River shingles in Devon: a valuable habitat for invertebrates?
In recent years the conservation value of exposed riverine sediments (ERS) has begun
to be recognised, particularly for invertebrates. ERS are an important, largely natural
habitat within the river corridor, although they are also vulnerable to disturbance and
damage from a number of activities.
Along natural rivers the processes of erosion and deposition create a range of features.
Of the depositional features, point and side bars are the most important for ERS
invertebrates. The sediments that form them may be of any calibre, mainly
determined by catchment geology.
Various studies over the last fifteen or so years have shown that the habitat supports a
number of rare and nationally scarce invertebrates, with shingle ERS, in particular,
providing a home for rare Coleoptera. In those rivers that are predominantly
unmodified, the habitat tends to have a patchy but regular distribution that assists
dispersion by associated species. However, on many rivers, management activities
such as erosion control, flood defence, flow regulation or aggregate extraction alter
the geomorphological processes and cause a reduction in the extent or value of the
habitat.
As part of the recognition of the conservation value of ERS and its associated species,
several action plans have been produced as part of the UK BAP. These include a
grouped species action plan for six river shingle beetles (Coleoptera: Carabidae,
Staphylinidae and Hydrophilidae), two plans for stiletto flies (Diptera: Therevidae),
and a further plan for a diving beetle (Coleoptera: Dytiscidae).
The Environment Agency is the lead partner for these action plans, while Countryside
Council for Wales is the contact point. These bodies, together with English Nature
and Scottish Natural Heritage, have jointly funded R&D projects to review the
national resource, develop a robust and replicable survey method and provide
ecological criteria for the assessment of ERS invertebrates.
Relatively little strategic survey work has been carried out at a regional level.
However, Devon has fared reasonably well to date. The Environment Agency
commissioned a study of the parts of the Teign and Bovey around their confluence
(Hammond 1998) in connection with a proposal to divert the rivers for mineral
extraction. Subsequently, further survey (Sadler and Bell, 2000) included eight other
Devon rivers plus the Camel in Cornwall, with the intention of assessing the regional
significance of the Teign and Bovey.
These surveys have shown that Devon’s rivers support some areas of high quality
ERS, together with associated invertebrate faunas. Although only two of the UK
BAP species (Perileptus areolatus and Hydrochus nitidicollis) have been recorded in
Devon in recent years, there is a pre-1980 record for the endemic rove beetle Meotica
anglica from East Devon and Lionychus quadrillum has been recorded from East
Cornwall. Generally, a diverse beetle fauna was recorded, indicating that the
County’s rivers are among the best so far surveyed in England.
The surveys were carried out at sites selected using existing knowledge of
Environment Agency staff. Selected sites met the following criteria: three bars
(preferably large ones) in a one kilometre reach; a range of sediment types including
sand; diverse riparian vegetation. The chosen Devon rivers were the Yarty, Otter,
Exe, Creedy Yeo, Teign, Erme, Taw, Torridge and Thrushel.
For the ten rivers, approximately ten pitfall traps were set at each of three bars for two
two-week periods in May and June, supplemented by timed hand searching, splashing
and sieving of sediments. Unfortunately the first phase of pitfall traps were almost
completely destroyed by floods – an occupational hazard in this type of work. One bar
was completely removed by the landowner, and two sites were vandalised.
Despite these problems 313 beetle species, from 31 families, were recorded. Pitfall
trapping provided 251 species, while 170 species were collected by hand searching.
Staphylinids were the most numerous (120 spp.) followed by Carabids (68 spp.). The
Teign held the most species (133), with the Otter (121) not far behind. Most other
rivers supported between 84 and 105 species, although survey problems meant that
the Taw only produced 52 species.
The species recorded showed a range of fidelity to the ERS habitat. They are grouped
into grades depending on their dependence:
Grade 1 - species being tied to the habitat at some stage in their life cycle;
Grade 2 - strongly associated at some stage but also found in other habitat types; and
Grade 3 - associated with ERS but also occurring in a wide range of habitat types.
These three grades are referred to as exhibiting moderate to total fidelity, while
Grades 4 and 5 may be found on ERS but are not ERS specialists. A total of 130
species in Grades 1 to 3 were recorded, with the Teign at the top of the list again, with
72 species. The other rivers contained from 40 to 57 species in Grades 1 to 3. Hand
searching tends to produce more high fidelity species than pitfall trapping.
Of the BAP species, Perileptus areolatus was recorded from the Teign, Torridge and
Exe, while Hydrochus nitidicollis was found on the Teign. In total, the survey turned
up 38 species that are either nationally notable or rarer, with 26 of these showing
moderate to total fidelity to ERS. Four of these are RBD species, although none are
RDB1 (there are only two RDB1 ERS Coleoptera!). Once again, the Teign was the
best river with 15 of the higher fidelity species with conservation status followed by
the Exe with 12. All Devon rivers contained at least 5 species. The Camel was rather
disappointing, with just one species found, although more recent work has found a
further three species.
The regional picture, therefore, looks encouraging: good numbers of specialist
species, with reasonably well-distributed species with conservation status. In an
attempt to give a national (England and Wales) picture, the latest phase of the
Environment Agency’s R&D (Sadler and Bell, 2002) has attempted to pull together
all the information from the regional surveys. The view that emerges is that the best
sites are found where the shingle resource is greatest. The sites on the Welsh rivers
(the Tywi, Ystwyth and Wye in particular) are very large, diverse sites that support a
large number of species, including those of conservation interest.
The Teign and the Exe, although not scoring as well as these, are the best purely
English rivers considered. This seems to place the best Devon sites as nationally
important – possibly justifying consideration for SSSI status. The full reports for the
work are generally available – see references below.
The Environment Agency is planning a further survey of selected areas of ERS. The
project, in collaboration with Devon County Council, will look at good quality ERS
across the county, on rivers of various sizes. We will concentrate on areas where ERS
is abundant, with large, unshaded bars, especially those to which access by stock and
people is limited and where the riparian strip is fairly natural.
We expect that the project will confirm what we already believe; that the exposed
riverine sediments of Devon’s rivers are a valuable invertebrate habitat that deserves
greater recognition. The information will also assist in developing improved guidance
on management of the habitat and the processes that create it. To the untrained eye,
gravel bars do not look to be a particularly wildlife rich environment; as a result they
are frequently subject to damaging activities. A better understanding of the habitat’s
conservation value will be very helpful in reducing the impact.
Mike Williams
Environment Agency, Exminster House, Miller Way, Exminster EX6 8AS.
3 March 2004
Key References:
Hammond, P. (1998) Survey of the Invertebrates on Exposed Riverine Sediments on
the Rivers Teign and Bovey near their confluence. Unpublished report, Environment
Agency, Exeter.
Sadler, J. and Bell, D. (2000) A comparative site assessment of Exposed Riverine
Sediment (ERS) beetle faunas in South-West England. English Nature Research
Reports, No. 383, Peterborough
Sadler, J. and Bell, D. (2002) Invertebrates of Exposed Riverine Sediments – Phase 3.
R&D Technical Report W1-034/TR. Environment Agency, Bristol.