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The influence of conifer plantations on the distribution of the golden ringed dragonfly Cordulegaster boltoni (Odonata) in Upland Wales
Biological Conservation 53 (1990) 241-251
The Influence of Conifer Plantations on the Distribution of the Golden Ringed Dragonfly Cordulegaster boltoni (Odonata) in Upland Wales S. J. Ormerod,* N. S. Weatherley & W. J. Merrett Catchment Research Group, School of Pure and Applied Biology, University of Wales College of Cardiff, Cardiff CF1 3NX, UK (Received 3 September 1989; revised version received 6 November 1989; accepted 17 November 1989)
ABSTRACT The distribution of larvae of the dragonfly Cordulegaster boltoni was assessed in eighteen streams in contrasting land use in the upper catchment of the River Tywi, mid-Wales, in each year during 1985-89, inclusive. Larvae occurred abundantly in moorland streams and in a deciduous woodland stream, but were never found in five streams draining plantations of conifer forest. Although the latter were the most acidic, transplant experiments and distributional data showed that larvae were tolerant of the water conditions found there. Conifer forest streams were also generally cooler than moorland streams, but experimental removal of bankside trees to a width of over 10 m did not lead to colonization by Cordulegaster despite increased temperature. Dietary studies also revealed no likely restriction by food availability in conifer forest streams, though we could not exclude the possibility that foraging conditions for adults were impaired in conifers. However, larvae were strongly associated with habitats at the stream margins which were highly eroded and often absent from conifer forest streams. These habitat features probably explained their absence under forest conditions. Further data are required to assess whether other aquatic resources of high conservation value are affected in similar ways by conifer forestry. The effectiveness of aspects of new forest design, such as buffer strips around streams, also requires consideration. * Present address: UWCC, c/o National Rivers Authority, 19 Penyfai Lane, Llanelli, Dyfed SA15 4EL, UK. 241 Biol. Conserv. 0006-3207/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
242
s.J. Ormerod, N. S. Weatherley, IV. J. Merrett INTRODUCTION
Approximately 11"5% of the land area of Wales is occupied by plantation forestry, most of which is concentrated in the uplands and consists of nonnative species such as sitka spruce Picea sitchensis. Such intensive management and use of exotic trees clearly have profound ecological consequences, not only for the terrestrial environment (Nature Conservancy Council, 1986), but also for the many river systems which now rise in catchments occupied by conifers. Acidification of base-poor streams, modified hydrology, alterations in energy inputs to the stream ecosystem, and changes in the structure of stream habitats, have all been discussed (Ormerod et al., 1987b, 1989; Rutt et al., 1989). However, there are few instances in which the influence of conifer afforestation has been considered with reference to riverine resources of high conservation value (e.g. Ormerod et al., 1985; Egglishaw et al., 1986; Mason & Macdonald, 1987). Invertebrate studies of this kind are particularly scarce, even though some groups are important to conservation. Dragonflies Odonata, for example, are given high priority by the Nature Conservancy Council in assessing the conservation status of aquatic sites. Besides their intrinsic value, they are also thought to be useful bio-indicators (Nature Conservancy Council, 1984). The golden ringed dragonfly Cordulegaster boltoni (Donovan) is one of the few British anisopterans found in rivers and streams. Its range in Britain is predominantly northern and western, following the distribution of streams of upland character, and consequently it is a species which could be influenced by catchment perturbation such as afforestation. In this paper, we examine the distribution of larval C. boltoni in streams draining land used in different ways in the upper catchment of the River Tywi in west Wales (52°07 ' N; 3°43 ' W). We assess its diet, habitat preference, and physiological tolerance to acid stress, and discuss whether these factors could explain its distribution in relation to the presence of conifer forest. The data were mostly collected as part of a wider study, at Llyn Brianne, of the influence of acid deposition and land use on the aquatic environment.
STUDY AREA The eighteen streams used in the study have been described in more detail by Weatherley & Ormerod (1987), whose nomenclature we use. All are within a radius of 10 km, and hence are generally similar climatologically. All are monitored chemically at fortnightly intervals, and 12 are subject to intensive remote monitoring of flow, temperature, conductivity and pH at intervals of
1 2 3 4 8 5 6 7 1 2 1 2 3 4 5 6 4 1
C C C C C M M M M M M M M M M M M D
Land use~
4'8 4.6 5.2 5.0 (6.6) 5.3 5-7 6.9 6-9 5.4 4-9 5.2 5-0 (6-8) 5.2 5.3 5-2 (6-4) 5.2 5.9 6.3
pI-I~
0"36 0.48 0.32 0.17 (0.09) 0.21 0.04 0.07 0.04 0.19 0-30 0.10 0.16 (0.11) 0.12 0-12 0.15 (0-06) 0.10 0-07 0.05
Al b (mg litre- 1)
$
•
April 1985
$
July 1985
$
•
April 1986
*
*
.
$
April 1987
Larvae present
a C, conifer forest; M, moorland; D, deciduous woodland. b The pH and aluminium concentrations are annual means, with post-liming values in parentheses.
LI LI LI LI LI LI LI LI TI TI CI CI CI CI CI CI UC GI
Site
$
*
~t
*
April 1988
TABLE 1 The Distribution of Cordulegaster boltoni in Streams in the Upper Tywi in Relation to Land Use
*
$
$
April 1989
t,,9
E"
244
S. J. Ormerod, N. S. Weatherley, I4. J. Merrett
15 min throughout the year. The streams are second-third order, situated at 250-500m OD, and all have relatively soft water (3.9-18.8 mg CaCO 3 litre- ~ total hardness), typical of large areas of Wales and upland Britain generally. Land uses are predominantly either conifer forestry (mostly Picea sitchensis), and rough moorland of Molinia, Festuca, Agrostis and Nardus. One catchment is occupied by deciduous woodland of sessile oak Quercus petraea (see Table 1). Streams draining the afforested catchments have generally lower pH (annual means 4.6-5.3) than those from moorland (pH 4-9-6.9), and the former also have elevated concentrations of dissolved aluminium (see Table 1). The catchments of streams LI 4, CI 2 and CI 5 have been limed since 1987, resulting in marked increase in pH and decrease in aluminium (see Table 1).
METHODS Patterns of distribution
Sites on the eighteen streams were sampled for larval Cordulegaster in spring (April 1985-89) and summer (July 1985 only). The procedure involved vigorous kick/sweeps separately in the margins (1 min) and riffles (2 min), using a hand net (0.9 mm mesh, 230 x 255 mm frame), usually over a distance of 20-30 m of stream. Riffle sampling involved disturbing the river bed to a depth of 10-20 cm. Areas sampled at the margins included emergent Juncus, the roots of herbaceous and woody vegetation, and banks of mosses such as Sphagnum. Deposited material at the river margins, such as silt and coarse organic detritus, was sampled coincidentally. Samples were preserved onsite in 4% formaldehyde and sorted by hand in the laboratory. Larvae were identified using Hammond (1983). Survival at low pH
In order to assess their survival under chemical conditions typically encountered in conifer forest streams, 12 larvae were transplanted to stream LI 3 and held in plastic containers during June 1987. The stream was dosed for 24 h with sulphuric acid and aluminium sulphate to mimic a natural, but severe, acid episode (pH 4.5, 0.9mg Allitre -1, 2.3mg Calitre-l; see Weatherley et al., 1989a). Survival was monitored during the episode and for 72 h following. A further study of longer-term survival at low pH was carried out in March and April 1989 when 8 larvae, of length 2-3 cm, were transplanted from a moorland stream to a highly acidic stream in conifer forest. The larvae were held in a plastic cage with a substratum of leaf and
Dragonfly distribution and conifer plantations
245
rush litter and a food supply of small invertebrates. Survival was assessed at intervals over a 4-week period during which five natural acid episodes occurred following rainfall, stream pH falling each time to pH < 4.8 for 1-3 days.
Dietary studies Aquatic invertebrates from several genera are scarce in conifer forest streams, though c o m m o n in adjacent moorland streams (Stoner et al., 1984; Weatherley & Ormerod, 1987). If these were important in the diets of Cordulegaster, food availability could restrict its distribution in afforested catchments. Diet was therefore assessed in a sample of Cordulegaster by analysis of their foreguts. Material was teased out and examined at magnifications of × 10 to × 40, and identification to family was possible either from whole animals or from recognizable structures such as mouthparts.
Bankside management At least some of the influences of conifer forestry on stream ecosystems operate through bankside effects such as altered heat and light inputs. Stream temperatures are affected as a result, possibly retarding the growth and development of stream organisms (Weatherley & Ormerod, in press). To aid assessments of the magnitude of effects of this kind, the banksides of the conifer forest streams LI 3 and LI 2 were cleared respectively in 1984 and 1986 as part of the wider research programme. Both sides of the streams were cleared along most of their lengths of 2-3 km to a width of 10 m (LI 3) and 10 m-50 m (LI 2). These management studies permitted assessment of the response of Cordulegaster to the changed conditions.
RESULTS
Patterns of distribution Larvae of Cordulegaster occurred almost exclusively in streams draining moorland, the exception being two records from the deciduous stream (Table 1). They occurred in both acidic and circumneutral conditions, including the limed streams before and after treatment. However, larvae were never recorded in streams draining through catchments occupied by conifers despite sampling on six occasions. More records were from marginal habitats than riffles (33 v 14), this difference being statistically
S. J. Ormerod, N. S. Weatherley, W. J. Merrett
246
significant (G test, p < 0.01). Quantitative data also indicated a habitat preference, larvae being significantly more abundant in margins than riffles on the 38 occasions when they were recorded (Wilcoxon Test p < 0"01). Occurrences in marginal habitats almost certainly involved larvae in vegetation in addition to those buried in organic detritus. Survival at low pH
Although they were absent from conifer forest streams, three lines of evidence indicated that Cordulegasterlarvae were tolerant of the highly acid conditions found there. First, all transplanted larvae survived throughout the duration of the experimental acid episode, and were subsequently returned to their original streams. Secondly, the results of chronic exposure showed that 62.5% survival occurred for at least 4 weeks under acid conditions. Mortalities occurred only amongst the smaller larvae and were probably due to cannibalism within the experimental cages. Thirdly, stream TI 2 was situated in moorland but had the upper part of its catchment in conifer forest. Its pH and aluminium concentrations were typical of forest streams (Table 1), but larvae were recorded there on four sampling occasions. Diet
The diet of Cordulegaster included oligochaete worms, stonefly and mayfly larvae (Leuctridae, Perlodidae, Baetidae and Leptophlebidae), caddis larvae (Rhyacophilidae, Polycentropodidae and Limnephilidae) and dipteran larvae (Simuliidae, Chironomidae and Tipulidae). However, 54
•
3¢L
~5
N
2-
.610-
oo
N
m m m t
oN
I
I
I
I
I
I
1"5
30
4.5
6"0
7.5
9"0
Head
Fig, 1.
O0
width
mm
The number of prey types (families) found in the foreguts of 39 individual
Cordulegaster boltoni larvae in relation to larval size (r = 0"69, p < 0"001). Each point is one
larva.
Dragonfly distribution and conifer plantations
loo-
!
!
247
A .
-
A
o~
oe 0I
I
i
1.5
3"0
4.5
I
6"0 Head width mrn
I
I
7.5
9-0
Fig. 2. The cumulative percentage of occurrences by Chironomidae (A), Simuliidae (B), stonefly larvae (C) and Polycentropopodidae (D) in the foreguts of 39 Cordulegaster bohoni larvae, arranged by increasing larval size.
dietary patterns varied between individuals. In particular, larger larvae ate a wider variety of prey types than smaller larvae (Fig. 1). There was strong evidence that this pattern reflected prey size, smaller Cordulegaster taking mostly chironomids (c. 0.4 mg dry weight), while simuliids (0.6 mg), stonefly larvae (c. 1.2rag) and polycentropodid caddis (4-6mg) occurred with increasing frequency as the dragonflies grew (Fig. 2). Bankside management Bankside clearance of stream LI 2 did lead to significant increase in stream temperature during spring and summer, while cleared stream LI 3 had temperature conditions intermediate between those of adjacent conifer forest and moorland streams (see Ormerod et al., 1987b; Weatherley & Ormerod, in press). Neither appeared to be colonized by Cordulegaster in the 3-5 years after treatment (see Table 1).
DISCUSSION Our data support previous studies in showing an influence by conifer plantations on the ecology of upland Welsh streams. Streams from afforested catchments, particularly in base-poor areas, often show markedly reduced diversity amongst invertebrates by comparison with adjacent moorland streams (Stoner et al., 1984; Ormerod et al., 1987b). Fish are often sparse or absent, and the reduced availability of fish and invertebrates may have consequent effects on animals such as dippers Cinclus ¢inclus and otters Lutra lutra which depend on the river for food (Ormerod et al., 1985; Mason & Macdonald, 1987). At least in the upper Tywi, C. boltoni provides a further
248
S.J. Ormerod, N. S. Weatherley, W. J. Merrett
example of an influence by plantation conifer forest on an animal group of high conservation value. In many cases, the absence of aquatic species from conifer forest streams reflects physiological effects by low pH and elevated aluminium (e.g. Stoner et al., 1984; Ormerod et al., 1987a). However, the absence of Cordulegaster from conifer forest streams in the Tywi was unlikely to reflect toxic stress for reasons outlined in the results. We can also rule out the possibility of fish predation as a feature restricting its numbers in the forest streams: while the abundance of large-bodied predators is often reduced in the presence offish (Schofield et al., 1988), the latter were absent under forest conditions. It also seems unlikely that larval Cordulegaster were restricted from afforested streams because of dietary requirements. Generally, the larvae appeared to be opportunistic predators, taking a wide variety of prey which were widely distributed irrespective of land use. All the prey used were widespread in all the streams in the study area, and densities in conifer forest streams were not significantly lower than those in moorland (see Weatherley et al., 1989b). Younger larvae appeared to feed particularly on small prey such as chironomids, but these are also abundant in forest streams. The direct impingement of conifers over the water course might impair the movement of adult dragonflies along streams either during patrolling, feeding, dispersal or egg-laying (Kaiser, 1982). However, the occurrence of Cordulegaster in deciduous woodland streams suggests no such restrictions. Also, bankside clearance of two forest streams at Llyn Brianne has opened a wide flight path, though no colonization of Cordulegaster has yet occurred. These features indicate that streams in conifer forest at Llyn Brianne limit Cordulegaster in ways other than physical exclusion of adults. One possibility we cannot exclude is that foraging conditions for adults are impaired in conifer. Recent work in upland Wales has shown that conifers differ from other site types in important respects during the flight period of Cordulegaster, with insect biomass significantly reduced beneath conifers (Ormerod & Tyler, in press). In the cases at Llyn Brianne where stream banks have been cleared, the habitat at the margins of the cleared streams at least in the wetted area, have remained similar to other conifer forest streams (Rutt et al., 1989). Indeed, the strongest possibility from our data was that larval Cordulegaster were restricted from conifer forest streams because of their habitat structure. Previous observations indicate that larvae are burrowers, dependent on soft sediments, though they range more freely under some circumstances (Corbet, 1980; Corbet et al., 1985). In our study, larvae were clearly most abundant in marginal habitats, where they occurred particularly in undercut banks, on the submerged parts of Juncus and Molinia, and buried in organic detritus. These habitats are often scarce along forest streams, either because
Dragonfly distribution and conifer plantations
249
of shading or, possibly, because of the erosive characteristics of forest run-off (see Ormerod et al., 1987b). These features are apparent in the study area at Llyn Brianne, where forest streams have margins eroded to a 'harder' edge than moorland streams (Rutt et al., 1989). These authors showed by multivariate analysis that Cordulegaster boltoni is an indicator species for moorland streams with 'softer', well-vegetated margins. They speculated that species associated with marginal habitats could be restricted from conifer forest, streams because the marginal habitats available were below the minimum required to support viable populations (cf. Thiollay & Meybury, 1988). C. boltoni, with its clear habitat preference, conspicuous occurrence, ease of isampling and conservation value, represents a useful species in which to examine this hypothesis.
S U M M A R Y A N D M A N A G E M E N T IMPLICATIONS Our data suggest an influence by conifer plantations on the distribution of Cordulegaster boltoni in an area typical of much of its British range. This pattern probably reflects the scarcity of well-developed marginal habitats in the conifer forest streams, these habitats providing a refuge for larvae either burrowing into detritus, or clinging to vegetation. Given that dragonfly larvae are often useful indicators of biological conditions in freshwater ecosystems, further data are now required to assess whether any other important aquatic invertebrates are influenced in a similar way to Cordulegaster. Because the effect we detected appeared to reflect habitat conditions, there may be scope for designing and managing forest streams in a way which minimizes such impacts. Bankside clearance of existing conifer forest streams, on the basis of our evidence, was not effective. However, clear or broadleaved 'buffer strips' are now being left alongside streams as new forests are planted (Forestry Commission, 1988). Their effectiveness in protecting animals and plants with conservation value in streams now requires research attention.
ACKNOWLEDGEMENTS Our thanks are due to Graham Rutt and Stuart Thomas for help in the field. The study was undertaken as part of the Llyn Brianne project and we are grateful to the Department of the Environment and the Welsh Office for funding the project co-ordinated by the Welsh Division of the National Rivers Authority.
250
S. J. Ormerod, N. S. Weatherley, W. J. Merrett REFERENCES
Corbet, P. S. (1980). Biology of Odonata. Ann. Rev. Ent., 25, 189-217. Corbet, P. S., Longfield, C. & Moore, N. W. (1985). Dragonflies. Collins, London. Egglishaw, H., Gardiner, R. & Foster, J. (1986). Salmon catch decline and forestry in Scotland. Scot. Geogr. Mag., 102, 57-61. Forestry Commission (1988). Forests and Water Guidelines. Forestry Commission, Edinburgh. Hammond, C. O. (1983). The dragonflies of Great Britain and Ireland (revised edition). Harley Books, Colchester. Kaiser, H. (1982). Do Cordulegaster males defend territories? A preliminary investigation of mating strategies in Cordulegaster boltoni (Donovan) (Anisoptera, Cordulegasteridae). Odonatologica, 11, 139-52. Mason, C. F. & Macdonald, S. M. (1987). Acidification and otter Lutra lutra distribution on a British river. Mammalia, 51, 81-7. Nature Conservancy Council (1984). Nature Conservation In Great Britain. NCC, Shrewsbury. Nature Conservancy Council (1986). Nature Conservation and Afforestation in Great Britain. NCC, Shrewsbury. Ormerod, S. J. & Tyler, S. J. (in press). The influence of stream acidification and riparian land use on the feeding ecology of breeding Grey wagtails Motacilla cinerea in Wales. Ibis. Ormerod, S. J., Tyler, S. J. & Lewis, J. M. S. (1985). Is the breeding distribution of dippers influenced by stream acidity? Bird Study, 32, 33-9. Ormerod, S. J., Boole, P., McCahon, C. P., Weatherley, N. S., Pascoe, D. & Edwards, R. W. (1987a). Short-term experimental acidification of a Welsh stream: comparing the biological effects of hydrogen ions and aluminium. Freshwat. Biol., 17, 341-56. Ormerod, S. J., Mawle, G. W. & Edwards, R. W. (1987b). The influence of forest on aquatic fauna. In Environmental Aspects of Plantation Forestry in Wales. ed. J. Good. Institute of Terrestrial Ecology, Grange over Sands, pp. 37-49. Ormerod, S. J., Donald, A. P. & Brown, S. J. (1989). The influence of plantation forestry on the pH and aluminium concentration of upland Welsh streams: A re-examination. Environ. Pollut., 62, 47-62, Rutt, G. P., Weatherley, N. S. & Ormerod, S. J. (1989). Microhabitat availability in Welsh moorland and forest streams as a determinant of macroinvertebrate distribution. Freshwat. Biol., 22, 247-61. Schofield, K., Townsend, C. R. & Hildrew, A. G. (1988). Predation and the prey community of a headwater stream. Freshwat. BioL, 20, 85-95. Stoner, J. H., Gee, A. S. & Wade, K. R. (1984). The effects of acidification on the ecology of streams in the upper Tywi catchment in west Wales. Environ. Pollut., 35, 125-57. Thiollay, J. M. & Meybury, B. U. (1988). Forest fragmentation and the conservation of raptors: a survey of the island of Java. Biol. Conserv., 44, 229-50. Weatherley, N. S. & Ormerod, S. J. (1987). The impact of acidification on macroinvertebrate assemblages in Welsh streams: towards an empirical model. Environ. Pollut., 46, 223-40. Weatherley, N. S. & Ormerod, S. J. (in press). Forests and the temperature of upland
Dragonfly distribution and conifer plantations
251
streams in Wales: a modelling exploration of the biological effects. Freshwat. Biol. Weatherley, N. S., Rutt, G. P. & Ormerod, S. J. (1989a). Densities of benthic macroinvertebrates in upland streams of different acidity and land use. Arch. Hydrobiol., 115, 417-31. Weatherley, N. S., Thomas, S. P. & Ormerod, S. J. (1989b). Chemical and biological effects of acid, aluminium and lime additions to a Welsh hill-stream. Environ. Pollut., 56, 283-97.
The Influence of Conifer Plantations on the Distribution of the Golden Ringed Dragonfly Cordulegaster boltoni (Odonata) in Upland Wales S. J. Ormerod,* N. S. Weatherley & W. J. Merrett Catchment Research Group, School of Pure and Applied Biology, University of Wales College of Cardiff, Cardiff CF1 3NX, UK (Received 3 September 1989; revised version received 6 November 1989; accepted 17 November 1989)
ABSTRACT The distribution of larvae of the dragonfly Cordulegaster boltoni was assessed in eighteen streams in contrasting land use in the upper catchment of the River Tywi, mid-Wales, in each year during 1985-89, inclusive. Larvae occurred abundantly in moorland streams and in a deciduous woodland stream, but were never found in five streams draining plantations of conifer forest. Although the latter were the most acidic, transplant experiments and distributional data showed that larvae were tolerant of the water conditions found there. Conifer forest streams were also generally cooler than moorland streams, but experimental removal of bankside trees to a width of over 10 m did not lead to colonization by Cordulegaster despite increased temperature. Dietary studies also revealed no likely restriction by food availability in conifer forest streams, though we could not exclude the possibility that foraging conditions for adults were impaired in conifers. However, larvae were strongly associated with habitats at the stream margins which were highly eroded and often absent from conifer forest streams. These habitat features probably explained their absence under forest conditions. Further data are required to assess whether other aquatic resources of high conservation value are affected in similar ways by conifer forestry. The effectiveness of aspects of new forest design, such as buffer strips around streams, also requires consideration. * Present address: UWCC, c/o National Rivers Authority, 19 Penyfai Lane, Llanelli, Dyfed SA15 4EL, UK. 241 Biol. Conserv. 0006-3207/90/$03"50 © 1990 Elsevier Science Publishers Ltd, England. Printed in Great Britain
242
s.J. Ormerod, N. S. Weatherley, IV. J. Merrett INTRODUCTION
Approximately 11"5% of the land area of Wales is occupied by plantation forestry, most of which is concentrated in the uplands and consists of nonnative species such as sitka spruce Picea sitchensis. Such intensive management and use of exotic trees clearly have profound ecological consequences, not only for the terrestrial environment (Nature Conservancy Council, 1986), but also for the many river systems which now rise in catchments occupied by conifers. Acidification of base-poor streams, modified hydrology, alterations in energy inputs to the stream ecosystem, and changes in the structure of stream habitats, have all been discussed (Ormerod et al., 1987b, 1989; Rutt et al., 1989). However, there are few instances in which the influence of conifer afforestation has been considered with reference to riverine resources of high conservation value (e.g. Ormerod et al., 1985; Egglishaw et al., 1986; Mason & Macdonald, 1987). Invertebrate studies of this kind are particularly scarce, even though some groups are important to conservation. Dragonflies Odonata, for example, are given high priority by the Nature Conservancy Council in assessing the conservation status of aquatic sites. Besides their intrinsic value, they are also thought to be useful bio-indicators (Nature Conservancy Council, 1984). The golden ringed dragonfly Cordulegaster boltoni (Donovan) is one of the few British anisopterans found in rivers and streams. Its range in Britain is predominantly northern and western, following the distribution of streams of upland character, and consequently it is a species which could be influenced by catchment perturbation such as afforestation. In this paper, we examine the distribution of larval C. boltoni in streams draining land used in different ways in the upper catchment of the River Tywi in west Wales (52°07 ' N; 3°43 ' W). We assess its diet, habitat preference, and physiological tolerance to acid stress, and discuss whether these factors could explain its distribution in relation to the presence of conifer forest. The data were mostly collected as part of a wider study, at Llyn Brianne, of the influence of acid deposition and land use on the aquatic environment.
STUDY AREA The eighteen streams used in the study have been described in more detail by Weatherley & Ormerod (1987), whose nomenclature we use. All are within a radius of 10 km, and hence are generally similar climatologically. All are monitored chemically at fortnightly intervals, and 12 are subject to intensive remote monitoring of flow, temperature, conductivity and pH at intervals of
1 2 3 4 8 5 6 7 1 2 1 2 3 4 5 6 4 1
C C C C C M M M M M M M M M M M M D
Land use~
4'8 4.6 5.2 5.0 (6.6) 5.3 5-7 6.9 6-9 5.4 4-9 5.2 5-0 (6-8) 5.2 5.3 5-2 (6-4) 5.2 5.9 6.3
pI-I~
0"36 0.48 0.32 0.17 (0.09) 0.21 0.04 0.07 0.04 0.19 0-30 0.10 0.16 (0.11) 0.12 0-12 0.15 (0-06) 0.10 0-07 0.05
Al b (mg litre- 1)
$
•
April 1985
$
July 1985
$
•
April 1986
*
*
.
$
April 1987
Larvae present
a C, conifer forest; M, moorland; D, deciduous woodland. b The pH and aluminium concentrations are annual means, with post-liming values in parentheses.
LI LI LI LI LI LI LI LI TI TI CI CI CI CI CI CI UC GI
Site
$
*
~t
*
April 1988
TABLE 1 The Distribution of Cordulegaster boltoni in Streams in the Upper Tywi in Relation to Land Use
*
$
$
April 1989
t,,9
E"
244
S. J. Ormerod, N. S. Weatherley, I4. J. Merrett
15 min throughout the year. The streams are second-third order, situated at 250-500m OD, and all have relatively soft water (3.9-18.8 mg CaCO 3 litre- ~ total hardness), typical of large areas of Wales and upland Britain generally. Land uses are predominantly either conifer forestry (mostly Picea sitchensis), and rough moorland of Molinia, Festuca, Agrostis and Nardus. One catchment is occupied by deciduous woodland of sessile oak Quercus petraea (see Table 1). Streams draining the afforested catchments have generally lower pH (annual means 4.6-5.3) than those from moorland (pH 4-9-6.9), and the former also have elevated concentrations of dissolved aluminium (see Table 1). The catchments of streams LI 4, CI 2 and CI 5 have been limed since 1987, resulting in marked increase in pH and decrease in aluminium (see Table 1).
METHODS Patterns of distribution
Sites on the eighteen streams were sampled for larval Cordulegaster in spring (April 1985-89) and summer (July 1985 only). The procedure involved vigorous kick/sweeps separately in the margins (1 min) and riffles (2 min), using a hand net (0.9 mm mesh, 230 x 255 mm frame), usually over a distance of 20-30 m of stream. Riffle sampling involved disturbing the river bed to a depth of 10-20 cm. Areas sampled at the margins included emergent Juncus, the roots of herbaceous and woody vegetation, and banks of mosses such as Sphagnum. Deposited material at the river margins, such as silt and coarse organic detritus, was sampled coincidentally. Samples were preserved onsite in 4% formaldehyde and sorted by hand in the laboratory. Larvae were identified using Hammond (1983). Survival at low pH
In order to assess their survival under chemical conditions typically encountered in conifer forest streams, 12 larvae were transplanted to stream LI 3 and held in plastic containers during June 1987. The stream was dosed for 24 h with sulphuric acid and aluminium sulphate to mimic a natural, but severe, acid episode (pH 4.5, 0.9mg Allitre -1, 2.3mg Calitre-l; see Weatherley et al., 1989a). Survival was monitored during the episode and for 72 h following. A further study of longer-term survival at low pH was carried out in March and April 1989 when 8 larvae, of length 2-3 cm, were transplanted from a moorland stream to a highly acidic stream in conifer forest. The larvae were held in a plastic cage with a substratum of leaf and
Dragonfly distribution and conifer plantations
245
rush litter and a food supply of small invertebrates. Survival was assessed at intervals over a 4-week period during which five natural acid episodes occurred following rainfall, stream pH falling each time to pH < 4.8 for 1-3 days.
Dietary studies Aquatic invertebrates from several genera are scarce in conifer forest streams, though c o m m o n in adjacent moorland streams (Stoner et al., 1984; Weatherley & Ormerod, 1987). If these were important in the diets of Cordulegaster, food availability could restrict its distribution in afforested catchments. Diet was therefore assessed in a sample of Cordulegaster by analysis of their foreguts. Material was teased out and examined at magnifications of × 10 to × 40, and identification to family was possible either from whole animals or from recognizable structures such as mouthparts.
Bankside management At least some of the influences of conifer forestry on stream ecosystems operate through bankside effects such as altered heat and light inputs. Stream temperatures are affected as a result, possibly retarding the growth and development of stream organisms (Weatherley & Ormerod, in press). To aid assessments of the magnitude of effects of this kind, the banksides of the conifer forest streams LI 3 and LI 2 were cleared respectively in 1984 and 1986 as part of the wider research programme. Both sides of the streams were cleared along most of their lengths of 2-3 km to a width of 10 m (LI 3) and 10 m-50 m (LI 2). These management studies permitted assessment of the response of Cordulegaster to the changed conditions.
RESULTS
Patterns of distribution Larvae of Cordulegaster occurred almost exclusively in streams draining moorland, the exception being two records from the deciduous stream (Table 1). They occurred in both acidic and circumneutral conditions, including the limed streams before and after treatment. However, larvae were never recorded in streams draining through catchments occupied by conifers despite sampling on six occasions. More records were from marginal habitats than riffles (33 v 14), this difference being statistically
S. J. Ormerod, N. S. Weatherley, W. J. Merrett
246
significant (G test, p < 0.01). Quantitative data also indicated a habitat preference, larvae being significantly more abundant in margins than riffles on the 38 occasions when they were recorded (Wilcoxon Test p < 0"01). Occurrences in marginal habitats almost certainly involved larvae in vegetation in addition to those buried in organic detritus. Survival at low pH
Although they were absent from conifer forest streams, three lines of evidence indicated that Cordulegasterlarvae were tolerant of the highly acid conditions found there. First, all transplanted larvae survived throughout the duration of the experimental acid episode, and were subsequently returned to their original streams. Secondly, the results of chronic exposure showed that 62.5% survival occurred for at least 4 weeks under acid conditions. Mortalities occurred only amongst the smaller larvae and were probably due to cannibalism within the experimental cages. Thirdly, stream TI 2 was situated in moorland but had the upper part of its catchment in conifer forest. Its pH and aluminium concentrations were typical of forest streams (Table 1), but larvae were recorded there on four sampling occasions. Diet
The diet of Cordulegaster included oligochaete worms, stonefly and mayfly larvae (Leuctridae, Perlodidae, Baetidae and Leptophlebidae), caddis larvae (Rhyacophilidae, Polycentropodidae and Limnephilidae) and dipteran larvae (Simuliidae, Chironomidae and Tipulidae). However, 54
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The number of prey types (families) found in the foreguts of 39 individual
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Dragonfly distribution and conifer plantations
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dietary patterns varied between individuals. In particular, larger larvae ate a wider variety of prey types than smaller larvae (Fig. 1). There was strong evidence that this pattern reflected prey size, smaller Cordulegaster taking mostly chironomids (c. 0.4 mg dry weight), while simuliids (0.6 mg), stonefly larvae (c. 1.2rag) and polycentropodid caddis (4-6mg) occurred with increasing frequency as the dragonflies grew (Fig. 2). Bankside management Bankside clearance of stream LI 2 did lead to significant increase in stream temperature during spring and summer, while cleared stream LI 3 had temperature conditions intermediate between those of adjacent conifer forest and moorland streams (see Ormerod et al., 1987b; Weatherley & Ormerod, in press). Neither appeared to be colonized by Cordulegaster in the 3-5 years after treatment (see Table 1).
DISCUSSION Our data support previous studies in showing an influence by conifer plantations on the ecology of upland Welsh streams. Streams from afforested catchments, particularly in base-poor areas, often show markedly reduced diversity amongst invertebrates by comparison with adjacent moorland streams (Stoner et al., 1984; Ormerod et al., 1987b). Fish are often sparse or absent, and the reduced availability of fish and invertebrates may have consequent effects on animals such as dippers Cinclus ¢inclus and otters Lutra lutra which depend on the river for food (Ormerod et al., 1985; Mason & Macdonald, 1987). At least in the upper Tywi, C. boltoni provides a further
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S.J. Ormerod, N. S. Weatherley, W. J. Merrett
example of an influence by plantation conifer forest on an animal group of high conservation value. In many cases, the absence of aquatic species from conifer forest streams reflects physiological effects by low pH and elevated aluminium (e.g. Stoner et al., 1984; Ormerod et al., 1987a). However, the absence of Cordulegaster from conifer forest streams in the Tywi was unlikely to reflect toxic stress for reasons outlined in the results. We can also rule out the possibility of fish predation as a feature restricting its numbers in the forest streams: while the abundance of large-bodied predators is often reduced in the presence offish (Schofield et al., 1988), the latter were absent under forest conditions. It also seems unlikely that larval Cordulegaster were restricted from afforested streams because of dietary requirements. Generally, the larvae appeared to be opportunistic predators, taking a wide variety of prey which were widely distributed irrespective of land use. All the prey used were widespread in all the streams in the study area, and densities in conifer forest streams were not significantly lower than those in moorland (see Weatherley et al., 1989b). Younger larvae appeared to feed particularly on small prey such as chironomids, but these are also abundant in forest streams. The direct impingement of conifers over the water course might impair the movement of adult dragonflies along streams either during patrolling, feeding, dispersal or egg-laying (Kaiser, 1982). However, the occurrence of Cordulegaster in deciduous woodland streams suggests no such restrictions. Also, bankside clearance of two forest streams at Llyn Brianne has opened a wide flight path, though no colonization of Cordulegaster has yet occurred. These features indicate that streams in conifer forest at Llyn Brianne limit Cordulegaster in ways other than physical exclusion of adults. One possibility we cannot exclude is that foraging conditions for adults are impaired in conifer. Recent work in upland Wales has shown that conifers differ from other site types in important respects during the flight period of Cordulegaster, with insect biomass significantly reduced beneath conifers (Ormerod & Tyler, in press). In the cases at Llyn Brianne where stream banks have been cleared, the habitat at the margins of the cleared streams at least in the wetted area, have remained similar to other conifer forest streams (Rutt et al., 1989). Indeed, the strongest possibility from our data was that larval Cordulegaster were restricted from conifer forest streams because of their habitat structure. Previous observations indicate that larvae are burrowers, dependent on soft sediments, though they range more freely under some circumstances (Corbet, 1980; Corbet et al., 1985). In our study, larvae were clearly most abundant in marginal habitats, where they occurred particularly in undercut banks, on the submerged parts of Juncus and Molinia, and buried in organic detritus. These habitats are often scarce along forest streams, either because
Dragonfly distribution and conifer plantations
249
of shading or, possibly, because of the erosive characteristics of forest run-off (see Ormerod et al., 1987b). These features are apparent in the study area at Llyn Brianne, where forest streams have margins eroded to a 'harder' edge than moorland streams (Rutt et al., 1989). These authors showed by multivariate analysis that Cordulegaster boltoni is an indicator species for moorland streams with 'softer', well-vegetated margins. They speculated that species associated with marginal habitats could be restricted from conifer forest, streams because the marginal habitats available were below the minimum required to support viable populations (cf. Thiollay & Meybury, 1988). C. boltoni, with its clear habitat preference, conspicuous occurrence, ease of isampling and conservation value, represents a useful species in which to examine this hypothesis.
S U M M A R Y A N D M A N A G E M E N T IMPLICATIONS Our data suggest an influence by conifer plantations on the distribution of Cordulegaster boltoni in an area typical of much of its British range. This pattern probably reflects the scarcity of well-developed marginal habitats in the conifer forest streams, these habitats providing a refuge for larvae either burrowing into detritus, or clinging to vegetation. Given that dragonfly larvae are often useful indicators of biological conditions in freshwater ecosystems, further data are now required to assess whether any other important aquatic invertebrates are influenced in a similar way to Cordulegaster. Because the effect we detected appeared to reflect habitat conditions, there may be scope for designing and managing forest streams in a way which minimizes such impacts. Bankside clearance of existing conifer forest streams, on the basis of our evidence, was not effective. However, clear or broadleaved 'buffer strips' are now being left alongside streams as new forests are planted (Forestry Commission, 1988). Their effectiveness in protecting animals and plants with conservation value in streams now requires research attention.
ACKNOWLEDGEMENTS Our thanks are due to Graham Rutt and Stuart Thomas for help in the field. The study was undertaken as part of the Llyn Brianne project and we are grateful to the Department of the Environment and the Welsh Office for funding the project co-ordinated by the Welsh Division of the National Rivers Authority.
250
S. J. Ormerod, N. S. Weatherley, W. J. Merrett REFERENCES
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