Testing the "niche centroid hypothesis" in flea-mammal networks from the palearctic: species' centrality across their environmental niches.

A species' centrality in a network is expected to decrease with distance from the centroid of its environmental niche. We investigated the variation of a species' centrality in flea-mammal networks across the space of its niche for 30 flea and 21 small mammal species occurring in 10-46 networks from different localities in the Palearctic. We characterized the environmental niche of a flea or a host species using the ellipsoid envelope model, based on a species' occurrence points and climatic, soil, and vegetation data. We found significant relationships between centrality in a network and the distance from the centroid of environmental niche in 14 of 30 flea species. In seven of these species, the centrality in a network either increased or decreased with an increase in the distance from the environmental centroid. In another seven species, the pattern was either U-shaped or inverted U-shaped. Among 20 host species, significant centrality-niche centroid relationships were found in only two species, being either linear with a positive slope or quadratic with a negative slope. We explain the lack of a general tendency of flea and host species' centrality in flea-host networks to decrease with an increase in the distance from the environmental niche centroid by the variable effects of species-specific life history traits. We also conclude that the most likely reason behind the difference between fleas and hosts in the occurrence of this tendency is their differential functions in the networks (consumers versus resources, respectively).