Is population density a species character? Comparative analyses of the nematode parasites of mammals

An increasingly popular approach to the question of what determines population density is to compare the characteristics of common and rare species. However, if densities vary wildly between populations or through time, or are poorly sampled, the search for species level traits may be fruitless, and perhaps not even justified. For example, parasite densities have been considered too variable for comparative analyses. Here, we use repeatability analysis on data of 62 species of mammalian nematodes where population density of each species was measured in at least two different host populations, and analysed three measures of parasite density: intensity, abundance and prevalence (abundance = prevalence x intensity). About half of the variation in population intensity was found between parasite species rather than between populations within species. For abundance there were significant, but less pronounced differences between parasite species. Population intensity and abundance also differed significantly across the 25 host species sampled. For prevalence, interpopulation variation within both parasite and host species may be too dominating for cross-species analyses to be fruitful. In line with this, prevalence and intensity were only weakly correlated, and had different frequency distributions. Intensity followed a log-normal distribution across both population estimates and species means; population prevalence estimates were bimodally distributed, but species means were normally distributed. Thus, despite striking variation within species, differences in population intensity between mammalian nematode species are identifiable from literature surveys, suggesting that comparative studies may be important for understanding intensity variation. More generally, repeatability analyses may also guide meaningful comparisons of cross-species analyses made in different species assemblages.