Density-dependent selection closes an eco-evolutionary feedback loop in the stick insect Timema cristinae

Empirical demonstrations of feedbacks between ecology and evolution are rare. Here, we used a field experiment to test the hypothesis that avian predators impose density-dependent selection (DDS) on Timema cristinae stick insects. We transplanted wild-caught T. cristinae to wild bushes at 50 : 50 cryptic : conspicuous morph ratio and manipulated density by transplanting either 24 or 48 individuals. The frequency of the conspicuous morph was reduced by 73% in the low-density treatment, but only by 50% in the high-density treatment, supporting a hypothesis of negative DDS. Coupled with previous studies on T. cristinae, which demonstrate that maladaptive gene flow reduces population density, we support an eco-evolutionary feedback loop in this system. Furthermore, our results support the hypothesis that predator satiation is the mechanism driving DDS. We found no effects of T. cristinae density on the abundance or species richness of other arthropods. Eco-evolutionary feedbacks, driven by processes like DDS, can have implications for adaptive divergence and speciation.