Scaling down from species to individuals: a flower-visitation network between individual honeybees and thistle plants

Studies of ecological networks usually focus upon interaction patterns among species. However, linkage among species is mediated by their constituting individuals. Thus, every linked species pair in a network encapsulates a new network of interacting individuals. Very few studies outside the sociology of larger animals have investigated networks at the level of the individual. Here, we analyse the structure of a flower-visitation network of individual thistles Cirsium arvense and honeybees Apis mellifera in a small meadow patch in Denmark. We marked and numbered 62 honeybees and 32 thistle stems and monitored all floral visits. The constructed bipartite network of individual plants and bees had a high connectance and low nestedness, but it was not significantly modular. Frequency distributions of number of links per species (i.e. linkage level) had their best fit to a truncated power law, and interactions were asymmetrical. Unipartite networks of either plants or bees had exceedingly short average path length and high clustering. Linkage level of plants increased with their number of flower heads and height of inflorescence (floral display parameters). Overall, the individual network of honeybees and thistles was denser linked than what is known from species pollination networks. Characteristics of both plants (e.g. floral display) and animals (e.g. foraging behaviour) are likely to generate this intra-specific, inter-individual link pattern. Such features of individual-individual networks may scale up and become important drivers of the structure and dynamics of species-species networks.