Time-varying interactions drive species coexistence via rainfall variation

Species interactions are fundamental to maintaining biodiversity. However, how these interactions change over time in response to varying climatic conditions has been poorly explored. This gap limits our ability to identify how temporal changes in coexistence mechanisms sustain diverse communities. Here, we present a modeling approach, coupled with theory, to evaluate how temporal changes in species competitive and mutualistic interactions and intrinsic growth rates promote structural stability. We illustrate our framework using two highly replicated yet independent datasets: (1) annual plant grasslands and (2) wild bee assemblages in shrublands, both subjected to the same variation in rainfall over nine years. We found that both systems respond similarly to year-to-year rainfall changes despite their disparate evolutionary backgrounds. While rainfall variability fosters more diverse communities, prolonged drought leads to substantial biodiversity loss. Although the spatial context dependency modifies some responses, the structural stability of most communities persists through time, with clear winners and losers emerging under environmental variability. The main underlying mechanism determining the observed community stability is a combination of species-specific changes in their growth rates, an increase in the variability of interspecific interactions, and a reduction in the strength of self-limiting effects. Our results underscore the need to consider time-varying species interactions to understand community stability under contrasting climatic conditions and to predict how climate severity influences biodiversity loss.