The hydroecological controls and conservation value of beetles on exposed riverine sediments in England and Wales

BIOLOGICAL CONSERVATION

Biological Conservation 118 (2004) 41–56 www.elsevier.com/locate/biocon

The hydroecological controls and conservation value of beetles on exposed riverine sediments in England and Wales Jon P. Sadler a

a,*

, David Bell a, Adrian Fowles

b

The School of Geography, Earth and Environmental Sciences, The University of Birmingham, Edgbaston, Birmingham B15 2TT, UK b Countryside Council for Wales, Plas Penrhos, Bangor, Gwynedd LL57 2LQ, UK Received 24 January 2003; received in revised form 10 July 2003; accepted 16 July 2003

Abstract There are large numbers of records of uncommon beetles from exposed riverine sediments (ERS) (gravel, sand and silt shoals) in the UK. However, systematic surveys of their occurrence in relation to environmental controls are seldom undertaken. Data are presented here from a survey of 69 shingle ERS across England and Wales and used to establish which factors were important in constraining species assemblages. Over 480 species of Coleoptera were collected by a combination of pitfall trapping, hand searching and excavation techniques. A total of 81 species with conservation status of Vulnerable, Rare or Nationally Scarce were recorded during the work and this includes six species on the United Kingdom Biodiversity Action Plan (BAP) lists. A subset of 42 of these rare species can be classified as ERS specialists. The data were analysed using TWINSPAN, redundancy analysis (RDA), single factor ANOVA, general linear modelling (GLM) and correlation to examine the importance of selected environmental variables and their relationships with the species assemblages. The results indicate that: (i) ERS sites with large numbers of species that are either rare or exhibit strong fidelity to the sediments have a markedly western distribution in the UK and are found on unregulated rivers in Wales and a number of rivers in the south west of England, (ii) the type of substrate, habitat heterogeneity, the percentage of shade from trees, the percentage of fine sediments on the ERS, the amount of trampling and ERS size are important in determining invertebrate communities and (iii) ERS provide valuable and important habitats for rare beetles species in the UK. The importance of river regulation, engineering and trampling by stocking for ERS invertebrates is discussed. Ó 2003 Elsevier Ltd. All rights reserved. Keywords: Exposed riverine sediments; Rarity; Floodplain biodiversity; Coleoptera; England; Wales; Disturbance

1. Introduction River margins are extremely sensitive to the hydrological forces acting upon them (Malanson, 1995). The floodplain is in a state of constant flux with repeated erosional and sedimentational processes resulting from inundation events (Junk et al., 1989). The riparian habitat is thus strongly influenced both by channel kinetics and the frequency of flood events (Ward et al., 1999; Tockner et al., 2000), and this (often seasonal) disturbance is thought to maximise biological processes (Naiman and Decamps, 1997) and both in-stream and riparian biodiversity (Naiman et al., 1993; Naiman and *

Corresponding author. Tel.: +44-1484-422288; fax: +44-1484516151. E-mail address: [email protected] (J.P. Sadler). 0006-3207/$ - see front matter Ó 2003 Elsevier Ltd. All rights reserved. doi:10.1016/j.biocon.2003.07.007

Decamps, 1997; Ward, 1998; Poole, 2002; Ward et al., 2002). The complex interaction of the river with the floodplain leads to the creation of both longitudinal gradients (downstream) and lateral gradients (across the floodplain) in species diversity (Bell et al., 1999; Bonn and Kleinw€achter, 1999). Exposed riverine sediments (ERS) (riverine shoals and similar sedimentary deposits) are at the extreme end of this lateral gradient, where they are subject to repeated inundation events and thus sit at the interface between wholly aquatic and terrestrial environments. As a result they are characterised by communities of invertebrates that are adapted to very disturbed habitats (Plachter and Reich, 1998). Along natural rivers, ERS have a patchy but regular distribution and spacing that relates to geomorphological setting (Petts et al., 2000), sustains connectivity by providing stepping stones of similar habitat, and facilitates

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J.P. Sadler et al. / Biological Conservation 118 (2004) 41–56

the dispersal of organisms (Ward, 1998). However, river regulation and flood defence schemes can result in variations of the hydrological regime leading to vegetation colonization (Gilvear et al., 2000; Parsons and Gilvear, 2002). The resulting stabilisation of the sediments is a source of real concern to conservationists and river catchment managers (Hammond, 1998a). For example, in a recent study Brewer et al. (2001) estimated that during the last 50 years the amount of ERS on Welsh rivers has reduced significantly as a result of changes in the frequency and magnitude of flooding. Although not well documented, these activities may have an impact on ERS invertebrate communities, which rely on expanses of mobile, sorted and bare sediments as habitat. This is particularly important in the UK, where ERS are not only one of the few habitats that remain relatively untouched by the direct influence of humans, but also associated with a considerable number of rare and nationally scarce invertebrates (Hyman, 1992, 1994; Rotheray and Robertson, 1993; Godfrey, 1999). Work by Fowles (1989) illustrated the potential importance of shingle ERS in providing habitat for a wide range of rare and nationally scarce species of Coleoptera. Acting upon such issues the UK Biodiversity Steering Group has created grouped species action plans for six species of ERS Coleoptera and additional plans exist for two species of Diptera and one species of water beetle (Anon, 1995, 1999) that are seen as ERS specialist invertebrates. Indeed, Hammond (1998a) estimated that 3.5% of the total British beetle fauna are riparian specialists. Recent work on ERS beetles on rivers in Devon and Cornwall (Hammond, 1998b; Sadler and Bell, 2000), elsewhere in England and Wales (Sadler and Bell, 2000; Sadler and Bell, 2002; Sadler et al., 2001) and in Scotland and northern England (Eyre, 1998; Eyre et al., 2001a; Eyre et al., 2001b), have served to emphasize the importance of ERS habitats for beetle species. The latter work draws on a large dataset from the Rivers Carron, Spey, Nith and Tweed and highlights the importance of both longitudinal variation and ERS sediment and vegetation characteristics. Here we examine the beetle faunas of ERS from a wide range of rivers in England and Wales to establish: (i) the extent to which ERS provide habitats for rare and stenotopic beetle species and (ii) the important habitat variables that constrain their distributions.

2. Sites and methods 2.1. The sites The sites were located in England and Wales and range from Cornwall, Devon and Dorset in the southwest and Hampshire in the south-east into Wales, the Welsh borders and Derbyshire, West Yorkshire and the

Yorkshire Dales (Fig. 1). An attempt was made to sample ERS with different sediment characteristics ranging from silt through to boulder, vegetated to unvegetated and near pristine to heavily degraded by either urbanisation, trampling or gravel extraction. The beetle faunas from 69 shingle ERS were selected from the following surveys: (i) A survey of 17 sites sampled during June–September 1997 in Wales and northern England as part of a methodological study that aimed to assess the most appropriate techniques of sampling ERS (Sadler and Petts, 2000; Sadler et al., in press). (ii) A further 19 sites sampled across England and Wales in May–July 1998, to gain an understanding of the types of coleopteran assemblages associated with ERS of near pristine quality to severely degraded sites, which were either polluted or trampled by stock (Sadler and Bell, 2002). (iii) Twenty-seven sites distributed across nine rivers in Devon and Cornwall were sampled in May–July 1999 to provide a comparative assessment of the ERS resource in those counties (Sadler and Bell, 2000). (iv) An additional six sites in three 2 km reaches of the upper River Severn near Newtown in Wales (Sadler et al., 2001) were sampled during late May to July 1999. As the emphasis here is an examination of shingle ERS (i.e. sediment sizes between gravel to boulder) 13 silt and sand ERS sites were excluded from the analyses (Sadler and Bell, 2002). 2.2. Invertebrate sampling The beetles were sampled using pitfall traps (Spence and Niemela, 1994) and direct hand searching techniques (Andersen, 1969). The pitfall traps were small plastic cups with an internal diameter of 10 cm placed with the rim flush with the sediment surface of the ERS, with ca. 100 ml of 50% solution of commercial antifreeze to preserve the sample. At each site nine pitfalls were used on a standard 3 m
3 m grid modified where necessary to account for variation in the size and the shape of the ERS and to reduce the chance of flooding. If the ERS was large pitfall traps were placed in each of the main sediment and/or habitat types, though on very large ERS it was not possible to cover all available habitats. The traps were emptied and reset every 14 days during the sampling periods. In a study aimed at assessing which techniques were best suited to sample ERS invertebrates (Sadler and Petts, 2000) it was found that pitfall trap captures overrepresent large and mobile species (Andersen, 1995; Standen, 2000), many of which are habitat generalists and not strictly associated with ERS. More significantly, however, pitfall traps were shown to under-represent

J.P. Sadler et al. / Biological Conservation 118 (2004) 41–56

43

R.Wharfe R.Worth

+ 10 km grid point 1 site 2 sites 3 sites 4 sites

R.Aire R.Calder

R.Alport R.Ashop

R.Derwent

+ ++ + + + + + ++ ++ + ++ + ++ +

+

+

++ ++ + + + +

+

+

10km Grid

Highland Water

R.Yeo

Fig. 1. Location of the sample sites.

smaller, cryptic, and more specialist ERS beetles (Sadler and Petts, 2000). For this reason the pitfall data were supplemented with timed hand searches, excavations and sediment sieving. Two 25 min hand searches were carried out using the procedure outlined by Andersen (1969) and adapted by Plachter (1986) and Fowles (1989), concentrating on turning stones and capturing beetles using an aspirator. Coverage of the different sediment type across the ERS started from the waterside and worked laterally across the bar towards the upper bank. The hand searches were supplemented by collecting invertebrates dislodged from the sediments by sieving an area of ca. 1 m2 at regular intervals along the edge of the ERS and also those invertebrates floating on the water surface in two 1 m2 pits that were excavated through the upper sediments down to the water table. Pitfall trap catches were pooled over the sample periods and standardized to 16 days, which was the maximum time that any site was run without disturbance

caused by flooding and the activity of livestock or vandals. As the sampling effort was equal at each site, combined abundances were calculated by summing the data from pitfall traps and ancillary searches, thereby maximizing recovery of the species that exhibit high fidelity to the sediments. 2.3. Environmental variables Data on a total of 42 environmental variables were collected at each site (Table 1), using the River Habitat Survey methodology (RHS) employed by the Environment Agency in the UK (Fox et al., 1998; Raven et al., 1998), which was modified for ERS (Eyre and Lott, 1997; Sadler and Petts, 2000). All data were collected when the rivers were in non-spate conditions. Sediment size and the overall physical diversity (in terms of variety of sediment type and ERS structure) of the ERS are known to be important, as is the type and amount of vegetation

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J.P. Sadler et al. / Biological Conservation 118 (2004) 41–56

Table 1 List of environment variables, data type and usage Variable

Variable code

Variable type

Data type

Range

Method of scoring

Navigation Impoundment % boulders % cobbles % pebbles % gravels % sands % silts % organic materials Phi value

Nav Impound %bould %cobble %pebble %gravel %sand %silt %org. Phi

Landscape Landscape ERS ERS ERS ERS ERS ERS ERS ERS

Binary Binary Percentage Percentage Percentage Percentage Percentage Percentage Percentage Phi units

0–1 0–1 0–100 0–100 0–100 0–100 0–100 0–100 0–100 )6.0 to +3.3

0 ¼ absent, 1 ¼ present 0 ¼ absent, 1 ¼ present Estimated Estimated Estimated Estimated Estimated Estimated Estimated Calculated from percentages

Vegetation type on ERS % vegetation cover on ERS Length ERS (m) Width ERS (M) Mixed woodland Coniferous plantation Moorland Scrubland Bog and marsh Improved pasture Ungrazed improved grassland Arable land Urban land Trampling on ERS

VegType Vegcov

ERS ERS

Ordinal Percentage

1–3 0–100

Estimated

Length Width Mixwood Conifplan Moor Scrub Bog Imprograz Ungrazipm

ERS ERS Landuse Landuse Landuse Landuse Landuse Landuse Landuse

Integer Integer Binary Binary Binary Binary Binary Binary Binary

4–150 m 2–25 m 0–1 0–1 0–1 0–1 0–1 0–1 0–1

Measured Measured 0 ¼ absent, 0 ¼ absent, 0 ¼ absent, 0 ¼ absent, 0 ¼ absent, 0 ¼ absent, 0 ¼ absent,

Arable Urban Grazed

Landuse Landuse ERS

Binary Binary Ordinal

0–1 0–1 0–3

Gravel excavation

Grpoach

ERS

Ordinal

0–3

ERS profile

ERSprof

ERS

Ordinal

1–3

ERS topography

ERStop

ERS

Ordinal

1–3

ERS habitat heterogeneity

ERShet

ERS

Ordinal

1–3

% shade from trees Grass tussocks on ERS

% shade Hersgras

ERS ERS

Percentage Ordinal

0–100 0–2

Grass tussocks on banks

Hbangra

Landscape

Ordinal

0–2

Dead wood on ERS

Herswood

ERS

Ordinal

0–2

Dead wood on bank

Hbanwood

Landscape

Ordinal

0–2

ERS hibernation potential Bank full height (m) Bank management Channel management Dredging Weed removal Channel enhancement Fishing Boating

Hibpot

ERS

Ordinal

1–3

Bankfull Bankman Channone Chandregd Chanweed Chanenh Recfish Recboat

Landscape Landscape Landscape Landscape Landscape Landscape ERS ERS

Decimal Binary Binary Binary Binary Binary Binary Binary

0–2 m 0–1 0–1 0–1 0–1 0–1 0–1 0–1

0 ¼ absent, 1 ¼ present 0 ¼ absent, 1 ¼ present 0 ¼ absent, 1 ¼ light, 2 ¼ moderate, 3 ¼ heavy 0 ¼ absent, 1 ¼ light, 2 ¼ moderate, 3 ¼ heavy 1 ¼ flat, 2 ¼ gentle, 3 ¼ sloping 1 ¼ simple, 2 ¼ hummocky, 3 ¼ complex 1 ¼ low, 2 ¼ medium, 3 ¼ high habitat heterogeneity Estimated 0 ¼ absent, 1 ¼ scattered, 2 ¼ abundant 0 ¼ absent, 1 ¼ scattered, 2 ¼ abundant 0 ¼ absent, 1 ¼ scattered, 2 ¼ abundant 0 ¼ absent, 1 ¼ scattered, 2 ¼ abundant 1 ¼ low, 2 ¼ medium, 3 ¼ high Measured 0 ¼ absent, 1 ¼ present 0 ¼ absent, 1 ¼ present 0 ¼ absent, 1 ¼ present 0 ¼ absent, 1 ¼ present 0 ¼ absent, 1 ¼ present 0 ¼ absent, 1 ¼ present 0 ¼ absent, 1 ¼ present

(Andersen, 1978, 1983; Sadler and Petts, 2000; Eyre et al., 2001a; Eyre et al., 2001b). The percentage abundance substrate in each sediment class was estimated to within

1 ¼ present 1 ¼ present 1 ¼ present 1 ¼ present 1 ¼ present 1 ¼ present 1 ¼ present

5%. The topography of the ERS was considered: (1) simple if it was flat and had no break of slope, (2) hummocky if it had clear mounds of sediments and (3)

J.P. Sadler et al. / Biological Conservation 118 (2004) 41–56

complex if there was a combination of hummocks, flatter areas, channels, and back waters. The type and % vegetation cover was estimated together with the % of ERS that was shaded by trees. Vegetation type was graded as (1) predominantly bare, (2) ruderal vegetation (mainly annuals and short-lived perennial herbs) and (3) complex vegetation (exhibiting an abundance of perennial species and trees). ERS heterogeneity was estimated by counting the number of microhabitats on each ERS (e.g. silt fringes, sand toes and back channels). Particular attention was paid to the amount of vertical sediment sorting on the ERS. ERS with many microhabitats scored 3 and ERS with limited microhabitats scored 1. Assessing whether the ERS was trampled and/or excavated for aggregates provided an indication of the amount of ÔartificialÕ disturbance on the ERS (0 ¼ none, 1 ¼ light, 2 ¼ medium, 3 ¼ heavy). Invertebrate hibernation potential on the ERS was graded from 1 (low) to 3 (high) based upon an assessment of the following habitat variables: (i) the amount and diversity of buffer habitat in the river corridors (e.g. trees, shrubs, grass tussocks and dead wood), (ii) diversity and nature of the vegetation and dead wood on the ERS and (iii) the nature of substrate, as some small species of ERS beetle may over-winter in the sediments, and these require sandier substrates (e.g. Dieterich, 1996). The adjacent land use was documented as were any evident feature indicating bank management and/or engineering. 2.4. Data analysis The full data matrix comprised 489 species and 69 sites. The data set was log-transformed prior to analysis, which transforms the data towards statistical normality and reduces the influence of dominant and rare species (Jongman et al., 1996). The environmental variables (those that were not ordinal or binary) were checked for normality using the Kolmogorov–Smirnov test in SPSS for Windows (SPSS, 2000). 2.5. Species richness, fidelity and conservation status To examine the influence of the environmental variables on community structure, species richness (SR) and relative abundance (RA) values were calculated for each site. The fidelity of species to ERS habitat (SF) was also examined. Species were graded 1 or 2 on the basis of their association with ERS habitats in the UK, using information from a wide range of sources (e.g. Koch, 1989; Hyman, 1992, 1994). The classification is based on the ecological attributes of the species themselves (e.g. lifecycle, sediment preferences), rather than on the basis of their recorded presence and abundance on ERS habitats: Grade 1 species are dependent for at least some stage in their life cycle on bare or sparsely vegetated sediments on the banks of rivers. Some of these species may also

45

inhabit exposed lacustrine sediments, particularly where wave action forms banks of sediment that are ecologically similar to riverine shoals. Whereas, grade 2 species are strongly associated with exposed riverine sediments for at least some stage of their life cycle, but also characteristically found in other habitat types where extensive deposits of wet or dry bare sediments are present, such as sand dunes, soft rock cliffs, sand or gravel pits. The conservation statuses of the species follow Hyman (1992, 1994). The number of species in each conservation class was totalled for each site (CS) (Table 2). As not all the rare or nationally scarce species caught were ERS specialists a further calculation was carried out to determine the total number of species that exhibit fidelity to ERS and have some conservation status at each site (CF) (Table 2). These derived variables were examined using correlation and general linear modelling using SPSS for Windows (SPSS, 2000). 2.6. Community composition Samples and sites were analysed by using Two Way INdicator SPecies ANalysis (TWINSPAN) (Hill, 1979) to provide a classification of sites on the basis of the species present. Analysis of variance (ANOVA) was used to establish whether any significant differences in species richness, relative abundance, species fidelity and the number of species with conservation status were discernible between the TWINSPAN end groups. The importance of the environmental relationships was elucidated using Canoco for Windows (ter Braak
and Smilauer, 1998). In the first instance the data were run through indirect ordination using detrended correspondence analysis (DCA) to examine the patterning in the data set and to determine whether to use linear or non-linear methods in further analyses. As the gradient lengths on the ordination axes were short (<4) redundancy analysis (RDA) was the preferred ordination technique (ter Braak and Prentice, 1988). The significance of the environmental variables was assessed using forward selection routines within a RDA in Canoco for Windows 4.0 (ter Braak and Table 2 The numbers of rare or nationally scarce beetles recorded in the survey(s) and the number of these that have fidelity to the sediments Status

Number

Fidelity

% with fidelity

N Nb Na RDBK RDBI RDB3 RDB2 RDB1

13 40 8 8 4 5 3 –

8 15 5 5 3 5 1 –

62 38 63 63 75 100 33 –

Total

81

42

52

46

J.P. Sadler et al. / Biological Conservation 118 (2004) 41–56


Smilauer, 1998). During analysis each variable was added into the model in a stepwise fashion after the previous Ôbest-fitÕ variable had been analysed. MonteCarlo permutation tests (Manly, 1994) provided significance values on each variable (P values 6 0.05 indicate a variable that significantly contributes to the model).

Table 3 Twinspan groupings of the ERS sites End groups

N

1

9

2

3

3

11

4

11

5

12

6

16

7

4

8

3

2.7. Selection of environmental variables for analysis Each environmental variable was examined in turn and a number of these were removed as they were considered unlikely to have a major influence on the faunas (e.g. evidence of fishing and boating) or varied very little (e.g. impoundment) or were recorded at very few sites (e.g. gravel extraction) or were related to the adjacent landuse (Table 1). All remaining variables were examined for colinearity in the data using SpearmanÕs correlation coefficient. All covariables were removed leaving those that had the highest coefficients. Thereafter the remaining data were analysed using Redundancy Analysis, which indicated a large amount of multicolinearity in the substrate variables (ter Braak
and Smilauer, 1998). As the substrate variables are proportional in any one site (i.e. they total 100%), they had very high variance inflation factors (VIFs). This complexity was resolved by creating a composite variable (termed Phi) by multiplying the Phi units for each substrate class by the proportion of the substrate that was found on each ERS. The products of this were summed to create a variable that provides one independent measure of substrate on each ERS.

3. Results The total number of species recorded in the 69 sites was 489 and the total number of individuals 43071. Species richness (per sampling station) varied between 12 and 81, with an average of 44. The Staphylinidae dominated the assemblage with 177 species (36%). Carabidae, Curculionidae, water beetles (Dytiscidae, Haliplidae, Hydraenidae, Elmidae, Dryopidae and Scirtidae), Chrysomelidae and Elateridae had 119, 45, 37, 39 and 12 species, respectively. Eighty-four of the species (17%) exhibited grade 1–2 fidelity to ERS habitats. 3.1. Beetle assemblages The first and second divisions of the TWINSPAN classification (Table 3) divide the sites along a substrate axis separating the coarse cobble and boulder sites from the finer shingle ones. There is a clear cline from

Description Small upland cobble and boulder ERS in the Upper (Derbyshire) Derwent Small upland cobble ERS in the River Wharfe catchment Cobble-Gravel lowland ERS (rivers Ystwyth, Tywi, Wharfe and Severn). A range of upland and lowland sites of varying size. Largely unvegetated Large heterogeneous sandy ERS (rivers Wye and Usk). Largely unvegetated Medium-large unvegetated lowland ERS from sites in Devon (rivers Teign, Exe, Torridge, Otter and Creedy Yeo) Smaller vegetated ERS (Rivers Camel, Erme, Thrushel, Frome, Yarty and lower Wharfe) Polluted urban ERS on the rivers Calder and Worth Small, heavily shaded chert ERS (Highland Water in the New Forest)

upland heavily armoured cobble and boulder ERS (e.g. the Wharfe at Buckden and the Alport and Ashop in the upper Derwent catchment), through the coarser cobbles and pebble shingles (e.g. River Torridge, Upper reaches of the Severn) and to the sandier shingles on the Rivers Usk and Wye. The remaining divisions separate the heavily vegetated ERS (both fine and coarse substates) from those that are poorly vegetated. Here the mature vegetation that characterises many of the shingle sites in Devon (e.g. Rivers Camel, Thrushel and Yarty) finds parallels with the more dense vegetation of the wooded chert ERS in the New Forest (e.g. Highland Water) and the polluted cobbles of the Rivers Worth and Calder in Keighley and Elland in West Yorkshire. Single factor ANOVA was carried out to assess whether the TWINSPAN groups differed significantly in terms of the biotic metrics. Species richness (SR) varied significantly between the TWINSPAN end groups (df ¼ 7, F ¼ 5:459, P < 0:001), as did the number of species that exhibit grade 1–2 fidelity to the sediments (SF) (df ¼ 7, F ¼ 8:187, P < 0:001) (Fig. 2), although relative abundance (RA) did not vary significantly across the sample. Heavily vegetated and smaller ERS (groups 5–6) have the highest species richness (SR). SR is markedly reduced in groups 3–4, which include larger shingle sediment ERS and groups 7–8 the urban and wooded ERS. In contrast, the number of species that exhibit grades 1–2 fidelity to ERS is high across the larger lowland ERS found in groups 2–6 and is reduced in the group 1 (upland) and groups 7–8 (heavily vegetated) (Fig. 2).

Species with fidelity to sediments

J.P. Sadler et al. / Biological Conservation 118 (2004) 41–56

70

Species Richness

60

50

40

30

20 10

20

10

0

N=

9

3

11

11

12

16

4

3

1

2

3

4

5

6

7

8

(a)

N=

3

11

11

12

16

4

3

2

3

4

5

6

7

8

Twinspan Class

10

ERS species with Conservation Status

10

8

6 4

2

0 -2 N =

9

1

(b)

Twinspan Class

12

Species with Conservation Status

47

8

6

4

2

0

-2 9

3

11

11

12

16

4

3

1

2

3

4

5

6

7

8

(c)

N=

9

3

11

11

12

16

4

3

1

2

3

4

5

6

7

8

(d)

Fig. 2. Box and whisker plots of selected variables by TWINPSAN end group: (a) species richness; (b) number of species that exhibit fidelity to ERS; (c) number of species with conservation status; and (d) number of species with both fidelity and conservation status. N refers to the number of sites in each TWINSPAN group.

3.2. Conservation status Table 2 shows the numbers of beetles that are Rare and Nationally Scarce according to the conservation reviews (Hyman, 1992, 1994). The total number of species with conservation status (nationally scarce or above) is 81, of which 42 (52%) are considered ERS specialists. The highest number of rare species (36) was found in the staphylinids, followed by the carabids with 16, which together account for 64% of the total (Table 4). The occurrence of these species differs across the sites and TWINSPAN groups. The number of species with conservation status (CS) (df ¼ 7, 11.396, P < 0:001) and the number of species with conservation status and fidelity (CF) (df ¼ 7, F ¼ 14:766, P < 0:001) again varies significantly between TWINSPAN groups (Fig. 2). The highest values for CS is in groups 3–4 and for CF groups 2–6. 3.3. Biodiversity Action Plan species In total, six species recorded in this study, Bembidion testaceum, Perileptus areolatus, Lionychus quadrillum, Bidessus minutissimus, Thinobius newberyi and Hydrochus nitidulus are on the UK Biodiversity Action Plan

(BAP) list(s) (Table 5). It is clear that the ground beetles P. areolatus and L. quadrillum were the most widely distributed of the species encountered. Of the two P. areolatus is the most widespread species occurring on 16 sites on the Rivers Severn, Tywi and Wye and in nearly a third of all the 10 km squares sampled. L. quadrillum was recorded only on the River Usk (in all three sites), three out of the four on the Tywi and one on the Ystwyth. It is interesting to note that although both species were found at sites on the Tywi, only Perileptus was found on the Wye and only Lionychus on the Usk. The other four species were only recorded from single sites during excavations and/or hand searches. Bidessus minutissimus was found at 50 cm depth in the cobbles at TyÕn-yr-helyg on the Ystwyth and Thinobius newberyi was recorded in an excavation in a partially vegetated area on the sandy toe of the ERS at Llanwrda Station on the Tywi. Hydrochus nitidicollis was recorded from littoral silts at one site on the River Teign in Devon. Three individuals of Bembidion testaceum were recorded from the edge of a diverse ERS at Llangibby Bottom A on the River Usk. It is evident that that these species have relatively limited distributional ranges within the confines of this study.

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J.P. Sadler et al. / Biological Conservation 118 (2004) 41–56

Table 4 Species captured that have conservation status in the UK (nomenclature follows Lucht, 1987) Species Carabidae Miscodera arctica (Payk.) Perileptus areolatus (Creutz.) Tachys bistriatus (Duft.) Tachys parvulus Dej. Bembidion litorale (Ol.) Bembidion obliquum Strm. Bembidion monticola Strm. Bembidion saxatile Gyll. Bembidion stomoides Dej. Bembidion testaceum (Duft.) Pterostichus anthracinus (Panz.) Pterostichus cristatus Duf. Calathus ambiguus (Payk.) Amara fulva (M€ ull.) Chlaenius nigricornis (F.) Lionychus quadrillum (Duft.) Dytiscidae Bidessus minutissimus (Germ.) Gyrinidae Gyrinus urinator Ill. Hydraenidae Hydraena nigrita Germ. Hydraena rufipes Curt. Hydraena testacea Curt. Ochthebius bicolon Germ. Hydrophilidae Hydrochus nitidicollis Muls. Helophorus arvernicus Muls. Helophorus strigifrons Thom. Cercyon ustulatus (Preys.) Helochares lividus (Forst.) Histeridae Saprinus virescens (Payk.) Paralister obscurus (Kug.) Staphylinidae Proteinus crenulatus Pand. Deleaster dichrous (Grav.) Thinobius bicolor Joy Thinobius strandi Smet. Thinobius praetor Smet. Thinobius newberyi Scheer. Stenus asphaltinus Er. Paederus fuscipes Curt. Scopaeus gracilis (Sperk) Lathrobium angusticolle Bois. Lathrobium angustatum Bois. Lathrobium ripicola Czwal. Neobisnius prolixus Er.

Status

Species

Status

Nb Na Nb Nb Nb Nb Nb Nb Nb Nb Nb Nb Nb Nb Nb RDB3

Erichsonius signaticornis Muls. & Rey Philonthus atratus (Grav.) Gabrius astutoides Strand Gabrius velox Sharp Staphylinus caesareus Ced. Ocypus pedator Grav. Quedius ventralis (Arag.) Trichophya pilicornis (Gyll.) Sepedophilus bipunctatus (Grav.) Myllaena elongata (Matt.) Tachyusa coarctata Er. Dasygnypeta velata (Er.) Hydrosmecta fragilis (Kr.) Hydrosmecta thinobioides (Kr.) Hydrosmecta delicatula (Sharp) Hydrosmectina septentrionum Benick Aloconota (s.str.) planifrons (Water.) Atheta (Acrotona) obfuscata (Grav.) Ilyobates subopacus Palm Oxypoda exoleta Er. Aleochara (s.s) brevipennis Grav. Aleochara (Coprochara) verna Say Aleochara (Coprochara) binotata Kr. Pselaphidae Brachygluta pandellei (Saulc.) Cantharidae Rhagonycha translucida (Kryn.) Elateridae Fleutiauxellus maritimus (Curt.) Negastrius sabulicola (Boh.) Dryopidae Pomatinus substriatus (M€ ull.) Dryops nitidulus (Heer) Elminthidae Stenelmis canaliculata (Gyll.) Oulimnius troglodytes (Gyll.) Coccinellidae Coccinella quinquepunctata L. Chrysomelidae Phyllotreta vittata (F.) Mantura rustica (L.) Curculionidae Caenopsis fissirostris (Walt.) Barynotus squamosus Germ. Tanymecus palliatus (F.) Grypus equiseti (F.) Baris lepidii Germ.

Nb Na RDBI Nb RDBI Na Nb Nb Nb N N N N N RDBK N RDBK N N N N RDBK RDBK

RDB3 Nb Nb Nb Nb Nb RDB3 Nb Nb Nb Nb RDBK RDB2 Nb Nb Na N N RDBI RDBI Nb RDBK Nb Nb N RDBK

3.4. Habitat species relationships The RDA biplots showing the environmental variables and sites and species data are shown as Figs. 3 and 4. The first four canonical axes explained 22% of the variation in the species data and 75% of the species environment relationships (Table 6), although the low eigenvalues are indicative of a ÔnoisyÕ dataset. Both the first and all subsequent canonical axes were significantly related to the data. The forward selection procedure

RDBK Nb Na RDB2 Na RDB3 RDB2 Nb RDB3 Na Nb Nb Nb Nb Nb Na

identified the % shade, heterogeneity, Phi (substrate size) and ERS width as the most important environmental variables operating on the coleopteran community (all P ¼ 0:005). Other significant variables included the amount of trampling (P ¼ 0:015), the percentage of fines (silt and sand combined) (P ¼ 0:025), and vegetation type (P ¼ 0:02) (Table 7). Table 8 shows the intraset correlation scores and illustrates the relationship of the environmental variables to the canonical axes.

J.P. Sadler et al. / Biological Conservation 118 (2004) 41–56

49

Table 5 Records of UK BAP species recorded during the survey work BAP species

Status

Number of 10 km2

Number of sites

*Bembidion testaceum (Duft.) *Perileptus areolatus (Creutz.) *Lionychus quadrillum (Duft.) Bidessus minutissimus (Germ.) *Thinobius newberyi Scheer. *Hydrochus nitidicollis Muls.

Nb Na RDB3 RDB3 RDBI RDB3

1 10 3 1 1 1

1 16 8 1 1 1

3 61 84 6 3 1

17

28

158

Totals

Number of individuals

TWINSPAN group membership 4 3, 4, 5 3, 4 3 3 5

* indicates species are in the ERS Grouped Species Action Plan.

Pencareg

ERShet Llangib1

%fines ErmeR1 Glasbury Ll-Hall2

Tyn-yr ExeR1 Penlan LlandovA OtterR2 ExeR2 Width S-Land2 S-Lland1

Ll-Hall1

BuckR2

TeignR3

TeignR1

Bronydd

DolyddR3 Llangib2

ErmeR2 ErmeR3 C-YeoR3

VegType

Ferm-typ

M-Hall1

OtterR1 Llan-sta CamelR2

BuckR1 LlandovBEllandR2 Pallin3

CamelR3

TeignR2

TorrR2 YartyR1 LlangibB TorrR3 TorrR1 C-YeoR1 DolyddR2 Grogwyn U-AshR1 ExeR3

Grazed

Av.Phi

Castley

OtterR3

BuckR3

U-AlpR2

ThrushR1

C-YeoR2 YartyR3 LymmR2 YartyR2 LymmR1

%shade ThrushR3

CamelR1 DolyddR1 ThrushR2 L-AlpR3

L-AlpR2 LymmR3

U-AshR2 L-AlpR1 U-AlpR1

EllandR3

U-Ash3

M-Hall2

Fig. 3. Redundancy analysis biplot of site and environmental variable. Circles ¼ sites with >6 species of conservation status; triangles ¼ sites with 3–6 species; squares ¼ sites with 2–3 species; * 6 1 species. Only significant environmental variables are displayed. Site name abbreviations: River Alport: Derbyshire; Lower Alport (L-AlpR1, L-AlpR2, L-AlpR3); Upper Alport (U-AlpR1, U-AlpR2, U-AlpR3); River Ashop: Derbyshire; Upper Ashop (U-AshR1, U-AshR2, U-Ash3); River Calder: Yorkshire (EllandR2, EllandR3); River Camel: Cornwall (CamelR1, CamelR2, CamelR3); River Creedy Yeo: Devon (C-YeoR1, C-YeoR2, C-YeoR3); River Erme: Devon (ErmeR1, ErmeR2, ErmeR3); River Exe: Devon (ExeR1, ExeR2, ExeR3); River Frome: Dorset; Pallington (Pallin3); River Highland Water at Lymmington (LymmR1, LymmR2, LymmR3); River Otter: Devon (OtterR1, OtterR2, OtterR3); River Severn: South Llandinam (S-Lland1, S-Land2); Llandinam Hall (Ll-Hall1, Ll-Hall2); Maesmawr Hall (M-Hall1, M-Hall2); Dolydd Hafren (DolyddR1, DolyddR2, DolyddR3); River Teign: Devon (TeignR1, TeignR2, TeignR3); River Thrushel: Devon (ThrushR1, ThrushR2, ThrushR3); River Torridge: Devon (TorrR1, TorrR2, TorrR3); River Tywi; Fferm Typica (Ferm-typ); Penlan (Penlan); Llandovery (LlandovA, LlandovB); Llanwrda Station (Llan-sta); River Usk; Pencarreg Farm (Pencarreg); Llangibby Bottom A (Llangib1, Llangib2); Llangibby Bottom B (LlangibB); River Wharfe – Yorkshire (Bucken; BuckR1, BuckR2, BuckR3; Castley); River Wye; Bronydd Farm (Bronydd); Glasbury shingles (Glasbury); River Yarty: Devon (YartyR1, YartyR2, YartyR3); River Ystwyth; Grogwynion (Grogwyn); Tyn-yr-Helyg (Tyn-yr).

J.P. Sadler et al. / Biological Conservation 118 (2004) 41–56

+1.0

50

ERShet

%fines Av.Phi Ochtomal

Negasabu

Stencomm

Width Bembdeco

Bembatro

Hydrthin

BemblioAgonmica

Thinbico Lionquad

Carprivu

VegType

Coccquin Hydrdeli

Periareo

Thinarcu

Stenbima Stengutt

Fleumari Tachparv Agonalbi

Zoromini Lathangu Aloccurr

%shade

Lestpube

Hydrfrag Aloccamb

Hypnripa

Grazed Geodnigr Nebrgyll

Bembtibi

-1.0

Bembandr

-1.0

+1.0

Fig. 4. Redundancy analysis biplot of species and environmental variable of the beetle species and environmental variables (see also Table 4 for conservation status of species). Only significant environmental variables are displayed. Species abbreviations: Agonalbi ¼ Agonum albipes; Agonmica ¼ Agonum micans; Aloccamb ¼ Aloconota cambrica; Aloccurr ¼ Aloconota currax; Bembandr ¼ Bembidion andreae; Bembarto ¼ Bembidion atrocoeruleum; Bembdeco ¼ Bembidion decorum; Bemblito ¼ Bembidion litorale; Bembtibi ¼ Bembidion tibiale; Carprivu ¼ Carpelimus rivularis; Coccquin ¼ Coccinella quinquepunctata; Fleumari ¼ Fleutiauxellus maritimus; Geodnigr ¼ Geodromicus nigrita; Hydrdeli ¼ Hydrosmecta delicatula; Hydrfrag ¼ Hydrosmecta fragilis; Hydrthin ¼ Hydrosmecta thinobioides; Hypnripa ¼ Hypnoidus riparius; Lathangu ¼ Lathrobium angusticolle; Lestpube ¼ Lesteva pubescens; Lionquad ¼ Lionychus quadrillum; Nebrgyll ¼ Nebria gyllenhali; Negasabu ¼ Negastrius sabulicola; Ochtomal ¼ Ochthephilus omalinus; Periareo ¼ Perileptus areolatus; Stenbima ¼ Stenus bimaculatus; Stengutt ¼ Stenus guttula; Stencomm ¼ Stenus comma; Tachparv ¼ Tachys parvulus; Thinarcu ¼ Thinodromus arcuatus; Thinbico ¼ Thinobius bicolor; Zoromini ¼ Zorochros minimus.

Table 6 Eigenvalues, cumulative percentage of variance explained by axes (1–4) and significance of the first and all canonical axes in the redundancy analysis of the ERS dataset Axes Eigenvalues Species-environment correlations

Axis 1

Axis 2

Axis 3

Axis 4

0.099 0.846

0.058 0.706

0.036 0.712

0.024 0.744

Cumulative percentage variance Species data Species-environment relation

9.9 33.8

Significance of first canonical axis Significance of all canonical axes

F ratio ¼ 6.289 F ratio ¼ 2.177

The site and environment biplot (Fig. 3) shows a differentiation across axis 1 from larger unvegetated bars (of coarser substrate) at the far left of the plot (e.g.

15.9 54.3

19.5 66.5

21.9 74.8

P ¼ 0:005 P ¼ 0:005

Maesmawr Hall, Llandovery, and the Exe) to smaller finer substrates where the ERS have characteristically mature vegetation (e.g. sites on the Rivers Thrushel,

J.P. Sadler et al. / Biological Conservation 118 (2004) 41–56 Table 7 Forward selection of ERS variables for the Coleoptera Variable

F ratio

P value

%shade ERShet Phi Width VegType Grazed %fines Hibpot Vegcov ERStop Length

5.44 3.56 3.48 1.7 1.71 1.67 1.52 0.99 0.87 0.85 0.82

      

NS NS NS NS

Ns, not significant. P < 0:05. ** P < 0:01. *

Table 8 Redundancy analysis inter-set correlations of environmental variables against the canonical axes for the beetle data (variables in bold were found to be significant in the forward selection procedure) Variable

Axis 1

Axis 2

Axis 3

Axis 4

Phi VegType Vegcov Length Width Grazed ERStop ERShet %shade Hibpot %fines

0.3217 0.4297 0.3855 )0.4667 )0.4371 )0.4298 )0.2803 )0.29 0.7004 0.1662 0.325

0.3611 0.1576 0.1057 0.0396 0.1492 )0.2136 0.2624 0.5492 )0.0685 0.3319 0.4222

0.4463 )0.2519 )0.074 0.1444 0.0365 0.3058 )0.3005 )0.2544 )0.2332 )0.104 0.3622

0.2078 0.1896 )0.0287 0.1493 0.1499 0.4159 0.1131 0.0818 )0.0925 0.1162 0.0494

%fines is the combined total of the silt and sand fraction.

Camel and Creedy Yeo). Among this group are wooded ERS on the Highland Water in the New Forest and the polluted urban ERS on the Calder at Elland, sites with very few species of conservation interest. Significant variables correlated to axis 1 include vegetation type, grazing, Phi (substrate size), and % shade (Table 8). ERS habitat heterogeneity is important on axis 2, as is % fines in the matrix of the sediments and hibernation potential, although the latter is not significant at the 5% level (Table 7). The larger more heterogeneous ERS

51

with a large amount of fines in the cobbles (e.g. Glasbury, Teign 1 and sites on the River Usk such as Pencarreg) are found in the upper left section of the diagram and have more species of conservation interest. A group of heavily trampled cobble sites on the Upper Alport, Upper Ashop and Maesmawr Hall are situated to the bottom left. Fig. 4 depicts the species distributions in relation to the environmental variables. Species exhibiting high and total fidelity to shingle (such as Fleutiauxellus maritimus, Perileptus areolatus, Coccinella quinquepunctata, Hydrosmecta spp. and Bembidion atrocoeruleum ) are related to ERS habitat heterogeneity, coarser sediments and larger ERS, and are located in the top left of the biplot. Species that require finer substrates, such as Thinobius bicolor, Thinodromus arcuatus, Bembidion littorale and Negastrius sabulicola are more centrally placed extending along axis 2. To the far right one finds species associated with more mature vegetation communities and greater amounts of shade, such as the ground beetles Agonum albipes, and staphylinids Stenus bimaculatus and Stenus guttula. Species characteristically associated with northern upland cobble ERS, such as the ground beetles Bembidion andreae and Nebria gyllenhali and the staphylinid Geodromicus nigrita, are found at the bottom of the plot along axis 2. Table 9 shows the results from Spearman correlations between the environmental variables that were found to be significant in the Redundancy Analysis and indices of community composition. Species richness (SR) is positively correlated with the Phi value of the bars and the percentage of fines (i.e. sand and silt) in the sediments, whereas relative abundance is not correlated with any variable. The number of species that exhibit fidelity grades 1–2 (SF) is positively correlated with bar width, heterogeneity and the % fines. The numbers of species that have conservation status (CS) and conservation status coupled with ERS fidelity (CF) are negatively related to vegetation type (i.e. CF decreases with vegetation complexity) and to the amount of shade on the sediments, and are positively related to bar length, width, and heterogeneity. CF is also positively related to the amount of trampling. General linear modelling of CF using ERS length, vegetation type and ERS

Table 9 Summary of Spearman correlation coefficients between ecological and environmental variables Phi SR RA SF CS CF N * **

P < 0:05. P < 0:01.



0.32 )0.13 0.24 0.09 0.00 69

VegType

Length

Width

Trampled

Ershet

Shade

Fines

0.22 )0.10 0.03 )0.28 )0.34 69

)0.02 0.05 0.22 0.45 0.46 69

0.12 0.08 0.24 0.33 0.31 69

)0.23 0.14 )0.06 0.17 0.25 69

0.18 )0.20 0.54 0.53 0.54 69

0.15 )0.17 )0.16 )0.55 )0.55 69

0.39 )0.08 0.35 0.15 0.06 69

52

J.P. Sadler et al. / Biological Conservation 118 (2004) 41–56

heterogeneity as independent variables provided a model with an adjusted R2 of 0.731 (df ¼ 55, F ¼ 4:359, P ¼ 0:003).

4. Discussion 4.1. Sites and sampling The survey sites are drawn from a range of sites across England and Wales. The aim was to select the best ERS habitat for sampling using our understanding of the requirements of the beetle species concerned. For shingle sites this means that the sites had a markedly western and northern distribution, as this is where the ERS resource is most abundant (Eyre and Lott, 1997). Shingle and gravel ERS are present elsewhere in England East of the Midlands and further SE, but the habitat is far less abundant and the sediments tend to be finer (e.g. silt). The amount of ERS in river catchments is far greater in Scotland, where a number of areas are well known for records of ERS beetles, such as the catchments of the Spey, Nith and Feshie fan shingles (Rotheray and Robertson, 1993). As some ERS species have northern distributions (e.g. Thinobius major Kraatz, Stenus arcticus Sahl., Negastrius pulchellus L.), they were not recorded in this study, so there is some regional bias in the dataset. However, it should be stressed that although most ERS species are distributed throughout the UK, some also have very localized distributions in England and Wales (not Scotland) (e.g. Lionychus quadrillum and Hydrochus nitidicollis ). A fuller understanding of the status and distribution of species in the UK requires the synthesis of all national survey work. 4.2. Conservation value This work illustrates clearly the importance of ERS as habitats for Vulnerable, Rare and Nationally Scarce beetles in the UK. Almost one fifth of the species captured during the survey work have conservation status, ranging from low status (National Scarce) to higher status (Red Data Book 2 – Vulnerable) (Table 4). This complements the work carried out in northern England and Scotland (Eyre, 1998; Eyre et al., 2001a,b) and reinforces how important ERS habitats are for beetle species in the UK. Moreover, work in Europe (e.g. Niemeier et al., 1997; Manderbach and Hering, 2001) also highlights records of a large number of Red Data Book beetle species in their survey work. 4.2.1. Biodiversity Action Plan species Five of the six species in the ERS Grouped Species Action Plan species were recorded in this survey. This compares favourably with Eyre et al.Õs (2001a,b) work in northern England and Scotland, where they record only

one species. The difference is mainly attributable to the geographic locations of the surveys and the decline in species richness north to south across the country. Lionychus quadrillum, Bembidion testaceum and Hydrochus nitidicollis are only known from England and Wales, and Perileptus areolatus only has records from the Scottish borders. The two putative endemic staphylinids, Thinobius newberyi and Meotica anglica have old records from Scotland, but all current records are from England and Wales. However, one must be aware of the difference in sampling technique. Eyre et al.Õs (2001a,b) study was based solely on the results of pitfall trapping, which under-represents smaller, cryptic species. Perileptus aerolatus is the most widely distributed B.A.P. species recorded in this survey, where it has a markedly western distribution. The work shows it to be locally abundant on a number of rivers, such as the Wye, Exe, Torridge, Tywi and Severn and more widespread than previously thought. Notwithstanding, there are only sixteen 10 km2 with post-1980 records, including a recent (1999) record for the River Nith in Scotland (Eyre et al., 2001a). Earlier records exist for Wales, Devon and Cornwall, the Midlands, North Lancashire and Dumfriesshire (Hyman, 1992; Luff, 1998). L. quadrillum is less widespread and although Luff (1998) lists 18 records in pre-1970 10 km squares, it is less common now with only six post-1990 records, mostly from Wales (Anon, 1999). Its absence from the survey work in Scotland (Eyre et al., 2001a) is a result of its southern distribution pattern in the UK (Luff, 1998). Similarly, B. testaceum appears to be in decline. There are records from ten 10 km2 (Luff, 1998). It is currently (post-1990) only recorded from five 10 km2 . Clearly, its designation as a Notable B species on the Joint Nature Conservation Committee lists is in error and warrants revision. Both T. newberyi and H. nitidicollis were recorded from only one site during this work. The former only has three post-1980 records all from Wales and the latter is only known from three 10 km2 in Devon and Cornwall (Anon, 1999). 4.3. ERS habitat–species relationships Exposed riverine sediment habitats exhibit a great deal of heterogeneity in terms of their physical characteristics and this affects invertebrate diversity and rarity. Andersen (1969, 1983) illustrated how Bembidion spp. were distributed on gravel bars in Norway in relation to microhabitats, and the importance of sediment characteristics has been related to both carabid (Eyre et al., 2001b; Niemeier et al., 1997) and staphylinid (Eyre et al., 2001a) beetle communities but not the whole coleopteran community. The variables that were significantly related to Coleoptera species included substrate size (Phi), habitat heterogeneity, percentage of shade, vegetation type and ERS size (width and length).

J.P. Sadler et al. / Biological Conservation 118 (2004) 41–56

There is an evident gradient of species from upland ERS and those found in lowland situations and this is characterised by the particle size (Phi variable), which tends to be much coarser boulders and cobbles in upland areas. Eyre et al. (2001a,b) record similar divisions in their study of carabids and staphylinids in Scotland and northern England, whilst Hering (1998) highlights significant differences in food sources as a contributory factor. In earlier studies (Eyre and Lott, 1997; Sadler and Petts, 2000), it was argued that the amount of sand on ERS might be an important factor in determining the numbers of rare and high fidelity species on ERS. Certainly in this study, sites that have loosely compacted sediments with a high percentage of fines (sands and silts) appear to provide habitat for a diverse surface and subterranean fauna of ERS specialist beetles, although the numbers of species with conservation status and with conservation status and ERS fidelity appear independent of this factor. One reason for this is that ERS with finer sediments are much more likely to undergo vegetation succession, particularly in catchments where regulation has altered the flooding regime of the river. ERS habitat heterogeneity was significantly related to the beetle assemblages in the ordination. Clearly, more microhabitats provide greater potential for a diverse fauna, but the relationship is not a result of a scaling effect of increased area, as species richness (SR) is not affected by either length or width but correlated to substrate type (Phi) and % fines (Table 9). However, ERS heterogeneity is positively correlated with the number of species with fidelity to ERS (SF), the numbers of species with conservation status (CS) and conservation status and fidelity (CF), suggesting a link between rare and stenotopic species and habitat availability. The importance of ERS width in the ordination may have been a sampling artefact as most of the data are drawn from pitfall trapping, which is affected by species mobility as well as their density on the sampled habitat (Greenslade, 1964; Thiele, 1977). This means that on narrow ERS, with relatively little ÔcoreÕ ERS habitat, Ôedge effectsÕ are high and there is an increased possibility of capturing species from the surrounding adjacent landscape. However, the association is a positive one and width correlated with the number of species exhibiting fidelity grades (1–2) (SF), the total number of species with conservation status (NC) and the number of species with conservation status that exhibit fidelity to the sediments (CF) rather than species richness (SR). It appears, therefore, that ERS specialist beetles are more common on larger, more diverse ERS, with greater expanses of bare sediments. Eyre and Lott (1997) suggested that the number of hibernation sites was potentially an important factor for many ERS invertebrates. This will be species specific, however, and the requirements of many ERS species remain unknown. The literature on Coleoptera shows

53

that grass tussocks (Luff, 1966; Sotherton, 1984, 1985) and wood/shrubs (Zulka, 1994) are important sites, but for many specialist shingle ERS species, such as species of Bembidion and several staphylinids, over-wintering takes place within the sediments at drier locations higher up the ERS (Andersen, 1968; Dieterich, 1996). An attempt to consider this aspect in this project failed to highlight any significant relationships, partly because many riparian species move some distance away from the water to hibernate (Zulka, 1994). A factor of some concern is the density and impact of livestock on riparian habitats. Not only do livestock (particularly cattle) cause significant geomorphological modification to bank processes such as stabilisation and erosion (Trimble and Mendel, 1995), but they tend to damage riparian margins and affect habitat structure and diversity (Jansen and Robertson, 2001). The presence of livestock is particularly important on lowland ERS in England and Wales as many large ERS rivers are heavily stocked, particularly in parts of Wales and in Devon and Cornwall. It is possible that that livestock can have a serious affect on some orders of ERS invertebrates by compacting the substrate and destroying habitat and possibly refuge sites that might be used for over-wintering, especially for those that utilise interstitial habitats. Additionally, livestock defecation could lead not only to enhanced siltation of the interstitial cavities used by several species of ERS invertebrates but also to a general eutrophication of the sediments, perhaps increasing competition from non-specialist predatory species, and the rate of vegetation development. The impact of livestock trampling by animals was found to be a significant environmental variable in the redundancy analyses, but, interestingly, it was positively correlated with the number of species with conservation status that exhibit fidelity to the sediments (CF) (Table 9). This might be expected, particularly in river catchments where hydrological changes have led to the stabilisation of the system, as light trampling would reverse vegetation succession and provide more available bare substrate as habitat. Single factor ANOVA on the level of trampling (light or heavy) found no significant differences in community measures (SR, SA, SF and NC), although species richness was significantly lower on bars with trampling than those without (df ¼ 1, F ¼ 4:882, P ¼ 0:031). However, research on shingle-specialist spiders in the Upper Severn catchment suggested quite strongly that even moderate stocking levels have an affect on their diversity and abundance (Sadler et al., 2001). 4.4. Hydroecological dynamics The amount, type and structure of ERS in the catchment, is of considerable importance for invertebrate populations. Rivers with more ERS habitats appear to have more species that are ERS specialists and greater

54

J.P. Sadler et al. / Biological Conservation 118 (2004) 41–56

numbers of rarities. The reasons for this are complex and related to the variety of habitat within the catchment, stream flow, flood regime, and the amount of substrate available for reworking by the fluvial system (Petts and Foster, 1985; Petts and Thoms, 1987; Petts et al., 2000). Using a metapopulation model for the grasshopper Bryoderma tuberculata, Stelter et al. (1997) illustrated that large numbers of shingle bars are needed in a given catchment to support populations of this mobile invertebrate. They noted also that older more stable shingle bars, which were less prone to inundation, provided sources from which dispersal took place after severe flood events. In their study, Stelter et al. (1997) also note that dynamics of bar creation and vegetation succession on the bars is regulated by the hydrological regime and that most local extinctions of populations of B. tuberculata result from modifications caused by river regulation. The results of river regulation include alterations in channel form and sedimentation as a result of changes in the magnitude and frequency of flooding events (Petts, 1984, 1988; Petts and Pratts, 1983). This has major impacts on both the in-stream fauna (Greenwood et al., 1999; Petts, 2000) and implications for the dynamics of riparian invertebrates. On the river Isar in Germany, reservoir construction distinctly altered the carabid community of the lower floodplain suggesting that sediment transport during extreme flooding events was an important factor structuring ERS and hence the invertebrates there. Recent work illustrates that flow regulation can affect ERS communities by increasing bar stability and enhancing vegetation succession (Niemeier et al., 1997; Von Manderbach and Reich, 1995). It is quite clear that when catchment hydrology is modified by either regulation schemes or engineering aimed at reducing the instances of flooding and/or bank erosion, the shingle bars become less common and those that persist become more stable and undergo vegetation succession. The present work illustrates that the species assemblages on vegetated ERS (particularly shingle) differ significantly from those on ERS with simple (or ruderal) vegetation communities (Fig. 3; Table 3). Indeed, Eyre et al. (2001a,b) suggested that vegetation dynamics on their sites was also very important. Moreover, in our dataset there is a clear relationship between ERS communities and river regulation insofar as species fidelity and conservation status is higher on less regulated or unregulated rivers (e.g. Tywi, Ystwyth, Usk and Wye in Wales and the Teign, Exe and Wharfe in England). 4.5. Management implications Not only do many species of beetle exhibit high levels of fidelity to ERS, but a large proportion are uncommon and require protection of some kind. At the moment conservation policy protects only the six ERS B.A.P species. However, these species have a very

limited distribution in the UK and were only recorded in <50% of the 10 km2 surveyed during this study. This has implications for the conservation of ERS nationally, as large numbers of other Vulnerable, Rare and Nationally Notable species were recorded at sites where the B.A.P. species were absent, and at present these sites have no protection. There is an urgent need for the creation of a Habitat Action Plan for ERS. Although this research has improved the knowledge on species distributions there is still a lack of understanding about their ecology, particularly in respect to the fluvial processes that act to control their population dynamics. It is clear, however, that ERS are disturbance-dominated systems (Plachter and Reich, 1998) and that the hydrological regime is the engine that drives diversity of these systems. Any river management measures that alter this and reduce the flashy nature of the flow events will lead to a loss of conservation resource – the importance of this cannot be over-stated. Stabilization of ERS resulting from river management leads to vegetation succession and a reduction in the amount of bare and well-sorted substrates that ERS invertebrates require. Any large-scale management for water resources and flood engineering on these dynamic systems needs careful planning and evaluation. It is important also to stress that ERS are connected to the wider floodplain landscape and management of this ultimately has impact on the riparian zone itself (Nilsson, 1991; Ward and Stanford, 1995). Animal husbandry is a major form of income generation for farmers in many river catchments in the UK, yet high stock levels and stock access to shingle bars can cause compaction of substrate and degradation of ERS habitats, as well as enhancing siltation. Although this study suggests that the impact of trampling on ERS invertebrates may be limited, stocking levels still require careful monitoring and management.

Acknowledgements Funding for this work was provided the Environment Agency, English Nature, Countryside Council for Wales and WBB Aggregates plc. Viki Hirst (EA), Mike Williams (EA), Michel Hughes helped with site selection and permissions and Colin Welch and Peter Hammond verified many species identifications. The maps are the work of Kevin Birkhill and the authors acknowledge the helpful comments of two anonymous referees and Brian Davis.

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