Determinants of helminth aggregation in natural host populations: individual differences or spatial heterogeneity?

We assessed the importance of spatial heterogeneity for the aggregation of helminth populations in the bank vole Clethrionomys glareolus (Arvicolinae) using a previously published method which allows to analyse parasite aggre gation at two host population levels. i.e., within and between spatial samp les of the host population. In the main empirical material from Finnish Lapland (Pallasjarvi). all five helminth species were significantly aggregated within study sires, but onl y three rare species showed significant aggregation among sites. In all hel minth species. the total aggregation was primarily (79-98%) determined by a ggregation within sites, i.e., between host individuals. Despite the larger spatial scale and more heterogeneous landscape. the comparative material f rom South Finland (Luhanka) supported the generally high proportion of the total aggregation due to within-site heterogeneity. Despite the clear interspecific differences in patchiness of helminth popul ations, the proportion of the total aggregation due to among-sire heterogen eity did nor vary significantly among helminth species. Thr results indicat e a link between aggregation at two population levels; species that showed strong within-site aggregation were also characterised by pronounced spatia l heterogeneity and significant among-site aggregation.