Assistant Professor University of California Davis, United States
Background/Question/Methods Mutualisms are a diverse set of interactions that support a vast amount of biodiversity and ecosystem services. However, theoretical progress on the population dynamics of mutualistic interactions comparatively lagged behind that of trophic and competitive interactions, leading to the impression that ecologists still lack a generalized framework to investigate the population dynamics of mutualisms. Yet, over the last 90 years, abundant theoretical work has accumulated. Here, we analyze historical models of two-species mutualisms, identifying key assumptions and characteristic population dynamics. We argue that the current literature provides a strong foundation on which to build deeper ecological understanding in both empirical and theoretical settings. As a case study, we investigate the differences in population dynamics of pollination and seed dispersal mutualisms. We develop simple consumer-resource models detailed enough to distinguish different mechanisms by which plant populations benefit from animal pollination or seed dispersal.
Results/Conclusions Within the historical literature, we found that population dynamics of mutualisms are qualitatively robust across model derivations, including levels of detail, types of benefit, and inspiring systems. Specifically, mutualisms tend to exhibit stable coexistence at high density and destabilizing thresholds at low density. These dynamics emerge when benefits of mutualism saturate, whether due to intrinsic or extrinsic density dependence in intraspecific processes, interspecific processes, or both. We distinguish between thresholds resulting from Allee effects, low partner density, and high partner density, and their mathematical and conceptual causes. Different types of thresholds have different implications for population-level processes affecting mutualistic dynamics and may be distinguished empirically. For example, both pollination and seed dispersal mutualisms are stable at high density but exhibit different dynamics at low density, including partner-induced population collapse or bistable coexistence, depending on plant carrying capacity, animal foraging efficiency, and whether populations are obligate upon their partners for persistence. We present ecological scenarios under which these different dynamics may occur and discuss implications for mutualistic networks and broader ecological communities. Finally, we argue that there exists a robust population dynamic theory of mutualism that can be used to investigate the ecological and evolutionary dynamics of mutualism across scales.
