Spatial and temporal variation in recruitment and its effects on regulation of parasite populations.

Variation in recruitment rates of parasites to hosts possibly contributes significantly to fluctuations in parasite numbers, yet is almost never measured directly in the field. I measured the variation in recruitment rates of three species of parasitic mites living in two species of freshwater mussels over several spatial and temporal scales. I also examined separately the effect of spatial dispersion of hosts on mite recruitment. Uninfected hosts of both species were placed out each month, for a period of a month, for 2 years at one site and 12 months at another. Mussels of both species were collected simultaneously each month so that abundance of recruiting mites could be compared to mean abundances of mites in hosts at that time. To test the effect of host dispersion on recruitment rates, mussels were set out in clumped and regular patterns in a separate experiment. Overall, recruitment rates were often high but also varied substantially between sites, seasons, years and months. The likely impacts differed between mite species with one probably affected strongly by recruitment variation, while abundances of the other two were not. Populations of the latter two species are probably regulated by intraspecific competition for mates and egg-laying sites. Sampling data are often used to estimate recruitment rates but the latter should be measured, if possible, by exposing uninfected hosts for a known period of time. This direct method reveals patterns of recruitment that cannot be deduced from sampling data. The lack of information on recruitment variation represents a major gap in our knowledge of parasite populations.