Host regulation and seasonality generate population chaos in a fig-wasp mutualism

Mutualistic interactions between species play a key role in ecosystem function. Theoretical models and laboratory experiments suggest that mutualistic systems can display chaotic population dynamics. Evidence of mutualistic chaos in nature, however, is rare and poorly understood. Here, we show the existence of chaotic behaviour in a mutualistic system. We combine a mathematical model with field data to analyse population fluctuations and the stability of a host-mutualist-exploiter system. Our goal is to understand whether the system can evolve to a steady state and under what environmental conditions. We establish a discrete, seasonally explicit host-mutualist-exploiter model and estimate the parameters using data collected in the field. Model simulations show that the host decreases the ratio of extra reward to host sanction to maintain its higher population density when external environment variation (such as variation in seasonal temperature) is small. The model parameterized with field data predicts that host regulation and environmental variation together, but not individually, can lead to population chaos. Our work provides a new mechanism of population chaos in a mutualistic system and may inspire more interest in exploring chaotic dynamics in natural ecological systems.