Resource heterogeneity and the evolution of public-goods cooperation

Heterogeneity in resources is a ubiquitous feature of natural landscapes affecting many aspects of biology. However, the effect of environmental heterogeneity on the evolution of cooperation has been less well studied. Here, using a mixture of theory and experiments measuring siderophore production by the bacterium Pseudomonas aeruginosa as a model for public-goods based cooperation, we show that cooperation in metapopulations that were spatially heterogeneous in terms of resources can be maintained at a higher level than in homogeneous metapopulations of the same average resource value. The results can be explained by a positive covariance between fitness of cooperators, population size and resource availability, which allowed cooperators to have a disproportionate advantage within the heterogeneous metapopulations. These results suggest that natural environmental variation may help to maintain cooperation.

Methods
Three types of assay were used in the collection of these data: Growth assays, fitness assays, and per capita siderophore measures. Growth assays measure colony forming units after a period of growth, fitness assays compete a focal strain against a competitor, calculating the ratio of malthusian growth of the focal strain vs that of the competitior, and per capita sierophore assays measure the iron binding ability of the supernatent after a period of growth, scaled by the number of colony forming units that produced that same supernatent.