The seasonal dynamics of snow cover strongly affect ecosystem processes and winter habitat conditions, making them an important driver of terrestrial biodiversity patterns. Snow cover data from the MODIS Aqua and Terra satellites can capture these dynamics over large spatiotemporal scales, allowing for the development of indices with specific application in ecological research and predicting biodiversity. Here, our primary objective was to derive winter habitat indices (WHIs) from MODIS that quantify snow season length, snow cover variability, and the prevalence of frozen ground without snow. We calculated the WHIs for the full snow year (Aug-Jul) and winter months (Dec-Feb) across the contiguous US from 2003/04 to 2017/18 and validated them with ground-based data from 797 meteorological stations. To demonstrate the potential of the WHIs for biodiversity assessments, we modeled their relationships with winter bird species richness derived from eBird observations.
Results/Conclusions
The WHIs had clear spatial patterns reflecting both altitudinal and latitudinal gradients. Snow season length was generally longer at higher latitudes and elevations, while snow cover variability and frozen ground without snow were highest across low elevations of the mid latitudes. However, there was substantial regional variability. West of the Rocky Mountains, the WHIs were most driven by elevation, east by latitude. Snow season length and frozen ground without snow were most accurately mapped, and had correlations with station data across all years of 0.91 and 0.85, respectively. Snow cover variability was accurately mapped for winter (r = 0.79), but not for the full snow year (r = -0.21). Errors in the WHIs were most common at high elevations, outside of the winter months, and in the Aqua snow cover data. The WHIs were strong predictors of winter bird species richness patterns across the contiguous US. Regions with longer snow seasons generally supported fewer species. Snow cover variability and frozen ground without snow were positively associated with bird richness up to certain thresholds (40% and 25%, respectively), after which richness steeply declined. Our results show that the MODIS-based WHIs accurately characterized unique aspects of snow cover dynamics and thus winter habitat conditions, highlighting their vast potential for ecological research and conservation planning applications.