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Note: Jose Montoya is available for comment in Spanish, French or English at +33 (0) 5 61 04 03 60 or josemaria.montoyateran@sete.cnrs.fr. Ian Donohue is available for comment in English at +353 1 8961356 or donohui@tcd.ie. Stuart Pimm is available for comment in English at (+1) 646 489 5481 or stuartpimm@me.com.

DURHAM N.C. – The notion that human impacts on biodiversity are fine so long as we keep them within “planetary boundaries” – the thresholds at which irrevocable and widespread damage will occur – has gained adherents in recent years. 

But while it’s a seductive theory, it is deeply flawed scientifically and – worse still – it encourages harmful policies, three of the world’s leading ecologists argue in a peer-reviewed commentary published this month in Trends in Ecology & Evolution.

In their commentary, the scientists propose an alternative approach, based on increasing informed insights into the connections between biodiversity and how ecosystems work.

“A critical question is how should we manage human actions that harm biodiversity,” said Stuart L. Pimm, Doris 51���� Professor of Conservation Ecology at 51����’s Nicholas School of the Environment. “Do we really want to operate under the assumption – as the notion of a planetary boundary for biodiversity purports – that humans can go about business as usual so long as the impacts of our actions remain within some arbitrary ‘safe operating space’?”
 
The concept of planetary boundaries is based on the idea that there are at least nine global processes that regulate the resilience and stability of Earth, and that human actions since the Industrial Revolution have become the main driver of change in each of them. These processes include climate change; land use; freshwater use; ocean acidification; nitrogen deposition; phosphorus deposition; ozone depletion; chemical pollution; atmospheric aerosol loading; and biodiversity loss.

While this part of the notion is “clearly true,” Pimm said, some scientists take it further and argue that when the impacts of human activities exceed a system’s boundary, or tipping point, there will be a precipitous and irreversible environmental change that could undermine Earth’s functioning and, ultimately, human survival.

“That idea is certainly not true,” Pimm said, “at least with the global scope and ubiquity its originators propose. The Floreana mockingbird, for example, now numbers fewer than 150 individuals. Its extinction would be a tragedy not least because it was one of four species of mockingbird in the Galapagos that gave Charles Darwin his first clues about evolution.  But were it to become extinct, it is not clear how this unfortunate event would lead to massive ecosystem collapse on the far side of the planet.”

This example underscores why including biodiversity loss in the planetary boundaries framework is meaningless and counterproductive, Pimm and his colleagues argue.

“The notion of a planetary boundary for biodiversity adds no insight into our understanding of the threats to biodiversity and how ecosystems function, has no empirical evidence to support it, is too vague for use by those who manage biodiversity, and can promote harmful policies,” said Jose M. Montoya, senior scientist at the French National Centre for Scientific Research.  Beyond that, what parameters should be used to set the planetary threshold of acceptable losses?

“The original idea was to use extinction rates. Certainly, there would be local consequences of species loss, but why a precipitous and global collapse of ecosystems?” Montoya said. “Neither theory nor empirical data support any threshold of biodiversity below which ecosystem function is compromised. Confronted with these fatal flaws to the original idea, there has been a proliferation of new indices. They add no useful insight. Even if we were able to estimate the necessary numbers, the definition of the threshold is entirely arbitrary.”

In addition to this scientific ambiguity, “there are acute moral hazards associated with this flawed worldview,” said Ian Donohue, associate professor of ecology at Trinity College Dublin. “Because there is no operational definition of what our ‘safe operating space’ is, policymakers and resource managers may assume that species and their habitats can recover from just about any human activity or development within limits,” he said. “Worse still, if we suggest a catastrophe has happened and the consequences are not evident, then how will managers and policy makers trust the science we do? If the planet isn’t obviously collapsing around us, then surely, we can continue to deplete it!”

The new commentary’s authors contend that a more sensible and practicable approach to setting these sort of boundaries is to focus on how biodiversity loss affects key aspects of Earth’s natural ecosystems.  

“There is mounting evidence that biodiversity loss alters the provision of functions and the stability of ecosystems upon which many species, including humans, depend,” Pimm said. “We should focus on this, and not some arbitrary and ill-defined boundary, to determine the effectiveness of environmental policies. The focus must be on appropriate scales and variables we can measure operationally and that tie to pressing practical problems. For instance, how can the functioning of ecosystems and their associated services to humans persist in the face of climatic change, particularly when local extinctions reduce the resistance of ecosystem productivity to climate extremes? This is an illustrative example of how ecosystem change is gradual, and inextricably tied 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. Merely 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,” Montoya said.

CITATION: “Planetary Boundaries for Biodiversity: Implausible Science, Pernicious Policies”, Jose M. Montoya, Ian Donohue, and Stuart L. Pimm, Trends in Ecology and Evolution, online publication: 8-NOV-2017, DOI: 10.1016/j.tree.2017.10.004.

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