Spatial scaling laws do not structure strongyloid nematode communities in macropodid hosts

The characteristics of species within a community can influence the number of species that can coexist within that community. In particular, body size can constrain how many individuals can 'fit' into a community, and overlap in resource use between species depends on differences in their body sizes. Here, using data from 18 communities of strongyloid nematodes living in the stomachs of macropodid marsupials, we test key predictions derived from spatial scaling laws regarding the minimum similarity in body size between coexisting species believed to control how many species can coexist in a community. These communities are ideal systems for such a test: they consist of huge numbers of individuals from numerous species, all belonging to the same family (Chabertiidae) and living in the same host organ. Within these communities, we found that mean abundance correlated negatively with body size across all nematode species, whether body size was measured as length or volume. However, we found no support for the predictions of spatial scaling laws. First, the size ratios of pairs of adjacent-sized species did not decrease as a function of the size of the largest species in a pair. The few significant relationships observed were all positive, suggesting that the relative difference in size between adjacent species in the size hierarchy may in fact increase toward the upper end of the size spectrum. Second, the frequency distributions of body sizes were predominantly right-skewed amongst the communities investigated: within the size spectrum observed in a nematode community, small-bodied species greatly outnumber large-bodied ones, in sharp contrast to the predictions of spatial scaling laws. Nematode body size may thus determine the abundance achieved by a species but not how many species can coexist; the limiting similarity between coexisting species must depend on other biological traits.