Centrality measures and the importance of generalist species in pollination networks

Studies of complex networks show that nodes with high centrality scores are important to network structure and stability. Following this rationale, centrality measures can be used to (i) identify keystone species in ecological networks, a major issue in community ecology, and (ii) differentiate the keystone species concept, e.g. species may play a key role in a network for different topological reasons. In 34 pollination communities we examine the relationship between the generalization level of species (ND) and two complementary centrality indices: closeness (CC) and betweenness centrality (BC). CC measures the proximity of a species to all other species in the community, while BC describes the importance of a species as a connector. Most networks had a linear ND–CC relationship with a minimum CC value of 0.41. Hence, species were close to each and will be likely to be rapidly affected by disturbances. Contrarily, in most networks, the ND–BC relationships were power-law distributed with exponents larger than one. Only 59% of the species were connectors (BC > 0). In particular, there was a connector threshold value of ND = 0.46. Species above this threshold represent ~40%, almost all of which were connectors. These results indicate that in pollination systems the most generalized species are usually network keystone species, playing at least two roles: (i) interact closely with most other species (high CC) and (ii) connect otherwise unconnected subnetworks (high BC). We discuss the implications of centrality measures to community-based conservation ecology.