With a circumference of 24,092 miles (40,075 kilometers) at the equator, Earth seems like a big enough planet for people, animals and plants to share. But according to the American Association for the Advancement of Science, humans are using about half the available land.
Eleven percent has become farmland, and another 11 percent is used for forestry. Twenty-six percent of the land has become pasture. Another piece, 2 or 3 percent, houses people's homes, businesses, transportation and other services we use to live and work.
All that expansion comes at a price: Habitat loss is the No. 1 contributor to extinction.
While it's easy to picture habitat loss on land, major changes in and near oceans can also play a role in a mass extinction. According to conservationist Tundi Agardy, almost half of the coral reefs on Earth have been destroyed. The same is true of about a third of mangrove forests, which house and protect land and sea animals. Development and industry have led to major changes in the world's coastlines over the last 100 years. Pollution is also a big factor in the survival of marine species.
But habitat loss isn't the only problem — another is overfishing. According to the American Association for the Advancement of Science, two-thirds of the world's marine fisheries are fished close to or beyond their limits. Fish populations can collapse, which can affect the rest of the ecosystem.
On top of that, it's tricky to chart exactly what's going on in the Earth's oceans. Scientists have discovered and classified only a fraction of the species that live there. And sometimes, it's a specific population of fish — not a whole species — that is in danger.
Loss of Biodiversity
In 1845, potato plants started dying in Ireland. The next six years became known as the Irish Potato Famine. An airborne fungus called Phytophthora infestans killed the potatoes, and more than a million people starved. A major factor in the famine was a lack of biodiversity.
Although this might sound like something that couldn't happen in today's industrialized world, a lack of biodiversity can still threaten specific plant and animal species. One of today's examples is the Cavendish banana. The most popular banana choice in the U.S., the Cavendish, like all bananas, is a clone of one parent. And all the Cavendish plants in the world are genetically identical. Two blights currently threaten it — although there will still be other bananas should it become extinct.
The drop in biodiversity also ties in to habitat loss. A smaller space in which to live leads to a smaller population size, which leads to less genetic diversity. Human activities like industrialized farming play a role, too, as large fields of one type of plant replace a diverse population of plants and animals.
If you've ever driven through the American Southeast, you've seen the effects of an invasive species firsthand. Kudzu is a climbing vine that was introduced to the U.S. in the late 1800s. The goal was to control erosion. But kudzu quickly took over parts of the landscape, displacing the plants that used to grow there. Kudzu is just one example. Other invasive species — like pythons in the Florida Everglades — make their way to new habitats unintentionally.
While non-native species can make some contributions to an ecosystem, they also use the food and resources that native species need to live. At the same time, invasive species often have no predators in their new environment. This means their population can grow unchecked. According to paleontologist Niles Eldredge, invasive species are contributing to the decline of 42 percent of the world's threatened and endangered species.
Due for Impact
So far, the Earth has experienced five notable mass extinctions. Scientists don't know the causes for all of them, but asteroid impacts probably made a contribution to at least two. One is the infamous Cretaceous-Tertiary extinction, which led to the end of land-walking dinosaurs. Other factors, including volcanic eruptions, likely played a role in that event as well.
Asteroids collide with the Earth all the time, but most burn up in the atmosphere. The bigger the asteroid, the bigger the threat — but the smaller the chance it will hit the planet. According to NASA, an asteroid big enough to seriously affect life on Earth collides with the planet once or twice per million years. NASA and other organizations are surveying the sky to find the locations of all near-Earth asteroids (NEAs) that might pose a threat to the planet.
According to New Scientist writer David Chandler, the biggest known threat is an asteroid called 1950 DA. This asteroid has a 1-in-600 chance of colliding with the Earth in 2880 — not in our lifetimes, but in the foreseeable future.
The Temperature Is Rising
The concept of global warming and whether it's caused by people is still a topic for debate in some circles. But most scientists agree that the temperature is rising — and human activity is the cause.
Critics point out that some animals have adjusted to changes in climate before and even thrived. But other researchers have created different models and equations for exactly how many animals and plants will die out because of global warming. A quarter of all plants and land animals could become extinct, according to a study by Chris Thomas of the University of Leeds. The U.N. Climate Panel states that a 1 degree Celsius (1.8 degree Fahrenheit) increase in temperature will lead to an increasing risk of extinction for 30 percent of species on Earth.
We See It in the Fossil Record
Many reports on climate change and extinction focus on the last hundred years or so. But it could be that we're in the middle of an extinction that started about 40,000 years ago, as humans started to settle new areas of the planet. This includes the period a lot of people think of as the last big extinction: the end of the last ice age, when animals like the woolly mammoth and saber-tooth cat became extinct.
It's easy to think of the ice age as its own, isolated event. But people who study the fossil record follow a much longer timeline — one of millions of years instead of thousands. It can take about 100,000 years for a mass extinction to play out in the fossil record. In other words, future scientists may look back on the fossil record and see a pattern that begins tens of thousands of years ago and extends through today. We can see some of this in the fossil record already, and in more recent archaeological evidence.
The Numbers Add Up
Extinction is a normal part of life on Earth. And by analyzing the fossil record, scientists have come up with a number known as the background rate of extinction. It's the rate at which species go extinct under normal circumstances, without the influence of asteroid collisions, climate changes, massive volcanic eruptions or other extraordinary factors. According to Peter Douglas Ward, the background rate of extinction is one to five animals per year.
Most researchers agree that today's extinction rate is far beyond the background rate of extinction. According to the U.N. Convention on Biological Diversity, three species are becoming extinct each hour — which is faster than new species are discovered. According to the American Museum of Natural History, this extinction rate is the fastest in the Earth's history.
The Biologists Agree
To some minds, the question isn't whether we're in a mass extinction but how big it is. According to the American Museum of Natural History, seven out of 10 biologists surveyed believe the Earth is undergoing a mass extinction.
Researchers on the other side of the fence point out that mass extinctions take time to develop and progress, making it too soon to determine exactly what's happening. Another argument is that a lot of research focuses on negative aspects of invasive species, environmental changes and other factors while ignoring possible benefits.
Extinction Is Inevitable
Over its 4.5-billion-year history, the Earth has been home to a diverse range of plant and animal life. A study of the fossil record uncovers everything from tiny trilobites to enormous dinosaurs. Some fossilized animals and plants bear little resemblance to anything living on Earth today. Others share an uncanny resemblance with their modern descendants.
Regardless of their appearance, the one thing all these fossilized creatures have in common is that they are extinct. Earth has experienced an ongoing cycle of extinction and development throughout its history. Most of these extinctions are separated by 100 million years or so.
The difference between past extinctions and the one most scientists agree is happening today is the cause. Previous extinctions came about because of factors that life on Earth had little control over. But most of the research suggests that an ongoing mass extinction taking place now would have its roots in human activity.