Species extinctions: what can we learn from the past?

Introduction

Millions of species live on Earth. But many species have also gone extinct. Humans have caused many recent extinctions. There are five main causes of extinction related to humans.
Habitat loss
Invasive species
Overharvesting
Pollution
Climate change

The number of extinctions can depend on the group of species and where they live. For example, species that live on islands are especially vulnerable. Many island species went extinct because of other species brought by humans from the mainland (like rats, cats, and snakes). This includes mainland predators that island species did not recognize as a threat.

Studying past extinctions might help predict future ones. This is called extrapolation. We set out to find out what past extinctions can tell us about extinctions today and in the future.

Methods

We used data about 163,022 species from the International Union for Conservation of Nature (IUCN). This database has information about plant and animal species, especially those that are already extinct or are now threatened with extinction. We looked at data from the past 500 years to explore our questions.
Who? We calculated the proportion of species that have gone extinct among different groups of species to see which groups have been most impacted. These groups included plants, insects, snails, mammals, birds, and amphibians.
Where? We looked at extinctions in three different habitat types: land, marine, and freshwater. We also looked at extinctions on islands and on the mainland.
When? We figured out when each extinction most likely happened. We especially wanted to know if extinction rates were increasing or decreasing closer to the present.
Why? We looked at the causes of extinction within each group of species and for each habitat. Next we compared this with information on the threats to currently endangered species. We could then see if the causes of past extinctions were the same as the threats to living species today.


Results

So, what did we find?
Who? We found evidence for 912 extinct plant and animal species over the last 500 years. Extinctions in vertebrates were highest for turtles, birds, and mammals. Extinctions were lower for plants and for invertebrates in general, but high for mollusks.
Where? We found that 65% of extinctions were on islands. On the mainland, the majority of extinctions were among freshwater species (like fish and snails). Extinctions were relatively rare among marine species.
When? We found that extinctions generally increased in each century over the last 500 years. Surprisingly, we found that extinction rates have not been increasing over the last 100 years across most groups. In some groups, like fishes and amphibians, extinctions have increased over the last 100 years. But most groups had their highest extinction rates more than 50 years ago (Figure 1). For some groups, like plants, insects, birds, and mammals, the highest extinction rates were about 100 years ago or more.
Why? We found that the most common cause of extinction in the last 500 years was from invasive species (Figure 2). However, invasive species are currently only a threat to 10% of endangered species. The most important threat now is habitat loss. This threatens more than 50% of endangered species.


Discussion
Our results revealed the major patterns of extinctions in plants and animals over the last 500 years. We found that extinctions vary greatly among different groups of species. For example, turtles and mollusks have very high extinction rates compared to other animals and to plants. Extinctions also vary among habitats. They are very common on islands and in freshwater, but much less common in the ocean and on land on the mainland.

Extinction rates also vary over time. We found that over the last 500 years, extinction rates generally increased in more recent centuries. However, for many groups (like plants, insects, and birds), the highest extinction rates were 100 years ago, and have declined more recently. We think this might be because of recent efforts towards conservation, especially for birds and mammals.

Our results suggest that past extinctions may not be a good way to predict future extinctions. For example, invasive species were the cause of most past extinctions, but habitat loss is the most important threat to endangered species today.

Our findings do not suggest that extinctions are no longer a problem. Currently, more than 40,000 species are threatened with extinction, many more than have gone extinct in the last 500 years. Instead, scientists must be careful when extrapolating data. The causes of extinction in the past (like invasive species on islands) may be very different from current threats to species (like habitat loss) and those in the future (like climate change). Focusing on the present and future will help us work out the best conservation strategies for endangered species.

Conclusion

Maintaining biodiversity on Earth is important to all species, including humans. Protecting species can help us have a healthy planet.

Everyone can do something to help. Here are a few ideas.
Dispose of trash properly. Don’t leave it where it can harm plants and animals.
Plant pollinator gardens with native plants in your yard or in other spaces.
Avoid foods that have strong negative impacts on climate and the environment, like beef.
Donate to or volunteer for organizations that help protect species, like the Nature Conservancy and the World Wildlife Fund.
Write to your government representatives about supporting biodiversity initiatives.