Higher establishment success in specialized parasitoids: support for the existence of trade-offs in the evolution of specialization

Most animals do not feed on all the resources available to them, but the mechanisms behind the evolution of dietary specialization are still debated. A central but unanswered question is whether specialists generally gain fitness advantages on their resource compared to generalists, experiencing a trade-off between the ability to use a broad range of resources and the fitness reached on each single one. Empirical tests so far suffered from difficulties in measuring fitness; they were restricted to few species, and results were equivocal. This lack of support for the importance of trade-offs gave rise to theories explaining the evolution of specialization without such trade-offs. Using a large dataset of intentional biological control introductions of 254 species of parasitoids from 15 families to locations outside their native range, we show that establishment success, a measure of total fitness, is higher in specialized species. This result holds when controlling for possible confounding factors such as the number of introduced individuals (propagule pressure). The outcome of this study provides robust evidence that dietary specialization implies fitness advantages in an entire species-rich taxon, indicating that trade-offs might be widely involved in the evolution of specialization.