Influence of flow regime on habitat selection by tadpoles in an Australian rainforest stream

Abstract

This study examined how spatial variation in flow regime affects patterns of tadpole distribution in Birthday Creek, an upland rainforest stream in northern Queensland. Tadpoles of Litoria genimaculata and Mixophyes schevilli are restricted to habitats with little or no water flow, and L. nannotis and Nyctimystes dayi occur only in rapidly flowing and turbulent riffle habitats. The abilities of L. genimaculata, M. schevilli and L. nannotis to resist flow rates typical of riffle habitats were compared in a laboratory flow chamber and in an artificial riffle in the field. In the flow chamber, large tadpoles of L. genimaculata were more resistant to flow rates of 25 cm/s than were smaller individuals, but at 50 cm/s all tadpoles were dislodged irrespective of size. Large M. schevilli tadpoles were significantly more resistant than were smaller individuals only at the fastest flow rate. Large (> 25 mm body length) M. schevilli were resistant to the highest tested flow rates (50 cm/s), but in an artificial riffle with turbulence typical of a riffle both Litoria genimaculata and Mixophyes schevilli were rapidly dislodged irrespective of their size. Fast water speeds had little impact on L. nannotis in the flow chamber or in the artificial riffle. Litoria nannotis and other tadpoles living in riffles have evolved a range of morphological and behavioural traits that minimize exposure to, and maximize resistance to, fast-flowing turbulent water within these habitats. The results of this study indicate that L. genimaculata and M. schevilli cannot live in riffles because they are unable to withstand the flow rates typical of these habitats.