The Removal of Connectivity from the BNG Metric
Biodiversity Net Gain (BNG) has become a key part of how we protect and enhance habitats through development. But there’s one critical factor that often gets missed in the process: connectivity.
When the connectivity multiplier was dropped from Biodiversity Metric 4.0, the reasoning made sense: it was inconsistently applied and difficult to audit. But that doesn't mean it wasn't important.
One of the core issues I explored during my EngD many moons ago was how to incorporate/measure/visualise fragmentation as an impact biodiversity particularly from linear infrastructure. I used GIS to model the impacts using a combination of patch-level and class-level landscape metrics. Things like Core Area, Nearest Neighbour Distance, Mean Patch Size, and Patch Density. These are standard ecological tools. They’re measurable, and they show how quickly habitats become ecologically disconnected even when they still look intact on a map.
In practice, these kinds of metrics let us understand whether habitats are actually usable by wildlife, not just how big they are. For example, two patches of grassland might look identical in condition and area, but one is surrounded by hedgerows and watercourses, and the other is ringed by roads and fences. Without measuring that difference, we can’t see the risks to biodiversity resilience.
It’s not just about species moving from one patch to another. Fragmentation can quietly undermine ecological value; species isolated in small patches often face reduced genetic diversity, higher risk of local extinction, and fewer opportunities to recolonise after disturbance. We’ve seen this play out again and again in landscapes broken up by roads, fences, or development with no wildlife corridors. Let's also not forget that suitable connectivity looks different for all species, opening the door for discussions around species-weighted distinctiveness (let's save that for another article).
What worries me is that the current metric, despite its strengths, doesn’t reflect this. A site can score well numerically and still be ecologically fragmented. I think that’s a risk, especially on projects where net gain outcomes are supposed to be long-term and robust.
There’s also a case to be made that connectivity is simply too nuanced to fit comfortably within a generalised, metric-based approach like the Biodiversity Metric. Connectivity is species-specific, spatially variable, and influenced by land use and landscape context in ways that can’t easily be reduced to a universal score. In trying to force it into a standardised system, we may risk oversimplifying or misrepresenting what’s actually going on. That doesn’t mean it isn’t important, it just means we might need to evaluate it separately, using more targeted spatial analysis tools and ecological interpretation.
However we do have the tools to explore this. In my thesis, I applied fragmentation metrics using existing GIS habitat data and spatial analysis modelling. It’s not difficult, especially when you already have mapping in place for condition and baseline. And while it’s not the job of the FRIDAS checklist to prescribe ecological analysis, maybe there’s room for something like FRIDAS+, a way to encourage best practice in interpreting what your spatial data is actually showing you.
