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Ben Thornes

Effects of Swan grazing on Macrophytes

Tuesday 15th July 2014

The effects of mute swan grazing on a keystone macrophyte in the River Frome, Dorset.

This study describes the early summer foraging behaviour of mute swans (Cygnus olor) on the River Frome, a highly productive chalk stream in Dorset in which Ranunculus penicillatus pseudofluitans is the dominant macrophyte. Previous investigations into the dynamics of freshwater eco-systems have traditionally highlighted the importance of submerged macrophytes to ecosystem function. These submerged macrophytes have a number of key ecological roles, including the provision of habitat and refugia for invertebrates and fish, contributing to chemical processes such as nutrient absorption and oxygenation of the water column and maintaining water clarity by increasing deposition of suspended sediments within the vegetation stands. As such, the herbivory of such submerged macrophytes is an issue that has received much attention, particularly when addressing strategic decisions within aquatic habitat management. Grazing by waterfowl is generally considered to be of greater significance than that of fish and invertebrates, with many previous studies investigating this. There has however, been a dearth of studies for one important grazing system’ Ranunculus penicillatus pseudofluitans exploited by the mute swan Cygnus olor.

Ranunculus pseudofluitans is a perennial, submerged macrophyte in chalk streams. It is a keystone species as it forms a fundamental structural component of these systems, influencing flow patterns and silt deposition, maintaining river depth during summer low flow and supporting high densities of invertebrates, enhancing system productivity to the benefit of fish populations. It is a protected species under the European Union Habitats and Species Directive (92/42/EEC), and the UK Chalk Stream Biodiversity Action Plan recognises both the need to maintain a macrophyte community dominated by R. pseudofluitans, and the need to ensure that a sufficient proportion of the R.pseudofluitans standing crop flowers and sets seed. The mute swan C. olor is protected by the European Union Wild Birds Directive (79/409/EEC), and is a large native herbivore which has seen a 59% population increase in Britain over the last 25 years. This increase in population has seen a perceived increase in swan grazing on R. pseudofluitans has caused concern among anglers and conservationists. This paper uses a combination of fieldwork and modelling to quantify the impact of mute swan grazing on R. pseudofluitans.

A study-reach area was identified, 1.1km long and containing abundant R. pseudofluitans and a daily maximum of 41 � 2.5 non breeding, yearling or adult swans were present during the study period between late May and the end of June. The river was the primary feeding habitat, with feeding activity on the river at dawn and dusk being much lower than during daylight ‘ however the possibility of the swans feeding during darkness cannot be ruled out.

Through observations, the effects of herbivory on R. pseudofluitans biomass and morphology were quantified. Results showed that biomass was lower in grazed areas and swans grazed selectively on leaves in preference to stems. A lower proportion of stems from grazed areas possessed intact stem apices and flowering of the plant was reduced in the grazed areas.

Through the observations made, a model based on the swans’ daily consumption was used to predict the pressure of swans on R. pseudofluitans. The model accurately predicted the number of bird days supported by the study site, only if grazing was assumed to severely reduce R. pseudofluitans growth. The proportion of the initial R. pseudofluitans biomass consumed by a fixed number of swans was predicted to be greater when the habitat area was smaller, when initial R. pseudofluitans biomass was lower and when R. pseudofluitans was of lower food value.

As the dominant macrophyte, R. pseudofluitans plays a key role in chalk stream ecosystems, providing cover for fish and invertebrates, influencing nutrient dynamics and increasing wetted habitat by retaining water within the channel during the growing seasons. It was concluded that the flux of Nitrogen and phosphorous through the study reach was largely unaffected by swan activity. The quality of R. pseudofluitans mesohabitat (the plant as a habitat for invertebrates and fish) was significantly reduced by grazing, which also indirectly contributed to reduced roughness (Manning’s n) and by inference water depth. Wetted habitat area for fish and invertebrates would also be lowered over the summer period as a consequence of the reduction in water depth. Grazing at this level would also lower the wetted habitat area for fish and invertebrates over the summer period, as a result of reduction in water depth. Additionally, it was estimated that, while grazing, an individual swan may eat the same mass of invertebrates per day as a 300-g trout.

Therefore, there is a need to manage the conflict between mute swans and the keystone macrophyte, R. pseudofluitans, in chalk streams, and the modelling approach used in this paper offers a potentially useful tool for this purpose.

Read the full article here.

Find out more about Professor Richard Stillman here.

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