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 14 / 20 species [80]. The pattern of increasing replacement rates and decreasing species loss rates with elevation found in North American Deserts is consistent with the upward range shifts of low-elevation species tracking their thermal niches [81]. Even small changes in the proportion of LULC can have large effects in relatively species-poor ecoregions. In North American Deserts, fast increases in barren land were associated with low rates of species loss, since likely the number of species present there was low to begin with due to the lack of vegetation characteristic of barren lands. Forest cover in this ecoregion is minimal, and its proportion is not changing significantly with time. Our models, however, detected that declines in forests in this ecoregion were associated with increased species turnover. Losing the habitat and resources that trees provide, the replacement of existing species by others better adapted to harsher environments is likely. In the Northern Forests, rapid increases in urban LULC (even if small in quantity) were related to lower rates of species replace- ment. In this case, we argue that, with the rapid habitat degradation that accompanies growing cities, there were fewer species to replace. Effect of the rate of change of the environment on species richness. We found significant relationships between species richness and the environmental variables for the Eastern Temperate

Forest, the Northern Forests, and Northwestern Forested Mountains. In addition, the effects of climatic and LULC predic- tors on the rate of change of avian assemblage species richness, when significant, differed across ecoregions (similar to findings by [82]. We explore these relationships further, below. Factors driving species richness were mainly changes in LULC. The only exception to this result was found in the Eastern Temperate Forests. There, fast increases in richness were associated with rapidly warming locations. This result is consistent with the northward range expansion of southerly-distributed, warm-affiliated species reported for eastern North America [62,83,84]. In contrast, other ecoregions saw increase in species richness driven by changes in LULC. In the Northern Forests, rapid increases in richness correlated with rapid increases in forests and urban LULC. An increase in richness with an increase in a particular LULC occurs when this change favors a particular group of species and immi- gration events exceed local extinction events [85]. Increases in forests may favor forest specialist birds with the increase in potential niches and habitat complexity. In contrast, rapid increases in urban LULC (even if small in quantity) may favor the increase of generalist, urban- or disturbance-tolerant species [86,87] in the avian assemblages of the Northern Forests. Decreases in crop/pasture were associated with fast increases in richness in the Northwestern Forested Moun-

tains. The decrease in agricultural lands can potentially result in the increase of natural vegetation and the reduction of the effects of pesticide/fertilizer exposure, improving the quality of habitat for less disturbance-tolerant, late‐successional, and insectivore species [88–90]. Assemblage dissimilarity is better predicted by climate and/ or LULC than is species richness. Our models performed reasonably well at predicting total assemblage dissimilarity for the northern and latitudinally more restricted ecoregions (Northern Forest and Northwestern Forested Mountains), though the better models were at the 25 km (Northern Forest) and 50 km (Northwestern Forested Mountains) scales. Models of the rate of change of species rich- ness did not detect a significant relationship with the rate of change in the environment for the avian assemblages in the Great Plains and North American Deserts. In contrast, models of the rate of change of the three assemblage dissimilarity metrics detected significant relationship with at least one environmental predictor for all ecoregions. However, for some ecoregions, the models of the rates of change of the component of assemblage dissimilarity performed poorly since they explained a small percentage or none of the variance in the response variable. This may be an indication that the varia- tion in the data was very small or that it would be explained by unmeasured variables (for example, [88], and the results for these models should be interpreted with caution. Interestingly, the variance in the response explained by the predictor

variables was higher for the rate of change of total assemblage dissimilarity than for the rate of change of species rich- ness. This is an indication that assemblage dissimilarity is more sensitive to changes in the environment than species richness, as other studies have also demonstrated [91,92]. Eventually, if the current rates of environmental change do not slow down, the effects of these changes will be reflected in the rate of change of species richness as well.