A major extinction is underway across most of the planet. From the Amazon forests to the suburbs of the American hinterland, from the depths of the oceans to the mangrove swamps of Southeast Asia, millions of species are being lost. And when species disappear, the populations of the remaining individuals also plummet. North America has lost more than a quarter of its birds since 1970.
And yet the situation is more complex than at first glance. Many local ecosystems—at least as measured by “species richness”—appear to be in good health, given the number of species they contain. The most comprehensive count of species counts in local ecosystems last month yielded two unexpected and controversial results.
First, the analysis of data from some 50,000 sites on land and in the oceans found no evidence of an overall decline in the number of species living in individual ecosystems. Local “species richness” persists, as some sites lose species while gaining them, the study showed. And second, species come and go from these study sites at a rate never before imagined. On average, study sites exchanged an astonishing 28 percent of their species every decade, many of which had an even higher turnover.
“Biodiversity is changing everywhere, but we’re not losing biodiversity everywhere. Some places are recovering and adapting,” says Maria Dornelas of the University of St. Andrews, one of 23 authors of the International Study, published in Science . “Our work doesn’t contradict the extinction crisis,” she emphasizes. “But it does show that what happens at one scale isn’t necessarily reflected at another.” She concludes that there are “winners and losers in the Anthropocene.”
Species numbers remain high locally while collapsing globally. Generalists take over—a process the study authors call “homogenization.”
Britas Eriksson of the University of Groningen and Helmut Hillebrand of the University of Oldenburg wrote a commentary in the Science of Fuel accompanying the paper, controversially arguing that “the global biodiversity crisis, at least for now, is not primarily about decline, but rather about extensive reorganization on a large scale.”
Some ecologists question the reliability of the results—since most of the data comes from functioning ecosystems rather than urban areas and farmland, where most species have been lost. Others fear that they may be used to denying the mass extinction.
But most addressed the findings for this article, while fearing that most of the losers in the “large-scale reorganization” are rare, endangered, and endemic species, while most of the winners are common, generalist, and invasive species—rats, mosquitoes, water hyacinth, and the like. This would explain the central puzzle raised by the results—that species numbers remain high locally and collapse globally. The generalists take over—a process the paper’s authors call “homogenization.”
A debate has been brewing among ecologists for some time about the true state of biodiversity at various spatial scales. It’s been more than two decades since researchers first pointed out that the dramatic global loss of species is not reflected in a few local ecosystems, many of which have more species than in the past. Such places range in size from small oceanic islands to North America, which have had many more plant species since the arrival of Europeans.
Researchers have tried to determine whether such sites are outliers or typical. And they have debated whether species richness—the number of species in a given site—is a useful measure of biodiversity. This implies that alien invaders have equal status to native species, when this may be far from the case.
The latest analysis, led by Shane Blowes of the German Centre for Integrative Biodiversity Research in Leipzig, uses a database called Biotime, hosted at St. Andrews University, of the world’s ecosystems.
Many of the largest changes have been seen in marine ecosystems, particularly in the tropics, where in some places species composition shifted by as much as 20 percent in a single year. Bubbles tells Yale Environment 360 that two of the largest fluctuation hotspots are in the western tropical Atlantic and off the coast of northwest Australia.
It’s likely that the rapid local species turnover is recent—likely due to human activities such as clearing forests, draining marshes, accentuating prairies, damming rivers, and altering the climate. But this isn’t a given, says Mark Vellend of the University of Sherbrooke in Quebec, a co-author of the new study. “These pristine ecosystems have always changed compositionally over time,” he says.
And while human activity clearly often harms local biodiversity, at certain times and places it can also drive local increases in species numbers—by creating new habitats, altering existing ones, or physically introducing new species.
“In North America, we have a greater variety of habitat types than before, such as agricultural areas, pastures, forests of different ages, roadsides,” says Vellend. “The result may be a greater diversity of species locally, as those that thrive in disturbed landscapes are [for publication] using forest inventory data spanning hundreds of years to support this.”
Some ecologists argue that the study ignored places like farms and cities where almost all biodiversity has been lost.
Ornithologists have found the same counterintuitive local trend, he says. In Ontario, the number of bird species in individual forest areas increases with deforestation against a backdrop of a continent-wide decline in bird populations, as the open ground attracts new specialists. Only as deforestation approaches 50 percent of the study area do numbers decline. Whether these additional species are a good thing for the ecosystems they invade is, of course, another question.
The same can happen in mountains. Even if some cold-loving species disappear from summits, more species move in from below. In a study of over 300 mountain peaks across Europe last year, Manuel Steinbauer, now at the University of Bayreuth, Germany, found an increasing number of species, “with five times the species enrichment between 2007 and 2016 compared to 50 years ago.”
Sometimes such trends mark recovery from past environmental damage. The UK, for example, has seen significant increases in species richness over the past 20 years across a network of closely monitored sites in upland marshes, rivers, wetlands, and woodlands. The main cause, says Rob Rose of the Centre for Ecology and Hydrology in Lancaster, is recovery from acid rain. Most researchers contacted for this article rate the biggest driver of more diverse local ecosystems as the import of non-native species by humans.
Many of the criticisms of the new findings are technical. Some ecologists argue that the database of local studies used in the analysis is unrepresentative and ignores places like farms and cities where almost all biodiversity has been lost.
“There’s no debate that biodiversity is increasing in some locations,” said Bradley Cardinale of the University of Michigan, a leading critic. “The debate is about whether these trends represent the generality. I’ve argued that the data sets aren’t globally representative because biologists don’t spend time monitoring areas that have been disturbed… in cornfields, clear forests, or urban habitats.” Such biases “preclude meaningful conclusions about local biodiversity change,” he and Andrew Gonzalez of McGill University argued in a critique of an earlier study.
Vellend and Dornelas have hit back, saying the critiques themselves have “fundamental flaws.” But Cardinale says his opponents’ papers “are a poor example of how to do science. When a group of people is so tied to their ideas or data sets that they can’t step back and look at things differently, their publications become more about advancing ideology than about testing hypotheses.”
However, some of the protagonists are now finding common ground. Gonzalez has gone from critic to co-author of the new bubble paper. “My position has shifted a bit over the years,” he told Yale Environment 360. He now accepts the evidence for dramatic species turnover, and that localities losing species are largely counterbalanced by others gaining them. But he questions any conclusion that humans “cause gains and losses equally. I’m not comfortable with that attribution. We simply don’t know,” he said.
Some researchers fear that the findings, without a loss of local biodiversity, could lead to the false impression that the global biodiversity crisis is a myth. “We can all easily imagine certain subgroups cherry-picking the results and trumpeting the public’s ‘non-existent, non-existent,'” the Science paper wrote in a blog post discussing some earlier findings in 2014.
“I don’t think our study can be used to conclude that everything is fine with the world’s biota,” says one of the paper’s co-authors.
However, the authors insist that the local diversity results are fully consistent with the fact that species are rapidly disappearing worldwide. They explain the paradox by ecological reorganization, which results in global species homogenization. “I struggle with this myself,” says Dornelas. “But it makes sense.”
The question remains whether we should view rapid turnover in disturbed ecosystems as an indication of the good health and adaptability of these ecosystems or of stress that could lead to their eventual collapse. Strong disagreements remain here.
Georgina Mace of University College London, a leading ecologist not involved in the study, is concerned about what’s emerging. The gains among species in many local ecosystems are “largely non-native, so I think we should be very cautious about assuming this is just outmigration or adaptation,” she said. “We don’t know how these rapidly changing communities will fare over time.”
But Vellend is more optimistic. “I certainly don’t think one can conclude from our study that everything is fine with the world’s biota,” he said. “But my experience in plant communities is that rapid compositional changes are adaptive.” And Dornelas is of a similar mind. “We should stop thinking about biodiversity as if it were art in a museum,” she said. “It’s not static. We need to accept changes in biodiversity as the rule, not the exception.”