Coming: The sixth mass extinction?

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.

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.

Power Words

biodiversity  The number and variety of organisms found within a geographic region.

carbon  The chemical element having the atomic number 6. It is the physical basis of all life on Earth. Carbon exists freely as graphite and diamond. It is an important part of coal, limestone and petroleum, and is capable of self-bonding, chemically, to form an enormous number of chemically, biologically and commercially important molecules.

diversity     (in biology) A range of different life forms.

ecology  A branch of biology that deals with the relations of organisms to one another and to their physical surroundings. A scientist who works in this field is called an ecologist.

ecosystem  A group of interacting living organisms — including microorganisms, plants and animals — and their physical environment within a particular climate. Examples include tropical reefs, rainforests, alpine meadows and polar tundra.

extinction  The state or process of a species, family or larger group of organisms ceasing to exist.

fossil  Any preserved remains or traces of ancient life. There are many different types of fossils: The bones and other body parts of dinosaurs are called “body fossils.” Things like footprints are called “trace fossils.” Even specimens of dinosaur poop are fossils.

geologic time     The span of time that covers Earth’s 4.5 billion-year history. Scientists divide geologic time into successively briefer intervals of time, called eons, periods, epochs, eras and ages.

geologic record     Mineral deposits and fossils that form in rock. Geologists can “read” these minerals to decipher what Earth’s climate and geology was like (such as dry spells, earthquakes or volcanic eruptions) when the rock’s ingredients were laid down. Fossils and other mineral can signal what life may have existed at the same time.

geology  The study of Earth’s physical structure and substance, its history and the processes that act on it. People who work in this field are known as geologists. Planetary geology is the science of studying the same things about other planets.

global warming  The gradual increase in the overall temperature of Earth’s atmosphere due to the greenhouse effect. This effect is caused by increased levels of carbon dioxide, chlorofluorocarbons and other gases in the air, many of them released by human activity.

mass extinction    Loss of 75 percent of the world’s species in a short time period, typically defined as 2 million years or less. Our planet has experienced 5 known mass extinctions.

microorganism,or microbe  A living thing that is too small to see with the unaided eye, including bacteria, some fungi and many other organisms such as amoebas. Most consist of a single cell.

Quaternary Period     A time in the distant past that was defined by regular ice ages. It began about 2.58 million years ago. Scientists refer to its earliest period as the Pleistocene. Then, about 11,700 years ago, a second period began, known as the Holocene. It continues through today.

species  A group of similar organisms capable of producing offspring that can survive and reproduce.