New study challenges old thinking about extinctions worldwide

For some time, the accepted wisdom has been that biodiversity is declining worldwide. A new study challenges that assumption and suggests a different dynamic is at work.

Although humans are driving other species to extinction at high rates and biodiversity is declining globally, researchers have found that in many locations around the world, the number of species in a wide range of habitats has either increased or remained virtually stable during the past 40 years or more.

However, these local habitats are experiencing turnovers in species at a breathtaking pace – some 10 percent per decade on average, according to a new study, with hard-to-predict consequences.

That means that in many places, in the span of a human lifetime at least half the species in a backyard, wood lot, or local meadow will be replaced, notes Peter Kareiva, an ecologist and chief scientist for the Nature Conservancy in Arlington, Va.

The pace of change is likely being driven by a range of factors, including climate change, the arrival of invasive species, or the expanding range of creatures such as mosquitoes who see humans as fly-in diners.

The results suggest that conservation planning must focus on more than loss of biodiversity. It also must plan for the emergence of novel groupings of species that may alter the ecological services a patch of landscape once provided and that people have grown to rely on, the study's authors say.

The new study updates the prevalent narrative regarding biodiversity, says Dr. Kareiva, who did not take part in the study.

That narrative is founded on the premise that large decreases in biodiversity are underway everywhere. To counter those losses, conservationists have set up conservation areas and otherwise sought to safeguard habitats that would serve as safe havens for species. More recently, corridors between protected areas have been recognized as important elements in efforts to rebuild healthier populations of threatened or endangered plants or animals.

"That strategy has been effective in averting a number of extinctions," says Kareiva.

But the local story is more subtle.

"Yes we are losing species" in the global aggregate, he says. "Locally, you wouldn't see a horrible decline in biodiversity. I don't think most people realize that. What you would see is remarkable change."

For the study, an international team of scientists led by marine ecologist Maria Dornelas at St. Andrews University in Scotland analyzed more than 6 million data points contained in 100 studies. These studies tracked species richness over time in a range of habitats from the polar regions to the tropics. The data the team analyzed covered more than 35,600 species. While most of the studies tracked changes during the past 40 years, others reached as far back as 1874.

The first question: How had diversity – essentially the number of species and the abundance of each – changed with time in each of the studies? The answer surprised them, they wrote. Some studies showed increases, others decreases, and still others showed no changes. Taken together, the collective change in biodiversity in these studies was statistically no different than zero. There was no consistent decline or even an average decline in biodiversity.

One hint as to why: Many species may continue to thrive in habitats humans have disturbed.

A second study published recently could shed further light on that phenomenon. For years, many conservation scientists had assumed that once at-risk species leave the reserves set up to protect them, their prospects for survival are significantly diminished. This was based on observations of extinctions on islands. 

But the second study suggests that the dynamics at work on islands are different from those at work on land. It compared the diversity of the bat population in and around forest reserves in southern Costa Rica, where reserves often are surrounded by coffee plantations and other forms of agriculture, with those on the islands of Gatun Lake in Panama. More bat species survived habitat encroachment in Costa Rica compared with the island bat species that lost habitat, according to the team led by Stanford University PhD student Chase Mendenhall.

The research adds credence to the idea that landscape outside reserves can play a key role in conservation.

The second question posed by the global study: What were the changes to the mix of species? The answer was that the changes were dramatic across the range of ecosystems covered in the study. The results appear in the current issue of the journal Science.

Some scientists caution against reading too much into the study. It doesn't cover important ecosystems in tropical South America or Africa – basically because much less work has been done in these areas over the time periods Dr. Dornelas and colleagues tried to cover. Yet the tropics are where many of the extinctions are being recorded, says Volker Radeloff, an ecologist at the University of Wisconsin at Madison.

On a more basic level, the study of necessity treats all species as equal, when they aren't, he says. In some cases the species lost to invasives or other replacements may be rare or culturally important. In others, the species that vanish may be top predators. They may be few in number but their disappearance can radically alter the nature of an ecosystem.

"It's the rare species that we are worried about," he says.

Moreover, in the eastern US and in Europe, where many studies have been conducted, most of the biodiversity loss took place before the time span covered in the team's study, he says.

For Dr. Radeloff, the biggest surprise came from the global study. It produced a map of locations where scientists have been able to track their study sites long enough to monitor changes in biodiversity.

The yawning, empty regions on the map show "how biased our sampling has been as ecologists," he says. When it comes to truly representing changes in Earth's biodiversity, "as a global community, we don't have our finger on the pulse."

Even so, taken together, both studies leave room for "an enormous amount of optimism," adds Kareiva.

They indicate that "agricultural landscapes, once thought of as having nothing to do with conservation, really are a key part of conservation," he says. "That gives us a lot more land to work with."