Functional relationships and microhabitat distributions of enteric helminths of grebes (Podicipedidae): the evidence for interactive communities.

The significance of interspecific interactions as a structuring force in the enteric helminth communities of 4 species of grebes (Aechmophorus occidentalis, Podiceps grisegena, P. nigricollis, and P. auritus) was evaluated. Patterns of microhabitat distribution revealed that helminths resided in predictable locations along the length of the small intestine. Individual species of helminths occupied a high proportion of the intestine and varied in position in different host species suggesting a broad tolerance for conditions along the intestine. However, in individual birds, helminths were much more restricted in distribution, overlapped considerably less than their overall ranges would suggest, congeners had complementary distributions, and there was evidence of interference by at least 1 core species, all suggesting that interactions are important in these communities. Nevertheless, vacant niches were present in most birds, and core species were not distributed more evenly than would be expected through random placement. This suggests that important resources may not be distributed evenly along the intestinal gradient.