Many ecosystem functions result from mutualisms, yet mutualism-based functions have rarely been studied at the scale of whole mutualist networks. Thus, it is unclear how much biodiversity is needed to provide function to an entire network of partner species. Here we use 23 plant-pollinator networks to ask how the number of functionally important pollinator species depends on the number of plant species studied. We found that, because of complementarity among pollinators in the plants they pollinate, 3-13 times as many pollinator species were needed to pollinate an entire network as compared with a single plant species. Furthermore, many pollinator species that were rare within the network as a whole, and therefore not important pollinators on average, were important to the pollination of particular plant species. By not measuring function across entire mutualist networks, ecologists have likely underestimated the importance of biodiversity, and particularly of rare species, for ecosystem function.
Most studies of plant--animal mutualistic networks have been temporally static. This approach has revealed many general patterns in the structure of complex webs of mutualistic interactions, but limits our ability to understand the ecological and evolutionary processes that shape these networks, and to predict the consequences of natural and human-driven disturbance on species interactions. The growing availability of temporally explicit data is allowing ecologists to move beyond this static perspective. We review the growing literature dealing with temporal dynamics in plant--animal mutualistic networks including pollination, seed dispersal and ant defence mutualisms. We identify general patterns of temporal variation in these networks across temporal scales. We discuss potential mechanisms underlying variation in interactions, ranging from behavioural and physiological processes at the narrowest temporal scales to ecological and evolutionary processes operating over much broader temporal scales. We conclude by discussing priorities for future research, including an improved understanding of the abiotic and biotic factors driving temporal network change, and further development and refinement of analytical tools. Our review highlights the key role of the importance of considering the temporal dimension for our understanding of the ecology and evolution of complex webs of mutualistic interactions.