Peering Into the Past Century of Mountain Diversity Change by Uniting Two Modes of Remote Sensing.

Mountain ecosystems are particularly susceptible to climate change and biodiversity loss as altitudinal diversity generates rare habitats and adapted specialist species, both sensitive to change. Mountain songbird diversity can be especially telling of land cover changes given breeding songbirds' strong patterns of habitat preference. However, most records of bird populations go back only a few decades, affecting baselines. Our aim was to examine changes in mountain diversity using a novel approach to analyze historical data that reaches nearly a century back in time. We repeated 46 historical survey photographs and used image analysis tools to quantify landscape change. In parallel, we generated species distribution models for 15 breeding songbird species in the study area. Based on the paired photographs, we modeled changes in bird occurrence. We then analyzed changes in Shannon diversity in terms of both land cover and bird occurrence. Forest cover increased over the past century at the expense of rarer alpine and riparian land covers, leading to decreased landscape diversity. This landscape homogenization resulted in declines in 5 species of songbirds (including 4 that breed in rare habitats), while 9 abundant forest-breeding species were positively impacted, without substantial changes to the diversity of species in the community. We highlight shifts in species occurrence over a time interval not often captured by other methods. Historical photographs linked with species distribution modeling have potential for inferring global change for conservation and landscape management in mountain environments-some of the most challenging places to monitor.