The latest analysis, led by Shane Blowes of the German Center for Integrative Biodiversity Research in Leipzig, uses a database called BioTIME, hosted at St Andrews University, to review 239 studies that have recorded the number of species through time at around 50,000 places in most of the world’s ecosystems.
Many of the biggest changes turn out to have been in marine ecosystems, especially in the tropics, where in some places species composition changed by as much as 20 percent in a single year. Blowes told Yale Environment 360 that two of the biggest turnover hotspots are in the western tropical Atlantic and off the coast of northwest Australia.
It seems likely that the rapid local species turnover is new—probably a result of human activity such as chopping down forests, draining marshes, concreting prairies, damming rivers, and changing climate. But that is not self-evident, says Mark Vellend, of the University of Sherbrooke in Quebec, a co-author of the new study. “The most pristine ecosystems have always changed in composition over time,” he says.
And while human activity clearly often damages local biodiversity, it can also, at certain times and places, drive local increases in species numbers — by creating new habitats, by changing existing ones, or by physically bringing in new species.
“In North America, we have a greater variety of habitat types than we used to, such as agricultural fields, pastures, forests of different ages, roadsides,” says Vellend. The result can be a greater variety of species locally, as those that thrive on disturbed landscapes move in. “I have a study about to be submitted [for publication] using forest inventory data over hundreds of years that supports this.”
Some ecologists argue that the study ignored places such as farms and cities where almost all biodiversity has been lost.
Ornithologists have found the same counterintuitive local trend, he says. In Ontario, against a backdrop of a continent-wide decline in bird populations, the number of bird species in individual woodland areas increases with deforestation, because the open ground attracts new specialists. Only as deforestation approaches 50 percent of the study area do the numbers begin to decline. Whether those extra species are a good thing for the ecosystems they invade is, of course, another question.
The same can happen on mountains. Even as some cold-loving species disappear from summits, more species move in from below. In a study of over 300 mountain summits across Europe last year, Manuel Steinbauer, now at the University of Bayreuth, Germany, found a rising number of species, “with five times as much species enrichment between 2007 and 2016 as fifty years ago.”
Sometimes such trends mark recovery from past environmental damage. Britain, for instance, has seen a significant increase in species richness over the past 20 years in a network of closely monitored sites in upland moors, rivers, wetlands, and woods. The main cause, says Rob Rose of the Center for Ecology and Hydrology in Lancaster, is recovery from acid rain. But most researchers contacted for this article assess 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 not representative, ignoring places such as farms and cities where almost all biodiversity has been lost.
“There is no debate that biodiversity is increasing in some locations,” Bradley Cardinale of the University of Michigan, a leading critic, said. “The debate is about whether these trends represent the generality. I have argued that the datasets are not globally representative, because biologists don’t spend time monitoring habitats that have been destroyed… in corn fields, clear-cut forests, or urban habitats.” Such biases “preclude meaningful conclusions about local biodiversity change,” he and Andrew Gonzalez of McGill University, argued in a critique to an earlier study.