Why is the pale-headed brushfinch more at risk of extinction than the osprey?

Every animal or plant species has its own range. Some species are found worldwide, while others live in only one place. For example, the osprey is found worldwide, whereas the pale-headed brushfinch lives only in a small area in southwestern Ecuador. It is a well-known fact that species with a small range like this are at greater risk of extinction than those that are widespread.

On average, there are simply fewer individuals of these species, and their populations are smaller. This makes them more vulnerable to disturbances in their habitat, such as the effects of climate change. "To predict the risk of extinction for species and to coordinate conservation efforts properly, it is crucial to understand which factors determine the range of species", says Renske Onstein, researcher at the Naturalis Biodiversity Center.

More time, larger range?

Researchers from the German Centre for Integrative Biodiversity Research (iDiv), Leipzig University, and Naturalis Biodiversity Center compared the evolutionary age and range of 26,000 species of palm trees, birds, reptiles, amphibians, fish, and mammals. On average, older species were found to have larger ranges. "Since their origin, they have had more time to reproduce, expand their habitat, and adapt to it. Sometimes millions of years more than younger species", explains Adriana Alzate, a guest researcher at Naturalis.

For example, the African palm swift is widespread in Africa and very old. "The species has existed for about thirteen million years and can spread very well. You can see this in the shape of its wings." Other birds and palm trees with large fruits can also quickly establish themselves over a large area. "The effect of the evolutionary age of these species on their range is smaller than that of less effective dispersers, such as amphibians." Marine mammals like whales, despite their young evolutionary age, are often still widespread. "They can spread quickly over long distances because they are large and can go without food for a long time", Alzate explains.

Early island colonizers spread faster

Geography also plays a role. On islands, a species' maximum habitat is limited. For example, the Juan Fernandez firecrown is found only on Robinson Crusoe, an island over 600 kilometers off the coast of Chile. "This species is about 4 million years old and should be able to spread well. But the island where it lives is so isolated that it's impossible to reach new areas", Alzate explains.

The researchers also observed that on islands, the difference in range between young and old species is greater than on the mainland. "The absence of predators and competitors may have helped early island colonizers to conquer larger habitats than would be expected based on their age alone", explains Roberto Rozzi, a paleontology collection manager at Martin Luther University and iDiv alumnus.

Future research

The researchers' results confirm the relationship between the evolutionary age of species and their range and provide more insight into the factors that determine the range of island species. However, there are still more factors to investigate. "It's possible that older species have the genetic predisposition to adapt and survive more easily in their large habitats. This offers exciting possibilities for future research", Onstein concludes. 

More information

• The article 'Evolutionary age correlates with range', published in Nature Communications.
• The ICUN Red List.

Text: Naturalis Biodiversity Center
Images: Niels Andeweg (lead image: osprey); Derek Keats; Arthur Grosset