Island syndrome

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.

Ecological driving factors

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, which mostly removes the selection pressure for morphologies, ecologies and behaviours that help to evade large predators. 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. This means that insular species experience greater sexual selection and less natural selection compared to continental species, which contributes to the evolution of an island syndrome in these species. Finally, there is reduced parasite diversity in insular ecosystems which reduces the level of selection acting on immune-related genes.

Features of Island Syndrome in Animals

Body size

A coconut crab, an example of insular gigantism. It is sat 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. 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 Hippopotamus). These are examples are insular gigantism and insular dwarfism respectively.

Although, the giant tortoises of the Galápagos Islands and the Seychelles are sometimes given as examples of insular gigantism, 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 (Hesperotestudo) and South America (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 populated 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. This has occurred in several ratites including the kiwi and the Cassowary as well as in the dodo after invading island habitats. 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. Insular populations of barn owl have shorter wings, representing a transitional stage in which their capacity for flight is being reduced.

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  Skeleton and model of the dodo, an exctinct flightless bird from Mauritius
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  The Okarito kiwi, a small flightless ratite native to the South Island of New Zealand
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  The Southern cassowary, a flightless ratite native to Indonesia, New Guinea and northeastern Australia

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. As a result, insular bird species often exhibit duller, sexually monomorphic plumage.

Several insular species acquire increased melanin colouration. Male white-winged fairywrens living on mainland Australia exhibit a blue nupital plumage, wherease two island subspecies (Malurus leucopterus leucopterus from Dirk Hartog Island and Malurus leucopterus edouardi from Barrow Island) exhibit a black nupital plumage. A subspecies of the Chestnut-bellied monarch, Monarcha castaneiventris obscurior endemic to the Solomon Islands 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.

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 recieves greater parental investment, maximising their fitness. Lizards endemic to island ecosystems lay smaller clutches that give larger offspring compared to continental lizards of a similar size. However, it should be noted that, due to increased frequency of laying in insular lizards, continental and insular lizards produced offspring at a comparable rate.

Brain Size

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 without increases in predation risk. The endocranial volume of the extinct Malagasy dwarf hippos is 30% less than that of an equally sized continental ancestor while the early human Homo floresiensis had a similar sized brain to significantly earlier Australopithecus specimens from mainland Africa.

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.

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). 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.

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.

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

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. This may be due to reduced interspecies 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.

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.

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. 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.

Reversed Island Syndrome

Reversed island syndrome (RIS) was first used by Pasquale Raia in 2010 to describe the differences in morphology, ecology and behaviour observed in insular species when population density is either low or fluctuating. 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. The male lizards exhibit elevated α-MSH levels relative to mainland populations, which increases the basal metabolic rate, strengthens immune responses, produces darker blue coloration and raises 5α-dihydrotestosterone levels. The latter improves male reproductive success by increasing the likelihood of winning sexual conflicts over females and augmenting sperm quality. 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.

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  The insular subspecies of Italian wall lizard,Podarcis siculus klemmeri, exhibits darker blue coloration.
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  A mainland phenotype of Italian wall lizard with a lighter green-brown coloration

See Also

• Island ecology