8 - Bark beetle outbreaks alter biotic components of forested ecosystems

Abstract

Bark beetle (Coleoptera: Curculionidae: Scolytinae) outbreaks can be spectacular, killing trees across hundreds to thousands of hectares or more of forest land. As bark beetles are a natural disturbance agent, it is hypothesized that ecosystems are adapted to such periodic disturbances. Under climate change, however, these beetle outbreaks have become more severe, persistent, and chronic on many landscapes, bringing into question the long-term resilience of these forests and the biota that depend on them. Our chapter objective is to review and generalize the effects of bark beetle outbreaks on flora, fauna, and soil microbiota across diverse landscapes worldwide. Through loss of canopy trees, there is an increase in forest gaps and dead organic material that alters resource availability for many organisms. As organisms respond to altered resources, they may create feedback loops to themselves and their environment. Tree regeneration dynamics may set the stage for dominance by the same or different canopy species, retriggering, advancing, or otherwise altering successional trajectories. Herbs and shrubs often become more abundant and diverse under postbark beetle outbreaks due to increased light, water, and nutrient availability. Faunal species that rely on open habitat conditions and greater availability of newly available resources (e.g., coarse woody debris, bark beetles as prey items, and understory plant release) tend to benefit from bark beetle outbreaks; vice versa for other species; and variable responses are indicated for other species. Both positive and negative impacts on red-listed or endangered species have been reported. Ectomycorrhizal and saprophytic fungi, respectively, tend to decrease and increase after bark beetle outbreaks. There may be an increase in soil bacteria with different bacterial species dominant in beetle-killed trees. Responses of biota are therefore species-specific, and there are winners and losers in systems based on habitat alterations and life-history requirements. Longer-term and comprehensive studies of cascading, interacting, and simultaneous ecological impacts for multitaxa are recommended for maintaining the resilience of these disturbed landscapes under climate change.