The Balance of Public and Private Benefits Determines Sensitivity to Catastrophic Population Collapse in a Microbial Volunteer’s Dilemma

The Balance of Public and Private Benefits Determines Sensitivity to Catastrophic Population Collapse in a Microbial Volunteer’s Dilemma

Cooperation plays important roles in bacteria, plant, animal, and human ecosystems. Cheaters, who garner public benefits without paying the associated costs, often outcompete cooperators. One mechanism of cooperation is the production of a public good. This seems evolutionarily unstable, because cooperation tends to decrease the fitness of the individual, while promoting the fitness of cheaters. However, cooperation can persist even in the presence of cheaters, as long as a private benefit is retained by the cooperator. While theoretical and experimental studies have examined the use of public goods in cooperation, research has yet to determine how the ratio of public and private benefit affects the growth and survival of populations consisting of cooperators and cheaters. To address this question, we used a mathematical simulation alongside a microbial cooperator-cheater system to demonstrate that the balance between the public and private benefit of a public good determines the sensitivity of the population to catastrophic population collapse in a well-mixed environment. Populations consisting of both cooperators and cheaters are most sensitive to collapse at intermediate initial percentages of cheaters. This sensitivity increases as the ratio of public to private good decreases. Furthermore, if the public good provides a sufficiently large private benefit and a weak public benefit, we find that increasing the amount of a stressor can counterintuitively increase total population growth. Overall our results contribute to our understanding of the principles that guide the maintenance and stability of cooperation, and may have implications for infectious diseases.