A new paper in Oikos by Lam Weng Ngai (who recently defended his PhD thesis in Hugh Tan‘s lab at NUS) and Ryan looks at theoretical conditions under which one species—a facilitator—can promote the persistence of another—a recipient—by providing it with resources. A classic example of a facilitator species is a “nurse” plant, which creates an environment suitable for the germination of other plant species under its canopy. In the paper, we were particularly interested in examining the stress gradient hypothesis, which predicts that such positive species interactions should be stronger in environments suffering greater resource stress.

We analysed two simple dynamical mathematical models of resource facilitation and found that positive interactions between a facilitator and a resource recipient species occur only when the facilitator-mediated resource conversion rate is higher than the background rate. We found limited support for the stress gradient hypothesis in our two models—the hypothesis holds only when certain mathematical conditions on the model parameters are satisfied. Our work on these simple models establishes a mathematical framework on which future studies can build and explore the robustness of our conclusions about when and how resource facilitation operates.

W. N. Lam and R. A. Chisholm. Resource conversion: a generalizable mechanism for resource-mediated positive species interactions. Oikos (in press)

The introduced plant Gorse (Ulex europaeus) in New Zealand can act as a nurse plant for regenerating native forest by stabilising the soil and providing an understorey environment that shelters seedlings from excessive wind and sun.