Calleja Solanas, V. | Search bioRxiv

bioRxiv2025

Time-varying interactions drive species coexistence via rainfall variation

Species interactions are fundamental to maintaining diversity. However, it remains unclear how environmental variability modifies the structure, sign, and strength of species interactions that ultimately affect coexistence mechanisms. Here, we combined structural stability theory with time-varying population models to study the temporal variation of niche and fitness differences over nine years of annual plants in Mediterranean grasslands. Rainfall variation modulated species-specific responses in performance, and changed the magnitude of self-limitation and the variability in the strength of interspecific interactions for both competition and facilitation. These changes implied substantial variation in yearly niche and fitness differences, which ultimately impacted community stability. Additional simulations show that this full biological reorganization due to rainfall variation can maintain diversity by promoting temporal shifts in winners and losers compared to a single climatic year. Our results highlight the need to incorporate time-varying interactions to understand species coexistence under contrasting environmental conditions.

ecology↗

bioRxiv2025

A general framework for cycles in ecology

Theory predicts that indirect interactions in ecological networks sustain species diversity through oscillatory dynamics. However, a framework linking interaction structure to these cycles is lacking. We develop an analytical toolbox combining assembly graphs with a mathematical decomposition of interaction matrices into symmetric and anti-symmetric components. We find that assembly cycles--closed loops of species invasions--are suppressed when symmetric interactions dominate, reflecting strong intraspecific competition. Conversely, anti-symmetric dominance, indicating competitive asymmetries, leads to cycles like rock-paper-scissors and novel multispecies invasion patterns. As asymmetries increase, new cycles emerge involving both sequential and simultaneous invasions. Applying this approach to 21 plant community interaction matrices, we find few cycles due to prevalent self-limiting effects. Our work clarifies when indirect interactions drive cycles and introduces a simple ratio assessing symmetric versus anti-symmetric contributions, constraining cycle emergence, and species coexistence in nature. Significant statementUncertainties of the fundamental blocks that maintain biodiversity hinder our understanding of the dynamics we can observe in ecological communities. By applying a well-known mathematical approach to decomposing interactions among species within ecological communities, we discover great potential for observing rock-paper-scissors and more complex cyclic dynamics in nature. However, we predict and confirm analyzing multiple datasets that cyclic dynamics are rare because of two phenomena that pervade ecological communities. These are differences in species performance and the dominance of self-limiting effects. Our framework underscores the importance of studying simple properties of biotic interactions to predict complex ecological dynamics.

ecology↗