Evolution in response to an abiotic stress shapes species coexistence

Adaptation to abiotic stresses is pervasive and generally relies on traits that are not independent from those affecting species interactions. Still, how such evolution affects species coexistence remains elusive. We addressed this using populations of two herbivorous spider mite species (Tetranychus urticae and T. evansi) evolving separately on tomato plants hyper-accumulating cadmium, a stressful environment for the mites, or on control plants with no cadmium. Combinations of phenotypic analyses with structural stability theory predicted that adaptation of both species to cadmium allow them to coexist in that environment, whereas in cadmium-naïve mite populations the most likely outcome is exclusion. The shift from competitive exclusion to coexistence was due to an increase in niche differences accompanied by a decrease in fitness differences. A parallel population experiment validated these predictions. However, such changes were environment-specific, as spider-mite adaptation to cadmium did not affect species interactions and competitive outcomes in the cadmium-free environment. Our results suggest that evolution of single species in a new environment, even in absence of interspecific competitors, shapes species coexistence. Therefore, population shifts to novel environments may have unforeseen evolutionary consequences for community composition and the maintenance of species diversity.