Every once in a while, a species that hasn't gone extinct will disappear from the fossil record. Sometimes, this is because a life form has evolved into a new species -- this is known as pseudoextinction. Life forms can also disappear from the fossil record and reappear later. These Lazarus species may have experienced a dip in population, or they may not have died in conditions that lead to fossilization.
But most of the time, when a species disappears from the fossil record -- or from the face of the planet today -- it's because it's on its way to extinction. Typically, these small-scale extinctions happen because of some kind of change in the environment where a life form lives. While some species can adapt to the changes, others die, and if enough die, the species becomes extinct. These are some of the biggest factors in the extinctions of particular species:
- Habitat loss
- Competition with new species
- Human hunting
- Contaminants in the environment, such as pesticides
The loss of one species can also lead to the loss of many others. For example, flowering plants rely on pollinators, like bees and butterflies, to reproduce. If the pollinators disappear, the flowering plants can die, too. The same is true with changes in the food chain. If an animal relies on a specific plant for food and that plant becomes extinct, the animal will soon follow unless it's able to change its diet. A good example is the extinctions that happened at the end of the last ice age, about 11,000 years ago. Small mammals began to go extinct, probably because of climate change and changes in geography. This caused the extinction of bigger animals, like saber-tooth cats, which lost their food source.
Typically, little changes like this happen all over the world all the time, leading to the extinction of a few interrelated species. But sometimes the stress on an ecosystem is so large that not many life forms survive. Next, we'll look at some of the world's biggest extinctions.