COS 293-1 - Partner fidelity impacts plant performance in changing community contexts

Abstract: Ecological communities around the world are under a myriad of pressures from invasive species to habitat alterations which are disrupting species assemblages and changing community structure. Understanding how biotic interactions adapt to these changes is critical for predicting community response. For example, species which are more flexible in their interactions may be more resilient to changing community composition, altering interacting partners with little or no cost to their fitness. Species which show greater partner fidelity, however, may outperform their more opportunistic counterparts in their native communities, but pay the price in novel communities. And, these patterns may be interaction type dependent. For example, meta-analyses comparing multiple interaction types have demonstrated that plants tend to have higher fidelity to their mutualistic partners than their antagonistic ones, suggesting that plants bereft of their mutualistic partners will perform more poorly than if they are lacking their antagonistic partners.

Here we leveraged data from 160 experimental mesocosms, each comprising eight plants from a pool of 39 species with differing gradients of woodiness, exoticness, herbivory, and soil condition to examine how the biotic factors impact plant fidelity to their mutualistic and antagonistic partners and then how that fidelity impacts plant performance. Our experimental mesocosms returned almost 10 000 unique interactions across the 160 networks including 323 antagonistic and 378 mutualistic fungal OTUs and 189 antagonistic and 49 mutualistic bacterial OTUs. We found considerable variability in plant biomass (mean = 163 g, SD = 264 g) across communities. We observed that despite the changes in the biotic community including bacterial and fungal soil assemblages, plants showed high partner fidelity to both mutualist and antagonist partners. Counter to previous suggestion, we found that antagonistic interactions were significantly more conserved than mutualistic interactions. As the number of exotic plant species within the community increased, however, plants showed decreasing partner fidelity. This result was most pronounced for antagonistic interactions demonstrating an overlooked impact that invasive species have on the structure of biological communities. While studies suggest that individuals can switch their direct interaction partners, it is unclear how these changes to a species’ network role impact fitness. Here we provide that link, demonstrating that high partner fidelity is not always a good thing. In particular, a high fidelity to bacterial pathogens is significantly correlated with a lower plant biomass, while high fidelity to bacterial mutualists is significantly correlated with a higher plant biomass.