Trophic mediation and ecosystem stability: An assessment using qualitative network models

Nontrophic interactions can contribute to negative and positive feedbacks within a community, thus affecting likelihood of regime shifts; however, assessing the nature and importance of these effects in a network remains challenging, especially for pelagic ecosystems. Here, we present a qualitative modeling approach for assessing the importance of different effects and resultant feedbacks for community stability, using a Southern Ocean example. A potentially important positive feedback in the Southern Ocean ecosystem involves production of a chemical cue, dimethyl sulfide (DMS), by some phytoplankton. Production of DMS can promote phytoplankton growth by attracting predators of phytoplankton-grazers, and nutrients released as feces from those predators help fertilize the water column. We explored how uncertainties in the nature of this feedback affect community stability in a set of small, community models. We found that stability varied substantially depending on how the community was modeled, but that the interactions most important for determining stability were consistent across all models. Model stability was sensitive to the strength of phytoplankton competition, controls on phytoplankton, DMS production and release, and predator attraction to DMS, suggesting that the community could be destabilized by perturbation affecting these interactions. Incorporating DMS-mediated feedbacks into a larger Southern Ocean network had a moderate impact on stability characteristics and altered the trophic level at which the system would be most vulnerable to perturbation.

Automated summary

Non-trophic interactions, such as the production of DMS by phytoplankton in the Southern Ocean ecosystem, can play a crucial role in community stability by attracting predators and releasing nutrients that fertilize the water column. Modelling studies show that uncertainties in these interactions can significantly impact stability, with factors like phytoplankton competition, DMS production and predator attraction to DMS being key determinants of community stability. Incorporating DMS-mediated feedbacks into larger networks can have a moderate impact on stability characteristics and alter vulnerability to perturbation at different trophic levels.