An ecological law and its macroecological consequences as revealed by studies of relationships between host densities and parasite prevalence

Epidemiological models predict a positive relationship between host populat ion density and abundance of macroparasites. Here I test these by a compara tive study. I used data on communities of four groups of parasites inhabiti ng the gastrointestinal tract of mammals, nematodes of the orders Oxyurida. Ascarida. Enoplida and Spirurida, respectively. The data came from 44 mamm alian species and represent examination of 16 886 individual hosts. I studi ed average prevalence of all nematodes within an order in a host species. a measure of community level abundance, and considered the potential confoun ding effects of host body weight, fecundity, age at maturity and diet. Host population density was positively correlated with parasite prevalence with in the order Oxyurida, where all species have direct life cycles. Consideri ng the effects of other variables did not change this. This supports the as sumption that parasite transmission rate generally is a positive function o f host population density. It also strengthens the hypothesis that host den sities generally act as important determinants of species richness among di rectly transmitted parasites and suggests that negative influence of such p arasites on host population growth rate increase with increasing host popul ation density among host species. Within the other three nematode orders. w here a substantial number of the species have indirect life cycles, no rela tionships between prevalence and host population density were seen. Again, considering the effects of other variables did not affect this conclusion. This suggests that host population density is a poor predictor of species r ichness of indirectly transmitted parasites and that effects of such parasi tes on host population dynamics do not scale with host densities among spec ies of hosts.