The impossible quest for a biodiversity metric to rule them all

Will a new metric save biodiversity? This recent opinion piece in Science magazine argues for just that: “a single, simple indicator”, an annual biodiversity target based on species extinction rate. The idea is that the metric will encourage people and policy to protect nature.

The authors justify their argument based on two assumptions:

1) extinction fully incorporates the most fundamental aspect of biodiversity loss

2) extinction is widely understood and easy to communicate

This all sounds well and good. But…

1. How exactly does extinction fundamentally represent biodiversity loss?

Extinction is pretty hard to measure in the first place. And the meaning of biodiversity is elusive. It is one of those terms that is recognised by most people, but also undefinable by most people. There is no single scientific standard to measure biodiversity, because it depends on so many things.

There are four main aspects to biodiversity, all of which are equally important but hard to measure (as an aside, most textbooks will only tell you about the first three, because there is no standard across scientific disciplines).

  • Species diversity: there are more than 1 million described species on Earth (and many more still to be discovered) and all of them are part of biodiversity. But what is a species anyway? There are over 26 proposed species concepts and still no agreed way to define a species…especially if you work with insects! However, this aspect of biodiversity is a lot easier to measure than the next three, which is probably why most of our conservation policy and planning is focused on ‘species’ rather than ecosystems.
  • Genetic diversity: populations of species need different genomes to maintain resilience as environmental conditions change.
  • Ecosystem diversity: we need different types of ecosystems for nature to do her thing. Mangroves to clean water and buffer storm damage, peatlands to store carbon, grasslands to support wildlife etc.
  • Interaction diversity: ecosystems exist because of multiple interactions across time and space. Imagine an ecosystem that was literally just slow-growing plants and a single herbivore that ate them…it would very soon run out of juice!

So measuring biodiversity loss is complicated and not always associated with species extinction.

We can lose entire ecosystems from land clearing, meaning the local environment will suffer from loss of ecosystem function and loss of ecosystem services…but the species that lived in that patch may still exist somewhere else. This doesn’t make the removal of that particular ecosystem okay.

Biodiversity loss is fundamentally so much more than just counting species.

2. Is extinction widely understood and easy to communicate?

Extinction literally means the death of the last individual of a species. If everyone always definitely knows when that death occurs, then sure…you could say that extinction is widely understood.

In reality, this is rarely the case. The only time we can be sure that the last individual of a species has died is when we’re talking about charismatic species that we are watching so closely that we know for certain how many individuals are left in the world. This is usually vertebrates, which make up about 6% of global species diversity. And this is usually because the last few individuals are living in captivity, or because the last known population is being documented constantly by funded researchers or local expert naturalists.

The vast majority of species are not being observed that closely, or have not been observed at all. For most invertebrates, we don’t even know their names, let alone where they live and how many are left on earth. An invertebrate could go extinct tomorrow and we would never know!

Some species have been claimed to be extinct, only to be found again many decades later. The Lord Howe Island stick insect is a classic example. This is not a miracle, it’s just a sign of how little we know about the world.

In most cases, when we talk about extinction, we talk about the criteria defined by the IUCN Red List, which is the best available collation of evidence on global biodiversity. It’s important to note that the criteria to define extinction is almost impossible to meet for the majority of species.

“A taxon is Extinct when there is no reasonable doubt that the last individual has died. A taxon is presumed Extinct when exhaustive surveys in known and/or expected habitat, at appropriate times (diurnal, seasonal, annual), throughout its historic range have failed to record an individual. Surveys should be over a time frame appropriate to the taxon’s life cycle and life form.”

IUCN Red List Categories and Criteria

So I would argue that most people do not understand these nuances about how extinction is defined. And it goes without saying that nuance is not “easy to communicate”.


Governments, corporations and individuals (every single one of us) must take immediate action to stop biodiversity loss. This is a fact, and is backed up by decades of evidence.

But there is no “single, simple indicator” that will inspire people and governments to take the necessary responsibility to reduce extinction and biodiversity loss.

And if there was, extinction rate is definitely not the best approach. There are many more effective concepts and ecological metrics that can drive change and inform effective policy, like ecosystem services. Sadly, the term has been misused and misrepresented so much that many people avoid it purely because of the confusion surrounding it. It is fundamentally an indicator for how we impact nature, and how we can change those impacts. It provides the tools we need to value and communicate the importance of biodiversity, and it provides a pathway to effective environmental policy.

We just need to engage more with science and invest more in understanding and protecting the environment.

© Manu Saunders 2020

5 thoughts on “The impossible quest for a biodiversity metric to rule them all

  1. Walter Mansfield July 22, 2020 / 8:41 PM

    What’s your thoughts on the Australian Farm Biodiversity Certification Trial (due 2021)? I imagine it’ll be a difficult framework to develop due to the complexity of ecosystems across Australia’s landscape and, as you state, the metrics are difficult to standardise – the easier option may be to ignore the metrics for now and follow the ‘organic’ certification route by prescribing practises that have to be adhered by and then let research studies show if it addresses biodiversity loss over time. However, research states that landscape heterogeneity can be the most important factor and a whole-of-landscape approach is needed – maybe the NFF should offer incentives on whether you can sign up a neighbouring farmer! ‘Refer a Farm’


    • Manu Saunders July 23, 2020 / 8:01 AM

      It will be interesting to see how the scheme works. Landscape approaches definitely more effective for addressing loss of biodiversity & ecosystem services


  2. ecoquant July 24, 2020 / 12:21 AM

    I do not know much about this, but diversity indices have been offered in other fields, e.g., econometrics, and they are used in algorithms work, e.g., when building random trees. There’s the famous Gini index, but it’s been overshadowed if not supplanted, if I understand the literature, by the Lorenz curve and ratios of quantiles. See:

    Yves TILLE´ and Matti LANGEL (2012), The American Statistician, November.

    Merritt LYON, Li C. CHEUNG, and Joseph L. GASTWIRTH (2016), The American Statistician, February.

    Luke A. Prendergast and Robert G. Staudte (2018), The American Statistician, 72(4), 328-343.


  3. Anarchanthropus crapuloideus July 25, 2020 / 6:25 PM

    Thanks for that. I have selected it for discussion in my Biological Conservation course next year. Thank you, in general, for your inspiring and motivating texts.

    Liked by 3 people

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