Explaining co-occurrence among helminth species of lesser snow geese (Chencaerulescens) during their winter and spring migration

The digestive tracts of 771 lesser snow geese (Chen caerulescens) collected from January to May 1983 from 12 locations (27 samples) were examined for helminth parasites to determine whether parasite species present in winteri ng geese or in spring migrants occurred independently of each other. Nine h elminth species were identified. Seven had mean prevalences >5% and were th e focus of this study. Six of those species were waterfowl generalists, one was a goose specialist. Our primary objective was to assess the potential contribution of factors, other than species interactions, in determining pa tterns of co-occurrence between helminth species. There were few negative r elationships between helminth species, regardless of whether presence-absen ce or abundance data were used. However, some species pairs showed recurren t and significant co-occurrences. There were similar and significant effect s of timing of sampling, host gender, and host age, on prevalence and mean abundance of particular species. Co-occurrences were found for those specie s that showed seasonal declines in prevalence, for those expected to have h igh colonizing ability based on host age profiles (using abundance data), a nd for abundant species that may have shared vectors or environmental condi tions favorable for transmission. Thus, similarities between parasites in t heir abundance, transmission biology, and phenology seem sufficient to expl ain species co-occurrences without invoking other processes such as species interactions.