Dwarf and giant species are most at risk of extinction

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Skeleton of dwarf mammoth from the Pleistocene of Sicily (Source: wikimedia)
Skeleton of dwarf mammoth from the Pleistocene of Sicily (Source: wikimedia)

Islands are biodiversity hotspots and are home to animal species with unique characteristics, including dwarf specimens, which have evolved to very small sizes compared to their mainland relatives, and giants. An international study now reports that these species are at higher risk of extinction. The findings are supported by software developed by Daniele Silvestro of the Swiss Institute of Bioinformatics (SIB) at the University of Fribourg. The study, published in Science, also reveals a dramatic increase in extinction rates of island mammals worldwide after the arrival of modern humans.

Islands cover less than 7% of the Earth’s land area, but are home to up to 20% of the world’s total terrestrial species. They are also extinction hotspots, with 50% of the threatened species on the current IUCN list originating from them and hundreds of island species lost in recent history.

In response to the unique characteristics of island environments, many organisms undergo dramatic evolutionary changes, the most dramatic of which include extreme changes in body size. This phenomenon, known as gigantism or dwarfism, generally results in a tendency for island relatives of large continental species to shrink, while their smaller counterparts are inclined to grow. Some of these species are evolutionary marvels that are now extinct, such as dwarf mammoths and hippos that were reduced to one-tenth the size of their continental ancestors, and some rodents that increased in size by more than 100 times.

Increased extinction risk for dwarf and giant species at the extreme ends of the spectrum
An international team of researchers, including Daniele Silvestro, SNSF Professor and Group Leader at the University of Fribourg’s SIB, has observed that evolution towards these characteristics often goes hand in hand with an increased propensity for extinction. ’On the one hand, phyletic giants could be bigger hunting trophies,’ explains Dr. Roberto Rozzi, Curator of Paleontology at the Martin-Luther University in Halle-Wittenberg, Germany, and first author of the paper. On the other hand, dwarf species seem to have less deterrent power, thus facilitating hunting or predation by introduced predators’.

Inferring extinction rates from the fossil record
To quantify the impact of evolution toward dwarfism and gigantism on extinction risk and rate (both pre- and post-human), the researchers used data from more than 1,500 species - from fossils of the last 23 million years to living island mammals - from 182 islands around the world. They analyzed this data using software developed by Daniele Silvestro’s team. To quantify the extent of human impact on island animals, we needed a model that could estimate extinction rates in the distant past, long before humans appeared, and in recent history," explains Daniele Silvestro. The new model, developed for this purpose, has revealed a previously unknown result that species undergoing more extreme changes in body size, from the largest to the smallest, are the most at risk of being endangered or threatened with extinction on the islands.

Overlap of human colonization and increased mammal extinction rates i
By analyzing the global fossil record of island mammals over the past 23 million years, the authors also found a clear correlation between island extinctions and the arrival of modern humans. Using our new model, we found that the arrival of humans - which occurred at different times on different islands - was linked to a 10-fold increase in extinction rates," explains Daniele Silvestro. These results underscore the importance of understanding past extinction patterns to assess the current status of biodiversity and the threats to it. This work also highlights the urgent need for conservation action and the importance of prioritizing protection of specific species that have evolved to extreme sizes.

> Rozzi R. et al, Dwarfism and gigantism drive human-mediated extinctions on island , Science