Double mutualism describes the situation where two species interact for the benefit of both in more than one type of interaction. For instance, if a given plant species is both pollinated and has its seeds dispersed by the same animal species. Double mutualism is thought to be more present in ecosystems where organisms are generalized, such as on oceanic islands. In the Caribbean islands, double mutualism has only been reported four times, of which only one involves a bird, between the widespread Bananaquit (Coereba flaveola) and the plant Cordia bicolor (Boraginaceae) on the continental island of Tobago. Here, for the first time, we report an observation of double mutualistic behaviour of a bird on Caribbean oceanic islands: The Black-crowned Tanager (Phaenicophilus palmarum), a bird endemic to Hispaniola, and the widespread plant Beach Creeper (Ernodea littoralis, Rubiaceae). Whereas hummingbirds and insects were more frequent visitors to the flowers of E. littoralis compared to P. palmarum, only P. palmarum was observed as a seed-disperser of E. littoralis. Based on our observations, we conclude that P. palmarum and E. littoralis show double mutualistic behaviour, though more detailed studies are needed to quantify the importance of this mutualistic behaviour, both from the bird's point of view and from the plant's point of view regarding the effectiveness of P. palmarum as pollinator and seed-disperser, respectively. We believe that double mutualism has been overlooked in the Caribbean, and, thus, we hope our observations will stimulate research into the importance of double mutualism across the Caribbean.
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Vol. 51 • No. 1