Answers to the reading theory exercise
Make sure you have read your handout (or download the handout here) for the summary of this paper.
It is often thought that mutualisms are susceptible to cheaters (a species that takes its benefit but does not provide a benefit in return will come in and turn a mutualism into a parasitic relationship). The authors show that, in this case, due to the cycling, the mutualism prevents the invasion of cheaters.
This question is asking whether all stable mutualisms can be reduced to this sort of cyclic one way sharing scenario. Scenarios where the results may apply: bacterial communities where cross-feeding mutualisms are common or other communities where a third resource limits the growth of the community and prevents a population explosion. Scenarios where they may not apply: communities where a third limiting resource is not present. Further, communities where the cycling does not emerge might be susceptible to a cheater.
The model helped them figure out the specific mechanism that could cause the cycles. The authors used the model to reduce complexity to arrive at the essential mechanism that can lead to the cycles. The model also allowed them to investigate a range of scenarios for which experiments were not done, such as investigating whether a cheater can invade the community.
Both mutualists in the model are limited by glucose, a resource they compete for. This prevents runaway population growth in both strains.