Biome-scale disturbances by eruptive herbivores provide valuable insights into species interactions, ecosystemfunction, and impacts of global change. We present a conceptual framework using one system as a model, emphasizing interactions across levels of biological hierarchy and spatiotemporal scales. Bark beetles are major natural disturbance agents in western North American forests. However, recent bark beetle population eruptions have exceeded the frequencies, impacts, and ranges documented during the previous 125 years. Extensive host abundance and susceptibility, concentrated beetle density, favorable weather, optimal symbiotic associations, and escape from natural enemies must occur jointly for beetles to surpass a series of thresholds and exert widespread disturbance. Opposing feedbacks determine qualitatively distinct outcomes at junctures at the biochemical through landscape levels. Eruptions occur when key thresholds are surpassed, prior constraints cease to exert influence, and positive feedbacks amplify across scales. These dynamics are bidirectional, as landscape features influence how lower-scale processes are amplified or buffered. Climate change and reduced habitat heterogeneity increase the likelihood that key thresholds will be exceeded, and may cause fundamental regime shifts. Systems in which endogenous feedbacks can dominate after external forces foster the initial breach of thresholds appear particularly sensitive to anthropogenic perturbations.
Bibliographical noteFunding Information:
This work was supported by National Science Foundation grants DEB0314215 and EPS-0447689, the Natural Resources Canada Canadian Forest Service Mountain Pine Beetle Initiative, the Joint Fire Sciences Program, the US Department of Agriculture Forest Service’s Rapid Science Assessment Team, the British Columbia Forest Sciences Program, and the University of Wisconsin College of Agricultural and Life Sciences. We appreciate the critical reviews of Thomas Veblen (Department of Geography, University of Colorado at Boulder) and two anonymous reviewers, which improved this paper.
- Anthropogenic change
- Forest management
- Landscape disturbance
- Plant-insect interactions