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.