Asymmetrical facilitation ameliorates environmental conditions through positive feedback in partner traits

Abstract

AbstractPositive species interactions, including facilitation and mutualism, are important in shaping ecological communities through the amelioration of stressful conditions. Facilitation results from traits (e.g., plant growth form) that create benefits for one or more interacting partners. The outcomes of facilitation depend on positive or negative feedbacks between partners, which can determine the stability of interactions over time. We can expand our understanding of these dynamics through a bidirectional approach that identifies the specific mechanisms creating benefits for the associated species, the traits related to those benefits, and the potential for feedbacks to affect amelioration by modifying traits. We characterized an association between the salt marsh plant Limonium carolinianum and mussel Geukensia sp. in the northern Gulf of Mexico. We used a field survey and three field experiments to establish the bidirectional effects of the association, the underlying mechanisms, and the relevant traits for each partner species. Mussels benefitted from Limonium canopies, which ameliorated high temperatures and predation for mussels, increasing survival and recruitment. These positive effects were particularly evident under larger canopies. Mussels enriched sediment organic matter and thus over time are expected to alleviate nutrient limitation for Limonium. Changes in functional traits of both species contributed to facilitation. Limonium both benefitted from and promoted higher mussel density, and mussels both allowed Limonium to grow or retain more leaves and benefitted from stress amelioration under larger canopies. The benefits of the association appear asymmetrical: mussels consistently benefitted in both experiments, but we found only modest positive effects on Limonium, and only over the longer term. These differences likely reflect variation in the seasonality and/or timescale of the facilitative mechanisms. The bidirectional and mechanistic design of our study revealed positive feedbacks that reinforced the traits conferring benefits to each partner, potentially stabilizing this positive interaction. Understanding how feedbacks influence the outcomes of facilitation is increasingly important as anthropogenic stressors increase globally.