Asymmetric specialization:: A pervasive feature of plant-pollinator interactions

Although specialization in species interactions has usually been equated to reciprocal specialization, asymmetric specialization (i.e., a specialist interacting with a generalist) is also likely. Recent studies have suggested that asymmetric specialization in species interactions could be more common than previously thought. We contrasted patterns of asymmetric specialization observed in 18 plant-pollinator interaction webs with predictions based on null models. We found that asymmetric specialization is common in plant-pollinator interactions, and that its occurrence is more frequent than expected under a simple null model that assumed random interactions among species; furthermore, large assemblages with many pairs of,interacting species tend to have more asymmetric interactions than smaller assemblages. A second null model, which incorporated a correlation between species frequency of interaction and degree of specialization observed in most data sets produced patterns that were generally closer to those present in the data. At least three kinds of explanations could account for the observed asymmetric specialization, including random interactions among individuals (rather than species), adaptive consequences of specialization, and artifacts, such as data aggregation and sampling biases. Future studies should be aimed at understanding the relative importance of each of these alternative explanations in generating asymmetric specialization in species interactions.