Invertebrate movements within a small stream: Density dependence or compensating for drift?

The movements of invertebrates within a permanent first order stream in sou thern Ontario are described, including: drift, upstream migration and post- emergence flight. Large numbers of invertebrates drift within one year. Fec undity estimates of Nemoura trispinosa (Plecoptera) indicated that the upst ream flight of ovipositing females, in conjunction with the within-water up stream migrations of immatures and the immigration of mated females from ot her streams, was not enough to account for headwater densities depopulated through drift. However, we submit that drift does not threaten the maintena nce of headwater populations to the extent that has been suggested,and that drift acts on a density dependent basis.