Highlighting competition for space between Strongylus vulgaris and Strongylus edentatus in horse large intestine.

A statistical approach highlighting interspecific competition for spatial niche between intestinal parasites is presented. Two similar nematode species, Strongylus vulgaris and Strongylus edentatus, collected throughout the large intestine of 50 horses but more abundant in the caecum and ventral colon respectively according to their typical location, were studied. Ecological interspecific interactions are commonly considered to be absent because of the absence of negative pairwise correlations between helminth species abundances. However, the absence of these correlations does not demonstrate that competition is not occurring because similar epidemiology and biology could produce positive correlations and mask competitive interactions. Therefore, after a preliminary Spearman’s test highligting positive pairwise correlations only between S. vulgaris and S. edentatus collected from different intestinal locations, two negative binomial regressions were performed with S. vulgaris as the dependent variable, (these appear in the caecum and ventral colon respectively), demonstrating that the presence of S. edentatus in the caecum influences the abundance of S. vulgaris both in the caecum and in ventral colon. This kind of negative relationship has never been highlighted before because the positive relationship between the overall abundances of the two species, due to similar epidemiological frameworks, usually hides the phenomenon of spatial competition. This aspect was controlled for in the present regression model by the inclusion of the total number of S. vulgaris itself as the independent variable.