Downstream drift in madeiran levadas: Tests of hypotheses relating to the influence of predators on the drift of insects

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Abstract
On the Atlantic island of Madeira, mobile invertebrate predators such as perlid and perlodid stoneflies as well as a native fish fauna are absent, permitting tests of hypotheses concerning the influence of predators on the nocturnality of drift. Here, species and size composition of the drift, in addition to drift lengths, were studied in two man‐made waterways (levadas). It was found that the following insects dominated in the drift: Baetis rhodani (Ephemeroptera), Dixa tetrica (Diptera), Chironomidae (Diptera), and Simuliidae (Diptera). Wind increased the drift density, evidently by causing large amounts of terrestrial plant material to enter, especially in the more open streams. It seems likely that the drifting material caused the benthic animals to drift either through direct mechanical action or via indirect effects, such as visual disturbance. Rainbow and brown trout were successfully introduced into the island of Madeira in 1961. In a levada with trout, B. rhodani drifted significantly more by night. In another levada devoid of fish there was no difference between daytime and nighttime drift density. The size composition in the drift of A rhodani was skewed so that larger nymphs dominated by night in the presence of fish. Where fish did not occur there was no difference in size distribution between night and day. This is in accordance with an expected “risk of prédation”; being greater on larger insects in the light because of size‐selective feeding by the fish. Dixa tetrica drifted significantly more by night in the fishless levada in which the population had a size composition that was biased towards larger larvae. In the other levada containing fish there were no diurnal patterns in drift numbers. This is in accordance with a previous report in which a dixid has been shown to occur in the drift to a higher extent at pupation. A set of experiments demonstrated that the drift distance of & rhodani v/as less than 20 m in a channelized stream having a current velocity as high as 2.0 m s−1, and that nymphs of B. rhodani will drift upon a visual disturbance.

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