OOS 54-4 - Wildfire, climate, and biogeography influences on avian diversity across the US-Mexico sky islands

Effective conservation and management responses to mega-disturbance events and potential ecological transformation requires understanding variability in how vegetation and wildlife species are responding to these threats at multiple scales across broad contexts of climatic gradients and fire regimes. The Madrean sky islands provide a unique study area for addressing this challenge, where biogeographical location and elevational gradients interact to produce a globally recognized biodiversity hotspot. Our objective was to link multi-scale bird and vegetation responses to fire regimes across eighteen clusters of sky island mountain ranges to assess factors and mechanisms that drive vulnerability and resilience. Field observations for birds and vegetation were collected from eighteen clusters of mountain ranges (n = 1623 points along 222 transects) between 2009 and 2015. Samples represented variability across climatic gradients for two principal components (a general temperature gradient expressed as a latitudinal and altitudinal energy and phenology gradient, plus a heat-moisture and precipitation gradient). Fire history at study sites included unburned areas (fire data time frame 1985-2017); and locations that burned up to three times. We modeled relationships between avian richness (alpha diversity) and turnover and nestedness in avian species composition (beta diversity) and vegetation, fire and climate metrics, and biogeography using multi-species occupancy models and multivariate ordination. Vegetation and bird communities found in mesic environments at higher elevations and latitudes reflected increased fire frequencies and high severity fire in mixed conifer and spruce-fir forests. Specifically, we observed beta diversity differences in these areas, particularly with birds that rely on old growth forests. In contrast, fires were absent or infrequent in some pine-oak forests, oak woodlands, and grasslands, especially in the warmest, driest environments. These systems historically relied on frequent fire that maintained the structure and composition. Changes in fire regimes has impacted bird diversity, especially beta diversity with species that rely on frequent fire or resources such as snags and vegetation openings often created by fire. In these warmer, drier environments, land-use change and management are important drivers of change. Birds are often used as indicators of ecological integrity based on their reliance on forest structure for foraging and nesting. Understanding the drivers of avian biodiversity can provide insights into ecosystem sustainability, which is relevant to management in this diverse binational region. Our study across both latitudinal and elevation gradients provides a unique opportunity to evaluate potential future climate and fire responses, allowing for evaluation of adaptive strategies.