Biodiversity Off-setting and Invertebrate Conservation


in Biodiversity Offsetting

by Robert Homan (County Moth Recorder for East Gloucestershire)

The concept of biodiversity off-setting within the framework of planning decision making was introduced in the 2011 White Paper “The Natural Choice”  and subsequently refined in additional documents such as “Biodiversity offsetting: Guidance for offset providers” in April 2012.

Off-setting was one of a triumvirate of strategies and was envisaged as coming after the more preferable approaches to development of avoidance and mitigation.  A key element of the off-setting proposals was a group of six pilot schemes initiated in 2012 and set to run for two years.   Despite the welcome shown in some quarters for the White Paper e.g. Developing Devon’s Local Nature Partnership/Biodiversity Offsetting Pilot, recent activity by what might be  termed the buccaneering wing of the development industry suggests that avoidance and mitigation will be ignored and that off-setting will be the default mechanism used as a means of badgering local authorities into reaching development friendly decisions.

At an abstract level, biodiversity off-setting represents a tacit acceptance within the planning framework that a site has some ecological value and that this value is both known and appreciated by others.  Hence the willingness to bear the additional cost of off-setting, which can then be presented as a solution to the biodiversity issues relating to the development of the site.

Equally, off-setting represents a denial of any distinctive ecological value as the underlying assumption is that one group of organisms (an ecosystem) can simply be replaced by another.

At a more specific level, there are several reasons why off-setting from the perspective of  invertebrate conservation is doomed to fail.

There is a very high probability that the strategy is built on a foundation of ignorance. Great play has been made, for example, by the potential developer of Rodborough Fields (Gloucestershire) of  the appointment of a “a consultancy to undertake ecological surveys and ‘masterplan the proposals to respond as sensitively as possible to the species and habitats present.’ ”  (See  In practical terms, experience shows that this approach involves a “desktop” review (usually a request to the relevant environmental  record office for the records relating to the site with a view to establishing the presence or otherwise of any “protected” species) and a field visit of probably one day duration, where the quality of the results will reflect the timing of the visit and the skills and knowledge of the individual(s) involved.

Fieldwork for the forthcoming  BTO Bird Atlas 2007-11 was based on a long period of intensive, systematic recording, resulting in an accurate and geographically comprehensive picture of the distribution of breeding and over-wintering bird species.  However, this level of coverage is exceptional.   By contrast, recording of invertebrates is patchy, unsystematic and in places non-existent.  Even charismatic groups such as butterflies have received only partial coverage.  In Gloucestershire, the records of the local Butterfly Conservation group show that many peripheral areas of the county in particular have no butterfly records at all.  As insect groups become less appealing so the amount of attention they have received declines.  Fig 1 shows the number of moth species recorded by tetrad  in East Gloucestershire [1]

Fig 1. The number of moth species recorded by tetrad in East Gloucestershire (VC 33). The first record in the database is from 1880, the majority of records are much more recent, mainly post 1970.

Fig 1. The number of moth species recorded by tetrad in East Gloucestershire (VC 33).  The first record in the database is from 1880, the majority of records are much more recent, mainly post 1970.


There is a clear negative correlation, showing that a large number of tetrads have very few species, while a tiny number of areas have very large numbers of species.  The latter are  sites where keen moth recorders have run moth traps over a long period thus accumulating large numbers of records.  The former are not “moth deserts”, but simply the norm in that they have received little attention from the small group of proficient and very busy recorders active in the area.  The pattern shown in Fig 1 will be reflected in the number of records held by the county environmental record office such that a “desktop search” will reflect the spatial distribution of recording and expert recorders far more than the county’s biogeography.

The issue of low levels of moth recording is also apparent spatially.  Fig 2 echoes the geographical  distribution of butterfly records and shows significant parts of East Gloucestershire are seemingly moth free.  This is especially the case in the east of the region on the dip slope of the Cotswolds.

recorded sites

Fig 2. Sites in VC 33 with at least 1 moth species record shown at 1km square scale.  (The vice-county boundary is shown in green and each green square represents a “recorded” 1km square. The higher land of the Cotswolds is shown in shades of yellow and brown)


Despite the familiar example of the wasps that always find their way to a summer picnic, there are groups of invertebrates which have but limited mobility and a low propensity to spread.   Not all spiderlings can balloon on a thread of gossamer,  molluscs can make slow and steady progress within a vegetable patch but no further, and some insects are tied to a locality characterised by a complex and unique web of geomorphological, climatic and biotic factors.   Land-use change can obviously lead to the reduction in the total number of a species by loss of habitat, but it can then result in the fragmentation of the population and hence to isolation and ultimately genetic impoverishment and possibly local extinction.  Developing site  A and off-setting to site B means a geographical separation such that the new location will be beyond the pale for many invertebrate species.

Given the complexities of cataloguing the biodiversity of a site and the logistical and  financial burdens of recreating suitable micro-habitats and niches, the basis for off-setting calculations is likely to be crude and convenient rather than comprehensive or ecologically appropriate.   Currently the scheme rests on “biodiversity units” which give little or no regard to species level analysis.  In essence the the concept boils down to an eye for an eye and a field for a field.

Biodiversity off-setting is a policy created in the age of ecology when biologists are familiar with habitats, food chains and energy flows but have a poor grasp of field identification, taxonomy and autecology.  To such a mind-set it makes perfect sense to write  “Using the biodiversity offsetting approach means that an offset provider delivers a quantifiable amount of biodiversity benefit to offset the loss of biodiversity resulting from a development.  The losses and gains are measured in the same way, even if the habitats concerned are different.  In the biodiversity offsetting pilot, the measurement is done in ‘biodiversity units’, which are the product of the size of an area, and the distinctiveness and condition of the habitat it comprises,” Biodiversity Offsetting Pilots, Guidance for developers March 2012. With just three categories available to measure each of  distinctiveness and condition,  biodiversity units are never going to be rocket science, but equally they will never add up to an adequate basis for effective invertebrate conservation.


Robert Homan (County Moth Recorder for East Gloucestershire) August 2013


Read more articles about the issues with Biodiversity Offsetting

[1] A tetrad is a 2×2 kilometre square and is a widely used unit for biological recording.     “East Gloucestershire” is the name of Watsonian vice-county 33.  Vice- counties are another widely used spatial unit for biological recording..













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