MERRA-based trend analysis identifies complex, multidecadal patterns of changing climatic suitability for Cassin's Sparrow (Peucaea cassinii)

Cassins Sparrow (Peucaea cassinii) is a grassland resident of the American Southwest. Despite decades of study, there remains uncertainty regarding the conservation status of the species. The species past response to a changing climate may help explain this uncertainty, especially if patterns vary across the species range. In this study, we combine data from NASAs Modern-Era Reanalysis for Research and Applications, Version 2 (MERRA-2; M2) with field observations spanning the past 40 years to examine historical changes in climatic suitability for Cassins Sparrow across its full annual range within the continental United States. We examine two time- and variable-specific time series using MaxEnt. The M2 times series uses a mix of 30 microclimatic variables and ecosystem functional attributes related to energy and water fluxes for predictors; the MERRAclim (MC) time series uses 19 MERRA-2-derived bioclimatic variables for predictors. Trend analysis reveals complex patterns of slowly increasing climatic suitability over 69.5% of the study area in the MC time series accompanied by decreases over 24.4% of the area. Shifts in the study area-wide, weighted centroid for climatic suitability show a northwesterly, 40-year displacement of 1.85 km/yr. The M2 time series points to a less favorable history with increasing and decreasing trends over 54.9% and 40.1% of the study area, respectively, and a westerly weighted centroid shift of 2.60 km/yr. A clear subset of seven M2 and MC variables emerged as the most important determinants of suitability over the past 40 years. These variables also demonstrated complex patterns of non-constant trends across the study area. Suitability trends in both time series appear to have little in common with current, state-level, abundance-derived conservation status assessments. Increasing winds, drying land surface conditions, and variability in North American Monsoon rainfall appear to be dominating, climate-related influences on the species. We thus see complex patterns of historical change in climatic suitability depending on whether time series models are driven by bioclimatic variables alone or by variables more aligned with ecological function. This leads us to conclude that modeled estimates of climatic suitability for Cassins Sparrow can vary widely depending on the temporal frame, spatial extent, and environmental drivers considered, and that this variability mirrors the uncertainty in the literature regarding the species conservation status. Furthermore, these factors should be taken into account in future conservation assessments for the species, and retrospective ecological niche modeling, as applied here, offers a promising approach to addressing these issues.