As a general rule, forests dominated by angiosperms broadleaf forests are more species-rich than those dominated by gymnosperms conifer, montane, or needleleaf forestsalthough exceptions exist.
Advanced Search Abstract Biome-scale disturbances by eruptive herbivores provide valuable insights into species interactions, ecosystem function, 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 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. Interactions between plants and insects encompass half of all ecological relationships Strong et al. Understanding the dynamics of eruptive species can provide valuable insights into fundamental ecological processes such as ecosystem disturbance, multitrophic interactions, symbioses, chemical signaling, and the selective pressures driving coevolution.
Eruptive species are also important systems for studying economically and environmentally damaging consequences of anthropogenic activities.
To better understand these systems, we require more knowledge of how processes at different biological levels and spatiotemporal scales interact. In many cases, emergent patterns cannot be predicted even when lower-level mechanisms are well characterized Peters et al. Likewise, underlying mechanisms inferred from higher-level patterns can be obscured or incorrect when key cross-scale interactions and thresholds are not identified McMahon and Diez We approach this problem by exploring one system in depth, using information from biochemical- through landscape-level mechanisms to improve linkages of pattern with process.
We illustrate how this approach can serve as a general model for improved understanding of ecological processes by which a cross-scale interactions, feedback, and thresholds both contribute to and constrain eruptive dynamics, and b anthropogenic activities interact with endogenous drivers to alter system behavior and generate fundamental regime shifts.
Regime shifts have been defined as abrupt changes into different domains and trajectories beyond which prior controls no longer function Scheffer and CarpenterFolke et al. Scolytinae are major disturbance agents of western North American forests, often affecting a larger area than fire does see www.
Population eruptions by these native insects have occurred on numerous occasions throughout previous centuries Baker and VeblenBerg et al. Depending on one's perspective, bark beetles are major sources of economic loss, integral agents of ecosystem function, challenges to natural resource policy, or environmental threats arising from anthropogenic change.
In addition to causing widespread tree mortality, bark beetles substantially change forest structure, composition, and function. Colonized trees become hosts to dozens of arthropod, nematode, and vertebrate species. Reduced canopy cover releases herbs, shrubs, and grasses on the forest floor, and increased ratios of light-loving to shade-loving species may persist for more than 60 years.
In mixed-species stands, the conversion to nonhost tree species Veblen et al. Stand-level primary productivity declines initially, but growth of surviving plants accelerates e.
Additional effects include increased quantities of coarse wood on the forest floor, altered degrees of landscape heterogeneity, increased stream-flow, and significant interactions with other disturbances such as fire Veblen et al. Plant compositional changes to other tree species or herbaceous cover may follow severe outbreaks for decades Allen et al.
Extensive eruptions may even modify biome-scale biogeophysical processes such as carbon cycling and sequestration Kurz et al. In recent years, the magnitude of epidemics has increased, and epidemics have also expanded into persistent infestations in habitats that previously had only rarely been affected, and into previously unexposed habitats with new i.
For example, the moun tain pine beetle Dendroctonus ponderosae has affected more than 13 million ha of western Canada sinceincluding areas at higher elevations and more northern latitudes than indicated by past records. Large areas of sensitive whitebark pine Pinus albicaulis habitat are experiencing higher levels of mortality than are typically observed Logan et al.
Beetles have breached the geoclimatic barrier of the northern Rocky Mountains and invaded hybrid lodgepole-jack pine Pinus contorta var. In addition to larger eruptions by individual species, there may be greater temporal synchrony among species, as 47 million ha of nearly every region and coniferous type have been affected during the last 10 years figure 1.
These outbreaks have substantial ecological and economic ramifications, and have provoked calls for policy changes ranging from more aggressive harvesting and fire suppression to mitigation of global carbon emissions.
We present a framework for understanding current and future patterns of bark beetle outbreaks in conifer forests of western North America, and the challenges they pose to resource management. Our synthesis draws on an extensive literature to address three questions:Airbnb is a community marketplace for unique vacation spaces around the world.
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