Species‐specific traits constrain the ways organisms interact in nature. Some pairwise interactions among coexisting species simply do not occur; they are impossible to observe despite the fact that partners coexist in the same place. The author discusses these ‘forbidden links’ of species interaction networks. Photo: the sphingid moth Agrius cingulata visiting a Bauhinia Mollis flower; Las Yungas, Argentina. Courtesy of Andrea Cocucci. In Focus: Sazatornil, F.D., Moré, M., Benitez‐Vieyra, S., Cocucci, A.A., Kitching, I.J., Schlumpberger, B.O., Oliveira, P.E., Sazima, M. & Amorim, F.W. (2016) Beyond neutral and forbidden links: morphological matches and the assembly of mutualistic hawkmoth‐plant networks. Journal of Animal Ecology, 85, 1586–1594. Species‐specific traits and life‐history characteristics constrain the ways organisms interact in nature. For example, gape‐limited predators are constrained in the sizes of prey they can handle and efficiently consume. When we consider the ubiquity of such constraints, it is evident how hard it can be to be a generalist partner in ecological interactions: a free‐living animal or plant cannot simply interact with every available partner it encounters. Some pairwise interactions among coexisting species simply do not occur; they are impossible to observe despite the fact that partners coexist in the same place. Sazatornil et al. (2016) explore the nature of such constraints in the mutualisms among hawkmoths and the plants they pollinate. In this iconic interaction, used by Darwin and Wallace to vividly illustrate the power of natural selection in shaping evolutionary change, both pollinators and plants are sharply constrained in their interaction modes and outcomes.
