Fen Management Handbook - Scottish Natural Heritage

There are few published methods for the assessment of the significance of specific groups and
assemblages of invertebrates, and fewer that are relevant to fens. Foster & Eyre, (1992) describe
a system of water beetle assemblages which revise a numerical score (WETSCORE) indicative
of interest, and define a threshold for a “good” assemblage, but no indication of its actual level of
significance. Drake (2004) describes a revised, and simpler, scoring system, devised specifically for
grazing marsh ditch systems but of potentially wider value. There are two methods of assessment for
saproxylic Coleoptera assemblages (Alexander, 2004; Fowles et al. 1999) which provide values for
national significance and, in the former case, for lower levels of significance also, though the selected
values are all open to question.
Natural England are developing a system for the assessment and monitoring of invertebrate
assemblages which can be used across all habitats. It was primarily designed for Common
Standards Monitoring, but is potentially of wider application. Known as ISIS (Invertebrate Species
– Habitat Information System) this uses standard sampling protocols to collect invertebrates.
Assemblage types are identified from the species list thus obtained, and a numerical estimate
derived of the quality of each assemblage. Assemblages are recognised at two levels: the Broad
Assemblage Type (BAT) and the Specific Assemblage Type (SAT). Fens fall within the Mire
assemblages BAT (W31). Two specific fen assemblages are recognised, Mesotrophic fen (W313)
and Rich fen (W314). Fens will, however, often contain additional assemblages in accompanying
habitats and contained features.
Invertebrate survey
Features of potential invertebrate interest are relatively easily identified but past changes
in management, or fluctuations in water levels, may have resulted in the loss of less mobile
invertebrates. Determining whether features are in fact of significant invertebrate interest or value
depends on detailed expert survey. Methods for obtaining population estimates and comparative
counts for butterflies and dragonflies are outlined in Pollard & Yates, 1993, and Brookes, 1993.
Since the range of invertebrate species likely to occur on a fen is large, the effort needed to obtain
a comprehensive picture of the invertebrate fauna is substantial even on a quite small site. In most
cases, invertebrate survey will be selective and/or limited by available expertise and resources. For
general site assessment, it is preferable to examine a wide taxonomic range and habitat range. Drake
et al (2007) provide a range of standard protocols for sampling, lists key groups for the assessment
of invertebrate assemblages in fens and other habitats, and provides guidance in the use of the
data gained from such surveys in the ISIS application developed by English Nature for monitoring
invertebrate assemblages on SSSIs in England, which can provide an assessment of the quality of
the fauna as well as providing a basis for long-term monitoring. However, the ISIS application is still
under development, the list of methods incomplete, the protocols not uncontentious, and some key
groups not readily identified. It is, moreover, a general-purpose system, and it may be preferable to
design one for particular sites, based on available expertise, or to divide the site into assemblages
other than those identified by the ISIS programme.
A more pragmatic approach is to identify potentially contentious issues and areas at an early stage
and target survey at these. Simple questions such as “does the invasive scrub on this site support
a significant invertebrate assemblage, and if so how much scrub is needed to support it” or “how
important are the invertebrates of this field, and how will they be affected by a change from cutting
to grazing” may prove impossible to answer definitively, but survey aimed at answering them will
provide better guidance than an attempt to derive an answer to a specific question retrospectively
from the results of a general survey. Equally, if the management of a particular feature is not in doubt,
and conforms to general principles (for example, the maintenance of marginal willows by rotational
pollarding) then survey work may be very valuable for site assessment, but is unlikely to contain
useful information for management purposes.
315