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Abstract

The evidence shows 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. The results of attempted management 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.
February 2014 British Wildlife 171
The chalk streams of southern and eastern
England, with their crystal-clear, gently
flowing waters, are one of our most iconic
ecosystems and famous for game-fishing. They are
also among our most important wildlife habitats,
with many designated as Sites of Special Scientific
Interest (SSSIs) and Special Areas of Conservation
(SACs), owing to their abundant and diverse flora
and fauna. These conservation designations require
the UK to maintain or restore these rivers to favour-
able condition.
Sadly, these watercourses and their plant commu-
nities 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 reduce plant growth and encour-
age algal blooms which smother the plants, further
reducing the latter’s abundance. Low flows combine
with soil run-off from agriculture to cause siltation
of the gravel riverbed, which makes growing condi-
tions less suitable for aquatic plants. Algal blooms
are exacerbated by nutrient pollution from agricul-
ture and human settlements. These problems have
contributed to the observed decline in river condi-
tion, 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 anglers found that 15% ranked
grazing by swans in the top three factors contribut-
ing to chalk-stream malaise, placing it sixth overall
(Frake & Hayes 2001). 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’ (Elliott 2004).
What has been lacking from the debate is an exami-
nation of the evidence for these alleged impacts.
Chalk streams and
grazing Mute Swans
Kevin A Wood, Richard A Stillman, Francis Daunt and Matthew T O’Hare
River Frome, Dorset.
Nicholas and Sherry Lu Aldridge/FLPA
Chalk streams and grazing Mute Swans
172 British Wildlife February 2014
The chalk-stream aquatic plant community
One of the most common chalk-stream plants is
Stream Water-crowfoot Ranunculus penicillatus
subsp. pseudofluitans, a perennial, submerged
species which extends its branching stems up
to 2m downstream of its roots (Dawson 1976).
Other common species include Blunt-fruited Star-
wort Callitriche obtusangula and Spiked Water-
milfoil Myriophyllum spicatum. These plants fulfil
a range of key ecological roles, modify the river
and provide suitable conditions for wildlife. Plants
within the river channel physically hold back
the flow of water, creating a deeper wetted area
than an unvegetated channel; this function can
be vital for preventing low flows in late summer
and autumn. This also keeps the water table high
in the surrounding pasture fields and so increases
the drought resilience of the river and floodplain.
Plants increase and diversify the habitat available
to other species, as well as providing food and
creating cover from flow and predators.
Chalk streams are perhaps best known for their
Atlantic Salmon Salmo salar and Brown Trout
Salmo trutta, which in turn support game fisher-
ies. These salmonids are only part of the diverse
fish community of chalk streams, which in particu-
lar support abundant Bullhead Cottus gobio.
Chalk streams are famous for their rich inverte-
brate communities, including mayflies, stoneflies
and caddisflies, and may also include rare species
such as White-clawed Crayfish Austropotamo-
bius pallipes. In turn, these invertebrate and fish
species support recovering populations of Eurasian
Otter Lutra lutra, the top predator of chalk-stream
ecosystems. In recognition of the role of Water-
crowfoot in sustaining a productive and diverse
ecosystem of high conservation value, the plant is
protected under the EU Habitats Directive (92/43/
EEC), which requires the UK to maintain or restore
rivers with these plants to favourable condition.
The impacts of swan-grazing on plants
Grazing damage to plants is very obvious: the pref-
erences of swans for the more nutritious leaves and
stem tips reduces the normally bushy Water-crow-
foot to short cropped stems (O’Hare et al. 2007).
When food becomes scarce even these stems may
be eaten, leaving only the root network in the river
gravels. O’Hare et al. (2007) compared plant abun-
dance in reaches in early summer with and without
flocks of swans on the River Frome in Dorset, and
reported that abundance was 49% lower where
the flocks fed. Similar reductions were reported for
the River Wylye in Wiltshire (Porteus et al. 2008).
An ungrazed reach of the River Frome, Dorset, showing plentiful Water-crowfoot and its characteristic white
flowers. Kevin Wood
Chalk streams and grazing Mute Swans
February 2014 British Wildlife 173
In 2010 we investigated the
effects of swans on plants
between March and Septem-
ber across 20 sites on the River
Frome, taking into account the
effects of water temperature,
shading by riparian trees, and the
distance from the river source.
We found that swans reduced
plant biomass (i.e. quantity)
between July and September,
and reduced plant cover (i.e. the
proportion of the riverbed that
was vegetated) between May and
September (Wood et al. 2012).
For example, between July and
September a flock of 15 swans
could halve the plant biomass
in a typical reach with 15%
shading, whilst 30 swans could
remove all the above-ground
biomass in the same reach. We
found no effects earlier in the
year, perhaps because there were
fewer swans and because the
plants grow more vigorously during this period.
Crucially, the effects of grazing on the plant commu-
nity appear not to carry over into subsequent years.
From repeated plant surveys we found no relation-
ship between the grazing pressure in 2009 and
aquatic-plant biomass in 2010 (Wood 2012). This
is perhaps unsurprising, for two reasons. First, the
tough and complex root network of Water-crow-
foot means that swans struggle to uproot plants,
and so regrowth from roots can occur. Secondly, the
scouring winter floods wash many ungrazed plants
away and thus reset the plant community.
The number of Stream Water-crowfoot flowers
in early summer was reduced, both because swans
eat flowers and because they eat the tissues which
are growing to the surface to produce the flowers. A
flock of 13 or more swans could halve the proportion
of stands flowering, whilst a flock of 26 swans could
prevent Water-crowfoot from producing any flow-
ers (Wood et al. 2012). Crucially, 13 or more swans
were likely to reduce the percentage of stands flower-
ing below the 25% conservation target for this plant
community (JNCC 2005). In addition to flowering,
plant species composition and abundance are also
among the attributes used to assess the conditions of
SSSIs and SACs (JNCC 2005). Through their nega-
tive effects on the chalk-stream plant community,
swans could contribute to ‘unfavourable’ conserva-
tion status at SSSI and SAC sites.
The wider effects of swan-grazing
We do not yet fully understand the wider effects of
swan-grazing on the chalk-stream ecosystem, for
example on the invertebrate and fish communi-
ties. Certainly, there have been many complaints,
particularly from anglers, that grazing damage
caused by swans is associated with reduced inver-
tebrate and fish abundance. For example, anglers
on the River Kennet have observed a sudden
decline in invertebrate abundances which coin-
cided with a period of intense grazing. Unfortu-
nately, there have been no published studies of
the knock-on effects of grazing on chalk-stream
fauna. If swans do have an impact on fauna, we
need to know the size of this impact, which species
are affected, how frequently the impacts occur,
and for how long the impacts persist.
Whilst the effects of grazing on fish species are
unknown, the aesthetic damage caused by plant
loss due to swan grazing reduces the value of
chalk-stream reaches as fisheries (Fox 1994). Some
evidence has also emerged that grazing effects
An adult Mute Swan, which can reach up to 1m below the surface,
feeds on early growth of Water-crowfoot. Kevin Wood
Chalk streams and grazing Mute Swans
174 British Wildlife February 2014
on plants may affect river hydrology. A study by
Wessex Water (2008) reported that a period of
intense grazing by a flock of swans coincided with
a decline in river depth of approximately 30% at a
site on the River Wylye.
How widespread are the effects of
swan-grazing?
To understand the impact of swan-grazing, we need
to know where and when grazing damage could
occur. Swan-grazing damage to the chalk-stream
plant community has currently been reported for
larger chalk streams, including the River Frome
(Dorset), River Avon and its tributaries (Hamp-
shire and Wiltshire), River Test (Hampshire), River
Itchen (Hampshire) and River Kennet (Wiltshire and
Berkshire). The wider channels of these larger rivers
allow flocks of non-breeding adults and juveniles to
congregate and thus cause grazing damage. These
flocks are seldom seen on the narrower channels of
smaller rivers. Evidence suggests that swan flocks
use only part of a chalk river, and as a consequence
grazing damage will be localised. A survey of the
Hampshire Avon and its major tributaries in 1999
and 2000 found evidence of swan-grazing damage
at 33% of sites on the Avon itself, 38% of sites on
the Nadder, 40% of sites on the Wylye, 25% of sites
on the Till, but at no sites on the Bourne (Wheeldon
2003). On the River Frome, flocks use approxi-
mately 20% of the river course (Wood 2012). Given
that the British Mute Swan population appears
to have stabilised, it is unclear whether the grazing
problem will spread to other rivers.
The use of the river channel by flocks of swans
occurs predominantly between late spring and
autumn. River use by non-breeding swans seems
to be linked to water velocity, birds preferring
lower current speeds as these require less energy to
be expended on swimming. Consequently, flocks
typically enter the river between April and May,
when flow speeds are low enough to make river
feeding more efficient than pasture feeding. These
birds generally remain on the river until the first
heavy autumn rains of October cause the river
to become swift and murky (Wood et al. 2013a).
Some non-breeding swans may spend the July
moulting period on a nearby estuary, returning
to the river afterwards. Flock birds spend winter
and spring grazing the flooded improved pasture
fields, where they eat grasses (Trump et al. 1994;
Wood et al. 2013a). In the River Frome catch-
ment, the fertiliser-enriched pasture fields around
dairy farms were particularly popular with flocks.
This pattern of habitat use suggests that grazing
damage to the river is likely to occur only during
summer and autumn, which matches the pattern
of complaints from conservationists and anglers.
Management of the swan-grazing conflict
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. What management action
A pair of Mute Swans grazing on Water-crowfoot in the River Frome, Dorset. Kevin Wood
Chalk streams and grazing Mute Swans
February 2014 British Wildlife 175
could we take to reduce grazing damage where
it occurs? Mute Swans are native to Britain, with
fossils found from approximately 5,000 years ago
(Northcote 1980), and so management must recog-
nise that swans are a natural element on chalk
streams. Swans are protected under the EU Wild
Birds Directive (2009/147/EEC). These conserva-
tion designations present us with legal obligations
to look after both swans and chalk streams.
Improving the environmental condition of chalk
streams could make them more resistant to the
effects of grazing, for example by improving growth
conditions for Water-crowfoot. Water velocity has
been identified as a key variable, as flocks avoid
faster-flowing reaches (Parrott & McKay 2001).
Velocity regulates the periods when the swans can
enter the river and thus the length of the grazing
season: when the flow is too fast, the birds must
expend so much energy in swimming that river feed-
ing becomes inefficient (Wood 2012). Measures
to increase water velocity in affected areas could
be used to shorten the grazing season and displace
swans to other river reaches or adjacent habitats
such as pasture fields (Wood 2012). Increased flow
is also known to improve Water-crowfoot growth
while decreasing siltation and epiphytic algae, so this
option has a number of benefits. Similarly, reduc-
ing nutrient pollution would improve aquatic-plant
growth by preventing plants from becoming smoth-
ered by blooms of epiphytic algae.
Another strategy could be to reduce swan densi-
ties in affected areas, either by exclusion or by reduc-
ing the population size. River-managers have tried
unsuccessfully to fence off areas of river to exclude
flocks, and swans’ tolerance of people makes scar-
ing a labour-intensive option. Swans aggressively
defend their breeding territories from other swans,
and so, by encouraging nesting in an area, we could
prevent flocks from entering and causing graz-
ing damage. The density of pairs and their families
are typically too low to cause any serious damage
(Wood et al. 2012). Attempts to encourage nesting at
desired locations on the Hampshire Avon, however,
failed, not a single pair using the nesting platforms
provided (Parrott & McKay 2001). Furthermore,
territorial defence may fail to protect an area where
the flock is too large or the river too wide, as obser-
vations suggest that the breeding pair would be
overwhelmed. Removing the swans themselves is
unlikely to be a successful, or popular, management
strategy. In 1978, on the River Wylye, 70 swans were
killed illegally, yet the population recovered in fewer
than five years (Trump et al. 1994).
Translocations of swans away from affected
areas have been attempted at least twice. Maudsley
(1996) described 11 swans moved from the River
Wylye to the River Taymar in 1986, and 26 swans
from the River Kennet to the River Severn in 1988.
In view of the continued complaints of grazing on
both rivers, these translocations appear not to have
eased the problem. A population-modelling study
concluded that only by translocating 60% or more
of non-breeding swans each year could grazing
damage on the River Frome be prevented (Wood
et al. 2013b). Given the large and widespread
swan population in Britain, coupled with the risk
of transferring grazing problems, translocations are
unlikely to be a useful solution.
Chalk-stream habitat in early spring, River Frome, Dorset. Nicholas and Sherry Lu Aldridge/FLPA
Chalk streams and grazing Mute Swans
176 British Wildlife February 2014
Limiting nests to two cygnets each year by oiling
additional eggs has been suggested as a method
of reducing population size (Watola et al. 2003).
Results to date, however, have been disappoint-
ing. A trial on the River Wylye in the late 1990s
was abandoned after several years as the seasonal
movements of birds within and between different
river catchments, together with the large between-
year differences in swan breeding success, made it
difficult to detect any change in overall population
size (Watola et al. 2003). Two separate population-
modelling studies have concluded that, whilst it can
reduce numbers, it is unlikely to reduce them suffi-
ciently to prevent grazing damage, and so further
management would be needed (Watola et al. 2003;
Wood et al. 2013b).
The abundant and nutritious aquatic vegetation
found in chalk streams offers swans high-quality
feeding areas, and strategies which make rivers
less attractive for foraging or make another area
more attractive could therefore be worth explor-
ing. Such habitat management should focus on
shifting flocks of swans away from sensitive areas,
such as areas that support rare species or valuable
fisheries. Conflicts between swans and farmers on
the River Tweed in Scotland have been reduced by
the planting of sacrificial crops of Oilseed Rape
Brassica napus (Spray et al. 2002). Goose-grazing
has also been managed successfully by the use of
sacrificial crops. Unfortunately, because grasses
routinely grown near chalk streams, such as Peren-
nial Ryegrass Lolium perenne, are tough and
difficult to digest, models suggest that even high
densities of fertilised grass are unlikely to draw
swans away from Water-crowfoot (Wood 2012).
Arange of alternative plant species could, however,
be explored.
Conclusions
The evidence shows that grazing by the swans can
reduce plant abundance, prevent flowering, reduce
water depth and reduce fishery value. These effects
seem, however, to be limited to a small number
of sites on larger chalk streams. The results of
attempted management have been disappointing,
and we currently have no simple effective means
of preventing grazing damage. Nevertheless, 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 condi-
tion and move swans away from sensitive areas
could offer a way of managing grazing effects.
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Kevin Wood is a researcher at Bournemouth
University who recently completed his PhD on
Mute Swan grazing. Richard Stillman is Professor
of Conservation Sciences at Bournemouth
University. Francis Daunt and Matthew O’Hare
are senior ecologists at the Centre for Ecology &
Hydrology.
... A number of studies from Europe and North America have demonstrated that grazing by flocks of non-breeding mute swans (Cygnus olor Gmelin, 1789), a generalist avian herbivore [38,39], can damage aquatic plant communities of high conservation value [40,41,42]. In particular, mute swan grazing has been reported for shallow river ecosystems of southern England [42,43,44]; such grazing conflicts with a key conservation objective for such shallow rivers, the protection of the aquatic plant community which is designated for its high conservation value under the European Union Habitats Directive (92/43/EEC). The biological productivity and conservation status of these lowland river ecosystems is strongly determined by the aquatic plant community, and thus even small reductions in plant abundance can have negative effects on the ecosystem [44]. ...
... In particular, mute swan grazing has been reported for shallow river ecosystems of southern England [42,43,44]; such grazing conflicts with a key conservation objective for such shallow rivers, the protection of the aquatic plant community which is designated for its high conservation value under the European Union Habitats Directive (92/43/EEC). The biological productivity and conservation status of these lowland river ecosystems is strongly determined by the aquatic plant community, and thus even small reductions in plant abundance can have negative effects on the ecosystem [44]. Reported decreases in aquatic plant biomasses have ranged from 0 to 100% [42], yet even relatively small decreases in biomass reduce the habitat, as well as cover from flow and predators, available for other species [43,44]. ...
... The biological productivity and conservation status of these lowland river ecosystems is strongly determined by the aquatic plant community, and thus even small reductions in plant abundance can have negative effects on the ecosystem [44]. Reported decreases in aquatic plant biomasses have ranged from 0 to 100% [42], yet even relatively small decreases in biomass reduce the habitat, as well as cover from flow and predators, available for other species [43,44]. In this region mute swans are non-migratory [45], and feed in the river between May and October, and in adjacent pasture fields between November and April [45]. ...
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... Rivers in particular have been repeatedly overexploited by humanity (UK National Ecosystem Assessment, 2011). In the UK, rivers are at risk from direct pollution events (Wood et al., 2014), extractions, damming and channelisations (Van Looy, Tormos and Souchon, 2014), as well as the disruptions from invasive alien species (Jackson et al., 2014). Global climate change threatens drought in some areas (Wilby and Harris, 2006) and salinisation from sea-level rise in others (Meier et al., 2007). ...
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... In recognition of its keystone role in chalk rivers, water crowfoot is protected under the EU Habitats and Species Directive (92/43/EEC). Within the last 30 years, fisheries and conservation interests have become concerned that the foraging by mute swans Cygnus olor on water crowfoot degrades the habitat of invertebrates, fish and other animals (Wood et al., 2014). Mute swans are a native species and a natural part of the chalk river ecosystem, but the population in Britain has almost doubled since the 1970s. ...
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1. In the U.K. the population of Mute swans (Cygnus olor) has doubled since the early 1980s.Swans feed mainly on submerged aquatic macrophytes, especially water crowfoots (Ranunculusspp. subgenus Batrachium). The River Avon (a chalk-stream in southern England) is designated a Special Area of Conservation, in which Ranunculus-dominated habitat is required to be protected. This study focused on the River Wylye (part of the Avon catchment) to quantify the impact of grazing by swans on the biomass of chalkstream macrophytes.2. In spring 2004 the resident swan population in the Wylye valley comprised 21 pairs and approximately 65 non-breeders. 17 nest sites were found, of which only 3 fledged a total of 14cygnets. Swan presence was monitored at 46, 100 m sample sites along the river, from January to September 2004.3. Ranunculus biomass was estimated at the same sites using a low-impact destructive sampling technique, on 3 occasions (roughly May, July and September). Mean biomass estimates (with ranges) for sample 1, 2 and 3 respectively were 544 (0-2,752), 818 (0-3,328) and 781 (25-4,388)kg fresh weight per 100 m of river.4. Using a regression model, swan numbers at each site were found to be a significant influence on change in Ranunculus biomass between May and July, accounting for at least 15% of the variation in growth rate. No significant relationship was found between swan numbers and Ranunculus growth during July to September.5. Ranunculus is important both as a habitat in its own right, and because of its influence on hydrological processes. The scale of Ranunculus biomass loss due to swan grazing is presented in terms of fresh weight and volume to allow these ecosystem consequences to be considered.
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The seasonal maximum biomass of the flowing-water macrophyte Ranunculus penicillatus var. calcarȩus was 380 g dry weight/m2 in areas managed by severe cutting back of the plant in summer, in both a shallow stream and at a similar depth (0.2–0.5 m) in a larger river. Plant growth was reduced to about 25 g/m2 when growing in river areas of mean water depth of 2.5 m. The ratio of production to maximum biomass (P/B ratio) was 1.16. The annual loss of leaves was 0.085 as a fraction to the maximum biomass. This loss was determined by considering the seasonal changes in the number of nodes from which leaves had been lost and the mean weight of mature leaves. Most leaves were reduced to about half their mature size by fragmentation of the leaf filaments before the remainder was lost from the plant. The latter was further fragmented and decomposed near its site of production and was not washed downstream in a recognisable form.The loss of damaged stems as determined by labelling studies was predominantly from distal parts. The weight of material lost was determined by comparing the length to weight relationship of undamaged branches with that of damaged ones on the same plant. The ratio of this loss to the maximum biomass increased during the growing season to reach 0.77 by May. However, little of this material was lost from the community because it accumulated on other plants immediately downstream. Plant material, of between 0.06 and 0.23 g dry weight per m2 per day, which would normally have left the experimental reach was collected on screens (5-mm mesh) extending across the whole width of the stream.When management by removal of R. calcareus from the stream ceased, the seasonal maximum biomass declined to about half that previously present (200 g/m2) within a 4-year period. There was no concomitant progressive change in the physical or chemical environment.
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Abundant herbivores can damage plants and so cause conflict with conservation, agricultural, and fisheries interests. Management of herbivore populations is a potential tool to alleviate such conflicts but may raise concerns about the economic and ethical costs of implementation, especially if the herbivores are 'charismatic' and popular with the public. Thus it is critical to evaluate the probability of achieving the desired ecological outcomes before proceeding to a field trial. Here we assessed the potential for population control to resolve a conflict of non-breeding swans grazing in river catchments. We used a mathematical model to evaluate the consequences of three population management strategies; (a) reductions in reproductive success, (b) removal of individuals, and (c) reduced reproductive success and removal of individuals combined. This model gave accurate projections of historical changes in population size for the two rivers for which data were available. Our model projected that the River Frome swan population would increase by 54%, from 257 to 397 individuals, over 17 years in the absence of population control. Removal of $60% of non-breeding individuals each year was projected to reduce the catchment population below the level for which grazing conflicts have been previously reported. Reducing reproductive success, even to 0 eggs per nest, failed to achieve the population reduction required. High adult and juvenile survival probabilities (.0.7) and immigration from outside of the catchment limited the effects of management on population size. Given the high, sustained effort required, population control does not represent an effective management option for preventing the grazing conflicts in river catchments. Our study highlights the need to evaluate the effects of different management techniques, both alone and in combination, prior to field trials. Population models, such as the one presented here, can provide a cost-effective and ethical means of such evaluations.
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Understanding plant community responses to combinations of biotic and abiotic factors is critical for predicting ecosystem response to environmental change. However, studies of plant community regulation have seldom considered how responses to such factors vary with the different phases of the plant growth cycle. To address this deficit we studied an aquatic plant community in an ecosystem subject to gradients in mute swan (Cygnus olor) herbivory, riparian shading, water temperature and distance downstream of the river source. We quantified abundance, species richness, evenness, flowering and dominance in relation to biotic and abiotic factors during the growth-, peak-, and recession-phases of the plant growth cycle. We show that the relative importance of biotic and abiotic factors varied between plant community properties and between different phases of the plant growth cycle. Herbivory became more important during the later phases of peak abundance and recession due to an influx of swans from adjacent pasture fields. Shading by riparian vegetation also had a greater depressing effect on biomass in later seasons, probably due to increased leaf abundance reducing light intensity reaching the aquatic plants. The effect of temperature on community diversity varied between upstream and downstream sites by altering the relative competitiveness of species at these sites. These results highlight the importance of seasonal patterns in the regulation of plant community structure and function by multiple factors.
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Following declining numbers in some areas in the 1980s, Mute Swan numbers have shown a marked increase in recent years. In some areas, such as the Wylye Valley in Wiltshire, this increase has been accompanied by complaints of damage caused to farming and fishing interests. The population dynamics of this population of Mute Swans is described, including its habitat preferences, and these findings are related to possible management options. As the Wylye catchment already receives immigrants from unknown sources, culling and translocation are not appropriate solutions to prevent agricultural damage. A combination of scaring, repellents and the provision of alternative food sources could provide the most successful long‐term management. Whilst different techniques may be needed to resolve the fishing problem, it is considered that integration of selected management options is most likely to provide maximum benefit.
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Capsule: The Mute Swan, a large generalist herbivore, showed patterns of habitat use influenced by social grouping and season. Territorial swans showed strong preferences for river and lake habitat in all seasons, while the non-territorial birds known to cause grazing conflicts preferred river in summer–autumn and pasture in winter–spring. Aims: To quantify the habitat preferences across different seasons of two types of Mute Swan social group, territorial and non-territorial, and assess the implications for the grazing conflict between swans and valuable plant communities. Methods: Repeated surveys of the River Frome catchment, Dorset, UK, over a two-year period allowed us to record the numbers of swans in each habitat type. An electivity index was used to calculate habitat preference scores for territorial and non-territorial swans across different seasons. Results: We found strong seasonal switches in habitat use for both territorial and non-territorial swans, but preferences for some habitats differed between these social groupings. In particular, non-territorial swans preferred pasture in winter and spring, and river in summer and autumn, while territorial swans preferred river and lake habitats throughout the year. Conclusion: The Mute Swan, a large generalist herbivore that can cause grazing damage to river plant communities, showed patterns of habitat use influenced by social grouping and season. These seasonal patterns of habitat use suggest that the grazing conflict with the river plant community caused by flocks of non-territorial swans is currently limited to summer and autumn.
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Summary • The accidental or deliberate release of alien species may be very disruptive to native biota, principally through competition or predation. Naturalized populations of mute swans Cygnus olor in western Europe and North America have overgrazed native aquatic vegetation, competed with other waterbirds, and damaged arable and fodder crops. Numbers may be controlled by destroying or oiling a proportion of eggs in each clutch to prevent hatching (clutch reduction). • A difference equation model was used to examine the effectiveness of clutch reduction on a mute swan population in the Wylye Valley, Wiltshire, UK. Model parameters were derived mainly from a long-term study of individually marked birds. Survival and emigration were parameterized as a combined function. The model focused on the non-breeding subpopulation, considered to have a negative impact on local fisheries by overgrazing aquatic macrophytes. • The model was also parameterized for another swan population in the West Midlands, UK. This population was characterized by rapid growth in a much larger area, compared with the smaller, comparatively stable population in the Wylye Valley. There were insufficient data available to parameterize accurately the model for other areas. • Annual survival rates were high in both populations, ranging between 68% and 73% for juveniles, first-years and non-breeding adults, and between 72% and 90% for breeding adults. Immigration was an important factor in the dynamics of both populations. • The effects of different levels of clutch reduction on the Wylye Valley mute swan population were simulated. Reducing clutches to two eggs per clutch lowered non-breeding numbers by 30% over a 10-year period. Total destruction of all eggs in each clutch stabilized the non-breeding subpopulation but did not eradicate it. The effects of clutch reduction were offset by high survival rates and immigration. • In the West Midlands, the simulated restriction of clutches to two eggs stabilized the non-breeding subpopulation but did not affect breeding numbers. Total destruction of all eggs in each clutch markedly reduced the non-breeding subpopulation. However, immigration was underestimated in this model and may have further diluted the effects of total egg destruction. • Synthesis and applications. Clutch reduction is labour intensive, requires persistence to be effective, and its effects may vary between populations depending on immigration rates. An evaluation of the consequences of clutch reduction in advance of implementation in the field is therefore highly desirable. The population model described in this study provides the means to do this. This study has demonstrated that control of breeding output did not alleviate a localized conflict, in this case overgrazing by swans in rivers. Deterrent measures and habitat management at the site of conflict may be more effective. The model permits the exploration of a range of demographic manipulations to determine optimum population management regimes before they are implemented.
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1. This study describes the early summer foraging behaviour of mute swans (Cygnus olor) on the River Frome, a highly productive chalk stream in southern England in which Ranunculus penicillatus pseudofluitans is the dominant macrophyte. 2. A daily maximum of 41 ± 2.5 swans were present along the 1.1 km study reach during the study period (late May to the end of June). The river was the primary feeding habitat. Feeding activity on the river at dawn and dusk was much lower than during daylight, but we cannot rule out the possibility that swans fed during the hours of darkness. 3. The effects of herbivory on R. pseudofluitans biomass and morphology were quantified. 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 grazed areas. 4. A model, based on the swans’ daily consumption, was used to predict the grazing 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, initial R. pseudofluitans biomass was lower and R. pseudofluitans was of lower food value. 5. We concluded that the flux of N and P through the study reach was largely unaffected by swan activity. The quality of R. pseudofluitans mesohabitat (the plant as 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. It was estimated that, while grazing, an individual swan may eat the same mass of invertebrates per day as a 300-g trout. 6. 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 here offers a potentially useful tool for this purpose.
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Mute Swan Cygnus olor, Bittern Botaurus stellaris, Mallard Anas platyrhynchos and Common Crane Grus grus were the commonest among 14 species of birds from a neolithic to bronze age open site in Cambridgeshire. Presence or absence of remains of these birds in English late-glacial and early Flandrian deposits is accounted for by reference to their different habits.