PATTERNS OF DISPERSION OF AQUATIC INSECTS COLONIZING ARTIFICIAL SUBSTRATES IN A SOUTHERN ONTARIO STREAM

Modified artificial turf substrates were readily colonized by stream invertebrates, supporting 20 or more taxa after 10 or 22 days in the stream. Of the individuals present, 50-80% belonged to four taxa: Hydropsychidae, Hydroptila sp. (larvae and pupae), Rheotanytarsus sp., and Baetidae. All four taxa displayed significantly aggregated patterns of dispersion (as shown by Clark-Evans-Donnelly indices) on most of the 6 substrates. The hydropsychids and the Hydroptila sp. pupae appeared to have the highest degree of aggregation, especially at high densities; both taxa occupied about the same areas on the substrates, forming discrete bands of occupation at the upstream and downstream edges as well as along a narrow band near the middle of the substrates. Weakly positive (but generally nonsignificant) Phi(2) values (intertaxon association indices based on presence-absence data) coupled with negative I-ai values (intertaxon association indices based on joint abundances) for the Hydropsychidae Rheotanytarsus sp. and Hydroptila sp. - Rheotanytarsus sp. pairings indicate that Rheotanytarsus sp. occupies quite different areas of the substrates than either the hydropsychids or Hydroptila sp. pupae, suggesting that competition for space may be important amongst these sessile taxa.