Complex changes in climatic suitability for Cassin's Sparrow (Peucaea cassinii) revealed by retrospective ecological niche modeling

Conservation status assessments for Cassin's Sparrow (Peucaea cassinii) show considerable variability across the species' North American range. In this study, we combine data from NASA's Modern -Era Reanalysis for Research and Applications, Version 2 (MERRA-2; M2) with field observations spanning the past 40 years to investigate Cassin's Sparrow's response to multi-decadal changes in climatic suitability that could help explain this variability. We examine two time- and variable -specific time series using MaxEnt. The M2 time series uses a mix of microclimatic and ecosystem functional attributes; the MERRAclim-2 (MC) time series uses MERRA-2-derived bioclimatic variables. 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 suitability show a northwesterly, 40 -year displacement of 1.85 km/yr. The M2 time series indicates a less favorable history with increasing and decreasing trends over 54.9% and 40.1% of the study area, respectively, and a westerly centroid shift of 2.60 km/yr. Increasing winds, drying land surface conditions, and variability in North American monsoon rainfall appear to be dominating, climate -related influences on the species. These variables also demonstrate complex patterns of non -constant spatial and temporal trends across the study area. We conclude that modeled estimates of climatic suitability for Cassin's Sparrow can vary widely depending on the temporal frame, spatial extent, and environmental drivers considered; that the species' response to nonconstant trends in key environmental drivers is a potential source of this variability; that this variability mirrors the inconsistencies seen in the literature regarding the species' conservation status; and that retrospective ecological niche modeling that combines time and variable specificity, as we have done here, can be a useful adjunct to assessments of a species' conservation status.