Alternate attractors in the population dynamics of a tree‐killing bark beetle

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 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.

Geographically variable response of Dendroctonus ponderosae to winter warming in the western United States

“…Large mortality events due to winter cold can push populations back to endemic levels (i.e., halt outbreaks) and diminish positive feedback (Safranyik and Linton 1991;Martinson et al 2013). A reduction in the frequency of extremely cold winters over time then could favor positive feedback and not only permit outbreaks, but also prolong them especially in regions historically subjected to very cold winters.…”

“…Meddens et al (2012) estimated that 63 and 94 % of total bark beetle-caused tree mortality from 1997 to 2010 in the US and Canada was caused by D. ponderosae, respectively. Dramatic population increases of tree-killing bark beetles are believed to be the result of positive feedbacks that enable beetles to overcome host tree defenses through mass-attacks and thereby transition small local infestations to outbreak status Martinson et al 2013). Key factors that strengthen positive feedbacks include behavioral plasticity of beetles (Boone et al 2011) and weather effects on insect survival, developmental rate, seasonal rhythms of tree attacks, and tree defenses (Régnière and Bentz 2007;Powell and Bentz 2009).…”

Semiochemicals in the Natural History of Southern Pine Beetle Dendroctonus frontalis Zimmermann and Their Role in Pest Management

“…Tree death is assured by the girdling of the phloem tissue by the beetles' mining activities as well as the mass inoculation of weakly pathogenic fungi that the beetles carry into the host. Thus aggressive bark beetle species live double lives; persisting almost invisibly as scavengers in the environment when at low population densities but killing vast acreages of host trees as predators during outbreaks (Martinson et al, 2013;Raffa et al, 1993). The mass attacks required to kill a healthy tree are initiated, sustained, and potentially terminated through the beetles' pheromones (Byers, 1989b;Wood, 1982a), and semiochemicals are an essential mechanism underlying the epidemiology of these disturbance agents.…”

Forest pests and their management in the Anthropocene

“…This practice usually stops growth of the local infestation (by removing beetles from the forest and disrupting the pheromone plumes that catalyze mass attacks on trees). When most such infestations in a forest are successfully suppressed, the regional beetle population is reduced to such low levels that they can no longer employ mass attacks to kill trees, and then natural forces can maintain them at nonepidemic levels for many years without further suppression (Martinson et al 2013). Conveniently, this management practice can pay for itself while contributing to the local economy because the cut trees have value.…”