Population density is a fundamental prop-erty of a species and yet it varies among populations of the same species. The variation comes from the inter-play between intrinsic features of a species that tend to produce repeatable density values across all popula-tions of the same species and extrinsic environmental factors that diVer among localities and thus tend to pro-duce spatial variation in density. Is inter-population variation in density too large for density to be consid-ered a true species character? We addressed this ques-tion using data on abundance (number of parasites per individual host, i.e. equivalent to density) of Xeas ecto-parasitic on small mammals. The data included samples of 548 Xea populations, representing 145 Xea species and obtained from 48 diVerent geographical regions. Abundances of the same Xea species on the same host species, but in diVerent regions, were more similar to each other than expected by chance, and varied signiW-cantly among Xea species, with 46% of the variation among samples accounted by diVerences between Xea species. Thus, estimates of abundance are repeatable within the same Xea species. The same repeatability was also observed, but to a lesser extent, across Xea genera, tribes and subfamilies. Independently of the identity of the Xea species, abundance values recorded on the same host species, or in the same geographical region, also showed signiWcant statistical repeatability, though not nearly as strong as that associated with abundance values from the same Xea species. There were also no strong indications that regional diVerences in abiotic variables were an important determinant of variation in abundance of a given Xea species on a given host spe-cies. Abundance thus appears to be a true species trait in Xeas, although it varies somewhat within bounds set by species-speciWc life history traits.
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