- Biodiversity has been defined as one of nine planetary boundaries that help regulate the planet’s operating system. But humanity is crossing those boundaries, threatening life on Earth. The big question: Where precisely is the threshold of environmental change that biodiversity can withstand before it is destabilized and collapses planetwide?
- The planetary boundary for biodiversity loss was initially measured by extinction rates, but this, as well as other measurements, have proved to be insufficient in determining a global threshold for biodiversity loss. At present, a worldwide threshold for biodiversity loss — or biosphere integrity, as it is known now — remains undetermined.
- However, thresholds for biodiversity loss can be clearly defined at local or regional levels when an ecosystem goes through a regime shift, abruptly changing from one stable state to another, resulting in drastic changes to biodiversity in the changed ecosystem.
- While the planetary boundary framework provides one way of understanding biodiversity or biosphere integrity loss, there are many other measures of biodiversity loss — and all point toward the fact that we are continuing to dangerously destabilize life on Earth.
Biodiversity. When you hear this word, what do you picture? Iconic animals like African elephants, gray wolves and humpback whales? Or multicolored coral species that make up a reef system? Or bacteria and microbes that regulate nutrients in the soil, or oxygen-releasing phytoplankton that live in the ocean’s sunlit zones?
While biodiversity does embrace all these living things, the concept extends beyond mere species diversity or abundance. It also encompasses the infinite variety of genes that allow animals and plants to adapt and survive, as well as the diversity of planetary ecosystems, and the different functions that organisms and ecosystems play in our intricately connected world.
In short, biodiversity is the living web of species and ecosystems that form the basis of life on Earth. Humanity, of course, is part of biodiversity, but it is also a driver of biodiversity loss. Homo sapiens can negatively impact species and ecosystems through a multitude of actions, but we are also dependent on biodiversity for food, energy, medicine, economic security, and our overall well-being.
With biodiversity embracing such a wide breadth of organisms, ecosystems and genes, it can be challenging to understand the full extent of humanity’s impact on it, especially when one considers the fact that we only know about 20% of Earth’s species. There are, however, plenty of indicators to suggest that human activities are placing exceptional pressure on biodiversity.
For instance, the Intergovernmental Science-Policy Platform on Biodiversity and Ecosystem Services (IPBES) estimates that about 77% of the land and 87% of the ocean have been altered by humans, which has led to a loss of 83% of wild mammal biomass, and half of the world’s plant biomass. The IPBES also suggests that more than a million plant and animal species are currently threatened with extinction, potentially putting us on a path to what has been dubbed Earth’s sixth mass extinction.
The question bedeviling scientists: Will there come a point when humanity will have pushed biodiversity past a point of being able to recover, imperiling the very stability of Earth’s operating system and putting humanity and possibly all life as we know it at risk?
Researchers have been grappling with this question since the development of the planetary boundary framework, a theory that argues Earth has nine biophysical subsystems or processes with clear limits beyond which they cannot withstand anthropogenic pressure. If humanity stays within the “safe operating space” of these boundaries, life can thrive, the theory suggests. If the thresholds are crossed, humanity can push Earth into a new, dangerous state that isn’t as accommodating to life as we know it.
While there have been attempts to define a global threshold for the biodiversity planetary boundary, no conclusions have been drawn. One thing researchers do tend to agree upon, though, is that it is incredibly hard to measure and define biodiversity loss, particularly on a global scale. But that isn’t to say biodiversity isn’t in trouble; there are plenty of ways to see that humanity is placing extreme pressure on biodiversity, and in the process, imperiling the integrity of our world.
A global threshold for biodiversity loss?
In 2009, Johan Rockström of the Stockholm Resilience Centre, along with an international team of researchers, published a paper that introduced the concept of planetary boundaries. The theory suggests that there are nine Earth system processes that help regulate the planet, but that these processes have limits beyond which they cannot withstand environmental change. Originally, these were described as climate change, the rate of biodiversity loss, interference with the nitrogen and phosphorus cycles, ozone depletion, ocean acidification, global freshwater use, land use changes, chemical and other pollution, and atmospheric aerosol loading.
Some of the boundaries were shown to have clear global thresholds. For instance, the planetary boundary for climate change is primarily set by the atmospheric concentration of 350 parts per million by volume of carbon dioxide. Since this limit was already exceeded in the late 1980s, researchers say we have entered a danger zone.
Biodiversity loss within the planetary boundary framework was initially defined by extinction rates, which measured the number of species per million species that went extinct each year. While fossil records show that extinctions happen naturally, current extinction rates are estimated to be 100 to 1,000 times higher today than what is considered natural. This rate is even expected to increase tenfold over the course of the century.
The researchers used this metric to suggest that humanity has transgressed the biodiversity boundary and entered a “danger zone.” Yet, they acknowledge that using extinctions as a measure is imperfect since it is “very difficult” to identify a boundary for biodiversity loss that would push the Earth into a new state at both regional and global levels.
“Our primary reason for including biological diversity as a planetary boundary is its role in providing ecological functions that support biophysical sub-systems of the Earth, and thus provide the underlying resilience of other planetary boundaries,” the authors write. “However, our assessment is that science is, as yet, unable to provide a boundary measure that captures, at an aggregate level, the regulating role of biodiversity.”
Belinda Reyers, a professor of sustainability science at the University of Pretoria, South Africa, and a senior adviser at the Stockholm Resilience Centre, agrees that extinction rates are a poor indicator of global biodiversity loss for several reasons. For one, Reyers notes that biodiversity loss tends to focus on vertebrate species, which account for less than 2% of all described species. Second, extinction rates don’t account for important factors like differences in abundance and distribution between species, which affect ecosystems in complex ways. Third, extinction rates tend to be estimated long after extinctions occur, which means they can’t be used to forecast our approach to a critical threshold.
Then there’s the problem that extinctions have “little influence on ecosystem functioning at scales relevant to the safe operating space,” Reyers told Mongabay. For example, the loss of one salamander species may not have a huge impact on an ecosystem if the species had a small population and range, and if other species continue to fulfill similar functions within a natural community.
“Species are a really important aspect of biodiversity — they’re things that people care about, [and] species extinction rates are very high and very bad,” Reyers said. “But there’s no strong relationship between species extinction rates and the function of the biosphere.”
‘Building blocks of life’
In a 2014 paper, Reyers and fellow researchers argue that biodiversity loss could be more accurately measured by three concepts: genetic diversity, functional diversity, and biome integrity. When the planetary boundary framework was refined in a 2015 paper co-authored by Reyers, the biodiversity boundary was reframed as “biosphere integrity” to embody all of the world’s ecosystems, organisms and their relationships to each other. Biosphere integrity was also divided into two components, defining it as genetic diversity and functional diversity.
Genetic diversity, in the simplest sense, is the variation of genes found in life-forms. It’s the total number of genes, but also how they are different from one another. As Reyers puts it, genetic diversity is the “building blocks of life.”
“The more diversity you have, the more options you have,” Reyers said. “So for example, we used to eat 75 species of crops. We now mostly eat five species of crops, and those other 70 species are sort of disappearing over time … and that gives us less of a resource base with which to cope with climate change, because we don’t know whether some of those other 70 crops might have been better in a water-scarce or hotter environment.”
Functional diversity refers to the roles organisms play in an ecosystem. For instance, some species decompose waste. Others support plant generation by dispersing seeds. If species that perform certain functions are lost, there can be devastating ecosystem consequences, says Sarah Jones, a biodiversity expert and researcher on the Food and Land-Use Coalition’s (FOLU) Food, Agriculture, Biodiversity, Land-Use and Energy (FABLE) Consortium.
“If we lose all the soil microorganisms, for example, that are really improving the carbon in soil, this could have a catastrophic impact on plant diversity,” Jones told Mongabay. “So looking at the functional diversity, it’s a useful way to capture when things are going really downhill and we’re going to go off the edge of a cliff for certain groups of species that perform a role.”
The 2015 planetary boundaries paper suggests that functional diversity could be measured using the Biodiversity Intactness Index, an indicator that measures species abundance based on human pressures, while taking species functionality into account. However, the authors note that there isn’t a clear relationship between BII and Earth-system responses, so they only use it as an interim control variable for functional diversity.
As for genetic diversity, the authors continue to use extinction rates. Reyers says that while extinction rates do incorporate genetic diversity and therefore can act as a “decent enough substitute,” it is still not a great measure since it “misses all the genes that you lose even before you lose the whole species, and it misses all the genes that exist in species we don’t know anything about.”
“It assumes that if you lose an elephant, it has the same impact on genetic diversity as if you lose a wombat, [but] their genes are so different, and each plays such a different role,” she said.
In other words, a global threshold for biodiversity loss — or biosphere integrity, as it is now known — remains entirely uncertain.
“Whether or not we’ve crossed a threshold, we can’t say that,” Reyers said. “But whether we are losing too much biodiversity, we can definitely say [that we are]. If you look at the sub-global scale of biomes [and] look at the risk that we face in losing coral reefs, in losing other biomes like the Arctic tundra … it’s probably appropriate to say we’re very close to exceeding some boundaries that are going to be quite detrimental to the [Earth] system and our lives on it.”
‘It is not a gradual process’
While it is difficult, if not impossible, to identify a global threshold for biosphere integrity, many researchers suggest that biodiversity boundaries can be defined at local or regional scales through something called “regime shifts,” also known as “tipping points.”
A regime shift is an abrupt change that fundamentally alters the structure and function of an ecosystem, changing it from one state to another. In most cases, such shifts are irreversible.
“It is not a gradual process,” Ingo Fetzer, a researcher at the Stockholm Resilience Centre who studies how Earth systems interact, told Mongabay. “The system stays stable until you reach a certain tipping point, and then it rapidly shifts into something new.”
Human pressures on a coral reef offer a good example of a regime shift. A reef system can remain resilient after multiple climate change-driven bleaching events, regaining its life-sustaining algae after warmer-than-usual waters force the algae to be expelled. But the corals may reach a point where they can’t recover, and the ecosystem collapses, quickly turning a biodiverse reef into a disease-ridden system that only hosts a limited number of organisms.
Another example of a regime shift can occur in a freshwater lake. When its aquatic ecosystem becomes inundated with too much phosphorus from untreated sewage, the lake becomes eutrophied, with the lack of life-giving oxygen killing almost everything that lives there.
Garry Peterson, a professor of environmental sciences at Stockholm University, says that while plenty of research has been conducted on regime shifts, it’s still difficult to predict when one might occur, or what will drive an ecosystem to tip from one state to another. In most cases, regime shifts aren’t identified until after the process has occurred, he said.
“You can detect a regime shift a while after it’s happened because something big happens, and then it doesn’t go back,” Peterson told Mongabay. “Like when the Canadian cod [fishery] collapsed. They stopped fishing, the cod didn’t come back. You still stop fishing, the cod still isn’t back … something had changed.”
Ecosystems that undergo regime shifts usually cannot shift back to their original state, but those that are healthier and more resilient are more able to cope with change, he added.
How do regime shifts at local or regional levels translate to the entire world? Experts say there isn’t a straight answer. While localized shifts can aggregate in consequences at a global scale, what these consequences will be isn’t fully known. As a result, biodiversity thresholds crossed at local or regional scales have not necessarily brought us closer to determining a global threshold for biodiversity loss, Reyers said.
Despite all these uncertainties, researchers argue that biosphere integrity acts as a core boundary in the planetary boundary framework, providing capacity for the planet to adjust to changes that occur in other boundaries, such as elevated levels of ocean acidification, and the onslaught of plastic pollution and other man-made chemicals. But if the biosphere becomes too compromised through anthropogenic pressures, other boundaries will weaken as a result.
“If the biosphere boundary gets worse, the other boundaries get worse, too,” Reyers said. “If the biosphere boundary gets better, they get better too. In that way, [the biosphere] has a bit of a controlling role. It’s not a terribly strong relationship at a global level, but it gets really strong at local or biome levels.”
‘A radical proposal’
The challenges in defining a biosphere integrity threshold has led to some criticism of using the planetary boundary framework to measure biodiversity loss. For instance, Jose Montoya, research director at the French National Centre for Scientific Research (CNRS by its French acronym), told Mongabay that treating biodiversity loss as a planetary boundary is “fundamentally wrong” since most biodiversity metrics tend to look at local rather than global changes, and since most changes in biodiversity that happen at a local scale may not have global consequences.
“It makes no sense establishing a boundary beyond which the Earth system is doomed,” said Montoya, who co-authored a paper on this topic. “There is a lack of scientific basis to define a boundary for biodiversity at the global scale.”
Others, including those who’ve worked on the planetary boundary concept, acknowledge the uncertainties in using this theory to define biodiversity loss. However, many see the overarching theory as a powerful way for humanity to visualize our planet’s ecological health, and our capacity for pushing Earth’s stability in a positive or negative direction. Fetzer compares each boundary to a “clock that tells us how far we are away from a complete dysfunctioning Earth.”
Reyers calls the planetary boundary concept “a radical proposal” for thinking about biodiversity or biosphere integrity at a global level, but stresses that it offers just one approach to measuring biodiversity loss.
“It doesn’t do away with the fact that we have multiple other approaches for managing biodiversity and for setting targets at national levels and global levels,” she said. “And I think that’s where there’s a bit of tension because everyone thinks you’ve got to choose one or the other.”
Still another method for measuring global biodiversity loss is to calculate habitat degradation and destruction, which can transform ecosystems and extirpate species. A 2012 paper led by Reed Noss from the University of Central Florida, Orlando, argued that we need to protect at least 50% of land in order to conserve biodiversity, an idea built upon the work of other researchers including E.O. Wilson, author of Half Earth. Only about 16% of the world’s terrestrial and freshwater regions and 8% of the marine environment are currently protected, which, by this metric, puts biosphere integrity in a perilous position.
The Convention on Biological Diversity (CBD), the multilateral treaty responsible for conserving biodiversity and ensuring sustainable and equitable use of biodiversity, draws on the concept of protecting half of land and sea in the draft of its post-2020 global biodiversity framework. The framework, which was negotiated in Geneva in March, maps a route for “living in harmony with nature” by 2050. One proposed means for achieving this target is for countries to protect at least 30% of land and ocean by 2030, extended to 50% by 2050. Besides land and sea conservation, other global biodiversity protection aims include reductions in invasive species, pesticides, and incentives that harm biodiversity. (While negotiators agreed upon the draft text at the recent meetings, little progress has been made to halt the ongoing destruction of biodiversity.)
Conservation biologist Henrique Pereira, from the German Centre for Integrative Biodiversity Research (iDiv), told Mongabay that he favors using multiple metrics for calculating biodiversity change, such as those tracking alterations occurring in natural community composition, while treating extinction rates as a primary metric for biodiversity loss.
Pereira added that while extinction events that happen on a global scale can’t be reversed, we do have the ability to respond proactively to changes due to local or regional extinctions through rewilding efforts.
“Nature is really resilient,” he said, “and it will reinvent itself and it will come back.”
‘Very radical change’
Despite the many uncertainties surrounding biosphere integrity and the approaches to measuring it, experts agree that biodiversity loss is happening now at unacceptable rates.
But Peterson of the Stockholm Resilience Centre said he’s encouraged by collaborations happening between researchers as they try to measure biodiversity, and by the progress made in this field. “There’s a whole bunch of uncertainty about how you define the right level for [maintaining] biodiversity,” he said. “But the research on this is moving [forward] like anything, and I think we’re going to be in a really different place in five years, and a really different place in 10 years.”
Will it be possible to ever define a global threshold for biosphere integrity? That remains to be seen. But for now, indicators provide compelling evidence that we need to act quickly to protect what remains in our biosphere.
“Global economies are destroying the web life, ripping all these threads out,” Peterson said. “Has it passed some boundary or not? What does that even mean? There’s no question … we’re on a terrible trajectory with biodiversity, and the only way of dealing with this is very radical change.”
Banner image: A red-eyed tree frog in Brazil. Image by Rhett A. Butler/Mongabay.
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Elizabeth Claire Alberts is a staff writer for Mongabay. Follow her on Twitter @ECAlberts.
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