Evolutionary history as the main driver of cohesive groups’ hierarchical organization in flea-mammal interaction networks

Cohesive species groups (components, sectors, modules, and subgraphs) represent parts of an ecological network with a substantially higher density of interactions than the surrounding parts. We investigated cohesive groups in 108 flea-mammal networks from all over the world and tested whether these groups are hierarchically organized, that is, whether groups at the higher level are composed of groups at the lower level, thus representing a network structure. We measured congruence between groups, using congruence coefficients, and asked whether the extent of hierarchical organization differs between biogeographic realms, different biomes, and different climatic zones. We also tested whether coefficients of congruence between cohesive groups are affected by environmental variables (amount of green vegetation, precipitation, and air temperature). We found that (i) cohesive groups of species in these networks are hierarchically organized and (ii) the strength of this organization differs significantly between networks from different biogeographic realms but is not generally affected by surrounding environmental conditions such as vegetation type and climate. In other words, the structure of flea-mammal networks, in terms of the hierarchical organization of cohesive groups, seems to be determined, first and foremost, by the evolutionary history of flea-mammal interactions, that is, by processes and events of the past. We conclude that the impact of evolutionary history on the network structure appeared to be stronger than that of the contemporary environment.