Precipitation regime drives key alterations in subtropical stream fish community assembly and functional trait distribution across a major rainfall gradient

Shifting precipitation patterns associated with global climate change are significantly impacting lotic ecosystems worldwide. To understand how these changes influence stream fish community assembly, we conducted a space-for-time study under a steep natural rainfall gradient in the coastal plain region of Texas (USA). Leveraging multi-year intra-annual fish surveys and environmental data from nine streams, we assessed the effects of precipitation regime on functional diversity and trait distributions, focusing on the relative importance of environmental filtering, interspecific interaction, and individual environmental drivers. Fish communities transitioned from functionally underdispersed to overdispersed with increasing precipitation rate, suggesting that filtering was a key assembly mechanism in more arid communities, while species interaction played a more important role under wetter climates. Deviations in functional dispersion across the rainfall gradient were best explained by changes in relative distributions of traits (functional evenness) as opposed to losses or additions of traits (functional richness) between sites, and the best predictors of these deviations were mean annual rainfall, low-flow frequency, and prevalence of hypoxic conditions. Proportional abundances of hypoxia-tolerant, herbivorous taxa within communities were associated mainly with the same predictors, illustrating the importance of these fishes’ increasing prevalence with increased aridity. Relationships between their abundances and benthic primary producer densities may also point to changes in grazing pressure, possibly stemming from top-down trophic processes. The shift from communities with low functional diversity dominated by hypoxia-tolerant, herbivorous taxa to more diverse assemblages as rainfall rates increased was non-linear, which may represent an important precipitation-driven threshold in community assembly. Our observations across this spatial rainfall gradient may translate to similar community-level changes in lotic ecosystems experiencing temporally shifting precipitation patterns, ultimately leading to increased understanding of relationships between functional trait distributions and climate, as well as providing valuable knowledge for predicting the impacts of changing precipitation regimes in freshwater ecosystems worldwide.