How to Measure Biodiversity and Species Richness

The UK government's biodiversity net gain (BNG) policy aims to markedly improve wildlife habitats across England, and knowing which creatures and plants call a development site home is crucial. There are roughly 8.7 million¹ different species on the planet. We can't be more precise than that because scientists don't know the true number, a situation not helped by the shifting sands of what defines a species. We can say there is a great deal of diversity among life on Earth, and ecologists, conservation biologists, and environmental scientists have been using species richness as a way to measure biodiversity for a long time².

Species richness is defined as³ the total number of unique species present in an ecosystem, whether that's a field, patch of woodland, a river, or an entire region. For landowners and developers in England, this term is increasingly becoming part of their vocabulary, thanks to the key role species richness plays in BNG. But what does it mean, why is it so important, and if scientists are stumped, can species richness really be properly assessed?

Species Richness vs Abundance and Evenness

Species richness may be a tangible measure of an area's biodiversity - but it doesn't give us the whole story. Two additional metrics are also needed. Species abundance⁴ reveals how many individuals of each species are in a given landscape, while species evenness⁵ is the relative abundances of those species.

For example: a patch of grassland that's home to 30 plant species, one of which dominates 90% of the ground, demonstrates species abundance. Another, where all 30 species are roughly equally represented, would be considered more even.

Measured by species richness alone, both grasslands appear the same, but viewed through the lens of abundance and evenness, they are very different indeed.

Measuring Species Richness

Assessing biodiversity on any scale is a complex task, and even the most comprehensive survey would have a hard time detecting every species in an ecosystem.

It's why measuring observed species richness is almost always an underestimation⁶ of the true figure, because of sampling size, species are rare or simply missed during sampling. Nonetheless, several methods have been developed to achieve the most accurate measurement possible.

Species accumulation curves⁷ use samples to estimate the number of species in an area. Usually, the curve rises steeply at first before levelling off and eventually flattening as the maximum species are detected.

Rarefaction curves⁸ show the rate of new species being found as the samples are collected - the higher the curve, the more species are identified.

Both the Chao1 Index⁹ and the Abundance-based Coverage Estimator (ACE¹⁰) use abundance patterns to estimate the number of species likely to be present but were not sampled or observed.

Indices such as Simpson's Diversity Index¹¹ and the Shannon-Wiener Index¹² provide a measure of biodiversity by calculating how many different species are present and how evenly they are distributed.

What Can Affect Species Richness?

Species richness can be influenced by a range of ecological and environmental factors divided into abiotic, such as latitude, temperature and disturbance, and biotic¹³, covering the interactions and competition between species.

Stable environments¹⁴, untouched by dramatic climatic disruption such as glaciation, have had more time for species to evolve, diversify, and settle into stable ecological communities, enabling species richness. In addition, regions with a mix of soil types, landscapes, and microclimates¹⁵, will likely be home to more biodiversity.

Some disturbance can actually help species richness¹⁶. In areas with little or no disturbance, one or two species may crowd out others. Traditional hay cutting, controlled grazing, or periodic flooding, allows a more biodiverse community to flourish within a habitat.

The condition of the soil can also impact species richness - but not in the way you might think. Nutrient-poor soils¹⁷, such as those found under calcareous grassland or lowland heath, tend to support higher plant species richness than their nutrient-rich counterparts, because competitive, fast-growing species are less able to dominate.

It's why habitats on unimproved, low-fertility soils are often the most species-rich in the UK.

Climate Change and Species Richness

Climate change is one of the most significant threats to species richness globally, and its effects are already being seen in UK habitats. However, the relationship between climate change and species richness is far from black and white.

In some areas, warming may initially increase species richness¹⁸ by making conditions suitable for new arrivals, while in others, it may drive local extinctions¹⁹ faster than new colonisations can replace them.

The ability of species to move between suitable environments is critical in a changing climate. This is why the UK government's BNG framework places such importance on creating and connecting habitats so species can thrive²⁰.

Species Richness and Biodiversity Net Gain

In the UK planning system, species richness is central to how Biodiversity Net Gain (BNG) is assessed.

The statutory (official) biodiversity metric calculation tool²¹ (SBMCT) scores habitats including grassland, lakes, woodland, hedgerows, and watercourses, on their size, condition, strategic significance, and type²².

The small sites metric (SSM) tool²³ offers a streamlined version for smaller-scale projects that meet the following criteria:

• Where only the habitats available in the SSM are present on-site. Any site containing any additional habitats (including riparian zones) not included in the SSM must use the SBMCT.
• Where no priority habitats are present on-site. Some hedgerows and arable field margins are excluded from the above as these are medium distinctiveness habitats and are included in the SSM.
• Where no statutory protected sites or habitats are present.
• Where no European protected species are present.

Both tools calculate the number of biodiversity units in a habitat²⁴ - its biodiversity value - and how much of that value could be impacted or lost through development.

The SBMCT helps developers compare and contrast BNG proposals²⁵, enabling them to meet the legal requirement of achieving a minimum 10% net gain in biodiversity, as well as informing and supporting habitat management decisions.

We Can Help You Find the Best Way Forward

If you're a landowner thinking about enhancing the species richness of your land or want to know more about its potential BNG value, we can help you understand the benefits.

If you're a developer new to BNG requirements, our team can give you all the information you need to move forward.

If you're working on Nationally Significant Infrastructure Projects and are looking for effective BNG solutions, talk to us.

Email sales@greenshank-environmental.com or fill in this contact form, and we'll get back to you straight away.

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Sources

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