Disentangling the Components Shaping Aquatic Richness Over Spatial Scales in Extreme Climatic Events

Species richness in a region can be partitioned into three components: (1) spatial aggregation of individuals, (2) variation in total individual density and (3) species abundance distribution (SAD). However, it remains unclear how the influence of these components on richness changes under extreme climatic events. In floodplains, extreme droughts isolate aquatic ecosystems, while extreme floods connect them, affecting organism distribution and potentially altering the relative importance of each component. The data used in this study were collected from phytoplankton, zooplankton and fish communities in 12 lakes in a subtropical region in South America, the Upper Paraná River floodplain. Data were collected during La Niña (2000–2001) with prolonged low waters and El Niño (2010–2011) with extended high waters. We evaluated how the contributions of these three components of species richness changed over time across different spatial scales for each community during both extreme climate events. We also quantified how the contributions of density, spatial aggregation, and SAD to species richness varied over time during each extreme event. Species richness was consistently higher during the flood period across all communities. The relative importance of each component varied with habitat connectivity. During floods, increased connectivity promoted species gains at both local and regional scales, driven by aggregation, density and SAD. In phytoplankton, connectivity was associated with reduced aggregation and density, likely due to dilution effects. Zooplankton displayed greater uniformity, possibly reflecting the homogenising influence of flood pulses on dominant species. Fish showed lower density and more uniform distributions during floods, likely as individuals dispersed in search of resources and refuge. Conversely, during droughts, zooplankton and fish communities experienced richness declines, primarily due to reduced density and increased dominance, as harsher conditions favoured more tolerant species. The variation in components during floods highlights the impact of habitat connectivity on species dispersal, while local variables, such as environmental conditions, play a comparatively minor role in these processes. Our results show that changes in water levels affect richness by driving the components responsible for the distribution of individuals. These findings underscore the importance of hydrological connectivity in shaping species distributions, while highlighting the comparatively minor role of local environmental variables during high‐water phases. Moreover, anthropogenic and climatic disturbances, such as prolonged droughts and water level regulation by hydroelectric infrastructure, which occurs in our study region, can undermine aquatic biodiversity by disrupting richness dynamics.