Island gigantism

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The large Haast's eagle and moa from New Zealand (both extinct)
The extinct Tenerife giant rat

Island gigantism or insular gigantism is a biological phenomenon in which the size of animals isolated on an island increases dramatically in comparison to their mainland relatives. Island gigantism is one aspect of the more general "island rule", which posits that when mainland animals colonize islands, small species tend to evolve larger bodies, and large species tend to evolve smaller bodies. With the arrival of humans and associated predators (dogs, cats, rats, pigs), many giant island endemics have become extinct.

Possible causes[edit]

Large mammalian carnivores are often absent on islands because of insufficient range or difficulties in over-water dispersal. In their absence, the ecological niches for large predators may be occupied by birds or reptiles, which can then grow to larger-than-normal size. For example, on prehistoric Gargano Island in the Miocene-Pliocene Mediterranean, on islands in the Caribbean like Cuba, and on Madagascar and New Zealand, some or all apex predators were birds like eagles, falcons and owls, including some of the largest known examples of these groups. However, birds and reptiles generally make less efficient large predators than advanced carnivorans.

Since small size usually makes it easier for herbivores to escape or hide from predators, the decreased predation pressure on islands can allow them to grow larger.[a] Small herbivores may also benefit from the absence of competition from missing types of large herbivores.

Benefits of large size that have been suggested for island tortoises include decreased vulnerability to scarcity of food and/or water, through ability to survive for longer intervals without them, or ability to travel longer distances to obtain them. Periods of such scarcity may be a greater threat on oceanic islands than on the mainland.[3]

Thus, island gigantism is usually an evolutionary trend resulting from the removal of constraints on the size of small animals related to predation and/or competition. Such constraints can operate differently depending on the size of the animal, however; for example, while small herbivores may escape predation by hiding, large herbivores may deter predators by intimidation. As a result, the complementary phenomenon of island dwarfism can also result from the removal of constraints related to predation and/or competition on the size of large herbivores.[4] In contrast, insular dwarfism among predators more commonly results from the imposition of constraints associated with the limited prey resources available on islands.[4] As opposed to island dwarfism, island gigantism is found in most major vertebrate groups and in invertebrates.

A further means of establishing island gigantism may be a founder effect operative when larger members of a mainland population are superior in their ability to colonize islands.[5]

Examples[edit]

Examples of island gigantism include:

Mammals[edit]

Archaeoindris, an extinct gorilla-sized sloth lemur of Madagascar.

Many rodents grow larger on islands, whereas carnivores, proboscideans and artiodactyls usually become smaller.

Birds[edit]

The kakapo of New Zealand is the heaviest, and only flightless, parrot.
The dodo, an extinct flightless pigeon of Mauritius.
The largest tortoises in the world are found on the Seychelles and Galápagos Islands (example of the latter shown).

Reptiles[edit]

The Komodo dragon of Flores and nearby islands is the largest living lizard.

Arthropods[edit]

Flora[edit]

Island plants often also exhibit "insular woodiness".[16]

See also[edit]

Notes[edit]

  1. ^ The reduction in predation on islands often also leads to tamer behavior of island prey species, a trend that has been analyzed in lizards.[1][2]

References[edit]

  1. ^ Cooper, W. E.; Pyron, R. A.; Garland, T. (2014-01-08). "Island tameness: Living on islands reduces flight initiation distance". Proceedings of the Royal Society B: Biological Sciences 281 (1777): 20133019. doi:10.1098/rspb.2013.3019.  edit
  2. ^ Yong, E. (2014-01-08). "Islands make animals tamer". Nature. doi:10.1038/nature.2014.14462.  edit
  3. ^ Jaffe, A. L.; Slater, G. J.; Alfaro, M. E. (2011-01-26). "The evolution of island gigantism and body size variation in tortoises and turtles". Biology Letters 7 (4): 558–561. doi:10.1098/rsbl.2010.1084. PMID 21270022.  edit
  4. ^ a b Raia, P.; Meiri, S. (August 2006). "The island rule in large mammals: paleontology meets ecology". Evolution 60 (8): 1731–1742. doi:10.1111/j.0014-3820.2006.tb00516.x. Retrieved 2011-11-27. 
  5. ^ Lomolino, M. V. (2005-09-05). "Body size evolution in insular vertebrates: generality of the island rule". Journal of Biogeography 32 (10): 1683–1699. doi:10.1111/j.1365-2699.2005.01314.x. Retrieved 2011-11-19. 
  6. ^ a b Naish, Darren (2008-01-28). "Titan-hawks and other super-raptors". Tetrapod Zoology blog. ScienceBlogs LLC. Retrieved 2011-03-02. 
  7. ^ Meijer H. J. M.; Due, R. A. (2010-11-04). "A new species of giant marabou stork (Aves: Ciconiiformes) from the Pleistocene of Liang Bua, Flores (Indonesia)". Zoological Journal of the Linnean Society 160 (4): 707–724. doi:10.1111/j.1096-3642.2010.00616.x. 
  8. ^ a b Hansen, D. M.; Donlan, C. J.; Griffiths, C. J.; Campbell, K. J. (April 2010). "Ecological history and latent conservation potential: large and giant tortoises as a model for taxon substitutions". Ecography (Wiley) 33 (2): 272–284. doi:10.1111/j.1600-0587.2010.06305.x. Retrieved 2012-03-02. 
  9. ^ Cione, A. L.; Tonni, E. P.; Soibelzon, L. (2003). "The Broken Zig-Zag: Late Cenozoic large mammal and tortoise extinction in South America". Rev. Mus. Argentino Cienc. Nat., n.s. 5 (1): 1–19. ISSN 1514-5158. Retrieved 2011-02-06. 
  10. ^ Harrison, T. (2011). "Tortoises (Chelonii, Testudinidae)". Paleontology and Geology of Laetoli: Human Evolution in Context, Vol. 2: Fossil Hominins and the Associated Fauna. Vertebrate Paleobiology and Paleoanthropology. Springer Science+Business Media. pp. 479–503. doi:10.1007/978-90-481-9962-4_17. ISBN 978-90-481-9961-7. 
  11. ^ Hocknull, S. A.; Piper, P. J.; van den Bergh, G. D.; Due, R. A.; Morwood, M. J.; Kurniawan, I. (2009-09-30). Turvey, Samuel T, ed. "Dragon's Paradise Lost: Palaeobiogeography, Evolution and Extinction of the Largest-Ever Terrestrial Lizards (Varanidae)". PLoS ONE 4 (9): e7241. doi:10.1371/journal.pone.0007241. PMC 2748693. PMID 19789642. Retrieved 2012-02-09. 
  12. ^ Pregill, G. K.; Worthy, T. H. (March 2003). "A New Iguanid Lizard (Squamata, Iguanidae) from the Lare Quaternary of Fiji, Southwest Pacific". Herpetologica (The Herpetologists' League) 59 (1): 57–67. doi:10.1655/0018-0831(2003)059[0057:ANILSI]2.0.CO;2. ISSN 0018-0831. 
  13. ^ Maca-Meyer, N.; Carranza, S.; Rando, J. C.; Arnold, E. N. & Cabrera, V. M. (2003-12-01). "Status and relationships of the extinct giant Canary Island lizard Gallotia goliath (Reptilia: Lacertidae), assessed using ancient mtDNA from its mummified remains". Biological Journal of the Linnean Society (Linnean Society of London) 80 (4): 659–670. doi:10.1111/j.1095-8312.2003.00265.x. Retrieved 2010-04-03. 
  14. ^ a b Keogh, J. S.; Scott, I. A. W.; Hayes, C. (January 2005). "Rapid and repeated origin of insular gigantism and dwarfism in Australian tiger snakes". Evolution 59 (1): 226–233. doi:10.1111/j.0014-3820.2005.tb00909.x. 
  15. ^ Keppel, Gunnar; Lowe, Andrew J.; Possingham, Hugh P. (2009). "Changing perspectives on the biogeography of the tropical South Pacific: influences of dispersal, vicariance and extinction". Journal of Biogeography 36 (6): 1035–1054. doi:10.1111/j.1365-2699.2009.02095.x. ISSN 0305-0270. 
  16. ^ Bowen, Lizabeth; Vuren, Dirk Van (1997). "Insular Endemic Plants Lack Defenses Against Herbivores". Conservation Biology 11 (5): 1249–1254. doi:10.1046/j.1523-1739.1997.96368.x. ISSN 0888-8892. 

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