The contribution of headwater streams to the taxonomic richness of river basins

Headwaters represent an extensive linear resource in the countryside in terms of total numbers, watercourse length and conservation value. However, they are particularly prone to impacts from agriculture, climate change and acidification. A study of the distribution of 508 taxa at 861 sites of 1st - 9th order showed that 12% of them were either confined to (n=35) or significantly associated with (n=26) headwaters. Detailed sampling in four contrasting river basins showed that headwaters contributed approximately 20% of the total basin taxonomic richness. Although headwaters collectively support high numbers of taxa, the richness of individual sites is relatively low in comparison with higher order streams. Marked longitudinal zonation occurs over the first kilometre of streams. Within a river basin, many taxa, including those of conservation importance, are confined to a small proportion of headwaters and/or to short stream reaches. These data indicate the vulnerability of many specialised headwater taxa to environmental impacts. In recent years UK environment agencies have focussed increasing attention on protecting and enhancing the biological condition of streams in the rural environment. National surveys of the state of the British countryside revealed that the biological conditions of 53.9% of headwater streams improved significantly during the past decade.