University of Wyoming Laramie, Wyoming, United States
Ecological theory has highlighted the importance of environmental variation in shaping ecological communities and driving species coexistence. One approach to quantifying variation as a driver of species dynamics is modern coexistence theory, which historically has been predominantly focused on annual species. However, this approach overlooks the role that perennial species play in many communities. In order to integrate perennial species into our theoretical understanding of coexistence dynamics, we extended the Ellner et al (2019) simulation approach to modern coexistence theory, using mutual invasion criteria as an indicator of coexistence, to communities with perennial species. We partitioned fluctuation-dependent mechanisms of coexistence to investigate the role of the storage effect (traditionally associated with seedbanking annual species) in driving patterns of perennial - annual dynamics. We simulated scenarios to quantify coexistence mechanisms across different life history types and investigated how these vary with the patterns of environmental variation, including the relative frequency of different environmental conditions.
We show that fluctuation-dependent mechanisms of coexistence in general and the storage effect in particular can be equally important in coexistence patterns for perennial species, with an adult "storage" stage, as for seedbanking annual species. Fluctuation dependent mechanisms contribute to stable coexistence for perennials on a similar magnitude as for seedbanking annuals. Relative nonlinearity in species' intrinsic growth rates have a stronger contribution when one environment is favorable for both species, while fluctuation-independent mechanisms become dominant when opposite environments are favorable. Our work additionally highlights the role of environmentally-variable per-capita competitive effects (versus quantity of competitors) as an often overlooked contributor to coexistence dynamics. Small changes in these parameters flip the contribution of the storage effect from positive to negative, and shift species' growth rates when rare from positive (persistent coexistence) to negative (competitive exclusion). Our results indicate that coexistence dynamics for annual and perennial species are dependent on the combination of species' direct responses to environmental variation, quantity of competition, and environmentally variable per-capita competitive effects. This work sets the stage for applying coexistence theory and fluctuation-dependent partitioning frameworks to a variety of perennial systems, facilitating understanding of how environmental variation drives species dynamics beyond annual plant systems.