The question of why species should specialize, i.e. using only a fraction of all resources available to them, has puzzled scientists and non-scientists alike. The proverbial saying "the Jack of all trades is master of none" provides an intuitive explanation. This saying implies that there should be a trade-off between the ability to use many different resources and the efficiency of using each one singularly. Therefore, the capacity of generalists to use a large number of different resources comes at the cost of having a decreased ability of using each one efficiently. By contrast, specialized species whose diets are restricted to a few food items should be able to use their resources more efficiently. However, empirical tests have had difficulty showing such trade-offs, making the importance of these trade-offs for the evolution of specialization still debatable.
A step forward
Published in the scientific journal Functional Ecology, the results obtained by Silvia Rossinelli, Ph.D. student, and Dr. Sven Bacher, Senior Lecturer (maître d'enseignement et de recherche), at the Department of Biology of the University of Fribourg provide the first robust support for the general importance of trade-offs in the evolution of specialization. The two researchers analyzed the survival of parasitic wasps which were released worldwide to control pest insects. Parasitic wasps, so-called parasitoids, are one of the largest insect groups with approximately more than 1 million species. They parasitize other insect species by laying their eggs upon or inside the host's body. These eggs then develop into larvae and eventually kill the host. For more than 100 years, parasitic wasps have been widely used for the biological control of insect pests worldwide. Within this insect group, all degrees of diet breadth are found, however, narrow host ranges are the most common.
The study carried out by Silvia Rossinelli and Sven Bacher is the most exhaustive to-date. The scientists found that specialized wasps, i.e. those that can only parasitize a few pest species, have a greater chance of establishing a stable population than generalist species with a wide host range. Thus, these results support the hypothesis that specialists are generally more efficient at utilizing their hosts than generalists. This conclusion strongly suggests that trade-offs play a major role in the appearance of specialists during the evolutionary history of this mega-diverse group of parasitic wasps. However, Silvia Rossinelli and Sven Bacher also point out that specialization does not always bring advantages; in environments where their preferred host is not available, generalists will profit from being able to utilize a variety of alternative hosts.
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