Competition between mobile species using patchy resources: an example from a freshwater, symbiotic assemblage.

Three species of freshwater mites that are symbiotic with mussels in St Mark's River, north Florida, have consistently high rates of colonization to and occupy high proportions of mussels. The mites Unionicola poundsi and U. serrata are territorial and have limited numbers/host, whereas the non-territorial U. abnormipes has highly variable numbers/host. U. abnormipes and U. serrata are most common in Villosa villosa and Uniomerus declivis respectively, patterns that can not be explained by host species preferences, whereas U. poundsi is equally abundant in both host species. Field experiments showed that both U. poundsi and U. serrata were limited most by intraspecific competition between adult mites, presumably for access to food and oviposition sites. Additionally, U. serrata did not remain within small hosts, most of which were V. villosa. In contrast, numbers of U. abnormipes were limited by both other mite species although the nature of the interactions differed. U. serrata may prey on U. abnormipes when they co-occur, whereas U. poundsi probably only excludes U. abnormipes from certain areas within hosts. Hence, U. abnormipes occurs mostly in V. villosa because most of these mussels do not contain U. serrata, but even so its numbers are still depressed by U. poundsi. The results were consistent with the general expectation of Holmes and Price (1986) that parasite assemblages where species have high colonization levels should be organized primarily by biotic interactions. However, specific outcomes of competition between mites were consistent with the more general model of Levins (1979) for competition between species using variable resources. Failure of other models to apply to Unionicola pinpointed at least five key biological characters that may form a better basis of comparison than taxonomic or habitat-based contrasts.