Eco-evolutionary dynamics is a burgeoning discipline at the interface of ecology and evolutionary biology that seeks to demonstrate how evolution can be responsible for large-scale ecological phenomena. Here, I combine theory with empirical research in the stick insect Timema cristinae – a well-developed field system in evolutionary biology – to investigate eco-evolutionary dynamics of populations and communities. Using field manipulations of camouflage (mal)adaptation, I show that natural selection and gene flow in T. cristinae have profound impacts on the plant-dwelling arthropod assemblage of which they are a part. I show that poorly camouflaged populations attract avian predators and drive reductions in the abundance of T. cristinae and non-Timema arthropods, arthropod species richness, and herbivory. I develop detailed, mechanistic verbal theory to formalize these findings in the context of island biogeography, demonstrating how it is possible for the evolution of (mal)adaptation to modify classical predictions of equilibrium species richness. Two observational studies test the evolutionary amendment to classical island biogeography, largely finding the classical theory to succeed alone in explaining patterns of species richness in this system. These observational studies furthermore show mixed support for patterns demonstrated in the field experiments, whereby T. cristinae population size is reduced in association with poor camouflage, but arthropod abundance and richness appear to be positively influenced by maladaptation. Lastly, I use a field experiment to test for density-dependent selection, finding that the strength of natural selection on a camouflage trait decreases with increasing population density. Negative density- dependent selection argues for an eco-evolutionary feedback loop in T. cristinae, with potential consequences for adaptive divergence and speciation.