Ecological consequences of mountain pine beetle outbreaks for wildlife in western North American forests

Background/Question/Methods

Fire suppression, logging, grazing, and climate change have altered many disturbance regimes since Euro-American settlement. After decades of fire suppression and even-aged management of forests followed by climate warming, elevated tree densities in many forests have increased the probability and size of mountain pine beetle (Dendroctonous ponderosae) (MPB) outbreaks. In general, multiple outbreaks of several bark beetle species have led to widespread tree mortality in conifer forests since the early 1990s in western North America. These outbreaks have led to marked ecological change in pine (Pinus spp.) forests with corresponding implications for wildlife. We reviewed published studies examining wildlife responses to MPB outbreaks and post-outbreak salvage logging to inform forest management and guide future research. Our literature search revealed 15 studies describing MPB outbreak relationships with 89 bird species and four studies describing relationships with 12 mammalian species, but no studies of reptiles or amphibians. We summarized population responses as positive (increases in abundance or fitness with outbreak conditions, or selection for beetle-affected habitats), negative, mixed (variable among years or study sites), or not statistically significant for each study. We also report three case studies of woodpeckers, red squirrel (Tamiasciurus hudsonicus), and southern red-backed vole (Clethrionomys gapperi).

Results/Conclusions

Avian studies reported positive responses to MPB by cavity nesters, shrub nesters, and bark-drillers more frequently than other groups. Twenty-five avian species were studied in beetle-killed forests characterized as either lodgepole (P. contorta) pine or ponderosa pine (P. ponderosa) dominant, disregarding forests described as “mixed conifer” with no particular dominant tree species reported. Of the 25 species, an equal number exhibited positive relationships (either significant or apparent) with MPB epidemics in ponderosa and lodgepole pine forests (n = 10 species each). Mammalian responses to MPB outbreaks were mixed. Studies consistently reported negative associations by red squirrels, although our case study suggested red squirrels can persist following an outbreak under some conditions. MPB effects on other, understory-associated small mammals may be modulated by post-epidemic production of coarse woody debris, as suggested by our southern red-backed vole case study. Post-outbreak salvage logging studies (n = 6) reported inconclusive results. Post outbreak salvage logging may impact wildlife less than post-fire salvage logging likely because only host-specific tree species are removed after beetle outbreaks.