Island syndrome

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The southern cassowary, a ratite native to Indonesia, New Guinea and northeastern Australia which exhibits island gigantism and sexually monomorphic plumage, both features of island syndrome Kasuar prilbovy.jpg
The southern cassowary, a ratite native to Indonesia, New Guinea and northeastern Australia which exhibits island gigantism and sexually monomorphic plumage, both features of island syndrome

Island syndrome describes the differences in morphology, ecology, physiology and behaviour of insular species compared to their continental counterparts. These differences evolve due to the different ecological pressures affecting insular species, including a paucity of large predators and herbivores as well as a consistently mild climate. [2] [3]

Contents

Ecological driving factors

Features of island syndrome in animals

Body size

A coconut crab, an example of insular gigantism. It is placed atop a coconut for size comparison Birgus latro.jpg
A coconut crab, an example of insular gigantism. It is placed atop a coconut for size comparison

Interspecific competition between continental species drives divergence of body size so that species may avoid high levels of competition by occupying distinct niches. Reduced interspecific competition between insular species reduces this selection pressure for species to occupy distinct niches. [6] As a result, there is less diversity in the body size of insular species. Typically small mammals increase in size (for example fossa are a larger insular relative of the mongoose) while typically large mammals decrease in size (for example the Malagasy hippopotamuses are smaller insular relatives of continental hippopotamuses). These are examples of insular gigantism and insular dwarfism respectively. This observed effect is called Foster's rule. Conversely, birds and reptiles tend to exhibit insular gigantism, exemplified by the moa, cassowary and Komodo dragon.

Although the giant tortoises of the Galápagos Islands and the Seychelles (the Galápagos tortoise and Aldabrachelys respectively) are sometimes given as examples of insular gigantism, [2] they are now thought to represent the last remaining populations of historically widespread giant tortoises. The remains of tortoises of similar or larger size have been found in Australia ( Meiolania ), southern Asia ( Megalochelys ), Madagascar ( Aldabrachelys ), North America [7] ( Hesperotestudo ) and South America [8] ( Chelonoidis ). The extant giant tortoises are thought to persist only in a few remote archipelagos because humans arrived there relatively late and have not heavily predated them, suggesting that these tortoise populations have been less subjected to overexploitation.

Locomotion

Since insular prey species experience a reduced risk of predation, they often lose or reduce morphologies utilised in predator evasion. For example, the wings of weevils, rails and pigeons have become so reduced in insular species that many have lost the ability to fly. [2] This has occurred in several ratites including the kiwi and the cassowary as well as in the dodo and the kākāpō after invading island habitats. The extinct moa of New Zealand exhibit the most extreme known example of insular wing reduction; there is no osseous evidence of even vestigial wings and the pectoral girdle is reduced to a scapulocoracoideum which would be unable to bear a forelimb as it lacks a glenoid fossa. [9] Therefore, it is the only bird known to have completely lost its wings after a shift to insularity. Loss of flight allows birds to eliminate the costs of maintaining large flight-enabling muscles like the pectoral muscles and allows the skeleton to become heavier and stronger. [10] Insular populations of barn owl have shorter wings, representing a transitional stage in which their capacity for flight is being reduced. [11]

Adaptive coloration

Due to the reduced sexual selection of insular species, they tend to exhibit reduced sexual coloration so as to conserve the energy that this demands. Additionally, the low biodiversity of insular ecosystems makes species recognition less important so species-specific coloration is under less selection. [5] As a result, insular bird species often exhibit duller, sexually monomorphic plumage. [5]

Several insular species acquire increased melanin colouration. Male white-winged fairywrens living on mainland Australia exhibit a blue nuptial plumage, whereas two island subspecies ( Malurus leucopterus leucopterus from Dirk Hartog Island and Malurus leucopterus edouardi from Barrow Island) exhibit a black nuptial plumage. [12] A subspecies of the chestnut-bellied monarch endemic to the Solomon Islands, Monarcha castaneiventris obscurior, exhibits polymorphism in plumage color: some birds are black with a chestnut-colored belly while others are completely melanic. The frequency of the melanic phenotype increases on smaller islands, even when the relative proximity of the islands is accounted for. [13]

Reproduction

High levels of intraspecific competition between offspring selects for the very fittest individuals. As a result, insular parents tend to produce fewer offspring so that each offspring receives greater parental investment, maximising their fitness. [2] Lizards endemic to island ecosystems lay smaller clutches that give larger offspring compared to continental lizards of a similar size. Because of increased frequency of laying in insular lizards, continental and insular lizards produced offspring at a comparable rate. [14]

Brain size

Homo floresiensis skull demonstrating the reduced neurocranium Homo Florensiensis-MGL 95216-P5030051-white.jpg
Homo floresiensis skull demonstrating the reduced neurocranium

The expensive tissue hypothesis suggests that tissues with a high metabolic demand like the brain will become reduced if they confer little selective advantage and so do not help to increase food intake. The paucity of large predators means that insular species can afford to become slower and less alert without suffering from massively increased predation risk. As such, reduction in relative brain size is often seen in insular species as this reduces basal metabolic rate [15] without increases in predation risk. For example, the endocranial volume of the extinct Malagasy dwarf hippos is 30% less than that of an equally sized continental ancestor. [16] Similarly, the early human, Homo floresiensis , had a brain of similar size to that of the significantly earlier Australopithecus specimens from mainland Africa [17] [18] and 3.4 times smaller than that of Homo sapiens which evolved later (see Evolution of human brain size).

Poikilothermy

Due to low predation risk, insular prey species can afford to reduce their basal metabolic rate by adopting poikilothermy without any significant increase in predation risk. As a result, poikilothermy is far more common in island species. [10]

Behaviour

Due to lack of predation, insular species tend to become more docile and less territorial than their continental counterparts (sometimes referred to as island tameness ). [2] [19] Deer mice, song sparrows and bronze anoles all have smaller territories with greater overlap compared to their mainland conspecifics. They are also more tolerant to intruders. Falkland Island foxes and Tammar wallabies have both lost an innate fear of large predators including humans. [2]

In parasites

The nematode parasite Heligmosomoides polygyrus underwent niche expansion (by invading new host species) and a reduction in genetic diversity after invading ecosystems in seven western Mediterranean islands. The loss of genetic diversity was related to the distance between the contemporal population and the mainland origin. [20]

In plants

A megaherb community of Ross lilies and Cambell Island carrots exhibit yellow and pink flowers respectively to attract alternate pollinators. They both exhibit insular gigantism Megaherb community 2.jpg
A megaherb community of Ross lilies and Cambell Island carrots exhibit yellow and pink flowers respectively to attract alternate pollinators. They both exhibit insular gigantism

Plant structure

Plant stature and leaf area both follow the pattern of insular mammals, with small species becoming larger and large species becoming smaller in island populations. [2] [6] This may be due to reduced interspecific competition which would decrease the ecological drive for plants to occupy separate niches. Due to reduced biomass of large herbivores, several island plants lose protective spines and thorns as well as decreasing the amounts of defensive chemicals produced. The improbability of island fires also results in a loss of fire-resistance in bark, fruits and cones. Insular woodiness, the evolutionary transition from herbaceousness toward woodiness, is a very common phenomenon among island floras. [21]

Reproduction and dispersal

Due to a lack of dedicated pollinators on remote islands, insular plants often use small, inconspicuously colored and easily accessible flowers to attract a range of alternative pollinator species. Self-pollination is also more commonly used by insular plant species, as pollen does not have to travel so far to reach a receptive ovule or stigma. Seeds exhibit insular gigantism, becoming predominantly larger than mainland seeds, which is thought to improve mortality at sea during dispersal. [2] [6]

Consequences of island syndrome for conservation

The relaxed predation risk in island ecosystems has resulted in the loss of several adaptations and behaviours that act to evade or discourage predation. This makes insular species particularly vulnerable to exploitation by alien species. For example, when humans first introduced dogs, pigs, cats, rats, and crab-eating macaques to the island of Mauritius in the 17th century, they plundered dodo nests and increased interspecies competition for the limited food resources. [22] This ultimately resulted in the dodo's extinction. The limited resources in island ecosystems are also vulnerable to overexploitation if they are not managed sustainably.

Hațeg Island

Hatzegopteryx was the apex predator of Hateg island and one of the largest pterosaurs to have ever lived, pictured hunting Zalmoxes. Hatzegopteryx.png
Hatzegopteryx was the apex predator of Hațeg island and one of the largest pterosaurs to have ever lived, pictured hunting Zalmoxes .
The basal avialan Balaur bondoc, which resembles dromaeosaurid dinosaurs due to its shorter, stockier forelimbs. Balaur in Hateg environment.jpg
The basal avialan Balaur bondoc , which resembles dromaeosaurid dinosaurs due to its shorter, stockier forelimbs.
Size comparison of the dwarf titanosaur, Magyarosaurus dacus, and a human Magyarosaurus- human size.JPG
Size comparison of the dwarf titanosaur, Magyarosaurus dacus , and a human

Hațeg Island was a large offshore island in the Tethys Sea of the Late Cretaceous [23] and is often called "The Island of the Dwarf Dinosaurs" on account of the extensive fossil evidence that its native dinosaurs exhibited island dwarfism. The island's native titanosaur, Magyarosaurus dacus , had a body mass of only 900 kilograms (2,000 lb) [24] [25] while mainland titanosaurs like Patagotitan could reach up to 69 tonnes (76 tons). The pterosaur Hatzegopteryx took over the position of apex predator of Hațeg Island in the absence of any hypercarnivorous dinosaurs and likely hunted juvenile dwarf dinosaurs or even adults of the smaller species. Based on its robust jaw and cervical vertebrae, Hatzegopteryx is thought to have hunted in a similar manner to modern storks by attacking prey that are too large to swallow whole. [26] Its wingspan is estimated to have reached up to 10 to 12 metres (33 to 39 ft), making it one of the largest pterosaurs to have ever lived. As such, it is a potent example of island gigantism, in this case to fill the otherwise empty niche of apex predator. Balaur bondoc was originally classified as a dromaeosaurid dinosaur based on its retractable toe claws. Its forelimbs appeared to be too short and stocky for it to be a basal avialan, however, phylogenetic analysis later confirmed that Balaur was indeed a basal member of Avialae, a clade that includes modern birds. Its limbs were clearly incapable of powered flight and so Balaur is yet another example of the secondary loss of flight after invading an island niche, similar to ratites as well as the extinct moa and the dodo (See Insular reduction in flight capacity).

Reversed island syndrome

The term "reversed island syndrome" (RIS) was first used by Pasquale Raia in 2010 to describe the differences in morphology, ecology, physiology and behaviour observed in insular species when population density is either low or fluctuating. [27] This results in stronger natural selection and weaker intraspecific selection, leading to different phenotypes compared to the standard island syndrome.

RIS was first described in a population of Italian wall lizard endemic to the Licosa Islet where the unpredictable environmental conditions and highly fluctuating population density have selected for aggressive behaviour and increased reproductive effort. [28] The male lizards exhibit elevated α-MSH levels relative to mainland populations, which increases the basal metabolic rate, strengthens immune responses, [29] produces darker blue coloration and raises 5α-dihydrotestosterone levels. [28] The latter improves male reproductive success by increasing the likelihood of winning sexual conflicts over females and augmenting sperm quality. [28] Females produce similar numbers of eggs compared to mainland populations but the eggs of insular females are significantly heavier, reflecting increased reproductive effort. The unpredictable conditions produce high mortality rates so adults invest more effort into current broods since they are less likely to survive to produce subsequent broods i.e. there is low interbrood conflict.

Related Research Articles

<span class="mw-page-title-main">Ratite</span> Polyphyletic group of birds

Ratites are a polyphyletic group consisting of all birds within the infraclass Palaeognathae that lack keels and cannot fly. They are mostly large, long-necked, and long-legged, the exception being the kiwi, which is also the only nocturnal extant ratite.

<span class="mw-page-title-main">Megafauna</span> Large animals

In zoology, megafauna are large animals. The precise definition of the term varies widely, though a common threshold is approximately 45 kilograms (99 lb), with other thresholds as low as 10 kilograms (22 lb) or as high as 1,000 kilograms (2,200 lb). Large body size is generally associated with other traits, such as having a slow rate of reproduction and, in large herbivores, reduced or negligible adult mortality from being killed by predators.

<span class="mw-page-title-main">Giant tortoise</span> Several species of land tortoise

Giant tortoises are any of several species of various large land tortoises, which include a number of extinct species, as well as two extant species with multiple subspecies formerly common on the islands of the western Indian Ocean and on the Galápagos Islands.

<span class="mw-page-title-main">Bergmann's rule</span> Biological rule stating that larger size organisms are found in colder environments

Bergmann's rule is an ecogeographical rule that states that, within a broadly distributed taxonomic clade, populations and species of larger size are found in colder environments, while populations and species of smaller size are found in warmer regions. The rule derives from the relationship between size in linear dimensions meaning that both height and volume will increase in colder environments. Bergmann's rule only describes the overall size of the animals, but does not include body proportions like Allen's rule does.

<span class="mw-page-title-main">Flightless bird</span> Birds that cannot fly

Flightless birds are birds that cannot fly. They have, through evolution, lost the ability to fly. There are over 60 extant species, including the well-known ratites and penguins. The smallest flightless bird is the Inaccessible Island rail. The largest flightless bird, which is also the largest living bird in general, is the common ostrich.

<span class="mw-page-title-main">Foster's rule</span> Ecogeographical rule in evolutionary biology

Foster's rule, also known as the island rule or the island effect, is an ecogeographical rule in evolutionary biology stating that members of a species get smaller or bigger depending on the resources available in the environment. For example, it is known that pygmy mammoths evolved from normal mammoths on small islands. Similar evolutionary paths have been observed in elephants, hippopotamuses, boas, sloths, deer and humans. It is part of the more general phenomenon of island syndrome which describes the differences in morphology, ecology, physiology and behaviour of insular species compared to their continental counterparts.

Insular biogeography or island biogeography is a field within biogeography that examines the factors that affect the species richness and diversification of isolated natural communities. The theory was originally developed to explain the pattern of the species–area relationship occurring in oceanic islands. Under either name it is now used in reference to any ecosystem that is isolated due to being surrounded by unlike ecosystems, and has been extended to mountain peaks, seamounts, oases, fragmented forests, and even natural habitats isolated by human land development. The field was started in the 1960s by the ecologists Robert H. MacArthur and E. O. Wilson, who coined the term island biogeography in their inaugural contribution to Princeton's Monograph in Population Biology series, which attempted to predict the number of species that would exist on a newly created island.

<span class="mw-page-title-main">Island gigantism</span> Evolutionary phenomena leading to an increase of the size of species with insularity

Island gigantism, or insular gigantism, is a biological phenomenon in which the size of an animal species isolated on an island increases dramatically in comparison to its mainland relatives. Island gigantism is one aspect of the more general "island effect" or "Foster's rule", which posits that when mainland animals colonize islands, small species tend to evolve larger bodies, and large species tend to evolve smaller bodies. This is itself one aspect of the more general phenomenon of island syndrome which describes the differences in morphology, ecology, physiology and behaviour of insular species compared to their continental counterparts. Following the arrival of humans and associated introduced predators, many giant as well as other island endemics have become extinct. A similar size increase, as well as increased woodiness, has been observed in some insular plants such as the Mapou tree in Mauritius which is also known as the "Mauritian baobab" although it is member of the grape family (Vitaceae).

<span class="mw-page-title-main">Insular dwarfism</span> Form of phyletic dwarfism occurring on islands

Insular dwarfism, a form of phyletic dwarfism, is the process and condition of large animals evolving or having a reduced body size when their population's range is limited to a small environment, primarily islands. This natural process is distinct from the intentional creation of dwarf breeds, called dwarfing. This process has occurred many times throughout evolutionary history, with examples including various species of dwarf elephants that evolved during the Pleistocene epoch, as well as more ancient examples, such as the dinosaurs Europasaurus and Magyarosaurus. This process, and other "island genetics" artifacts, can occur not only on islands, but also in other situations where an ecosystem is isolated from external resources and breeding. This can include caves, desert oases, isolated valleys and isolated mountains. Insular dwarfism is one aspect of the more general "island effect" or "Foster's rule", which posits that when mainland animals colonize islands, small species tend to evolve larger bodies, and large species tend to evolve smaller bodies. This is itself one aspect of island syndrome, which describes the differences in morphology, ecology, physiology and behaviour of insular species compared to their continental counterparts.

<i>Nuralagus</i> Extinct genus of leporid

Nuralagus is an extinct genus of leporid, with a single species, Nuralagus rex, described in 2011. It lived on Menorca, one of the Balearic Islands in the western Mediterranean during the Pliocene epoch. It is the largest known lagomorph to have ever existed, with an estimated weight of 8–12 kilograms (18–26 lb), nearly double the weight of the average Flemish Giant rabbit. It likely went extinct at the Pliocene-Pleistocene transition when Mallorca and Menorca were united as one island, letting the mammalian fauna of Mallorca, including the goat-like ungulate Myotragus, colonize Nuralagus's habitat.

<i>Palaeoloxodon falconeri</i> Extinct species of elephant

Palaeoloxodon falconeri is an extinct species of dwarf elephant from the Middle Pleistocene of Sicily and Malta. It is amongst the smallest of all dwarf elephants at under 1 metre (3.3 ft) in height. A member of the genus Palaeoloxodon, it derived from a population of the mainland European straight-tusked elephant.

<span class="mw-page-title-main">Island tameness</span> Natural tendency of behaviour as a result of a lack of predators and prolonged geographic isolation

Island tameness is the tendency of many populations and species of animals living on isolated islands to lose their wariness of potential predators, particularly of large animals. The term is partly synonymous with ecological naïveté, which also has a wider meaning referring to the loss of defensive behaviors and adaptations needed to deal with these "new" predators. Species retain such wariness of predators that exist in their environment; for example, a Hawaiian goose retains its wariness of hawks, but does not exhibit such behaviors with mammals or other predators not found on the Hawaiian Islands. The most famous example is the dodo, which owed its extinction in large part to a lack of fear of humans, and many species of penguin.

<span class="mw-page-title-main">Hațeg Island</span> Prehistoric island

Hațeg Island was a large offshore island in the Tethys Sea which existed during the Late Cretaceous period, probably from the Cenomanian to the Maastrichtian ages. It was situated in an area corresponding to the region around modern-day Hațeg, Hunedoara County, Romania. Maastrichtian fossils of small-sized dinosaurs have been found in the island's rocks. It was formed mainly by tectonic uplift during the early Alpine orogeny, caused by the collision of the African Plate and Eurasian Plate towards the end of the Cretaceous. There is no real present-day analog, but overall, the island of Hainan is perhaps closest as regards climate, geology and topography, though still not a particularly good match. The vegetation, for example, was of course entirely distinct from today, as was the fauna.

<span class="mw-page-title-main">Seed predation</span> Feeding on seeds as a main or exclusive food source

Seed predation, often referred to as granivory, is a type of plant-animal interaction in which granivores feed on the seeds of plants as a main or exclusive food source, in many cases leaving the seeds damaged and not viable. Granivores are found across many families of vertebrates as well as invertebrates ; thus, seed predation occurs in virtually all terrestrial ecosystems.

<i>Candiacervus</i> Extinct genus of deer

Candiacervus is an extinct genus of deer native to Pleistocene Crete. Due to a lack of other herbivores, the genus underwent an adaptive radiation, filling niches occupied by other taxa on the mainland. Due to the small size of Crete, some species underwent insular dwarfism, the smallest species, C. ropalophorus, stood about 40 centimetres (16 in) at the shoulders when fully grown, while other species were relatively large and comparable in size to mainland deer species. Some species are noted for their peculiar, elongate club-shaped antlers, though other species have more normal antlers.

<span class="mw-page-title-main">Italian wall lizard</span> Species of lizard

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<span class="mw-page-title-main">Deep-sea gigantism</span> Tendency for deep-sea species to be larger than their shallower-water relatives

In zoology, deep-sea gigantism or abyssal gigantism is the tendency for species of deep-sea dwelling animals to be larger than their shallower-water relatives across a large taxonomic range. Proposed explanations for this type of gigantism include necessary adaptation to colder temperature, food scarcity, reduced predation pressure and increased dissolved oxygen concentrations in the deep sea. The harsh conditions and inhospitality of the underwater environment in general, as well as the inaccessibility of the abyssal zone for most human-made underwater vehicles, have hindered the study of this topic.

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<i>Balaur bondoc</i> Extinct species of dinosaurs

Balaur is a genus of theropod dinosaur from the late Cretaceous period, in what is now Romania. It is the type species of the monotypic genus Balaur, after the balaur, a dragon of Romanian folklore. The specific name bondoc means "stocky", so Balaur bondoc means "stocky dragon" in Romanian. This name refers to the greater musculature that Balaur had compared to its relatives. The genus, which was first described by scientists in August 2010, is known from two partial skeletons. Some researchers suggest that the taxon might represent a junior synonym of Elopteryx.

<span class="mw-page-title-main">Egg predation</span> Feeding strategy for many animals

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  22. ↑ Hume, J.P.; Walters, M. (2012). Extinct Birds. London: A & C Black. pp. 134–136. ISBN   978-1-4081-5725-1.
  23. ↑ Benton, M.J., Csiki, Z., Grigorescu, D., Redelstorff, R., Sander, P.M., Stein, K., and Weishampel, D.B. (2010). "Dinosaurs and the island rule: The dwarfed dinosaurs from Hațeg Island. Archived 2011-07-10 at the Wayback Machine " Palaeogeography, Palaeoclimatology, Palaeoecology, 293(3-4): 438–454.
  24. ↑ Carballido, J.L.; Pol, D.; Otero, A.; Cerda, I.A.; Salgado, L.; Garrido, A.C.; Ramezani, J.; Cúneo, N.R.; Krause, J.M. (2017). "A new giant titanosaur sheds light on body mass evolution among sauropod dinosaurs". Proceedings of the Royal Society B: Biological Sciences. 284 (1860): 20171219. doi:10.1098/rspb.2017.1219. PMC   5563814 . PMID   28794222.
  25. ↑ Stein, K.; Csiki, Z.; Rogers, K. C.; Weishampel, D. B.; Redelstorff, R.; Carballido, J. L.; Sander, P. M. (30 April 2010). "Small body size and extreme cortical bone remodeling indicate phyletic dwarfism in Magyarosaurus dacus (Sauropoda: Titanosauria)". Proceedings of the National Academy of Sciences. 107 (20): 9258–9263. Bibcode:2010PNAS..107.9258S. doi: 10.1073/pnas.1000781107 . PMC   2889090 . PMID   20435913.
  26. ↑ Witton, M.P.; Naish, D. (2015). "Azhdarchid pterosaurs: water-trawling pelican mimics or "terrestrial stalkers"?". Acta Palaeontologica Polonica. 60 (3). doi: 10.4202/app.00005.2013 .
  27. ↑ Raia, Pasquale; Guarino, Fabio M; Turano, Mimmo; Polese, Gianluca; Rippa, Daniela; Carotenuto, Francesco; Monti, Daria M; Cardi, Manuela; Fulgione, Domenico (20 September 2010). "The blue lizard spandrel and the island syndrome". BMC Evolutionary Biology. 10 (289): 289. Bibcode:2010BMCEE..10..289R. doi: 10.1186/1471-2148-10-289 . PMC   2949876 . PMID   20854657.
  28. 1 2 3 Monti, Daria Maria; Raia, Pasquale; Vroonen, Jessica; Maselli, Valeria; Van Damme, Raoul; Fulgione, Domenico (January 2013). "Physiological change in an insular lizard population confirms the reversed island syndrome". Biological Journal of the Linnean Society. 108 (1): 144–150. doi: 10.1111/j.1095-8312.2012.02019.x .
  29. ↑ Baeckens, Simon; Van Damme, Raoul (2018). "Immunocompetence and parasite infestation in a melanistic and normally-coloured population of the lacertid lizard, Podarcis siculus". Amphibia-Reptilia. 39 (4): 471–478. doi:10.1163/15685381-20181008. hdl: 10067/1550590151162165141 . ISSN   0173-5373. S2CID   52245927.

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