Résumé
The finding of invariant structures in species interaction webs is of central importance for ecology, with the greatest challenge remaining the elucidation of the processes governing these universal web patterns. Here we quantify the degree of intervality of seven fish-metazoan and 33 mammal-flea webs, i.e., the number of irreducible gaps in parasite diets along the host spectrum, and then challenge the idea that some invariant structures may emerge in host-parasite webs. Using a null model of random links between parasite and host species we find that empirical host-parasite webs exhibit a strong bias toward contiguity of parasite diet, i.e., toward intervality. Going one step further, we demonstrate that a null model with phylogenetic constraints on host-parasite links produced webs very similar to empirical ones, particularly when phylogenetic constraints occur at the family level, that is, when two hosts from the same family are more likely to be infected than two random hosts. In addition, we propose a new standardized measure of intervality which describes a novel "facet'' of natural networks as it is independent of connectance or web size. We suggest using this measure as a surrogate of web maturity or saturation as phylogenetic constraints can drive webs toward intervality.