Island rule

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The so-called island rule , also Foster's rule (English: Foster's rule ), is a hypothesis to body size of species on oceanic islands . The rule states that in kinship groups (or clades ) of species-average height, such as proboscis animals (Proboscidea), living on islands representatives are smaller than average, insular dwarfism called. In kin groups with species that normally remain small, such as tortoises , for example , the representatives living on islands are larger than the average, known as island gigantism .

According to the island rule, mainly medium-sized species have an advantage on islands. The giants and dwarfs are only striking in size when compared to closely related species on the mainland. In absolute terms, they are not particularly extreme in size; they can actually be almost the same size. According to the island rule, species whose typical representatives on the mainland are medium-sized should maintain this size on islands and show neither island gigantism nor dwarfism. That the variance (a measure for the deviation of the measured values ​​from the average value) of the body mass in mammals on islands is smaller than on the mainland, could also be statistically proven.

Research history

The hypothesis of island dwarfing was first put forward by paleontologists such as Dorothea Bate , who had found skeletons of extinct, dwarf elephant species on islands in the Mediterranean, of which numerous other species later appeared worldwide (always on islands) (such as the dwarf mammoth ). The small body size of otherwise gigantic sauropods , such as the (however, only later discovered) Europäischeaurus , was soon explained as well. Strangely enough, however, more and more living and fossil species appeared on islands, which were the largest representatives of their respective genera (or other relatives), such as the Komodo dragon or the (extinct) Haastadler of New Zealand.

The Canadian ecologist Bristol Foster was the first to investigate the phenomenon comparatively in some orders of a group of animals, the mammals. He found widespread island dwarfing among the carnivores (Carnivora), the hare-like (Lagomorpha) and the even-toed ungulates , while, according to his data, island gigantism predominated among rodents (Rodentia) and (uncertainly) among marsupials (Marsupialia). He speculated about the unifying regularity, which was first called "island rule" by evolutionary biologist Leigh Van Valen . The island rule was then examined by the biogeographer Mark V. Lomolino in numerous other groups and generalized by him to all vertebrate groups. Today it is partly applied to other groups such as insects.

reasons

Various hypotheses have been proposed for the observed trends in body size, which they relate to the special ecological conditions on islands, which in many cases seem to allow a much higher rate of (morphological) evolution there than on the mainland. Compared to regions of the mainland of comparable size, islands almost always have fewer species per area. Particularly highly specialized species with a narrow ecological niche can hardly colonize islands successfully because they depend on a special community for their existence or because the island is simply not big enough for a minimal survivable population . Islands also offer only a few food resources to many animal species , which means that smaller individuals with lower appetites are preferred. At the same time, however, there is a lack of specialized predators for many species , especially large, predatory mammals, so that their usual prey on islands can become larger and thus slower and more conspicuous. Since there are fewer species, the interspecific competition between species tends to be lower. As a result, species that only occupy a narrow niche on the mainland could become more generalist in the absence of competitors, and thus also become larger or smaller when the constraints imposed by the competition no longer exist. Long-haired eagles and Komodo dragons, for example, were able to develop better from smaller ancestors on islands where competition from hunting mammals was less. Another possible reason is that oceanic islands, especially volcanic or coral islands far from the mainland, can only accommodate species that could travel the distance from the nearest mainland or other already populated habitat. As a result, not only is the species composition partly more determined by chance, but the probability of colonization itself can also depend on body size.

Verification

The hypothesis has received a great deal of attention since its creation and has been tested by numerous researchers in the context of meta-studies. In addition to the classic examples, more and more groups were included in the analysis. The rule has also been confirmed for primates , for example . However, not all animal groups behave according to this expectation. In a large-scale analysis, Shai Meiri and colleagues have compiled a huge data set on the body mass of birds, mammals and scallops on islands and mainlands and thus checked the island rule. Anomalies in some groups were confirmed: The largest representatives of numerous species of birds and reptile families were actually living on islands more often than would be expected by chance. In the case of mammals, island dwarfing was easily detectable in numerous (extinct and living) groups (while there were hardly any clear cases of island gigantism). The best matches were found in groups that are generally larger, especially in extinct forms. In almost all cases, humans are either proven or highly probable as the cause of extinction of these special island saunas (such as New Zealand's moas or Madagascar's elephant birds ). In many groups, however, an effect of the islands on body size was not detectable at all. Some cases confirm gigantism and dwarfism, but can hardly be explained with the island rule in its usual form. In rodents, both extremely large and extremely small forms appear to be more common on islands than on the mainland, i.e. the variance is greater, while the island rule would predict a smaller variance.

So while there are some examples that can be explained well with the island rule, the evidence for the general validity of the rule is rather dubious. According to the critics, their success may even be based in part on a perceptual effect. The giants and dwarfs of exotic island saunas may simply attract more attention than similarly large or small species from remote mainland regions.

Individual evidence

  1. ↑ Trevor D. Price & Albert B. Phillimore (2007): Reduced major axis regression and the island rule. Journal of Biogeography 34: 1998-1999.
  2. ^ DMA Bate (1907): On elephant remains from Crete, with description of Elephas creticus sp.n .. Proceedings of the Zoological Society of London 77: 238-250.
  3. ^ F. Nopcsa (1914): About the occurrence of dinosaurs in Transylvania. Negotiations of the Zoological-Botanical Society Vienna 54: 12-14.
  4. ↑ JB Foster (1964): Evolution of Mammals on Islands. Nature 202: 234-235.
  5. ^ L. Van Valen (1973): Pattern and the balance of nature. Evolutionary Theory 1: 31-49.
  6. ↑ a b Mark V. Lomolino (2005): Body size evolution in insular vertebrates: generality of the Iceland rule. Journal of Biogeography 32: 1683-1699.
  7. ^ SL Chown & KJ Gaston (2010): Body size variation in insects: a macroecological perspective. Biological Reviews 85: 139-169.
  8. ↑ cf. Jonathan B. Losos & Robert E. Ricklefs (2009): Adaptation and diversification on islands. Nature 457: 830-836. doi : 10.1038 / nature07893
  9. ^ Peter R. Grant: Patterns on islands and microevolution. Chapter 1 in Peter R. Grant (editor): Evolution on Islands. Oxford University Press 1997. ISBN 978-0-19-850171-8
  10. ^ LR Heaney (1978): Island area and body size of insular mammals: evidence from the tri-colored squirrel (Callosciurus prevosti) of Southeast Asia. Evolution 32: 29-44. doi : 10.2307 / 2407408
  11. ^ John J Welch (2009): Testing the island rule: primates as a case study. Proceedings of the Royal Society B 276 (1657): 675-682. doi : 10.1098 / rspb.2008.1180
  12. ^ A b Shai Meiri, Pasquale Raia, Albert B. Phillimore (2010): Slaying dragons: limited evidence for unusual body size evolution on islands. Journal of Biogeography 38 (1): 89-100. doi : 10.1111 / j.1365-2699.2010.02390.x
  13. ^ Paul AP Durst & V. Louise Roth (2015): Mainland size variation informs predictive models of exceptional insular body size change in rodents. Proceedings of the Royal Society B 282: 20150239. doi : 10.1098 / rspb.2015.0239