Fen Management Handbook - Scottish Natural Heritage

grazing on vegetation composition and structure, and on molluscs.
Measures of vegetation structure and composition were recorded in 20 quadrats
in each of seven randomly located 15 x 30 m plots. One half of each plot was open
to grazing, and the other half fenced off from grazing. Densities of molluscs were
estimated by carrying out timed searches in eight 0.5 x 0.5 m quadrats in the grazed
and ungrazed halves of each of these seven plots.
The effects of grazing
Statistical significant differences in the composition and structure of S25 tall-herb
fen after four years of grazing were:
– Increased dominance of reed sweet-grass Glyceria maxima at the expense of
reed Phragmites australis.
– Increased plant species-richness.
– Reduced vegetation height
– Reduced biomass of common reed and total vegetation biomass.
– Reduced inflorescence density of common reed.
Cattle were frequently observed feeding on the bark of willow scrub in winter,
breaking willow branches with their horns and browsing their leaves, and so
probably also reduced the growth of low scrub. No differences were detected in
fine-scale (tens of centimetre) variation in habitat structure between grazed and
ungrazed areas. However, the large-scale variation in grazing pressure within the
fen increased large-scale (tens of metres) variation in habitat structure.
Plant species showing significant differences in frequency between grazed
and ungrazed plots after four years of grazing.
Species more frequent in grazed
halves of plots
Celery-leaved buttercup Greater pond-sedge
Common duckweed
Gypsywort
Pink/blue water-speedwell
Purple loosestrife
Reed sweet-grass
Tufted forget-me-not
132
Species more frequent in ungrazed halves
of plots
Grazing substantially decreased densities of Desmoulin’s whorl snail. This species,
though, remained widely distributed in areas within the grazing unit that the cattle
did not graze.
Conclusions
Light grazing by Highland cattle has proved a useful method for increasing the plant
species-richness and probably large-scale vegetation structure of derelict fen. It is
probably also likely to slow the rate of succession to scrub.
On the negative side, grazing reduced overall densities of at least some key mollusc
species. These population reductions, though, have to be set against losses which
would take place if the fen was left unmanaged to succeed to scrub. They also
have to be set against likely increases in abundance of other important invertebrate
species which benefit from more varied habitat structure. Breeding densities and