The re-emergence of the Environment Bill has heralded some considerable debate about the proposed statutory enshrinement of the concept of biodiversity net gain.  There have been arguments about whether this a good thing or a bad thing.  Arguably, those debates are slightly academic because the reality of the situation is that biodiversity net gain is already with us.

In terms of statute we have section 40 of the Natural Environment and Rural Communities Act 2006.  Section 40(1) provides as follows:
“(1) [A] public authority must, in exercising its functions, have regard, so far as is consistent with the proper exercise of those functions, to the purpose of conserving biodiversity.”
A “public authority” includes a Minister of the Crown, a public body, a government department, a local authority and a local planning authority: Section 40(4).
Section 40(3) then provides an extended definition of “conserving”:
“(3) Conserving biodiversity includes, in relation to a living organism or type of habitat, restoring or enhancing a population or habitat.” (emphasis added)
Therefore, the duty in the NERC includes not only conservation but also restoration and enhancement.
Section 40(2) provides that, in complying with subsection 40(1), a Minister of the Crown or government department must in particular have regard to the United Nations Environmental Programme Convention on Biological Diversity of 1992.
Those familiar with the "listed building duty" in section 66 of the Planning (Listed Buildings and Conservation Areas) Act 1990 will see a resemblance between the two provisions. Likewise, the "conservation area duty" in section 72 of the same Act.  The legal ramifications of a local planning authority's failure to have proper regard to either duty are well known.  The grant of a planning permission in such a circumstance can lead to a quashing of the permission by the High Court.  It is difficult to see why section 40 of the NERC should not carry the same weight.  Those who practised in the 1980s may recall that the predecessor to section 72[1] was often treated as being hortative or a broad generalisation.  This approach was soundly rejected by the courts in a series of cases beginning with Steinberg v. Secretary of State for the Environment (1988) 58 P. & C.R. 453.[2]  It is only a matter of time before we have "Steinberg Mark II" with section 40 being the provision before the court.  It is unlikely that it will be treated as a mere statutory puff.
Turning to matters of policy, the National Planning Policy Framework (2019) contains a number of policy statements which clearly not only refer to matters of enhancement but also, in part, also clearly imply or invoke the concept of measurable net gain (emphasis added):
“170. Planning policies and decisions should contribute to and enhance the natural and local environment by:
 . . . .d) minimising impacts on and providing net gains for biodiversity, including by establishing coherent ecological networks that are more resilient to current and future pressures;. . “

174. To protect and enhance biodiversity  . . , plans should:
b) promote the conservation, restoration and enhancement of priority habitats, ecological networks and the protection and recovery of priority species; and identify and pursue opportunities for securing measurable net gains for biodiversity.

175. When determining planning applications, local planning authorities should apply the following principles:
a) if significant harm to biodiversity resulting from a development cannot be avoided (through locating on an alternative site with less harmful impacts adequately mitigated, or, as a last resort, compensated for, then planning permission should be refused; . . .
d) development whose primary objective is to conserve or enhance biodiversity should be supported; while opportunities to incorporate biodiversity improvements in and around developments should be encouraged, especially where this can secure measurable net gains for biodiversity.

It therefore follows that the notions of enhancement to, and measurable net gains for, biodiversity are already embedded in the planning system.  If one is thinking in terms of enhancement, then it is implicit that this must be some form of net gain.
Some might argue that such enhancement is, inherently, un-measurable.  One cannot measure the un-measurable.  Therefore one has to take an impressionistic approach only.  Such approach is, frankly, not tenable.  Putting one’s figure in the air simply imports uncertainty for local planning authorities, developers and the public.  The notion is simply too vague.  The only viable approach is to provide something that is, in some way, measurable.
Greenpeace resist the notion of biodiversity net gain by arguing, in part, that the valuation of biodiversity is an impossible task.  They give the example of comparing a wasp to a bee.  Which has the most value?[3] Whilst this is not the creation of a false dilemma[4], it is somewhat too simplistic.  It is setting out a standard of perfection and then arguing that anything which fails to meet that standard is useless.  There is a well-known saying in this connection, which is that the pursuit of the perfect should not drive out the good.  One can sit and cogitate upon esoteric arguments ad infinitum, or do something about a problem.  Practical progress is often to be preferred over academic procrastination about how many angels fit on a pin head.
Nor is it right to say that scientific progress is based upon perfection.  It is quite the reverse.  Take for example Newton's theory of gravity.  The theory was not all encompassing because it failed to explain the apparently anomalous orbit of the planet Mercury.  Be that as it may, this methodology provided a sound basis for much scientific progress, up to and including the period heralded by Einstein's theory of relativity.  Indeed, it has been said that the first men to reach the Moon plotted their courses on the basis of Newtonian calculations.[5]  For another example, consider quantum mechanics.  Scientists are candid about saying that much of the quantum realm is unknown to them.  Notwithstanding this, progress is being made in such fields as quantum computing on the basis of what we do know in terms of cause and effect.
So it is not necessarily a case of packing up and going home.  The alternative is to adopt a medium which is not perfect yet allows one to make some progress in the real world.  Which brings us to the topic of biodiversity metrics.  Those producing these metrics have made it clear that it is a proxy system of evaluation made necessary by the heterogeneous nature of the subject matter of their studies.
Those familiar with business management may recognise the methodology in biodiversity metrics as a 'paired comparison' approach.  Business managers do not have the luxury of sitting around in contemplation.  They have to make forced decisions; for example, in job evaluations.  Managers cannot duck out of the task saying it is too difficult. Pay scales, promotions etc will ride on the products of such evaluations.  The assessment of skills, experience etc. could be dismissed as an exercise in the un-measurable, but this would be a sad excuse.  Those managing a job evaluation exercise must therefore seek to appropriate weight to these factors, as best they can, and then, once they have totals for each job description, carry out a comparison exercise to provide a ranking as a basis for pay scales and the like.  This is very similar to the way in which a biodiversity metric operates.
It is not a measure of absolute values but a ranking system which attributes relative weights against certain specified criteria.  The choice of the criteria and the attribution of weight are judgmental.  The outputs of this approach are then given numerical values.  In the case of a biodiversity metric, they are given a numerical value which is described as being a 'biodiversity unit'.
It is, before going on, necessary to place the concept of biodiversity units in context.
Despite the fact that they are expressed in numerical terms, this should not be seen as giving them a false precision.  They are the result of a judgmental approach.  Arguably, it would be a mistake to place too heavy a reliance on the outputs from the application of biodiversity metric.  Arguably, it is an heuristic tool which is helpful in guiding one's thinking and decision-making.  There is a great danger in playing around with numbers if this means losing sight of how those numbers were derived in the first place.[6]
Subject to this caveat, it is possible to use a biodiversity metric to provide some measure of habitat loss caused by a development scheme and to express it in biodiversity units.  It is then possible to express the post development situation in terms of biodiversity units again, and then to compare the two.  If an enhancement is sought, then the proposed post-development value must exceed the pre-development value.  Both expressed in terms of biodiversity units. If the developer is offering a compensatory off-site habitat as part of a package, then the value of the receptor site can be expressed in biodiversity units.
All of which takes us back to the original question; namely, that a reference to enhancement must be something more than a nebulous exaltation.  If so, then it is necessary that the developer, the local planning authority and the public should be addressing something which is tangible.  At the moment, the best way of doing so is by way of a forced decision-making process which involves an evaluation using a biodiversity metric.  Outputs can be expressed in biodiversity units and, therefore, enhancement can be shown to be more than a mere puff.  And that enhancement must, inevitably, be a net gain in biodiversity as a result of the development. 
The residual question is what quantum of net gain will provide a meaningful enhancement? 
The Environment Bill sets the level at 10 per cent.  Arguably, this is a very modest improvement.  Aside from this, it is a matter for the local planning authority.  Be that as it may be, it is possible to express the concept of enhancement in percentage terms.  Of course, one should always bear in mind that the biodiversity metric is, by its nature, not a precise empirical tool.  It is the product of a forced decision-making process and it is based upon criteria which will be selected by way of judgement and by way of a weighting process which is, again, an exercise in judgment.  Be that as it may be, it is difficult to see how the application of biodiversity metric, with a view to showing measurable net gains, is at odds with a statutory duty to enhance in the Natural Environment and Rural Communities Act 2006 and the various policy statements in the National Planning Policy Framework.
The propositions in this essay might be somewhat troublesome for those who promote infrastructure schemes under the Nationally Significant Infrastructure Project regime, because they appear to routinely argue that their only obligation is to show a net "no-loss" outcome when it comes to biodiversity.  So be it !

[1] Section 277(8) of the Town and Country Planning Act 1970.
[2] See also South Lakeland District Council  v Secretary of State for the Environment [1992] 2 A.C. 141
[3] Not an easy thing because the levels of research are not the same for both and they each bring with them very different impacts on ecological networks.
[4] Those interested in philosophy or logic might characterise it as the ‘Nirvana fallacy’, but that purported fallacy is a fallacy itself.  It is not a matter of logic, but of making pragmatic decisions on Planet Earth.
[5] Apparently, they are still accurate in predicting tides.
[6] One can foresee advocates at public inquiries becoming overly focused on number crunching.


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