A Further Ecological Study of Calcareous Streams in the `Black Mountain' District of South Wales

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Abstract
The Sawdde and its tributary, the Clydach, are swift, stony trout streams flowing on the northern side of a range of hills, and have cool, unpolluted, alkaline, moderately calcareous water, are very constant in flow and have a very stable bed. The fauna of the Sawdde, examined in July-Sept. 1948, included about 75 spp.; predominant animals are Atractides brevirostris, Perla carlukiana, Leuctra fusciventris, Ephemerella, notata, Baetis rhodani, Glossosoma boltoni, and Rhyacophila obliterata. Of the flora, Fontinalis antipyretica is widely distr. and abundant at some points. Ulothrix is the dominant green alga. There is a rich diatom flora; characteristic spp. are Diatoma vulgare, Ceratoneis arcus, Achnanthes spp., Coccoheis placentula, and Navicula viridula. About 600 specimens of the predominant insects were examined for gut contents. Perla carlukiana, Dinocras cephalotes, Rhyacophila obliterata and Plectrocnemia conspersa are the chief predatory spp.; Baetis, Ephemerella, Simulium, and chironomid larvae are their most frequent prey. The common mayflies are entirely herbivorous; Baetis feeds upon Ulothrix and other green algae, detritus, and diatoms; Ecdyonurus mainly upon detritus; Siphlonurus upon the leaves of higher plants and detritus; Ephemerella upon Ulothrix and Fontinalis. Ecdyonurus venosus and certain other common animals in the stream system are apparently able to escape being eaten by the predatory Plecoptera and Trichoptera. Of the common, larger insects Philopotamus montanus is the only sp. mainly dependent on diatoms.