Prominent research studies have suggested that our planet is currently experiencing another mass extinction, based on extrapolating extinctions from the past 500 years into the future and the idea that extinction rates are rapidly accelerating.
A new study by Kristen Saban and John Wiens with the University of Arizona Department of Ecology and Evolutionary Biology , however, revealed that over the last 500 years extinctions in plants, arthropods and land vertebrates peaked about 100 years ago and have declined since then. Furthermore, the researchers found that the past extinctions underlying these forecasts were mostly caused by invasive species on islands and are not the most important current threat, which is the destruction of natural habitats.
The paper argues that claims of a current mass extinction may rest on shaky assumptions when projecting data from past extinctions into the future, ignoring differences in factors driving extinctions in the past, the present and the future. Published in the journal Proceedings of the Royal Society of London, the paper is the first study to analyze rates, patterns and causes of recent extinctions across plant and animal species.
For their study, Saban and Wiens analyzed rates and patterns of recent extinctions, specifically across 912 species of plants and animals that went extinct over the past 500 years. All in all, data from almost 2 million species were included in the analysis.
"We discovered that the causes of those recent extinctions were very different from the threats species are currently facing," said Wiens, professor of ecology and evolutionary biology. "This makes it problematic to extrapolate these past extinction patterns into the future, because the drivers are rapidly changing, particularly with respect to habitat loss and climate change."
According to Saban and Wiens, the most direct information on species losses comes from recent extinctions over the past five centuries. However, studies extrapolating these patterns into the future generally assume that recent extinctions predict current extinction risk and are homogeneous among groups, over time and among environments, the authors argue.
"To our surprise, past extinctions are weak and unreliable predictors of the current risk that any given group of animals or plants is facing," said lead author Saban, who recently graduated from the U of A and is currently a doctoral student at Harvard University.
Extinction rates varied strongly among groups, and extinctions were most frequent among mollusks, such as snails and mussels, and vertebrates, but relatively rare among plants and arthropods. Most extinctions were of species that were confined to isolated islands, like the Hawaiian Islands. On continents, most extinctions were in freshwater habitats. Island extinctions were most frequently related to invasive species, but habitat loss was the most important cause (and current threat) in continental regions. Many species appeared to go extinct on islands because of predators and competitors brought by humans, such as rats, pigs and goats.
Somewhat unexpectedly, the researchers found that in the last 200 years, there was no evidence for increasing extinction from climate change.
"That does not mean that climate change is not a threat," Wiens said. "It just means that past extinctions do not reflect current and future threats."
The authors also considered threat levels – for example "threatened," "endangered" or "least concern," – for 163,000 species as assessed by the International Union for Conservation of Nature.
"The current threat level provides probably our best hint of what is currently happening and might happen in the near future," Wiens said. "We found the patterns of today's threats to be different from those of past extinctions. For example, most extinct species are mollusks and vertebrates on islands that were driven extinct by invasive species, but most threatened species today are mainland species facing habitat destruction."
Saban said she doesn't want the study "to be taken as giving people a carte blanche" to suggest that human activity does not present a significant and urgent threat to many species.
"Biodiversity loss is a huge problem right now, and I think we have not yet seen the kinds of effects that it might have," she said. "But it's important that we talk about it with accuracy, that our science is rigorous in how we're able to detail these losses and prevent future ones."
Contrary to many studies, the rates at which species are going extinct are not rapidly accelerating, the study found.
"We show that extinction rates are not getting faster towards the present, as many people claim, but instead peaked many decades ago," Wiens said.
For some groups, such as arthropods and plants and land vertebrates, extinction rates have actually declined over the last 100 years, notably since the early 1900s, he added. One of the reasons for declining extinction rates "is many people are working hard to keep species from going extinct. And we have evidence from other studies that investing money in conservation actually works."
According to Saban, the study was born out of a motivation to take a step back from doomsday scenarios.
"If we're saying that what is happening right now is like an asteroid hitting Earth, then the problem becomes insurmountable," she said. "By looking at the data in this way, we hope that our study helps inform our overall understanding of biodiversity loss and how we can come up with better ways to address it."
