Cienciaes.com: The Island Rule. We spoke with Ana Benítez López.

2023-04-24 10:24:13

The creatures evolve in a surprising way when they live for a long time on remote islands and without possible contact with other creatures of the same species existing on the continents. On the Indonesian island of Komodo, for example, lives the largest lizard in the world, so large that it is known as the “Komodo dragon”. It is not a unique case, it is known that huge birds existed, such as the Hawaiian moa, giant rats on the island of Tenerife, etc. In contrast to those examples of much larger-than-usual creatures, we know of others that evolved in the opposite direction, though most have gone extinct. This is the case of the dwarf elephant on the island of Sicily, which in an adult state barely reached 90 centimeters, that is, the size of a current newborn elephant. Remains of dwarf hippos have also been found, and even the genus Homo has its representative in this small family, Homo floresiensis, which inhabited the Indonesian island of Flores.

The Island Rule provides an explanation for the evolutionary trajectories that lead to gigantism in small animals and dwarfism in large animals when separated from parent species on islands.

Our guest in Talking to Scientists, Ana Benítez López, researcher in the Department of Biogeography and Global Change of the National Museum of Natural Sciences (MNCN–CSIC), has spent years studying the giant or dwarf species that inhabit or inhabited certain islands. In 2021 he published a paper in the journal Nature Ecology & evolution whose title we translate as “The island rule explains consistent patterns of evolution of body size in terrestrial vertebrates.” It is logical to think that the species that inhabit islands had their origin in other inhabitants of the continents that in remote times populated them and were later disconnected with their ancestors. Thus, Ana and a large group of collaborators searched for the existing correlation, in terms of body size, in a global sample of vertebrate populations that inhabit islands and have mainland members that share a common origin. The study reveals that the effects of the ‘island rule’ are widespread in mammals, birds and reptiles, but are less evident in amphibians, although they mostly tend towards gigantism.
The mechanisms that try to explain the evolutionary drift in the islands involve several factors. On the one hand, the absence of natural predators can lead to an increase in size in small animals because this increase in size gives them an advantage over members of their own species with whom they compete for resources. On the other hand, the limitation of existing resources favors the survival of the smallest members of large species since they have lower energy needs. Other factors, such as the climatic conditions of the place and relaxed competition, can lead to members losing unnecessary abilities, such as the ability to fly in some bird species.

Ana Benítez now participates again in another work, published this time in Science, which connects the cases of giant and dwarf island species that became extinct with the invasion by human beings of these unique ecosystems.

The study assesses how the evolution of body size in island mammals may have made them more vulnerable to the arrival of invasive species, especially when the island’s ecosystem is disturbed by the arrival of humans and all the species they bring with them. The study collects data on 1,231 extant and 350 extinct species from islands and paleoislands around the world spanning the past 23 million years. The results indicate that the probability of extinction is highest on the most extreme dwarf and giant islands. The arrival of modern humans accelerated extinction rates by more than 10 times, resulting in a near disappearance of these iconic wonders of island evolution.

I invite you to listen to Ana Benítez López, a researcher in the Department of Biogeography and Global Change of the National Museum of Natural Sciences (MNCN–CSIC).

References:

Benitez-Lopez A, Santini L, Gallego-Zamorano J, Mila B, Walkden P, Huijbregts MAJTobias JA. The island rule explains consistent patterns of body size evolution in terrestrial vertebrates. Nat Ecol Evol. 2021 Jun;5(6):768-786. doi: 10.1038/s41559-021-01426-y.

Rozzi R, Lomolino MV, van der Geer AAE, Silvestro D , Lyons SK , Bover P , Alcover JA , Benitez-Lopez A , Tsai CH , Fujita M , Kubo MO , Ochoa J , Scarborough ME , Turvey ST , Zizka A , Chase JM . Dwarfism and gigantism drive human-mediated extinctions on islands. Science. 2023 Mar 10;379(6636):1054-1059. doi: 10.1126/science.add8606.