  Social dilemmas are an integral part of social interactions. Cooperative
actions, ranging from secreting extra-cellular products in microbial
populations to donating blood in humans, are costly to the actor and hence
create an incentive to shirk and avoid the costs. Nevertheless, cooperation is
ubiquitous in nature. Both costs and benefits often depend non-linearly on the
number and types of individuals involved -- as captured by idioms such as `too
many cooks spoil the broth' where additional contributions are discounted, or
`two heads are better than one' where cooperators synergistically enhance the
group benefit. Interaction group sizes may depend on the size of the population
and hence on ecological processes. This results in feedback mechanisms between
ecological and evolutionary processes, which jointly affect and determine the
evolutionary trajectory. Only recently combined eco-evolutionary processes
became experimentally tractable in microbial social dilemmas. Here we analyse
the evolutionary dynamics of non-linear social dilemmas in settings where the
population fluctuates in size and the environment changes over time. In
particular, cooperation is often supported and maintained at high densities
through ecological fluctuations. Moreover, we find that the combination of the
two processes routinely reveals highly complex dynamics, which suggests common
occurrence in nature.
