Island syndrome

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The southern cassowary, a ratite native to Indonesia, New Guinea and northeastern Australia[1] 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]

Ecological driving factors

  • Reduced predation. Island ecosystems cannot support a sufficient biomass of prey in order to accommodate large predators. This largely relieves prey species of the risk of predation,[2] which mostly removes the selection pressure for morphologies, ecologies and behaviours that help to evade large predators.
  • Reduced biodiversity. Insular ecosystems tend to comprise large populations of a limited number of species (a state termed density compensation), therefore, they exhibit low biodiversity. This results in reduced interspecific competition and increased intraspecific competition.[2]
  • Reduced sexual selection. There is also reduced sexual selection in insular species, which is especially prominent in birds which lose their sexually dimorphic plumage used in sexual displays.[4]
  • Reduced parasite diversity. Finally, there is reduced parasite diversity in insular ecosystems[5] which reduces the level of selection acting on immune-related genes.

Features of island syndrome in animals

Body size

See also:
Island dwarfism, and Island gigantism
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

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

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

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

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

The

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

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

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

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

Main article: Hațeg Island
pterosaurs to have ever lived, pictured hunting Zalmoxes
.
dinosaurs
due to its shorter, stockier forelimbs.
Magyarosaurus dacus
, and a human

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

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
.

References

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  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.
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See also
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