Diversity of Cooperation in the Tragedy of the Commons

Common-pool resources are of fundamental importance in biology and the social sciences. The use of a shared limited resource is an exemplar of a situation in which individuals can behave cooperatively, through modest consumption, or selfishly, through excessive consumption (Hardin 1968, 1998; Berkes et al. 1989; Ostrom et al. 1999; Ostrom 1999; Frank 1989; Foster 2004). The essential paradox concerning the use of such common-pool resources is paraphrased by Hardin’s celebrated Tragedy of the Commons (Hardin 1968): each individual benefits from consuming the resource, but the incurring costs are diluted and shared among all interacting individuals. Hence the common resource is prone to exploitation and self-interest drives over-consumption of the resource to the detriment of all. Here we show evolutionary outcomes based on a continuous range of consumption levels of common-pool resources that are often strikingly different from the classic Tragedy of the Commons. In fact, a second tragedy is revealed: not only is the common resource overexploited, but selection may result in states in which high and low consumers stably coexist. In a different, but related context this has been termed the Tragedy of the Commune, according to which evolution in communal enterprises may favor mixed states with restraint and excessive individuals rather than egalitarian consumption levels (Doebeli 2004). At least in human societies this clearly runs against accepted notions of fairness and bears formidable risks for escalating conflicts. Public goods play an important role in biology and human society. All organisms depend on common-pool nutrients for their survival and complex human societies depend both on natural resources such as fossil fuels, the global atmosphere, or the world’s fisheries, as well as on man-made resources, such as social welfare or the Internet (Ostrom et al. 1999; Milinski et al. 2006). Such public resources benefit everyone but the costs of abuse are shared by all or even deferred to future generations. Therefore, excessive consumption is cheap for the individual but costs the community dearly. The classical Tragedy of the Commons (Hardin 1968, 1998) states that the shared and unregulated use of limited resources is bound to result in overexploitation despite the fact that this puts everybody in a worse position (Berkes et al. 1989; Frank 1998; Ostrom 1999; Ostrom et al. 1999). The Continuous Snowdrift game (Doebeli et al. 2004) describes the complementary situation in which individuals make costly contributions to a public good. The benefit that accrues to all interacting individuals is determined by their accumulated contributions. Thus, the individual determines the costs and the community shares the benefits as opposed to the Tragedy of the Commons, in which individuals specify their consumption while the community shares the costs. One possible evolutionary outcome in the Continuous Snowdrift game is diversification in levels of contribution. This leads to coexistence between highly cooperative individuals that make large contributions to the public good, and defectors that make little or no contributions. This outcome was termed the Tragedy of the Commune (Doebeli et al. 2004) because in human societies