Coming: The sixth mass extinction?

Species are dying at an unprecedented rate, scientists say

Humans, without really intending to, brought down within decades the global number of passenger pigeons — from at least 3 billion birds to just one. And that last passenger pigeon died Sept. 1, 1914. Human changes to Earth’s ecosystem and climate now imperil many other species, putting them, too, at risk of extinction.

Louis Agassiz Fuertes/Wikimedia Commons

Five times in Earth’s history, some three-quarters of all living species disappeared forever — and within a short period of time. These mass-extinction events marked the boundaries between different periods in geologic time. Those species losses reflect a major shift in the planet’s ecology. Clues to these shifts can be seen in the fossils and rock layers that form part of the geologic record.

Today, human activities are driving species to go extinct at a rate never before seen.

And this loss of species really does matter, says Anita Narwani. She’s an ecologist at the University of Michigan in Ann Arbor. That’s because the diversity of species in an ecosystem — the breadth of different organisms present — provide people with all types of services we can’t do without.

Trees provide oxygen for us to breathe. At the same time, they remove carbon from the air. That carbon is a contributor to global warming. Plants clean pollutants from air and water. Microbes break down waste. Animals disperse seeds that keep forests thriving. A broad range of living resources also provides people with food, shelter and medicine.

Narwani wanted to find out just how important biodiversity is for people. So she teamed up with other ecologists to review more than 1,700 studies. They found that more diverse parts of the globe tend to excel at things like removing carbon and providing us with wood and other natural resources. They also were better at keeping fisheries large and healthy so that they could feed many people.

saber-toothed cat
This saber-toothed cat is among the ice age animals that went extinct about 12,000 years ago. They were part of a mass die-off called the Quaternary extinction. Cicero Moraes/Wikimedia Commons (CC BY-SA 3.0)
The findings suggest people benefit greatly from there being a diverse range of species.

Unfortunately, biodiversity is in steep decline.

It is too soon to declare that Earth is undergoing a sixth mass extinction, Narwani says. She defines a mass extinction as the loss of 75 percent of species over 2 million years or less. We haven’t lost that many — at least not yet. But if current rates of species losses continue, such a mass extinction could occur in just 300 years.

“This is a very short time relative to the time frame for the previous mass events,” she points out. Such an event would leave a telltale absence of many species in the fossil record. From that point on, fossils of the vanished species would no longer appear in the pages of Earth’s rock-based diary.

The new findings highlight the potential for humans to erase many of the resources on which we now depend. As people learn more, however, they can take steps to lessen those risks. For example, by using natural resources more efficiently, says Narwani, people can preserve ecosystems so that they can continue to provide us with their services.

Alison Pearce Stevens is a former biologist and forever science geek who writes about science and nature for kids. She lives with her husband, their two kids and a small menagerie of cuddly (and not-so cuddly) critters.

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