycles model. The model of alternate attractors was further supported by the existence of positive density-dependence at intermediate beetle abundances. Experimental manipulations show that interactions with competitors and shared enemies could create a locally stable equilibrium in small populations of D. frontalis. High variation among regions and years in the abundance of predators and competitors could permit switches between alternate states. Dendroctonus frontalis now provides the strongest case that we know of for alternate attractors in natural population dynamics. The accompanying demographic instability appears to underlie spatially extensive outbreaks that have lasting impacts on forest ecosystems. Understanding feedbacks in populations with alternate attractors can help to identify thresholds underlying regime shifts, and potentially manage them to avoid undesirable impacts. Keywords Alternate stable states Dendroctonus Multiple equilibria Pinus Regime shift Shared predators
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