A Survey of the Aquatic Invertebrates of RSPB Otmoor Reserve, Oxfordshire
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A survey of the aquatic invertebrates of RSPB Otmoor Reserve, Oxfordshire C. Martin Drake 2009 Dr C. M. Drake Orchid House Burridge Axminster Devon EX13 7DF 0 Summary Aquatic invertebrates in all major groups except Odonata were sampled at 25 water-bodies at Greenaways field at RSPB’s Otmoor Reserve on 24 – 25 July 2009. A total of 139 distinct taxa (nearly all species) were identified. These included 61 species of water beetles, 20 bugs and 16 molluscs. Twenty-one nationally scarce or rare species were found. Three beetles (Dryops similaris, Berosus signaticollis and Enochrus nigritus) are not characteristic of grazing marshes and all three had good populations here. The rare soldierfly Odontomyia ornata is an exceptional inland record for a species rarely found away from coastal grazing marshes. The remaining species are often found in ditch systems. Three distinct assemblages were identified using ordination. These corresponded closely to the three main ditch types: gutters, new shallow ditches, and old or deep ditches. Gutters were characterised by many species and individuals of beetles but a paucity of many other groups, and by high species quality scores and many uncommon species. New shallow ditches had high quality scores and high species richness in many groups and many uncommon species. Old or deep ditches had high species richness in many groups except beetles and markedly lower species quality scores and numbers of uncommon species. Creating gutters and shallow ditches was highly beneficial to the aquatic invertebrate fauna at Otmoor. Introduction Otmoor is a large grazing marsh on the floodplain of the River Ray in Oxfordshire. Large parts of it were drained in the 1960s and 1970s for arable farming. In 1997, RSPB started restoration back to pasture and reedbed. A dense network of ditches, gutters and scrapes has been dug over several years primarily to benefit waders. This survey was commissioned to investigate the aquatic invertebrates of the Greenaways field which is the first area to have the new water-bodies. Many of the ditches here are now between 5 and 8 years old [CHECK AGES] and have well established aquatic and marginal vegetation. The project brief required samples from 25 water-bodies. After discussion with the reserve staff, it was agreed that all samples should be taken from the Greenaways field rather than include some on other parts of the reserve. Methods Samples were taken using a pond-net. Three to four net-hauls were taken at each point and the material was inspected on a polythene sheet and in a white tray containing shallow water. A separate procedure was used to find small molluscs that were often difficult to see among the weed on the sheet and tray. After sorting, each batch of weed and debris was put in bucket, 1 dunked about and most weed removed. The remaining material was washed crudely while allowing molluscs to sink to the bottom. Most water was decanted and the residue inspected in the tray. As 12-13 samples had to be collected on each day, 40-45 minutes was allocated to each point. A few environmental features were noted but these included only a small number of those that the author normally collects at ditches, since filling the forms takes too much time which was better spent in sampling. They are givenin Appendix 2. Ditches were selected to cover most of the Greenaways field and to include the range of types from old to new, deep to shallow, and gutters. Many gutters and scrapes were dry or nearly dry but samples were obtained from plenty of them by gently stamping the vegetation to create a pool from which animals were netted. Sampling points are shown in Figure 1. The site was visited on 24-25 July 2009. The weather was good for most of the visit, with only one very heavy shower. Rarity statuses were obtained from Recorder 3.3 and checked using the JNCC website (updated in 2008). Some groups have been revised using the more recent IUCN criteria, but one draft review that still awaits publication by JNCC have been used for water beetles (Foster, in prep.). The geographic distribution of most scarce species was obtained using the NBN Gateway. Definitions of the status categories are given in Appendix 4. Species Quality Score was calculated for all aquatic grazing marsh species listed in Palmer et al. (2007); this excludes wetland species such as donacine reed beetles. The values used are given with the raw data in Appendix 1. Odination used DECORANA on presence/absence data with no downweighting of species that were scarce in the dataset, using the software Community Analysis Package 4 (Pisces 2007). Median values rather than means were calcuated as the samples had not been selected randomly. TWINSPAN was used to classify the samples. Results Species richness A total of 139 distinct aquatic taxa were identified, all but five of which were species, and excludes possible duplicates such as larvae that were identified only to genus. These included 61 species of water beetles, 20 water bugs, 16 molluscs (not including Pisidium pea-mussels which were not identified), nine flies (larvae), five mayflies (larvae), eight species and three genera of caddisflies, five leeches and the remainder comprising alderfly, two crustaceans and water spider. Dragonflies were excluded in the project specification as there was good information from adults already, but these were noted in any case; this added another six species and one genus. Both common sticklebacks were present. Another 32 species of ‘terrestrial’ beetles, flies and bugs were identified incidentally; some of these are wetland species and a few have aquatic larvae but are not traditionally included in ‘aquatic’ groups. Appendix 1 gives the raw data. 2 Rare and scarce species There were 21 species with nationally rare or scarce status using the old JNCC system (Table 1). Four of the water beetles are likely to be down-graded when a long-awaited review using the IUCN criteria is published (Foster, in prep.); these are shown with the IUCN status of ‘Lower Risk, least concern’ (LRlc) in Table 1. Odonata have recently been re-evaluated using the IUCN criteria and Hairy Dragonfly, Brachytron pratense, is now classed as ‘Least Concern’ (LC) (Daguet et al., 2008; Taylor, 2008). Figure 1. RSPB Otmoor Reserve, Greenaways Field, showing invertebrate sampling points. Many of these rare and scarce species are frequently found in ditches on grazing marshes and, despite their rarity status, are among the expected fauna. Species that are not part of the usual grazing marsh fauna are the beetles Dryops similaris, Berosus signaticollis and Enochrus nigritus. All three have been recorded previously either on Otmoor SSSI or nearby. Dryops similaris is found mainly in lowland England and coastal Wales, and is regarded as a ‘moss-edge dweller’ characteristic of temporary fens, although can also be found at mossy edges of man-made pools. Its distribution shows no hint that it occupies the large grazing marsh 3 systems of England and Wales. At Otmoor it was identified at four places in shallow or deep new ditches, although possible females (which cannot be identified) of a large dark Dryops were present at more sites here. Berosus signaticollis was moderately frequent at Otmoor in gutters and shallow new ditches. It is widespread in lowland England with many records following the distribution of some clay formations and sandy soils. It is normally associated with shallow temporary pools. Enochrus nigritus was one of the most interesting species found. It is moderately frequent in northern East Anglia where it lives in mesotrophic and base-rich fens and pools. Outside of this area it is scarce and appears to have been lost from some parts of the south-east. It was present at 20 sampling points, was sometimes numerous and showed no preference for any type of ditch. Records on the NBN Gateway indicate that it has been frequently recorded here. The most unexpected record was for the large aquatic soldierfly Odontomyia ornata. This fly can be regarded as a flagship species for grazing marsh ditch systems where it can be quite a common insect, but it is almost confined to them. Its distribution follows almost exactly that of the major marshes of England and Wales. Its discovery so far inland and away from old marsh was completely unexpected. Well grown individuals were found at three new deep or shallow ditches, and this agrees with the species’ known preference for fairly large ditches. Small individuals that may have been this species were at another ten ditches. There is some doubt about their identification as they had characters used for O. ornata in keys but resembled small O. tigrina. Clearly some taxonomic work is needed on small larvae to be sure of their identity. These were found widely at Otmoor and were found in all types of ditch which militates against them being O. ornata. Several other scarce water beetles have been recorded from the adjacent Otmoor SSSI before: Cercyon tristis, Enochrus melanocephalus and, Limnebius papposus (frequently), and Berosus affinis and Peltodytes caesus have been recorded nearby at the edge of Otmoor. There are no records in the NBN Gateway for the water beetles Cercyon sternalis, Dytiscus circumflexus, Hydaticus seminiger, the reed beetle Donacia thalassina, the tiny but distinctive rove beetle Stenus fornicatus or the soldierflies Odontomyia tigrina and Stratiomys singularior. All these species are within their geographic range with the possible exception of Berosus affinis which may be close to its north-west limit in Oxfordshire. Some other species deserve special mention, even though they are not especially uncommon nationally. The corixid bug Cymatia bonsdorffi was frequent at Otmoor and has been recorded on the RSPB reserve before.