According to the stress-gradient hypothesis, mutualistic interactions between species can become stronger in harsher environments. For example, mosquito larvae in water-filled tree hollows can benefit from the presence of beetle larvae—the beetle larvae shred leaves into smaller pieces that the mosquito larvae can consume—and the benefit is greater when the environment is harsher, i.e., when there is less overall leaf litter available. In a previous study, we failed to find theoretical support for the stress-gradient hypothesis in a mathematical consumer–resource model. But in a new study, we show that the stress-gradient hypothesis can hold if there is a sufficient leakage rate of the downstream resource (the small pieces of leaves in the mosquito larvae example). We additionally show that our previous results are robust to the precise mathematical assumptions about how a species’ consumption rate increases with resource availability.
The new study was led by undergraduate student Sim Hong Jhun, and came out of his Honours project, which was supervised by Lam Weng Ngai (now at Nanyang Technological University) and Chong Kwek Yan (now at the National Parks Board, Singapore). The paper has just been published in the Journal of Theoretical Biology:
Sim, H. J., W. N. Lam, R. A. Chisholm, and K. W. Chong. 2022. Downstream resource leakage a necessary condition for the stress-gradient hypothesis in processing chain commensalisms. Journal of Theoretical Biology 538:111043