Browsing by Author "Noble, Robert Graham"

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    Relation between tolerances and distribution of two species of Ephemeroptera
    (1970) Noble, Robert Graham; Brown, Alec C
    In this study, tolerance limits for a number of environmental factors, taken both singly and in combination, have been determined in the laboratory for aquatic nymphs of two mayflies, Baetis harrisoni Barnard 1932 and Choroterpes (Euthraulus) bugandensis (Kimmins 1956). These limits have been examined in the light of available information on the ecology of the nymphs, in order to see to what extent the distribution of each species might be limited by intolerable environmental conditions. Baetis nymphs were found to be dislodged from rocks in the stream flow by current speeds in excess of 0.5-0.6 m/sec actually impinging on the nymphs and estimated 0.1 cm from the substratum surface. Choroterpes nymphs were dislodged by current speeds in excess of 0.3-0.4 m/sec estimated 0.1 cm from the substratum surface. Possible effects of flooding on nymphal populations of each species have been discussed in relation to the behaviour and physical habitat of the nymphs. Baetis nymphs live on rocks exposed to the current and numbers of them are washed away even in moderate floods. Choroterpes nymphs live on the under surfaces of rocks and numbers of them are only washed away by floods sufficiently strong to roll these rocks.