Colonization of an introduced substrate by stream macroinvertebrates [Baetis, Capnia, Nemoura, Mink Creek, Idaho, USA, mountain stream]

Trays (625 cm('2)) filled with uniform-sized pebbles were allowed to become colonized (1 to 64 d) to determine the time required to establish a stable macroinvertebrate community. Changes in both numbers and biomass were followed to compare their response as indicators of colonization. Effectiveness of the trays in representing natural conditions and their usefulness in reducing sample variance and number of samples necessary for an accurate sample mean also were evaluated. Colonization rates differed by taxon and by season. Baetis intermedius, Capnia, and Nemoura were among the most rapid colonizers. The number of taxa increased from 21 and 18 on day 1 to 46 and 37 on day 64 in two experiments. Colonization was more rapid in summer than in winter. Numbers and biomasses for individual taxa corresponded reasonably well in each colonization period. Numbers of many organisms peaked before 64 d and then fluctuated. At least 11 and 18 taxa in two experiments reached peak numbers only at 64 d and many of these increased steadily throughout that period. However, results from comparable substrates colonized for 224 d suggest that colonization much beyond two months would not greatly enhance the community. Total number and total biomass yielded contradictory results and provide the least certain methods of assessing completeness of colonization. Trays colonized for 64 d collected similar numbers of taxa (46, 37) compared to a Hess sampler (36, 40) and dip net (36, 40) in two separate tests. They also contained 1.3 and 6 times greater total numbers and biomasses of invertebrates than did Hess samples from a riffle. Of taxa found in both sets of samples, 58 and 74% were more abundant in trays than in the riffle in two experiments. Compared to samples taken on the stream bed, trays were effective in reducing sample variance but did little to reduce the clumping of organisms. In general, use of trays reduced the number of samples needed to obtain a standard error of the mean within +/-5 to 20% of the sample mean by 2/3 to 3/4. Since the trays did not collect a fauna representative of the riffle community in terms of relative or absolute abundance, they cannot be recommended for studies requiring quantitative data directly relatable to the natural environment. However, because of their ability to control or eliminate extraneous variables and thus reduce sample variance, their use is appropriate for experimental or monitoring studies.