Revisiting ecological specialization: the case of plant–frugivore interactions

Understanding the extent to which organisms exhibit specialization when interacting with or relying on distinct aspects of their biotic and abiotic environment is a central tenet in ecology and evolution. Despite the relevance of the concept and theoretical advances, specialization has and continues to be ambiguously defined. Here we review the uses of the term ‘specialist' using as a case study the field of frugivory and seed dispersal. Our goal is to identify points of clarity and ambiguity to promote a more consistent and coherent use of specialization in plant–frugivore interactions and similar fields. Frugivory and seed dispersal research presents a good case study because specialist–generalist concepts have guided the theoretical framework of the field since its beginnings. Our analyses show that definitions of ‘specialist frugivores' have changed over time and tend to vary also depending on the studied animal taxa, such as birds, bats, or primates. At least some of the ambiguity arises from defining specialization at different scales such as evolutionary versus ecological timeframes. More critical is the finding that specialization is commonly used on frugivore species without knowledge or measure of the biological dependance of the species on specific types of fruit resources. Specifically, we found that most studies use specialization to describe fruit-preference patterns that – unlike specialization constrained by evolution – can be variable and context-dependent. We call for increased awareness to distinguish between contextual cases of preferential resource-uses, from specialized interactions that limit populations and singularly define species.