Invertebrate survey of pond

Report by P. Kirby to Amphibian and Reptile Conservation, November 2009

Introduction

Dacres Wood is one of several pond sites surveyed for their invertebrates for Froglife in 2009. The aim of the surveys is to establish the character and interest of the invertebrate fauna to inform management planning: in particular, whether there is existing interest which might need to be taken into account if inadvertent damage is not to be caused; and in what ways management might be used to benefit or improve the invertebrate assemblage. The invertebrate fauna was sampled on two dates, 24 April and 25 August, the first visit being largely devoted to the aquatic fauna, the second to the terrestrial fauna, especially wetland species and adults of species with aquatic larvae.

Sampling methods

Each station for aquatic survey was sampled using a standard pond net of side twenty-four centimetres and mesh size one millimetre. Water margins and dense vegetation in shallow water were sampled using plastic sieves: one of seventeen centimetres diameter, with a mesh size of one millimetre, and one of eight centimetres in diameter, with a mesh size of approximately 0.5 millimetre.

Representative bulk samples obtained by the larger pond net were examined in the net. Any fish or amphibians captured were released; large and readily identified animals were listed; and an approximate inventory of smaller species was recorded. The collected material was then spread on metal grids or in plastic sieves, of mesh size 8 millimetres, suspended over plastic trays, and active animals allowed to make their own way through. Material remaining in the sieve was then sorted for less active invertebrates, such as molluscs, and additional larger individuals unable to fit through the mesh of the grid. Meanwhile, sieves were employed in the capture of additional material from shallower areas and the pond net in a search for additional large and active species. Further pond-net samples were placed directly into trays of water, to search for small taxa ill-equipped to leave the sample via a grid, such as small caddis larvae and mayflies. Organisms were identified alive where feasible. Representative individuals of other taxa were preserved in 70% iso-propanol for later examination.

No precise length of pond margin nor fixed time period was used for sampling; sub-samples were taken from a number of points chosen both to reflect the character of the pond and to include the areas which seemed likely to hold the richest fauna. Sampling was continued until it appeared that no new species were being captured.

Though sampling was designed only for the recording of aquatic species, a small number of terrestrial species were also captured, especially those living close to water margins. These records have been included in the table of results, but are necessarily rather random in their occurrence.

The terrestrial fauna was sampled using a small range of active methods:

Sweep-netting of herbaceous vegetation and the foliage of trees and bushes
Sampling was done using a net supplied by Marris House Nets, with a lightweight folding circular frame 40 cm. in diameter and a long and relatively open-weave bag. Between twenty and fifty sweeps were taken before examining the catch. The contents of the net were initially examined in the net, noting or capturing large, fast-moving or readily identified species. The contents were then sifted through a 0.5cm. mesh sieve onto a white tray, and the material in the tray examined for smaller and slower animals.

Beating of dense tall herbaceous vegetation and low branches of bushes and trees
Samples were taken from tree and shrub foliage or dense tall herbaceous vegetation by holding a net under or against the foliage and tapping it sharply with a light stick. When sampling dense or low vegetation, the sweep-net in use on that sampling occasion was used. When larger and higher tree branches were being investigated, a larger net, with a lightweight folding frame 55cm. in diameter and a long bag was used: this net has the advantage that substantial amounts of foliage from projecting branches can be inserted into the net before hitting the branch. Material was initially examined in the net, then emptied onto a white tray for further sorting.

Suction sampling
Suction samples were taken using a petrol-driven garden leaf-blower, modified according to the method of Stewart & Wright (1998), by taping a fine-meshed net in the inlet tube. After three minutes of sampling, the collected material was sifted through a 0.5 centimetre mesh sieve onto a white tray, and invertebrates were sorted in the field.

Active search of other key features of value for invertebrates
Features of significance to invertebrates which were not sampled, or not necessarily adequately sampled, by sweeping, beating and suction-sampling were further investigated by close examination and hand searching. Attention was particularly paid to the undersides of stones and logs, flowering plants providing an important nectar source, and accumulations of wet plant litter.

In all survey methods, some readily identified species were noted in the field. Representative examples of most species were collected for subsequent identification or confirmation. Most terrestrial species were collected using a pooter. A dry pooter made from a flexible polythene sample bottle and a combination of rigid plastic and flexible polythene tubing was used to capture most insects and retain them alive until return to the laboratory; for spiders, some soft-bodied insects and predacious species which might do serious damage to other material if collected live into a pooter, a simple spider-pooter was used to gather up individual specimens which were then blown direct into a container of 60% iso-propanol. Water beetles were collected into glass tubes without preservative; representative specimens of other aquatic groups were placed into iso-propanol.

Target groups

The following invertebrate groups were identified from aquatic samples (in practice, some groups were not represented in samples):

• Tricladida (flatworms)
• Mollusca (water snails and mussels);
• Hirudinea (leeches);
• Oligochaeta (worms) – only presence of the group recorded;
• Larger Crustacea;
• Araneae (spiders);
• Hydracarina (water mites) – only presence of group recorded;
• Coleoptera (water beetles);
• Diptera larvae (to family only except Culicidae, Dixidae, Stratiomyidae);
• Ephemeroptera (mayflies);
• Hemiptera (water bugs);
• Lepidoptera (moths);
• Megaloptera (alder flies);
• Odonata (dragonflies);
• Trichoptera (caddisflies).

Survey of terrestrial invertebrates has been taxonomically wide-ranging, but has concentrated on those groups considered likely to be particularly informative as to conservation interest, and especially groups requiring wetland conditions, or with aquatic larvae. It was never the intention to attempt to generate comprehensive lists of any of these groups for the site, but rather to use taxonomically widespread recording to indicate the quality of the overall fauna of particular features and areas. The extent to which the various groups are represented in the records has been determined partly by the time of year of survey.

Assessment of statuses
Each species recorded during the invertebrate survey, and listed in the tables of results, has been assigned a status. The better-known groups of invertebrates were assessed for formal conservation status in Red Data Books and National Reviews from the mid-1980s onwards, using criteria from the IUCN for the rarest (Red Data Book) species, and defining species believed to occur in 100 or fewer 10-kilometres squares of the National Grid as Nationally Scarce. The earlier IUCN criteria have been superseded, but only a fraction of the fauna has as yet been assessed, in published reviews, under the newer criteria. The following formal statuses and abbreviations from the older system are used in this report:

Nationally Scarce category A (Na)
Taxa which do not fall within RDB categories but which are nonetheless uncommon in Great Britain and are thought to occur in 30 or fewer hectads of the National Grid or, for less well-recorded groups, within seven or fewer vice-counties.

Nationally Scarce category B (Nb)
Taxa which do not fall within RDB categories but which are nonetheless uncommon in Great Britain and are thought to occur in between 31 and 100 hectads of the National Grid or, for less-well recorded groups, between eight and twenty vice-counties.

Nationally Scarce (N)
For some less well-recorded groups and species, it has not been possible to determine which of the Notable categories (A or B) is most appropriate for scarce species. These species are assigned to an undivided Nationally Scarce category.

Formal conservation statuses have been taken from the most recent publications of the Nature Conservancy Council and the Joint Nature Conservation Committee, as follows:

Coleoptera Hyman & Parsons 1992
Diptera Falk 1991
Hemiptera Kirby 1992.

For species which do not fall into any formal conservation category, an estimate has been made of the current status of each species in the south-east of England. Statuses have been estimated on a four-point scale: common, frequent, occasional and rare. These estimates are to some extent matters of personal opinion and judgement, though documented evidence supporting changes of status is readily available for most species. No pretence is made that these statuses are assigned after rigorous assessment against precise criteria, but broad guidelines to their significance are as follows:

Common (c): species found in good numbers over substantial areas, usually in a number of habitats, and either having no very special ecological requirements or having requirements which are easily and widely met (restriction to a common foodplant, for example). Such species are expected or unsurprising in any sizeable tract of “wider countryside” within the central parts of their range.
Frequent (f): typically species with somewhat more specialised or infrequently met habitat requirements, but expected where these characteristics are met; such species may be restricted to a narrow habitat range or to particular soil types, require a particular foodplant of less than universal occurrence, or be associated with a widespread but erratic habitat resource, such standing dead wood of particular species or in particular conditions. Species in this category are expected or unsurprising wherever the habitat types with which they are associated is found.
Occasional (o): typically species with a very particular and infrequently met habitat requirement; or species of poor mobility whose presence may be heavily dependent on habitat continuity; or species which, though not obviously of highly restrictive requirements, are nonetheless very rarely recorded. Such species may be erratic in occurrence, and often require specific search of their specialist niches in order to be located; only in special circumstances are they expected merely on the grounds of broad habitat type.
Rare (r): Typically species with a particular and infrequently met habitat requirement, but sometimes merely highly geographically restricted. Such species are generally significantly less frequently found than apparently suitable habitat, and are expected, if at all, only when their very particular and special niche is found. Species falling into this category would usually be expected to have a formal conservation status, but recently discovered species which have not yet been included in a review are quite likely to qualify.

The list has been checked for any species included in the lists of threatened and declining species in the UK Biodiversity Action Plans (Biodiversity Reporting and Information Group, 2007).

The abbreviations for statuses used above are those employed in lists and tables elsewhere in this report.

Outline site character description

This elongate pond, approximately centred at TQ35407220, has two basins, one relatively deep with extensive open water, the other shallower with well-developed tall marginal swamp. The two are linked by a shallow seasonally dry area crossed by a boardwalk. The pond is sheltered by surrounding trees, and though there is good light penetration there is also significant shading in places. The structural range in vegetation is wide, the range of depths and profiles provides good conditions for invertebrates, and, except for the influence of shading, the location also favours invertebrate interests.

The recorded fauna

The recorded invertebrate fauna is rich and varied. The total of 59 recorded aquatic taxa is very respectable for a list largely based on a single visit, to a relatively isolated pond in an urban area, and the three Nationally Scarce species recorded include two, the soldier-fly Odontomyia tigrina and the water beetle Hydaticus seminiger, which are usually at best rather slow colonists of water bodies and suggest a degree of stability. Despite this richness, the site is sufficiently varied and interesting to suggest potential for additional species which were not found. Their absence from the samples probably in part reflect merely the isolated nature of the pond – it would be unreasonable to expect that it might have accumulated all the species which it could conceivably support. In part, however, it may result from minor, and perhaps correctable, deviations from perfection in habitat details.

A previous survey of the pond, carried out by R.A. Jones in 1996, recorded the Nationally Scarce water beetle Helochares lividus. This beetle is associated with well-vegetated, very shallow water in sunny conditions – shallowly flooded short grassy vegetation is especially favoured. This beetle could not be found in 2009, despite specific search, and it is likely that it was genuinely absent; vegetation in shallow water was mostly either too tall and dense, or too shaded by surrounding trees. The loss of the beetle is not serious: it is doubtfully worthy of its status, and is a mobile species which would almost certainly re-colonise if suitable conditions were re-created, but it points to a change in the character of the site since the earlier survey. The report of the 1996 survey refers to the pond having an “open aspect, not overshadowed by trees” which it would be a stretch to fit to its current state, and to the fauna of the “muddy margins”, which are now sufficiently restricted that they would scarcely warrant specific mention. Overall, however, the fauna of the pond appears to have improved since 1996: the single Nationally Scarce species of dubious worth from the 1996 survey can be compared in 2009 to four Nationally Scarce aquatic species, three of them arguably with stronger claim to their status, as well as two Nationally Scarce wetland and one Nationally Scarce woodland (especially damp woodland) species.

This pond qualifies as Priority Pond under the current guidelines for assessment for the pond Habitat Action Plan (Fairclough & Nicolet, 2008) on grounds both of presence of rare species (3 or more Nationally Scarce species) and of diversity (more than 50 species), though it must be admitted that the thresholds for qualification are currently set sufficiently low that this is not in itself a guarantee of high conservation value.

Notes on Nationally Scarce species

Cercyon convexiusculus (Coleoptera, Hydrophilidae)
Nationally Scarce category B
A small black beetle found at water margins and in marshes and swamps, where they occur on mud and amongst decaying plant material. It is of quite wide distribution, though more frequent in the south-east, and dubiously worthy of Nationally Scarce status.

Chorisops nagatomii (Diptera, Stratiomyidae)
Nationally Scarce
A small but brightly-coloured soldier fly with a shining green thorax and orange abdomen found in woodland and other tree-shaded places, especially where there are well-structured margins, clearings and rides. Larvae probably develop in the soil or in leaf litter. The British range extends through southern England and Wales, north to Cumbria.
Delphacodes capnodes (Hemiptera, Delphacidae)
Nationally Scarce category B
A small dark brown planthopper, which lives close to the ground in wet habitats, usually peatlands. Most records are from lowland bogs, but it has been recorded from at least one upland site. There are records away from bogs, and it may be that it will prove reasonably frequent in fens. The foodplants are not known, but all members of the family are associated with grasses, sedges and rushes. Cotton-grasses Eriophorum spp. have been suggested as hosts, but the bug has certainly been recorded in their absence. Its small size, secretive habitats and seemingly low population density may have led to under-recording in the past, but the current widespread use of suction samplers should correct this, and has not led to a massive increase in records.

Helius pallirostris (Diptera, Limoniidae)
Nationally Scarce
A delicate pale cranefly found in a wide range of non-acidic wetlands, including ditches, dune slacks, the margins of slow rivers, and marshes. It appears to be associated with tall wetland or emergent vegetation, and larvae have been found between the leaf sheaths of bulrush Typha sp. It has a wide but scattered distribution in England, Wales and Scotland.

Hydaticus seminiger (Coleoptera, Dytiscidae)
Nationally Scarce category B
H. seminiger is unknown from Wales and Scotland. It is especially associated with lowland fen pools with dense vegetation, often in shade, and often small and isolated. It does, however, also occur in ditches on coastal levels, but it is possibly restricted there to the shade afforded by reedbeds. In Poland, larvae were collected from May to September. Adults are capable of flight, overwinter on land, and return to breeding sites in spring. The life cycle is univoltine. The distribution falls into three main areas, the Cheshire Plain, northern East Anglia and the Home Counties plus Dorset.

Microvelia pygmaea (Hemiptera, Veliidae)
Nationally Scarce category B
A small surface-dwelling bug found on sheltered still lowland waters, usually small ponds or ditches, but occasionally at sheltered marginal fringes of larger water bodies. The bugs are usually found amongst emergent vegetation, though not if it is dense and tall enough to provide significant shade. It can occur in a wide range of macro-habitats, including heathland pools, calcareous fens, ditches in grazing marshes, and pools of relatively recent origin in disused mineral workings. It is a south-eastern species, with few records north of East Anglia. A relative abundance of recent records suggest a gradual increase in frequency and range, but past taxonomic confusion, and the ease with which the species may be overlooked, not only amongst emergent vegetation but also amongst the commoner M. reticulata with which it often occurs, lend uncertainty to its status and to trends in that status.

Odontomyia tigrina (Diptera, Stratiomyidae)
Nationally scarce
A medium-sized black soldier-fly whose larvae develop in well-vegetated shallow water. Recorded from fens, ponds, canals and ditches, this species appears to be found predominantly in areas of extensive and well-established wetland, especially in areas of grazing levels. Though the British range of the species is wide, with one old record for the Aviemore area of Scotland, most records are from southern England.

Management suggestions

The general character of the ponds, and the structure of the vegetation, is good. There is great variation in the depth of water and the density of emergent vegetation, the area is sheltered, the extent of shade varies, and the surroundings are an interesting mosaic of structure. Substantial management is not only unnecessary, but undesirable. It would be especially regrettable if the extent of reed sweet-grass swamp were to be significantly reduced: this is a valuable invertebrate habitat, and likely to contain significantly more interest than the 2009 survey has recorded. The ideal is to undertake minor corrections in the short to medium-term, then hold conditions reasonably steady for as long as possible to allow further development of the invertebrate assemblage. Three things are ideally needed in the short-term:
• Removal of fish, suspected of being grass carp, which are almost certainly preventing the establishment of submerged vegetation and disturbing the sediments.
• Management of selected areas of tall emergent/marginal vegetation by cutting to encourage low vegetation over shallow water, and hence warmer and sunnier conditions. The exact location of management does not matter greatly, except that it should be where there is good light penetration; an area of a few square metres, in the shallow central linking area of the pond, close to the boardwalk for ease of access, would be sufficient to support a significant assemblage of invertebrates.
• The amount of shade from woody vegetation needs to be controlled. There is not a great urgency for large-scale management, but shade appears to have increased, is increasing, and could easily become sufficient to limit the invertebrate fauna, as well as increasing leaf input. A programme of gradual piecemeal or small-scale control would be most appropriate, and would run the least danger of attracting criticism or causing inadvertent damage to other wildlife interests. Removal or coppicing of young woody growth near the ponds, which has the potential to cast shade in the future, and pruning of individual branches which overhang the pond or cast shade on it, should make a substantial difference. Such management, though small-scale on any one occasion, will need to be relentless, but could improve and maintain conditions in the long term without conspicuous or sudden change. Shade cast by taller trees could be a more insidious and difficult problem. Ideally, all trees to the south of the pond which have the potential to cast shade should be kept low, either by occasional surgery or active pollarding, but such management may not be either easy or universally popular.

In the longer term, apart from maintaining the process of woody vegetation management, it is likely to be desirable to undertake piecemeal management of tall swamp vegetation to maintain structural variety. In the shallow arm of the pond, in particular, tall swamp is now extensively dominant around the margins. This is not a bad thing: it is a useful vegetation type, shows a good transition at the moment to open water in the centre of the pond, and contrasts well with more open margins on the deeper arm. The proposed creation of an area of shorter vegetation, hopefully permanently maintained by regular cutting, will introduce some variety in the short term. In the longer term, however, a rotational process of vegetation management along the margin of the pond could increase variety: management should ideally take place when the water level is low in summer; standing vegetation, and accumulated dead leaves, would be removed as close to the ground as possible, and heaped in a part of the site of low intrinsic interest where it can act as habitat in its own right. Only a fraction of the margin should be managed in this way in any one year, and a portion should ideally be left permanently unmanaged. There is, however, no obvious need for such management to begin in the immediate future.

Fluctuations in water level in this pond are a beneficial feature. Such drying helps to limit the build-up of decaying organic material in the bed, an important factor in governing the invertebrate assemblage in well-vegetated shallow water; and many invertebrates associated with shallows are adapted to such seasonal drying: scarcer specialists of such conditions may be out-competed by common generalists if seasonal drying does not take place. However, there are other species on the list which require permanent or near-permanent water. It is strongly desirable that the lowest areas of both basins remain at least wet in all or most years. Topping up with mains water would, if essential, be preferable to complete drying. Such filling should, however, only be to a minimum “emergency” level.

One way of increasing the duration of wet conditions in the shallower basin would be to deepen it near its centre. Excavation should do no harm to existing invertebrate interest provided any deepened area occupied only a fraction of its area and did not unduly impinge on the marginal swamp; the area beyond the swamp fringe showed relatively little invertebrate interest. There is a possible additional benefit from excavation in slowing or preventing the spread of swamp vegetation into the central part of the pond, thereby maintaining an open-structured transition.

References

Fairclough, J. & Nicolet, P. 2008. Best practice for the identification and the assessment of UK BAP Priority Ponds. In Practice, March 2008: 1-5.

Falk, S. 1991b. A review of the scarce and threatened flies of Great Britain (part 1). Peterborough: Nature Conservancy Council. (Research and Survey in Nature Conservation, no. 39).

Hyman, P.S. & Parsons, M.S. 1992. A review of the scarce and threatened Coleoptera of Great Britain. Part 1. Peterborough: Joint Nature Conservation Committee. (UK Nature Conservation, no. 3).

Kirby, P. 1992. A review of the scarce and threatened Hemiptera of Great Britain. Peterborough: Joint Nature Conservation Committee. (UK Nature Conservation, no. 2).

Stewart, A.J.A. & Wright, A.F. 1998. A new inexpensive suction apparatus for sampling arthropods in grassland. Ecological Entomology, 20: 98-102.

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