Older species tend to have large ranges – unless they live on islands

(Press-News.org) Every living species on Earth has a unique geographical range, with some being widespread and others being very narrow. Several factors shape a species’ range size – and one of them is the evolutionary age of a species. To investigate how evolutionary age is related to present-day range size, a research team led by scientists from the German Centre for Integrative Biodiversity Research (iDiv), Leipzig University and Naturalis Biodiversity Center compared over 26,000 species of mammals, birds, reptiles, amphibians, reef fishes, and palms.

More than 40,000 species are facing extinction worldwide. Species with narrow geographical ranges are known to face a higher risk of extinction compared to widespread species: They tend to have lower overall abundance and smaller local populations, making them vulnerable to environmental perturbations. Although range size is one of the strongest determinants of extinction risk, the causes underlying the wide variation in natural range sizes remain poorly understood.

An international team of researchers led by Dr Adriana Alzate, alumna of iDiv and Leipzig University, compared data on the evolutionary age and range sizes of over 26,000 species from seven major taxonomic groups: birds, reptiles, amphibians, reef fishes, palms, and terrestrial as well as marine mammals.

More time, larger range?

The researchers show that, on average, older species have larger ranges across all groups except for marine mammals – a finding which did not come as a big surprise, but had not been proven so far. “Older species are expected to have larger distributions because they have had more time, sometimes several millions of years, to expand their ranges since first appearing,” explains first author Adriana Alzate, Guest Researcher at Naturalis Biodiversity Center. “Over evolutionary timescales, these species have had more opportunities to reproduce, disperse, colonize and adapt to diverse environments, allowing them to occupy broader geographical areas.”

Good dispersers quickly attain large ranges

But it is not only a species’ age that influences its range size. Some species are good dispersers and can move easily across barriers or over great distances, such as birds with long, narrow wings with pointed tips, and palms with large fruits, that are dispersed by wide-ranging large-bodied vertebrates. These species may attain large ranges faster than less dispersive species. Therefore, good dispersers might have larger ranges than expected based on age only. By contrast, the study shows that the effect of species age is likely more pronounced on less dispersive species, such as amphibians.

Geographical context also plays a major role. On islands, the maximum range size that native species can attain is geographically constrained. The new research confirmed that island-restricted species have smaller range sizes than species that are not restricted to islands – but it also brought to light an unexpected relationship: On islands, range size differences between young and old species are greater than on the mainland. Dr Roberto Rozzi, Curator of Palaeontology at the Central Repository of Natural Science Collections of Martin Luther University Halle-Wittenberg and iDiv alumnus: “Island dynamics and ontogeny modulate the relationship between age and range size. Release from predators and competitors may have enabled early island colonizers, typically ecological generalists, to achieve broader ranges than expected based on age only.”

Environmental change puts narrow-ranged species at risk

The smaller a species’ range, the higher the risk for it to go extinct. Understanding the dynamics that shape a species’ range size is crucial for predicting its vulnerability to extinction and tailoring conservation efforts to local conditions and needs.  “This is even more important in the context of changing environmental conditions, because not all species may be able to keep up with these changes”, says senior author Dr Renske Onstein, junior group leader at iDiv and group leader “Biodiversity Hotspots” at Naturalis Biodiversity Center. “Possibly, older species have the genetic makeup to more readily adapt and therefore persist in their relatively large ranges. This needs further testing with genetic data, for example, providing exciting possibilities for future research”.

 

The study was funded by the German Research Foundation (DFG; FZT-118), among others. Alumna Dr Adriana Alzate was a postdoctoral researcher at iDiv’s synthesis centre sDiv, which helps bring together existing but disparate data, methods, theories, and tools in new and sometimes unexpected ways.

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