PARASITE-INDUCED COLLAPSE OF POPULATIONS OF A DOMINANT GRAZER IN MICHIGAN STREAMS

The role of parasites in the population dynamics of lotic invertebrates is poorly known. Recently, populations of the caddisfly Glossosoma nigrior (Trichoptera: Glossosomatidae), a dominant periphyton-grazer in coldwater streams of eastern North America, have collapsed in several Michigan streams. All known cases of dramatic decline in Glossosoma density have been associated with the presence of the microsporidian parasite Cougourdella sp. (Microspora: Cougourdellidae). In two streams, reductions in Glossosoma abundance from > 2000 m-2 to < 10 m-2 were accompanied by marked increases in the proportion of larvae infected with Cougourdella. In a third stream in which the Glossosoma population had collapsed at least 2 yr earlier, an increase in Glossosoma density was accompanied by a lagged increase in Cougourdella infection levels in the population and a subsequent reduction in Glossosoma abundance to very low levels. Because Glossosoma density for several years pre-collapse was consistently about two orders of magnitude larger than post-collapse levels in all streams and because infected larvae were not detected in any stream until immediately prior to collapse, we suggest that Cougourdella has been recently introduced to Michigan streams.