Fig. 4.2 Patterns of aggregation in macroparasites. In all cases the bars represent frequency distributions based on the proportion of hosts with a given parasite intensity. The parameter k from the negative binomial distribution provides an index of the degree of aggregation; when k is large, the distribution is approximately random (Poisson), and when k is small (less than 1) the distribution is highly aggregated (Shaw and Dobson 1995). In these plots, s2 refers to the variance and m refers to the mean. (a) Theoretical distributions for a negative binomial process where the mean number of worms per host is 15, and the k parameter is either relatively high (gray bars; k = 5) or relatively low (black bars, k = 0.5). (b) Distribution of Schistosoma mansoni infection in a natural population of olive baboons (Papio anubis) in Gombe Stream National Park, Tanzania (Müller-Graf et al. 1997). Samples shown are worm eggs per gram of feces, from a total of 396 fecal samples collected from 206 known individuals from five different troops. In this case, the parasite shows a highly aggregated distribution (k = 0.16). Panel (b) reproduced from Müller-Graf et al., Parasitology vol. 115. Copyright (1997), Cambridge University Press.

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