Chalk streams and mute swans
Wednesday 16th July 2014
The chalk streams of southern and eastern England, with their crystal clear, gently flowing waters, are one of our most iconic ecosystems and are famous for game fishing. They are also among our most important habitats for wildlife, and as a consequence many chalk streams have been granted conservation designations such as SSSIs and SACs. These designations require the UK to maintain or restore these rivers to favourable condition. Sadly, these watercourses and their plant community face a number of threats to their value as conservation areas and fisheries. Water abstraction from the rivers and their aquifers contributes to low flows, which reduces plant growth and encourages algal blooms which smother the plants, further reducing abundance. Low flows combine with soil run-off from agriculture to cause siltation of the gravel river bed, which makes growing conditions less suitable for aquatic plants. Algal blooms are exacerbated by nutrient pollution from agriculture and human settlements. These problems have contributed to the observed decline in river condition, known as chalk stream malaise. More recently, conservationists and anglers have become concerned that flocks of non-breeding mute swans Cygnus olor reduce plant abundance, which in turn degrades habitat for invertebrates, fish and other animals. For example, a survey of 365 chalk stream anglers found that 15 % ranked grazing by swans in the top three factors contributing to chalk stream malaise, ranking it sixth overall. The media love conflict, and some national newspapers have reported concerns under sensationalist headlines such as ‘Anglers in a flap as swans wreak havoc on rivers’. What has been lacking from the debate is an examination of the evidence of the these alleged impacts.
In our recent article in British Wildlife we have summarised the available evidence on the effects of swan grazing on the chalk stream plant community. We also discussed what is currently known (and not known) about the knock-on effects of swan grazing on chalk stream fauna and river condition. The evidence suggests that, whilst swan grazing is not having the widespread destructive effect on chalk streams that factors such as abstraction, nutrient pollution and siltation are, swans may have localised impacts on the plant community which could reduce the conservation and angling value of affected sites. A growing number of field studies have shown that swan grazing can reduce plant abundance, prevent flowering, reduce water depth and reduce fishery value. However, these effects seem to be limited to a small number of sites on larger chalk streams where large flocks of up to 100 swans gather. The results of different attempts to prevent swan overgrazing of aquatic plants have been disappointing, and we currently have no simple effective means of preventing grazing damage. However, our understanding of the effects of swans on the chalk stream ecosystem has been growing rapidly, which gives us hope for future solutions. In particular, combining strategies which improve river condition and move swans away from sensitive areas could offer a way of managing grazing effects.
For more information please see the published article in the February 2014 issue of British Wildlife:
Wood, K.A., Stillman, R.A., Daunt, F. & O’Hare, M.T. (2014). Chalk streams and grazing mute swans. British Wildlife, 25 (3): 171-176.