Biotic and abiotic factors directly and indirectly shape mixed-species group structure across a rainfall gradient

Mixed-species animal groups are dynamic systems whose structure and functioning is contingent on biotic (e.g., food acquisition) and abiotic (e.g., climatic conditions) processes. Understanding the interactions among such factors, and the temporal scales at which they operate, can help to understand the mechanisms driving community structure and anticipate the consequences of human-driven alterations in ecosystem functioning. We used network analyses and Bayesian causal inference to study how direct and indirect effects of biotic and abiotic factors affected the structure of mixed-species groups of army ant-following birds (a bird aggregation specialized on feeding at army ant swarms). Our study revealed that the structure of interactions among birds visiting ant swarms was modulated by short-term rainfall, swarm traits, and the food resources they provide. Specifically, we found that ant-follower groups had 29% to 54.7% more species when swarms were larger, provided higher food resources, and the environment experienced high precipitation during the previous 15 days. Similarly, the ant-follower groups were 34% more cohesive (i.e., a tendency toward the formation of densely connected subgroups), attendant birds interacted with 55% more partners, and interactions in the groups were more evenly distributed among partners in swarms with high food resources. Our data suggest that rainfall effects have the potential to propagate through swarm size and food availability to affect the structure of ant-followers, but direct effects still occupy the primary role. Notably, the effects of biotic and abiotic predictors were contingent on the season and location along a rainfall gradient. Our study highlights three key points: (1) the factors modulating the structure of mixed-species animal groups are interlinked, with both abiotic and biotic factors influencing structure; (2) the effect of short-term rainfall is only detected at particular lagged time windows; and (3) the processes explaining the structure of mixed-species groups are strongly influenced by seasonality and location along a rainfall gradient. Our study emphasizes the importance of evaluating interdependent relationships between biotic and abiotic processes, their direct and indirect effects on interacting species, appropriate temporal scales, and the consequences of climatic cycles in understanding the mechanisms influencing the structure of mixed-species groups.