Math Biology Seminar Abstracts

Wednesday September 19, 2001

The Mathematical Basis for Modelling Insect Seasonality and Potential Climate Change-Induced Bark Beetle Invasions

James Powell (1), Jesse A. Logan (2) and Barbara J. Bentz (2)

(1) Department of Mathematics and Statistics, Utah State University (2) USDA Forest Service, Logan Utah

In this talk we explore the potential consequence of global warming on the distribution and outbreak status of mountain pine beetle (Dendroctonus ponderosae). Mountain pine beetle serve an important ecological role in western US pine forests, which have evolved with bark beetle disturbance as= an integral part of an adapted system. Lodgepole pine (Pinus contorta), for example, has co-evolved a relationship with fire and mountain pine beetle disturbances that serve to maintain it as a seral component of spruce/fir climax forests. Without the interaction of these two disturbance agents, lodgepole pine would be lost from much of its distribution. The reproductive strategy of mountain pine beetles requires new hosts each year, and pines under attack defend themselves strongly. There is consequently strong selective pressure for dispersed populations of beetles= to mature and emerge simultaneously (synchrony), and at an appropriate time of year (seasonality). Interestingly, the development and timing of mountain pine beetle development seems to be under direct control of the thermal environment, operating without the benefit of diapause, which often serves = to `time' the development of other insects. This makes it possible for bark beetles to invade new landscapes when it is thermally feasible. A very simple model for insect development uses a combination of linear developmental rates and temperature thresholds below which development can = not occur. In combination with seasonal temperature swings a natural consequen= ce of this simple model is that oviposition and emergence will occur in fixed, attractive cycles corresponding to one, two, or half generations per year (uni-, bi-, and semi-voltine populations), even in the absence of diapause. The dynamical properties of the thermal habitat are characterized by region= s of adaptive, synchronous seasonality separated by regions of maladaptive, asynchronous seasonality. Analytic and simulation results indicate that th= is mechanism for synchrony and seasonality is extremely robust. Global warming of the magnitude projected by current global circulation mod= els has the potential to significantly impact the geographic distribution of ma= ny species. Climate change translates to a latitudinal as well as an elevatio= nal shift in thermal habitat. Since the range of mountain pine beetles is not restricted by diapause, general warming is likely to open up the northern a= nd high-altitude boundaries of thermally feasible habitat. A latitudinal shif= t by an amount consistent with CO2 doubling would not only allow mountain pin= e beetles to occupy previously unoccupied lodgepole pine habitat (range expansion), but would also allow invasion of previously unattacked jack pin= e (Pinus banksiana), a commercial valuable species in Canada. Although model simulations of future scenarios must be considered speculative, currently observed phonological adaptations by mountain pine beetle populations are consistent with model predictions. (1) A well documented outbreak that occurred during the 1930s in high elevation whiteb= ark pine accompanied 10 years of exceptionally high temperature, on the order o= f those predicted by CO2 doubling. (2) Mountain pine beetle activity is currently being observed further north in Canada than previously recorded. This observation is concurrent with record setting warm temperatures of the past several years. (3) Model simulations predicted regional populations adapted to the prevailing regional climate. Laboratory experiments with beetles collected in central Idaho (44=81=BAN) and southern Utah (37=81=BAN= ) resulted in finding differences consistent with those predicted by the model.

For more information contact J. Keener, 1-6089