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The concept of planetary boundaries - or point of no return - for biodiversity states that global environmental processes typically have "tipping points" or thresholds. When these thresholds are crossed, a major shift in the functioning of ecosystems towards new states occurs, generally towards a state much less favorable for humanity.

This notion has been used to argue that there is no need to change anything as long as one remains below such thresholds, as in a "safe zone". The authors stress that this notion, which is used heavily to draw attention to global environmental problems, has in fact dangerous perverse effects:

« The problem is that the notion of planetary boundaries for biodiversity adds no insight into our understanding of the threats to biodiversity and ecosystem functioning, has no evidence to support them, is too vague for use by those who manage biodiversity, and can promote harmful policies » said Jose M. Montoya, from the French National Centre for Scientific Research, at the Theoretical and Experimental Ecology Station in Moulis.

An attractive approach but with limitations

The initial idea of the concept of a point of no return for biodiversity was based on studies of species extinction rates. However, neither theory nor empirical evidence supports the existence of a biodiversity threshold below which ecosystem function would be compromised. The authors take the example of the Floreana Mockingbird, a bird of which there are now less than 150 individuals. His extinction would be a tragedy, especially because it was one of four species of mocking birds on the Galapagos that gave Charles Darwin his first tracks on evolution. However, if this specie were to disappear, it is not clear how this unfortunate event would lead to the massive collapse of the ecosystem on the other side of the planet. The loss of species undoubtedly has local consequences, but why would it produce a collapse of ecosystems?

According to José Montoya and his collaborators, the many shortcomings of the concept of a point of no return triggered a proliferation of new indices meant to support it instead of leading to its questioning. However, the authors point out, these indices do not provide any useful information so that the definition of a limit threshold for biodiversity would remain entirely arbitrary, even if we were able to estimate the necessary data.

Is gradual degradation visible?

Although the concept of planetary boundary does not add anything new to what we know about the global impacts of human activities, one could imagine that its practical utility for environmental managers would justify its use. According to Ian Donohue, there are acute moral hazards associated to this flawed worldview, with some saying that « if global biodiversity does not collapse in an obvious way for us, then we can deduce that we can continue to exploit it intensively without worrying more. »

According to the authors, the use by politicians of such erroneous "scientific" conclusions also strongly penalizes the future credibility of science itself. If we suggest that a disaster has occurred while the consequences are not obvious, how will managers and policymakers trust the advice of scientists? How can environmental sciences adequately inform those who manage and define policies about something as complex as nature?

Taking into account the gradation of ecosystem changes

Stuart, Ian and José believe that the answer is to determine how the loss of biodiversity affects the different aspects of ecosystem change. There is growing evidence that biodiversity loss affects the functions and stability of ecosystems. It is possible to assess and monitor how the loss of biodiversity affects different ecosystems, and hence to assess the effectiveness of a given environmental policy. « The focus must be on appropriate scales and variables that we can measure effectively and that are linked to pressing practical problems » says Stuart L. Pimm, co-author of the study, whose joint work with José Montoya was facilitated in 2016-2017 by a "Visiting Scientist" funding from our LabEx.

For example, what is needed is to better understand how the functioning of ecosystems and the associated services that we benefit from can be maintained in a context of climate change where local extinctions increase the sensitivity of the productivity of ecosystems confronted with climatic extremes? This kind of question illustrates how ecosystem changes are gradual and inextricably linked to biodiversity loss.

« Good policy means we have no option but to understand the necessary complexity of nature in the environments we are starting to unravel. But acknowledging such complexities is not enough. We need the particulars: what aspects of ecosystem change we aim to minimize. Which species are vital to which processes and how these connect to human social and economic systems », concluded Jose M. Montoya. In other words, we first need to understand the mechanisms ruling a system to become able to effectively act on it.