Giant or dwarf, extreme sizes put species in danger of extinction

Dramatic changes often result in dead ends.
Nergis Firtina
Giant island bird concept
Giant island bird concept


Species that evolved to more extreme body sizes compared to their mainland relatives have a higher risk of extinction than those that evolved to less extreme sizes, according to a new study by the German Center of Integrative Biodiversity Research (iDiv) and Martin Luther University Halle-Wittenberg (MLU).

Their study also shows that extinction rates of mammals on islands worldwide increased significantly after the arrival of modern humans.

As iDiv explains, islands are hotspots for biodiversity, hosting up to 20 percent of all terrestrial species worldwide, taking up less than 7 percent of the planet's surface. Island native species make up 50 percent of the IUCN's endangered species today, making islands a hotspot for species extinction.

Dramatic changes often result in dead ends

Many creatures experience amazing evolutionary changes due to the particular traits of island settings, with enormous body size adjustments being among the most famous. In general, relatives of large mainland species tend to become smaller on islands while small species tend to become larger; this phenomenon is known as gigantism or dwarfism.

Giant or dwarf, extreme sizes put species in danger of extinction
Illustration of Sardinian Dwarf Mammoth, Sardinian Giant Otter, Deer, Sardinian Dhole and Giant Pica.

Some of these evolutionary wonders are already extinct, such as the dwarf mammoths and hippos, which shrank to less than a tenth the size of their mainland predecessors, and the huge rodents and gymnures multiplied by over 100.

Now, a group of scientists led by iDiv and MLU has verified that the greater susceptibility to extinction commonly coexists with the emergence of these traits.

The researchers used data on fossil and living island mammals, including over 1,200 extant and 350 extinct species of insular mammals on 182 islands and paleo-islands worldwide, to quantify how evolution towards dwarfism and gigantism may have affected the risk and rate of extinction.

These findings point to a previously unrecognized result: species on islands were more likely to become endangered or go extinct if their body sizes suffered more dramatic changes, whether larger or smaller.

The study was published in Science on March 9.

Study abstract:

Islands have long been recognized as distinctive evolutionary arenas leading to morphologically divergent species, such as dwarfs and giants. We assessed how body size evolution in island mammals may have exacerbated their vulnerability, as well as how human arrival has contributed to their past and ongoing extinctions, by integrating data on 1231 extant and 350 extinct species from islands and paleo islands worldwide spanning the past 23 million years. We found that the likelihood of extinction and of endangerment are highest in the most extreme island dwarfs and giants. Extinction risk of insular mammals was compounded by the arrival of modern humans, which accelerated extinction rates more than 10-fold, resulting in an almost complete demise of these iconic marvels of island evolution.

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