Mute Swan grazing in chalk streams

By Kevin Wood

Thanks to their abundant aquatic plants the chalk streams of southern and eastern England are an ecosystem of high conservation value, especially for invertebrates and fish. Over recent decades, fisheries and conservation interests have become concerned that the foraging by Mute Swans (Cygnus olor) on aquatic plants degrades the habitat of invertebrates, fish and other animals. However, there has been little research into swan grazing damage to plants.

Abundant aquatic plant growth in the River Frome, Dorset. © Kevin Wood

In 2008 a team of researchers from the Centre for Ecology & Hydrology and Bournemouth University, funded by the Natural Environment Research Council, began a four year research programme into the effects of Mute Swan grazing (Figure 2). This project quantified the effects of swan grazing on the plant community in the River Frome catchment, a chalk stream in southern England. Mute Swan grazing caused substantial reductions in plant cover and biomass, especially in late summer and at high densities swans could eliminate all of the above-ground biomass (Wood et al. 2012). The impacts on plants increased with the numbers of swans present, with non-breeding flocks having greater impacts than breeding pairs and family groups. However, swan impacts on aquatic plant biomass and cover did not persist into the following year, as scouring high-speed flows during winter removed the majority of uneaten plants, and caused swans to move to adjacent pasture fields (Wood et al. in press).

An adult Mute Swan feeding on submerged aquatic plants in a chalk stream. © Kevin Wood


Wood, K.A., Stillman, R.A., Clarke, R.T., Daunt, F. & O’Hare, M.T. (in press). Water flow limits the temporal extent of herbivore effects on aquatic plants in a lowland river. Hydrobiologia. DOI: 10.1007/s10750-016-2744-4

Wood, K.A., Stillman, R.A., Clarke, R.T., Daunt, F. & O’Hare, M.T. (2012). Understanding plant community responses to combinations of biotic and abiotic factors in different phases of the plant growth cycle. PLoS One, 7, e49824.