No Seniors Allowed: Biodiversity Net Gain

01.08.2023 6 min read

No Seniors Allowed is our brand new discussion series where we explore the views of Temple’s Leaders of Tomorrow. Tackling a range of subjects across the environmental sector, we take a look first at Biodiversity Net Gain (BNG).

With the exception of those blissfully living in a media vacuum, ecologists could not fail to register the concept of BNG and its importance to project delivery. Every aspect has been discussed in minute detail from all conceivable angles, perhaps unintentionally approaching a point where we risk losing the underlying purpose.

Importantly, do we know what our teams actually think? These are the people making decisions and doing the hard work day to day. Temple is committed to empowering its leaders of tomorrow and ensuring that the ideas, opinions, and knowledge of our teams shape how we approach our work.

Harry Jarvis and Kieran Ryan, key staff in Temple’s specialist ecology team, provide their take on BNG. So, let’s grab a cup of tea, take a step back from the complex rhetoric, and hear what they have to say.

Here’s what Keiran had to say:

Later this year Biodiversity Net Gain will soon become mandatory for developments in England. This has led to a lot of concern as ecologists question the lack of accuracy within the metric, and developers are left scratching their heads at the complicated nature of habitat design. In the coming years, I think we will see that there will always be a trade-off between these two points of view, but ultimately, they have to meet somewhere. If we don’t compromise on BNG, we will end up with either a system that reflects the complexity of British habitats extremely accurately but can only be understood by highly qualified individuals who may take years to train, or one that only serves as a box checking exercise for development, with no real benefits for biodiversity.

There are some aspects of metric that strike this balance very well, and help to discourage greenwashing, such as the system of trading rules, that will flag instances when an irreplaceable habitat is being lost to a habitat of lower value for nature. This is not only used for ancient woodland or hay meadows but is also applied to brownfield habitats like mixed scrub.

BNG is a constantly evolving system and the exact metric that will become mandatory later this year has not yet been confirmed. However, there are currently issues inherent in the system that will need to be addressed. One of these is a heavy overreliance on trees to maximise the biodiversity units a development can achieve. This could be an extension of our current thinking on how to mitigate climate change, as tree planting is viewed by many as a ‘one size fits all’ solution to the climate emergency. Undoubtedly trees can have an important impact on biodiversity, but do not suit every development, and we must remember that the job is not done once a tree is planted, years of maintenance and attention are required to get a mature tree. Another potential issue for the future of BNG is the ability to use the enhancement of off-site areas as mitigation for habitat losses on site. This could open a back door for developers and may eventually lead to one nature reserve having multiple developers pour money into it, creating extremely high-quality habitats, but leaving it disconnected and isolated lacking any space for nature. What we may need is a cap on how much enhancement a nature reserve can be subject to each year, or on how many organisations are permitted to use an area for offsite mitigation at any one time.

If systems like BNG are to be mandatory, then they must work for all parties involved, or they will be bypassed and superseded by less ambitious ones in the future. As ecologists, we have to remember that future developments will be built with or without our input, so we must seize the opportunity that BNG provides to change the way we think of new developments, as not just spaces for people to exist, but for people to live alongside nature and see it every day instead of fencing it off behind a nature reserve sign.

Here’s what Harry had to say:

Having worked on a variety of different BNG projects over the past 3 years, my overall review is positive. I think it’s a great way of communicating between different specialties and creates an easy-to-understand headline result that can be instantly understood by the client. I think you can get a sense of how important a site is from an ecological perspective, and you can also get an idea of what is needed to make the site more valuable. It also forces the developer to be responsible for their impact on the general environment and usually makes them leave the site more ecologically valuable than before.

I believe that BNG assessments will make a positive change for the environment, however, it does not come without issues. my main quarrel is one that’s shared with many other ecologists; you can’t quantify nature. The environment is infinitely complex, the connections and micro relationships between each species of animal, insect, plant, and micro-organism are so immense that you can’t break it down to a definitive score or percentage. In a BNG assessment you’re doing exactly this, and not taking into account the relationships between each habitat or what benefit these relationships have to the wider environment. Essentially, it’s a numbers game.

My second biggest issue with BNG comes when evaluating trees. Currently, the BNG calculation matrix puts an overexaggerated value on scattered trees. If one tree is taken out, almost three additional trees will need replacing to guarantee a neutral exchange of biodiversity units. For a lot of developments, this is physically impossible. If one hectare of scattered trees is being removed, 2.9 hectares of scattered trees will need to be replaced to achieve a positive net percentage change. If a site is only 3 hectares in size this leaves 0.1 hectares of land free to develop. I’m not saying trees aren’t amazing, they are very valuable and provide good habitats for a lot of species, however, they aren’t the be-all and end-all of ecology as the calculator shows. Putting such an emphasis on them means that a lot of developments fail to achieve a positive net change over a small number of scattered trees.  

Finally, there are many things that aren’t included in the BNG assessment that are essential to providing an ecologically valuable site. One of these is the sites’ geographical location. As every site is different, what is good for one region, can be bad for another. What could be essential to ecology in the north of England, could be less valuable in the south. As the same biodiversity metric is used nationwide, it doesn’t take into account regional differences which can be important to proving an ecologically valuable site.

My overall opinion is that it’s a great tool to help people understand the impact a development will have on the environment. However, for me its use stops there, it’s just a tool. The results cannot be taken in isolation, and you need to recognise the complexity behind the numbers to really understand a site’s ecological value.


Key Contacts

Harry Jarvis Ecologist
Kieran Ryan Assistant Ecologist