The speed of change in climate and land cover is associated with the speed of biodiversity changes in avian assemblages of the United States

PLOS One | https://doi.org/10.1371/journal.pone.0330153 August 29, 2025 13 / 20 for each ecoregion. Rapid biotic heterogenization in the Northern Forests was likely driven by high rates of species loss (for example, loss of forest specialist species), while in Northwestern Forested Mountains, this process was possibly linked to high rates of species replacement (for example, replacement of forest specialists by species more tolerant to disturbed habitats). In the Northwestern Forested Mountains, the rate of change in total assemblage dissimilarity lagged behind the rate of change of forest loss, indicating that either extinction debt or buffering mechanisms may be operating here [4,71]. Although the consequences of forest loss on vertebrate biodiversity may be severe not only for already frag- mented landscapes but also for relatively intact forested areas [72,73], the lagged response we found in the Northwest- ern Forested Mountains could be an indication of species assemblage resilience if critical habitat loss thresholds are not surpassed [14]. Decrease of within-ecoregion assemblage dissimilarity (biotic homogenization). Here, we provide some context on how homogenization in this study might compare to homogenization in other studies. Because we are examining homogenization inside of ecoregions, it is not necessarily the case that the most generalized species will become the most common, as the ecoregion becomes more homogenized. It is a question of the scale of the

analysis. For example, it could be that species typical of the ecoregion become more common within the ecoregion as homogenization progresses; these more common species might not be viewed as the most generalized, when compar- ing them to other species at a larger spatial scale. This is a nuance but is a result that is different from how other authors working at larger scales have conceptualized homogenization [4,61,74]. Rapid within-ecoregion homogenization of avian assemblages was mainly associated with fast changes in LULC. An exception was found in the Northwestern forested mountains, where fast increases in precipitation were associated with fast homogenization. Rapid increases in precipitation appear to be favoring ecoregion-specific species and slowing down their replacement by species tolerant of low precipitation levels, similar to results found by [62]. For Northern Forests and Northwestern Forested Mountains, the rapid change in LULC types less common than forest impacted the rate of change of total assemblage dissimilarity. For example, rapid increase of grassland/shrubland and wetland (within the Northern Forests) and rapid decreases in crop/pasture (within Northwestern Forested Mountains) were associated with accelerated rates of within-ecoregion biotic homogenization through the slowdown of species loss rates (Northern Forests) or declines in the species replacement rates (Northwestern Forested Mountains). Thus, rapid increases of natural LULC and declines of anthropogenic LULC appeared to be favoring habitat specialist species and

slowing down their loss or replacement by disturbance-tolerant, widespread, generalist species. This finding highlights the importance of the heterogeneity of landscapes in maintaining biodiversity and providing resilience and stability of ecologi- cal processes against environmental change [75]. In the Eastern Temperate Forest and the Great Plains, the higher the elevation, the more similar the species assem- blages within ecoregions. This occurred either because the species were less often replaced by others (Eastern Temper- ate Forests) or because they did not become a subset of richer species assemblages as fast as the species assemblages at lower elevation (Great Plains). These patterns may emerge from some species being restricted to certain areas and not colonizing whole gradients, likely due to environmental filtering linked to temperature and resource availability [76,77]. Effect of the rate of change of the environment on the components of assemblage dissimilarity. The rate of change in the components of total dissimilarity was mainly related to changes in LULC, except for the Eastern Temperate Forest where precipitation was the most important predictor. In general, we found that the change in environ- mental predictors had opposite effects on the rates of change of species replacement and species loss in the Eastern Temperate Forest, Great Plains and North American Deserts. Opposite and/or independent responses of species replace- ment and species loss to predictor variables have been found for multiple taxa along elevational and latitudinal gradients

[76,78,79], in agreement with what we found (S1-S3 Tables in S1 File). The pattern of decreased species replacement and increased species loss we identified in Eastern Temperate Forests and Great Plains is generally attributed to the presence of disturbance-tolerant or widespread species, and the local extinction of specialist or narrowly distributed