Floodplains as Refuges: High Bird Diversity During an Extreme Drought Is Driven by Wetland‐Dependent and Non‐Dependent Species

ABSTRACT Fluvial wetlands, such as floodplains, are particularly vulnerable to increasingly frequent extreme climatic events such as droughts because river flows are tightly coupled to atmospheric drivers such as precipitation. At the same time, these ecosystems can play a crucial role in supporting biodiversity resilience by providing refuge during regional droughts. The Paraná River Basin, the second largest in South America, has experienced unprecedented reductions in river flow since 2019. Here, we examine the effects of this multiyear hydrological and meteorological drought (2019–2024) on bird assemblages in the Paraná River floodplain. We compared two distinct hydrological contexts: a period with greater water availability in the floodplain (i.e., ‘wet’ period; 2011–2013) and another marked by drier and more recent conditions (i.e., ‘drought’ period; 2021–2024). Comparisons were based on point counts conducted at 60 sites along approximately 450 km. At each point, we also quantified habitat heterogeneity. Under drought conditions, regional and per‐point species richness, total abundance and Simpson diversity increased markedly, with non–wetland‐exclusive species showing the strongest response. Habitat heterogeneity also increased and was positively correlated with all bird metrics; however, drought effects remained significant after accounting for habitat variability. Beta‐diversity analyses revealed reduced species turnover during drought, and indicator‐species analysis identified 63 species associated exclusively with drought (22% wetland‐exclusive; 78% nonexclusive). Overall, our results suggest that although river levels reached historical lows and assemblages became dominated by non–wetland‐dependent species, the floodplain retained sufficient wetland refugia to sustain, and in some cases even enhance, populations of wetland‐dependent species during the drought.