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Tell me please about this murky concept of Potentially Disappeared Fraction (PDF). First, why is it written as "PDF.m2.yr" instead of PDF/m2*yr. Second, what is it, how to interpret it? How can it be in hundreds of billions [p. 24]? Does it refer to species loss (no certain species anymore) or to a reduction of their number (all species are in place, but there are fewer of them)? And what does it mean, "potentially"? So they may be fine after all?

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All the references I can find to this end up pointing at the 1998 book "Land use—the main threat to species. How to include land use in LCA" by Müller-Wenk, which I have been unable to access. Here is how it is described by Kok et al. in the paper "Balancing biodiversity and agriculture: Conservation scenarios for the Dutch dairy sector":

The impact of the different scenarios on biodiversity was assessed using the indicator potentially disappeared fraction (PDF) (De Schryver et al., 2010), in an attempt to compare the relative biodiversity value. The PDF expresses the species richness of vascular plants under a specific land use compared with a reference situation of semi-natural woodland (Knudsen et al., 2017). The mean impact on biodiversity across land use types in each scenario was expressed as mean PDF (per m2), calculated as:

PDF Formula

with CF being a characterisation factor expressing PDF of plant species richness, and A the area occupied (in m2) by land use i.

And by An M. De Schryver et al. in the paper "Uncertainties in the application of the species area relationship for characterisation factors of land occupation in life cycle assessment"

For ecosystem damage, Muller-Wenk (1998) proposes the potentially disappeared fraction (PDF) of species as endpoint indicator. This indicator measures the change in species diversity and is integrated over a certain time and area presented by the life cycle inventory.

So it is a fraction, with the number of species found in the test plot compared to those found in the reference plot (such as "semi-natural woodland"). One would be complete loss of all species, zero would be no loss at all. It is multiplied by land area and time. So a 10% loss over 1 square mile would be equal to a 1% loss over 10 square miles. It is not clear to me what potential refers to, I think it is that this is compared to a reference situation that is by definition potentially how this area could have been managed. Since the unit is square meters, and there are 148,940,000,000,000 square meters of land area on earth hundreds of billions is not that much. For example, a 1% PDF over the whole world in 1 year would be 1,489.4 billion PDF.m2.yr. It does refer to species loss, rather than a count of individuals.

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  • But how do you know what portion of species may disappear? Commented Aug 2, 2021 at 12:08
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    @SergeyZolotarev It is the ratio of species found in the test plot compared to the reference plot. So if they found 100 in semi-natural woodland and 50 in the test plot that would be a PDF of 0.5. I have tried to make this more clear in the answer.
    – Dave
    Commented Aug 2, 2021 at 13:48
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I found a very intuitive explanation in the ppt of 'EU B@B Platform – Webinar series “Measuring biodiversity for business and finance”' (in general a good overview for LCIA) (The link to the ppt: https://ec.europa.eu/environment/biodiversity/business/assets/pdf/EU_B@B_Webinar_2_Presentation_Slides.pdf)

PDF = Potentially Disappeared Fraction of species 10 PDF.m2.yr, can be interpreted as:

  • 10 m2 has lost all its species during a year
  • 100 m2 has lost 10% of its species during a year
  • 10 m2 has lost 10% of its species during 10 years

We only know the combined effect

Moreover:

PDF accounts for a fraction of species richness that may be potentially lost due to an environmental pressure (land use, ecotoxicity, climate change, eutrophication). The underlying environmental mechanism depends on the pressure being assessed.

  • Potential number of species in natural state: 4
  • Potential number of species in the modified state: 2
  • Potential disappeared fraction of species: 2/4 = 0.5

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