JSTOR is a not-for-profit service that helps scholars, researchers, and students discover, use, and build upon a wide range of content in a trusted digital archive. We use information technology and tools to increase productivity and facilitate new forms of scholarship. For more information about JSTOR, please contact support@jstor.org.. The American Society of Parasitologists is collaborating with JSTOR to digitize, preserve and extend access to The Journal of Parasitology.

This paper examines patterns in the linear distribution of helminths in the small intestine of each of 45 scaup. Over all birds, most common species of helminths were found in over half of the small intestine; their distributions overlapped broadly. In individual birds, each species occupied a more restricted, predictable part of the intestine; "core species" (those present in more than two-thirds of the birds) were more evenly distributed along the intestine than expected by chance and occupied almost all of it; "secondary species" (those in one- to two-thirds of the birds) were clumped in anterior and posterior portions of the intestine; and "satellite species" (those in less than one-third of the birds) were randomly distributed. For all common helminths, the range occupied in individual birds was significantly correlated with population size; however, overlap between adjacent species did not increase with increases in their total numbers. The intestinal helminths of scaup belonged to three guilds: small absorbers (paramucosal), large absorbers (mid-lumenal), and trematodes. The interactions noted above occurred both within and between guilds. The overall helminth community in scaup appeared to be saturated with species belonging to the two absorber guilds, but not with trematodes. Communities within individual scaup sampled this overall community and were often unsaturated. We conclude that the core and secondary species of absorbers provide a basic, interactive structure to the overall community and to most of the communities in individual birds. Trematodes and satellite absorber species provide stochastic elaborations.

The helminth communities of the small intestines of 45 lesser scaup ducks sampled from 13 lakes in Alberta, Canada, were examined for patterns of association among species. A core of eight frequent, numerous, and positively associated species provided a basic similarity across all host individuals. Six of these core species are specialists in lesser scaup, and the other two are generalists in waterfowl. A group of eight moderately frequent and numerous species, which were positively associated with the core species but not with each other, also contributed to this similarity; these secondary species included two specialists in scaup and three generalists in waterfowl. The 36 remaining "satellite species" appeared to be distributed randomly among birds. Variations among communities in individual birds were due largely to (i) differences in the numbers of two suites of helminth species, one using Hyalella azteca and the other Gammarus lacustris as intermediate host; (ii) differences in the numbers of the Hyalella suite, and in the presence or absence of some secondary or satellite species, associated with the lake from which the duck was taken; and (iii) differences in the total numbers of helminths per duck, which may be associated with differential susceptibility of individual ducks.

That parasites actively select specific sites in their hosts is well known. Some parasites respond to changing conditions within the host by making diel or other short-term migrations, which may be modified by the presence of other parasites.Evidence is presented to support the hypothesis that continued interactions between parasites lead to niche diversification, and that site segregation, and consequently narrow site specificity, is an important part of niche specialization. The paucity of cases of interactive site segregation as compared with those of genetically stabilized selective site segregation suggests that parasite faunas are mature communities, the diversity of which has been established to an important extent through biotic interactions.

No abstract