Biodiversity footprint : which metric for which information ?
Three essential dimensions for assessing the state of biodiversity
Biodiversity comprises three main aspects: genetic diversity, species diversity and ecosystem diversity. Each plays an important role in maintaining the balance of life on Earth.
🔹 Genetic diversity: This refers to the variety of genes within a given species. This diversity enables species to adapt by natural selection to the various changes taking place in their ecosystem. The greater the diversity, the greater the capacity to adapt. For example, a species with a high level of genetic diversity is more likely to resist disease and ensure its survival.
🔹 Species diversity: This corresponds to the number of species present in an ecosystem. Each species plays a unique role, for example in pollination or nutrient recycling.
🔹 Ecosystem diversity: This includes the variety of natural habitats such as forests, oceans and grasslands. Each ecosystem is home to complex interactions between species and their environment, sustaining life on Earth.
These three aspects are linked and essential to the proper functioning of ecosystems. Their disappearance or degradation directly affects the health of our planet.
Species, a key indicator of the biodiversity footprint
Biodiversity footprint measurements focus on species diversity, as these are easier to observe and count than genes or ecosystems. Their evolution can be tracked using indicators such as population abundance, extinction rates and changes in areas of presence. Some species even play a crucial role in maintaining the ecological balance; their disappearance can lead to major imbalances.
Genetic diversity is more difficult to assess on a large scale. Ecosystem diversity, while important, is complex to measure due to the many ecological interactions involved.
In short, species indicators are the easiest to use to assess the impact of human activities on biodiversity. They also provide a clear picture of the changes taking place
Time is of the essence
It is important to distinguish between the state of biodiversity and changes in biodiversity:
🔹 The state of biodiversity is a snapshot at a given moment of biological diversity, whether at genetic, species or ecosystem level. It includes indicators such as species numbers, abundance and habitat quality.
🔹 Change in biodiversity refers to the evolution of this state over time. It can indicate an improvement or a deterioration in biodiversity, under the effect of natural or human factors. This phenomenon, known as “biodiversity dynamics”, captures the trends and transformations that affect biodiversity over time.
Two concepts for assessing species : understanding the difference
Species abundance and species richness are essential concepts for measuring the biodiversity of an ecosystem. Both indices can be used to assess biological diversity, but each has a different focus.
🔹 Species richness refers to the total number of species present in a given ecosystem or habitat. The greater the variety of species present in an environment, the higher its species’ richness. This index is crucial for assessing the biological diversity of an environment and its ability to support different forms of life.
🔹 Species abundance, on the other hand, refers to the density of individuals within a particular species in an ecosystem. It measures the distribution of species and their relative frequency in relation to other species. While species richness assesses the diversity of species present, species abundance gives an idea of the distribution of these species within the environment.
These two indices are complementary. Species richness tells us about the variety of species, while species abundance indicates the quantity of each species. Together, they provide a good picture of the ecological health and stability of an ecosystem, showing both species diversity and distribution.
Assessing the biodiversity footprint: MSA and PDF, two key indicators
To understand the impact of human activities on biodiversity, two main indicators are used: MSA (Mean Species Abundance) and PDF (Potentially Disappeared Fraction of species). These metrics are essential for assessing the state of biodiversity and the risks it faces.
🔹 an abundance indicator: MSA (Mean Species Abundance)
This measures the abundance of species in an ecosystem, comparing it with a natural, preserved state. This index ranges from 0% (severely altered biodiversity) to 100% (totally preserved biodiversity). It can be used to assess the impact of human activities by showing how they affect the abundance of species in a given environment.
🔹 a wealth indicator: the PDF (Potentially Extinct Species Fraction)
assesses the risk of species extinction due to disturbances such as pollution, deforestation or urban sprawl. This indicator estimates the probability that some of the species present in an ecosystem are threatened with extinction, depending on the intensity of the pressures exerted on the ecosystem.
In short, the MSA focuses on species abundance and the ecological health of an environment, while the PDF measures species richness, indicating the risk of species extinction due to human pressures. Together, these two indicators provide a good overview of current biodiversity challenges.
Figure 1 below shows a fictitious example of the evolution of species in an area.
Figure1 : fictitious case of an ecosystem composed of 7 species observed at T0 (reference state) and T1 (after disturbance). One species disappears completely, indicating the loss of a species (red cross). Most of the others are made up of fewer individuals (loss of abundance).
Table 1 below shows the resulting calculations for the two indicators: MSA and PDF.
Table1 : description of the concepts of abundance and species richness applied to Figure1 above.
Here, the loss of species is one in 7, or 12%. Abundance at T1 is 60% of abundance at T0, corresponding to an MSA of 60%.
The loss of abundance is therefore 40%, while the loss of species is 12%. The MSA abundance indicator is therefore earlier than the PDF indicator.
/ !\ Please note, however, that the MSA represents species that remain, while the PDF represents species that have disappeared.
PDF: a multi-scale measure of the biodiversity footprint
PDF (Potentially Disappeared Fraction of species) is a key indicator for measuring the impact of human activities on biodiversity. It is assessed at three levels: local, eco-regional and global
- At local level, it quantifies the loss of species on a specific site, for example after deforestation or an industrial project. This unit is related to the surface area covered. Example: disappearance of a beetle over a distance of 1000 m2 during road construction.
- At the scale of the ecoregion (an area of ecologically similar ecosystems), the PDF helps assess the cumulative impact of different human pressures, such as intensive agriculture or urban sprawl. The calculation is independent of the ecoregion’s surface area. Example: disappearance of the Pyrenean bear, later reintroduced with bears from Slovenia.
- On a global scale, it estimates the proportion of species threatened by global factors such as climate change and habitat destruction. The calculation is independent of the ecoregion’s surface area.For example, the Dodo disappeared from the Indian Ocean in the mid-19s.
The table below describes these three levels.
Table 2 : description of the three PDF scales, and illustration of what each means for a 3 m2 parking facility.
Thanks to this multi-level approach, the PDF offers a comprehensive view of the risks to biodiversity, enabling companies and decision-makers to take more targeted action to limit their environmental footprint
See below the fictitious case of a world made up of two eco-regions, with variations in eco-regional and global biodiversity.
Figure 2 : fictitious case of a world composed of two ecoregions and containing only three species. Illustration of the difference between eco-regional species loss between two dates T0 and T1, expressed in PDF(reg), and the corresponding result in global species loss expressed in PDFglo.
MSA and PDF: two complementary indicators
Mean Species Abundance (MSA) and PDF, or “Potentially Disappeared Fraction of species”, are two complementary indicators of biodiversity, as they assess its transformations from distinct angles. Both focus on species, but the MSA is an early indicator, used at the initial stage of ecosystem degradation, while the PDF reflects a more advanced stage, when species have already (locally) disappeared. The MSA measures the average abundance of species in relation to an undisturbed reference state, thus signalling the first signs of pressure on biodiversity. In contrast, the PDF estimates the fraction of species that have disappeared as a result of human activities, highlighting a more irreversible loss. Together, they provide a more comprehensive assessment of changes in biodiversity at different scales and timescales.
The table below summarizes what we’ve just seen.
| Criteria | MSA (local) | Local PDF | Ecoregional PDF | Global PDF |
| Type of indicator | Measures the ecological status of a site via average species abundance | Measures the fraction of species potentially lost from a given site | Estimation of cumulative species losses over an entire region | Summary of species losses worldwide |
| Spatial scale | Local | Local | Entire ecological region | Worldwide |
| Pressures assessed | Local changes (urbanization, agriculture, pollution) | Idem, but measuring the potential disappearance of species | Cumulative pressures on a region (widespread deforestation, regional climate change) | Global pressures (climate change, habitat loss, global pollution) |
| Results | Score between 0 and 1 (0 = biodiversity destroyed, 1 = intact) | Score between 0 and 1 (0 = no species loss, 1 = total extinction) | Average score for local PDFs in an ecoregion | Aggregating regional PDFs for a global view |
| Temporality | Short to medium term | Short to medium term | Medium to long term | Long term |
We can see that the indicators provide different information on biodiversity.
- MSA and local PDF are local measures:
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- The MSA measures the general state of biodiversity on a site, based on the abundance of remaining species.
- The local PDF focuses on potential species losses in the same site.
- A high MSA does not necessarily mean a low local PDF (for example, an ecosystem may be in good condition overall but have lost one or more key species).
- Ecoregional and global PDFs are broader aggregations:
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- The ecoregional PDF is a sum of the local PDFs, taking into account all the sites in the same ecological region. It incorporates cumulative effects, such as habitat fragmentation.
- The global PDF summarizes all global losses, including effects that the local PDF does not capture (e.g. species migration, climate-related losses).
Conclusion
Thus, the joint use of these indicators is useful for understanding local and global biodiversity dynamics and avoiding the biases associated with an isolated analysis of a site or region.
- The MSA and local PDF are local indicators, but they measure different aspects (general condition vs. potential species loss).
- The Ecoregional PDF and Global PDF broaden the scale of analysis, providing a more comprehensive view of cumulative losses at regional and global levels.
However, current metrics are used in isolation. The MSA is mainly used for “corporate” footprints, at the level of companies’ CSR commitments. It is the metric for CDC Biodiversity’s Global Biodiversity Score (GBS). It is also the metric used for the Corporate Biodiversity Footprint (CBF) by Iceberg Data Lab and I-Care by Bearing Point.
The PDF, on the other hand, is favored in the “product” and consumer information footprints. There are links between the different levels of PDF: local, regional and global.
The research world is currently working on the links between MSA and PDF, but these are not trivial. More news soon!
How can Sayari help you ?
At Sayari, we know what each metric does and doesn’t measure, so we can advise you on the best metric for the question you’re asking. We can help you make the links between the different levels.
More generally, our design office regularly works on biodiversity footprints for companies or products. We accompany you through several stages:
- State of the art
- Dual materiality analysis
- Prioritizing actions
- Defining objectives and trajectory
- Identifying monitoring indicators
With this approach, we help companies to reduce their environmental impact in a targeted and effective way.
Member of the European PEF (Product Environmental Footprint) Technical Advisory Board
Member of the French ADEME environmental labelling working group
