exchange between Thomas Lepeltier and Tatiana Giraud, Causeur, from 27 June to 22 September 2026
A critique of the way the biologist Tatiana Giraud, a research director at the CNRS and member of the Académie des sciences, presents biodiversity in the media, her reply, then a rejoinder: a reasoned public exchange, now a rare thing.
In brief
- 27 June 2026: for Thomas Lepeltier, Tatiana Giraud turns biodiversity into an absolute value. In his view, not all biodiversity is desirable, no past state can serve as a norm, and the decline of living things must be set against the benefits of human transformations.
- He also challenges the word ‘collapse’ and the image of an aeroplane’s rivets: studies describe declines and reconfigurations, not the disappearance of life.
- 16 September 2026: in her right of reply, Tatiana Giraud points out that her work rests on dated data series, that she presents biodiversity as dynamic, with no ‘golden age’, and that she discusses costs and benefits; ‘collapse’ refers to declines of 75% to 90% for certain groups.
- 22 September 2026: Thomas Lepeltier welcomes this reply and agrees with her on several points, but maintains that the disagreement concerns the interpretation of the data, not the data themselves.
1. The critique: a sacralised biodiversity?
Thomas Lepeltier, Causeur, 27 June 2026 · Read the article ↗
Starting point: an appearance on LCI on 24 May 2026, in which Tatiana Giraud laments that humanity is the only species to poison its own food. The author sees this as overlooking a major fact: humanity has never been so well fed or in such good health, largely thanks to agricultural progress.
At bottom, biodiversity simply means the diversity of living things, which varies from place to place and from one era to another: we need a certain level of diversity, but nobody wants lice, bed bugs or rats. There is no ideal state to refer to: why should the biodiversity of the year 1000, a time of famine, be preferable to that of today? Farming, building or draining marshes reduces certain forms of life, but it is also what allowed us to escape famine and to live longer. Declines in biodiversity must therefore be judged in terms of costs and benefits.
The word ‘collapse’ suggests a generalised disappearance of living things, whereas the studies describe species extinctions, sometimes drastic declines in populations, and ecosystems that reorganise themselves. As for pesticides, plants produce them themselves (nicotine, caffeine, pyrethrum), and synthetic ones, though not all are harmless, have contributed to the decline of undernourishment. Finally, the author criticises the opposition between natural and artificial that underpins the preference for organic food and the rejection of GMOs.
‘not all biodiversity is equally desirable’Thomas Lepeltier, Causeur, 27 June 2026
2. The reply: measured declines, not a belief
Tatiana Giraud, right of reply, Causeur, 16 September 2026 · Read the reply ↗
Tatiana Giraud disputes the portrayal of her approach as ‘quasi-religious’. Her research, books and teaching, notably at the Collège de France, rest on the scientific literature and on empirical data; in them she presents biodiversity as dynamic and evolving, not as an ideal state to be preserved unchanged.
She refers to no ‘golden age’: the declines she presents come from dated data series, such as monitoring of vertebrate populations since 1970 or measurements of flying insect biomass between 1989 and 2016. These dates are the starting points of the observations, not ideal periods. She also discusses the costs and benefits of preserving biodiversity and of supporting farmers, for example in the Chaleur humaine podcast from Le Monde (April 2026).
She does not deny the need to feed populations: discussing the effects of certain pesticides is not condemning agriculture, which itself depends on the services provided by biodiversity. And the word ‘collapse’ refers to quantified declines, reaching, for some of the groups studied, 75% to 90% or more within a few decades. She accepts criticism, but asks that it be distinguished from her actual positions.
‘One may debate the choice of this term; but one cannot present its use as independent of the quantitative data to which it refers.’Tatiana Giraud, Causeur, 16 September 2026
3. The rejoinder: a disagreement of interpretation
Thomas Lepeltier, Causeur, 22 September 2026 · Read the article ↗
The author welcomes her approach, which has become rare, and thanks Tatiana Giraud for replying. He agrees with her on several points: she does present biodiversity as dynamic, the expression ‘golden age’ was his own, and his statement that she would ‘never’ reason in terms of costs and benefits was not precise enough. He never disputed the data she cites.
But data establish a trend; they do not say which level of biodiversity is preferable. And the cost-benefit analysis she invokes concerns the cost of protecting or restoring, not the benefits of the transformations that cause biodiversity to decline: clearing a forest produces food, and draining marshes made it possible to gain land and to limit certain diseases. Nor does the fact that an alternative to pesticides is effective mean that it replaces them everywhere, at no cost and with equal yields.
On ‘collapse’, he mainly targets the image of the rivets, which announces a tipping point: in his view, the literature describes ecosystems that change state, sometimes profoundly, not ones that collapse. His conclusion: one may prefer biodiversity to human development, or the reverse, but this must be stated openly, without presenting that hierarchy of values as a scientific result.
‘Our disagreement is therefore not about the scientific data, but about how to interpret them.’Thomas Lepeltier, Causeur, 22 September 2026
Why this debate interests us
It is rare for a publicly criticised figure to agree to enter the debate; Thomas Lepeltier points this out himself. This exchange shows what APALA stands for: discussing theses and public positions, not individuals, and distinguishing facts from their interpretation. From one text to the next, the disagreement narrows to two questions of values: the reference state, and the weighing of losses and benefits. On the substance, APALA shares Thomas Lepeltier’s position, which we have defended in ‘Il n’y a pas de crise de la biodiversité !’.
Questions for debate
- How should we choose the reference state against which to judge changes in biodiversity?
- Should a cost-benefit analysis count the benefits of transformations that reduce biodiversity?
- Does the word ‘collapse’ help us understand measured declines, or does it dramatise them?
- When a researcher speaks in the media, how can we distinguish scientific findings from value judgements?
To go further on apala.fr
Summary written by Claude (Opus 5.5 model), an artificial intelligence, for APALA, based on the three texts published on the Causeur website on 27 June, 16 September and 22 September 2026. The positions expressed are those of their authors. An error? Write to us.
Read the critique on Causeur ↗Read Tatiana Giraud’s reply ↗Read the rejoinder on Causeur ↗
0 comments