Look at this classic Dorset landscape. In a gap in the Purbeck Ridge, sits the majestic remains of Corfe castle. In the foreground is the shallow valley of the Corfe river,  wending its way to Poole harbour. What look like former water meadows along the river still support a diverse plant community, as does the slope on which we sat, those wild plants supporting diverse insects, our idyllic spot humming with bee and bird song. Enough of the prosaic already! I hear you mutter. We are indeed very lucky to live in Dorset.

With this in mind, I ponder of the latest “Biodiversity Offset Summit” which happened last week. Did you not see it fanfared in the press? No, there was almost no press coverage, the press weren’t informed or invited. The ENDS report did report it – thanks Simon.

Fortunately someone has published a note of the meeting – thanks to BES. Note that almost all of the speakers are pro offsetting and in the business. Why David Hill is described as representing the Ecosystems Market Task Force I do not know – given that he is “offsetting finder-general” in his capacity as chairman and co-founder of the Environment Bank. Or indeed as deputy chair of Natural England?

Anyway I digress, what I am really interested in is what is replaceable – for me this goes to the heart of offsetting.

Is any biodiversity replaceable? who decides and on what basis? Can whole SSSIs be replaceable?

There’s some Defra guidance on replaceability – produced last year, before the pilots started. Appendices A + B in this report .

Clearly a great deal of brain effort has gone into this work. According to the experts who drew it up, it’s highly difficult to recreate calaminarian grasslands, but easy to recreate open mosaic habitat on previously developed land (OMH for short). For those who are wondering what on earth these things are, calaminarian grasslands are grasslands that have developed on metal-contaminated land – metals such as lead, arsenic and so on, which are highly toxic to life, so only a few species can thrive there. OMH on the other hand, also known colloqioally as brownfield land important for biodiversity, was often created as a result of industrial processes – you can see where this is going – that led to the contamination of land by things like – yes you guessed it, toxic metals like lead and arsenic. Indeed there is cross over between the two priority habitats, and calaminarian grassland occurs within a wider mosaic of OMH.

Some habitats are innately irreplaceable – limestone pavement is the obvious one, as it was created as a result of actions that only happen immediately after the end of an ice age. Blanket bog has been oozing across the uplands for millennia following neolithic forest clearance – that’s not an easy one to do again in this interglacial.

Others are clearly replaceable in principle, since they have been created and destroyed, created and destroyed again and again over the 10 millennia since the end of the last ice age, in Britains’ ever-changing landscapes. A mediaeval arable field is abandoned after the Black Death and becomes a piece of chalk downland, surviving for 700 years, only showing us its arable history through the presence of strip lynchets. A heathland is ploughed during the war, and is slowly recovering towards moderately interesting acid grassland 70 years later. The three factors that determine whether a habitat can return to an area are: how much time has elapsed since the last major disturbance; what is the availability of wildlife nearby to recolonise; and is the area being managed in such a way that the wildlife can recolonise.

Now in the Alice in Wonderland world of biodiversity policy it is possible to create new priority habitat more or less instantly: plant some broadleaved trees in the ground – bingo – new priority woodland has been created. Sow some wildflower seeds on an arable field – manage it nicely – hey presto – new priority lowland meadow habitat created – very pretty. The only problem is that nursery-grown trees or sacks of wildflower seed don’t contain the insects, the fungi, or indeed the history of long-established habitats. You can’t sow ridge and furrow or a plague village.

Would anybody with ecological integrity really claim, and keep a straight face while doing it, that these “new” habitats are equivalent in any way, let alone adequately replace, ancient semi-natural woodlands or ancient meadows?