Comparative analysis of two methods used to show interspecific associations in naturally acquired parasite nematode communities from the abomasum of ewes

Abstract

The aim of our study was to compare a multivariate and a univariate method to evaluate the associations between nematode species in natural infections. We used a data set based on the 3-year study of the community of abomasal nematodes in ewes of the middle-Atlas region of Morocco. Frequency data (percentage of each species in the individual host community) were processed by principal component analysis. Euclidean distances between species ( D 2) were calculated from projections of principal component analysis on three axes and compared to a bivariate estimate (1−correlation coefficient R). Low distances were recorded between morphs of the same species ( Teladorsagia circumcincta and T. trifurcata, for example), which indicates a positive association. Only two species, Trichostrongylus axei and T. circumcincta, were negatively associated having D 2 values much over 1. The distances D 2, i.e., multivariate measure of association intensity were similar in most cases to 1− R estimates, indicating probably the absence of strong multispecies association. The D 2 fitted a Motomura geometric progression model indicating that associations were arranged in a continuum from low to high values, but they did not depart much from what could be expected from random associations for most species. The D 2 interest is mostly to relate associations with environmental variables, and comparing with 1− R, to assess the degree of interaction in the community.