When bigger isn’t better—Implications of large high‐severity wildfire patches for avian diversity and community composition

Abstract

Aim Wildfires increasingly create large high‐severity patches with interior areas far from less disturbed habitats. We evaluated how these trends impact bird communities by investigating the effect of internal distance from lower‐severity areas, high‐severity patch size, and years since fire on avian alpha and beta diversity. Location Sierra Nevada, California, USA. Methods Bird occurrence data were collected during 2009–2017 within high‐severity patches of 27 wildfires representing 1–30 years since disturbance. A two‐step multispecies occupancy method was used to account for imperfect detection of 94 species and estimate effects of patch characteristics on community richness and dissimilarity. Results Community richness decreased with distance from patch edge and with patch size. Richness increased with years since fire, but this pattern was dependent on distance from edge with higher peak richness (23 species) near edges than interiors (18 species). Community dissimilarity was not associated with distance, indicating that large high‐severity patch interiors contain subsets of, rather than complements to, edge communities. Dissimilarity peaked later with increasing patch size. Guild richness of tree and primary cavity nesters was negatively associated with distance and patch size. Richness of ground and shrub nesters was insensitive to distance, while shrub nester richness increased somewhat with patch size. Due to declines among other species, ground and shrub nesters made up a greater percentage of the avian community within the interiors of large high‐severity patches. Main conclusions As fire activity increases due to accumulating forest fuels and accelerating climate change, high‐severity patches and their resulting early‐seral habitats are becoming more extensive with less edge and more interior area. Such changes are likely to decrease avian diversity locally and shift community composition away from forest‐associated species. Management actions that promote the full range of fire effects but limit high‐severity patch size may best conserve bird diversity within fire‐adapted ecosystems.

Fire ecology for the 21st century: Conserving biodiversity in the age of megafire

“…Instead, features that could provide an internal refuge—rocks and large trees—were related to species’ occurrence within the fire footprint. In contrast to Hale—but encompassing a longer successional time‐scale—Steel et al (2022) identify a clear spatial gradient in richness and composition of Californian bird communities driven by distance to the edge of high severity patches: communities near the interior of high severity patches differed from those near the edge, with the former being comprised disproportionately of ground‐ and shrub‐nesting species, and relatively fewer tree and cavity‐nesting species. They also show that the richness of bird communities decreases with the size of patches that burned at high severity, due to the loss of tree and cavity‐nesting species.…”

“…They also show that the richness of bird communities decreases with the size of patches that burned at high severity, due to the loss of tree and cavity‐nesting species. Given the increasing regularity of large, severe fires in California (Keeley & Syphard, 2021), Steel et al (2022) warn of the substantial changes in bird community composition that shifting fire regimes could deliver. In an important contribution, Calhoun et al (2022) illustrate that such impacts have already spread well beyond the conifer forest ecosystems where the political and media focus of recent Californian fires has typically centred.…”

Ecological resilience and vegetation transition in the face of two successive large wildfires

“…Here, we use 75% canopy mortality as our cut‐off between low/moderate‐ and high‐severity. This threshold is often used to designate high‐severity fire effects in conifer‐dominated forests of California (Jones et al., 2020; Steel, Fogg, et al., 2021; Tingley et al., 2016; Welch et al., 2016), although other thresholds have also been used in the literature (e.g. Collins et al., 2017; Lydersen et al., 2016).…”

Design considerations for rapid biodiversity reconnaissance surveys and long‐term monitoring to assess the impact of wildfire

“…Wildfires generate a mosaic of patches across a landscape of varying fire severities, and survey designs should purposely consider the size and configuration of both low‐ and high‐severity patches within the fire footprint using remotely sensed fire severity maps. For example, targeting smaller high‐severity patches or patch edges might have a higher chance of detecting survivors than the interior of large high‐severity patches (Steel et al., 2021). This is because the edge of high‐severity patches potentially provides more resources within close proximity if low‐severity habitat is nearby.…”