Island Ecology
Encyclopedia
Island
ecology
is the study of island organisms and their interactions with each other and the environment. Islands account for nearly 1/6 of earth’s total land area, yet the ecology of island ecosystems is vastly different from that of mainland communities. Their isolation and high availability of empty niche
s leads to increased speciation
. As a result, island ecosystems comprise 30% of the world’s biodiversity hotspots, 50% of marine tropical diversity, and some of the most unusual and rare species. Many species still remain unknown.
The diversity of species on islands is highly impacted by human activities such as deforestation
and introduction of exotic species. In response, ecologists and managers are directing attention towards conservation
and restoration
of island species. Because they are simple systems, islands provide an opportunity to study processes of extinction
that can be extrapolated to larger ecosystems.
as species colonize the island (see theory of island biogeography
). New species cannot immigrate via land, and instead must arrive via air, water, or wind. As a result, organisms with high dispersal capabilities, such as plants and birds, are much more common on islands than are poorly dispersing taxa like mammals. However, some mammals are present on islands, presumably from swimming or riding on natural “rafts” that are washed away from the mainland.
Of the species that arrive, only some will be able to survive and establish populations. As a result, islands have fewer species than mainland habitats. Island populations are small and exhibit low genetic variability (see founder effect
), but are isolated from the predators and competitors that they initially evolved with. This can lead to a process called ecological release
, where a species is released from its ancestral community interactions and then colonizes new niches.
) or smaller (see island dwarfism). Some of these unique adaptations are reflected in charismatic ¬island species such as the giant tortoise
, Komodo dragon
, or pygmy mammoth
s.
After immigration, birds, and some reptiles or mammals, tend to become larger and predatory, showing relaxed intraspecific competition
. For mammals, small species will increase in size and large species will decrease in size. This is referred to as the “island rule,” and is suggested to minimize energy expenditure.
Other adaptations to life on islands include increased poikilothermy, relaxed anti-predator behavior, and reduced sexual selection
in animals, and loss of herbivore defenses and reduced dispersal in plants.
Endemism is often the result of adaptive radiation
. Adaptive radiation is when a single species colonizes an area and rapidly diversifies to fill all of the available niches. A common example is the assemblage of finch species documented by Charles Darwin
in the Galapagos Islands
. Darwin’s finches exhibited adaptive radiation by evolving different beak sizes to exploit the diversity of seeds present on the different islands.
Because the distributions of these populations are limited by their island habitats, they tend to have fewer individuals than their mainland counterparts and lower genetic variation. This, along with the behavioral and ecological factors mentioned above, makes island species more vulnerable to extinction.
, genetic variation, natural disturbances (hurricanes, volcanic eruptions) and human-caused disturbances (introduced species, habitat loss). Human-caused disturbances tend to be the greatest cause of mortality, and understanding the causes of extinction facilitates conservation efforts.
, or introduced species
.
, once found on the island of Mauritius
. It evolved to become large, flightless and docile, and was subsequently driven to extinction by humans and introduced predators.
, the depletion of the forest by humans not only resulted in widespread loss of species, but also the collapse of the island civilization. Today there are over 500 million people on islands, all dependent on local resources either directly (traditional use) or indirectly (ecotourism revenue). Population growth and development result in heavy deforestation
, pollution
, and over-exploitation. Overharvesting of ocean fauna is particularly troubling as yields of coral reef fish species are an important food source for island populations.
, the introduced brown tree snake
ate nearly all of the native vertebrate species to extinction. Feral cats and dogs have also greatly diminished native vertebrate populations on islands, through both predation and disease. Introduced ungulate
s are another major threat, as they graze on native vegetation and can destroy entire forests. Exotic grasses can out-compete native understory
species and increase the risk of fire. Lastly, social insects such as ants also cause major problems.
s, where direct effects on one species result in indirect effects on other species in the food web. An example is on Santa Cruz Island
of the California Channel Islands, where DDT
poisoning reduced bald eagle
populations. This, along with an abundance of introduced feral pigs for prey, allowed golden eagle
s to colonize the island and replace bald eagles. However, the golden eagles also ate native island fox
es. Fox population levels decreased to near extinction, while skunk
populations increased due to relaxed competition with foxes.
and do continued community education.
On land, island conservation focuses on the protection of species and their habitat. In some cases, conservation can be integrated with agricultural production. For example, the Acacia koa plantations and wooded pastures in Hawaii
are anthropogenically altered ecosystems, yet allow connectivity between forest fragments and thus maintain higher diversity than would open pasture. Other directions include habitat restoration, and eradication of introduced predators, ungulates, and exotic plants (via hunting, removal or biological control).
In marine ecosystems, there has been increasing establishment of “no-take” reserves. This allows for reestablishment of native species, and also augmentation of commercially harvested species. However, in both terrestrial and marine systems, these actions are expensive and do not always result in the desired outcomes. For example, some non-natives become keystone species
and their removal can cause more harm than good to the ecosystem. To be more effective, managers of island ecosystems should share information and learn from each other’s mistakes.
.
Active restoration on islands can be done for both animal species (translocations, induced breeding) and plant species (reforestation). Creating goals for restoration can be challenging because it is often impossible to restore the ecosystem to its “historic” or “normal” state, if that state can even be clearly defined. Restoration is never complete, as ecological communities are always in a state of change.
Island
An island or isle is any piece of sub-continental land that is surrounded by water. Very small islands such as emergent land features on atolls can be called islets, cays or keys. An island in a river or lake may be called an eyot , or holm...
ecology
Ecology
Ecology is the scientific study of the relations that living organisms have with respect to each other and their natural environment. Variables of interest to ecologists include the composition, distribution, amount , number, and changing states of organisms within and among ecosystems...
is the study of island organisms and their interactions with each other and the environment. Islands account for nearly 1/6 of earth’s total land area, yet the ecology of island ecosystems is vastly different from that of mainland communities. Their isolation and high availability of empty niche
Ecological niche
In ecology, a niche is a term describing the relational position of a species or population in its ecosystem to each other; e.g. a dolphin could potentially be in another ecological niche from one that travels in a different pod if the members of these pods utilize significantly different food...
s leads to increased speciation
Speciation
Speciation is the evolutionary process by which new biological species arise. The biologist Orator F. Cook seems to have been the first to coin the term 'speciation' for the splitting of lineages or 'cladogenesis,' as opposed to 'anagenesis' or 'phyletic evolution' occurring within lineages...
. As a result, island ecosystems comprise 30% of the world’s biodiversity hotspots, 50% of marine tropical diversity, and some of the most unusual and rare species. Many species still remain unknown.
The diversity of species on islands is highly impacted by human activities such as deforestation
Deforestation
Deforestation is the removal of a forest or stand of trees where the land is thereafter converted to a nonforest use. Examples of deforestation include conversion of forestland to farms, ranches, or urban use....
and introduction of exotic species. In response, ecologists and managers are directing attention towards conservation
Conservation biology
Conservation biology is the scientific study of the nature and status of Earth's biodiversity with the aim of protecting species, their habitats, and ecosystems from excessive rates of extinction...
and restoration
Restoration ecology
-Definition:Restoration ecology is the scientific study and practice of renewing and restoring degraded, damaged, or destroyed ecosystems and habitats in the environment by active human intervention and action, within a short time frame...
of island species. Because they are simple systems, islands provide an opportunity to study processes of extinction
Extinction
In biology and ecology, extinction is the end of an organism or of a group of organisms , normally a species. The moment of extinction is generally considered to be the death of the last individual of the species, although the capacity to breed and recover may have been lost before this point...
that can be extrapolated to larger ecosystems.
Ecological processes on islands
Islands are attractive sites for ecological research because they provide clear examples of evolution in action. They show interesting patterns of colonization, adaptation, and speciation.Colonization and establishment
Islands are surrounded by water, and may or may not exist as part of a continental land mass. Oceanic islands arise due to volcanic activity or reef growth, and usually subside over time due to erosion and changing sea levels. When islands emerge, they undergo the process of ecological successionEcological succession
Ecological succession, is the phenomenon or process by which a community progressively transforms itself until a stable community is formed. It is a fundamental concept in ecology, and refers to more or less predictable and orderly changes in the composition or structure of an ecological community...
as species colonize the island (see theory of island biogeography
Island biogeography
Island biogeography is a field within biogeography that attempts to establish and explain the factors that affect the species richness of natural communities. The theory was developed to explain species richness of actual islands...
). New species cannot immigrate via land, and instead must arrive via air, water, or wind. As a result, organisms with high dispersal capabilities, such as plants and birds, are much more common on islands than are poorly dispersing taxa like mammals. However, some mammals are present on islands, presumably from swimming or riding on natural “rafts” that are washed away from the mainland.
Of the species that arrive, only some will be able to survive and establish populations. As a result, islands have fewer species than mainland habitats. Island populations are small and exhibit low genetic variability (see founder effect
Founder effect
In population genetics, the founder effect is the loss of genetic variation that occurs when a new population is established by a very small number of individuals from a larger population. It was first fully outlined by Ernst Mayr in 1942, using existing theoretical work by those such as Sewall...
), but are isolated from the predators and competitors that they initially evolved with. This can lead to a process called ecological release
Ecological release
Ecological release occurs when a species is introduced to an environment other than its native habitat, the foreign species will either establish a local population, or die out from the region...
, where a species is released from its ancestral community interactions and then colonizes new niches.
Adaptation
In response to these changing ecological pressures, island species can become much more docile than their mainland counterparts, and may grow larger (see island gigantismIsland gigantism
Island gigantism or insular giantism is a biological phenomenon in which the size of animals isolated on an island increases dramatically in comparison to their mainland relatives....
) or smaller (see island dwarfism). Some of these unique adaptations are reflected in charismatic ¬island species such as the giant tortoise
Giant tortoise
Giant tortoises are characteristic reptiles of certain tropical islands. Often reaching enormous size—they can weigh as much as 300 kg and can grow to be 1.3 m long—they live, or lived , in the Seychelles, the Mascarenes and the Galapagos...
, Komodo dragon
Komodo dragon
The Komodo dragon , also known as the Komodo monitor, is a large species of lizard found in the Indonesian islands of Komodo, Rinca, Flores, Gili Motang and Gili Dasami. A member of the monitor lizard family , it is the largest living species of lizard, growing to a maximum length of in rare cases...
, or pygmy mammoth
Pygmy Mammoth
The Pygmy Mammoth or Channel Islands Mammoth is an extinct species of dwarf elephant descended from the Columbian mammoth . A case of island or insular dwarfism, M. exilis was only to tall at the shoulder and weighed about , in contrast to its tall, ancestor.Remains of M...
s.
After immigration, birds, and some reptiles or mammals, tend to become larger and predatory, showing relaxed intraspecific competition
Intraspecific competition
Intraspecific competition is a particular form of competition in which members of the same species vie for the same resource in an ecosystem...
. For mammals, small species will increase in size and large species will decrease in size. This is referred to as the “island rule,” and is suggested to minimize energy expenditure.
Other adaptations to life on islands include increased poikilothermy, relaxed anti-predator behavior, and reduced sexual selection
Sexual selection
Sexual selection, a concept introduced by Charles Darwin in his 1859 book On the Origin of Species, is a significant element of his theory of natural selection...
in animals, and loss of herbivore defenses and reduced dispersal in plants.
Speciation
The formation of new islands and their isolation from the mainland provides many unoccupied niches for species to adapt to. Since immigration of predators and competitors is limited, many organisms are able to persist in these new niches. This results in a high occurrence of endemism, where species are unique to a localized area. For example, 50% of endemic bird areas are found on islands.Endemism is often the result of adaptive radiation
Adaptive radiation
In evolutionary biology, adaptive radiation is the evolution of ecological and phenotypic diversity within a rapidly multiplying lineage. Starting with a recent single ancestor, this process results in the speciation and phenotypic adaptation of an array of species exhibiting different...
. Adaptive radiation is when a single species colonizes an area and rapidly diversifies to fill all of the available niches. A common example is the assemblage of finch species documented by Charles Darwin
Charles Darwin
Charles Robert Darwin FRS was an English naturalist. He established that all species of life have descended over time from common ancestry, and proposed the scientific theory that this branching pattern of evolution resulted from a process that he called natural selection.He published his theory...
in the Galapagos Islands
Galápagos Islands
The Galápagos Islands are an archipelago of volcanic islands distributed around the equator in the Pacific Ocean, west of continental Ecuador, of which they are a part.The Galápagos Islands and its surrounding waters form an Ecuadorian province, a national park, and a...
. Darwin’s finches exhibited adaptive radiation by evolving different beak sizes to exploit the diversity of seeds present on the different islands.
Survival
The continued survival of species on islands depends on factors such as natural selectionSelection
In the context of evolution, certain traits or alleles of genes segregating within a population may be subject to selection. Under selection, individuals with advantageous or "adaptive" traits tend to be more successful than their peers reproductively—meaning they contribute more offspring to the...
, genetic variation, natural disturbances (hurricanes, volcanic eruptions) and human-caused disturbances (introduced species, habitat loss). Human-caused disturbances tend to be the greatest cause of mortality, and understanding the causes of extinction facilitates conservation efforts.
Human impacts on island ecosystems
The movement of humans to islands has led to rapid extinction of native island species either from hunting, habitat destructionHabitat destruction
Habitat destruction is the process in which natural habitat is rendered functionally unable to support the species present. In this process, the organisms that previously used the site are displaced or destroyed, reducing biodiversity. Habitat destruction by human activity mainly for the purpose of...
, or introduced species
Introduced species
An introduced species — or neozoon, alien, exotic, non-indigenous, or non-native species, or simply an introduction, is a species living outside its indigenous or native distributional range, and has arrived in an ecosystem or plant community by human activity, either deliberate or accidental...
.
Hunting
Many large animals on islands have been hunted to extinction by humans. A well-known example is the dodoDodo
The dodo was a flightless bird endemic to the Indian Ocean island of Mauritius. Related to pigeons and doves, it stood about a meter tall, weighing about , living on fruit, and nesting on the ground....
, once found on the island of Mauritius
Mauritius
Mauritius , officially the Republic of Mauritius is an island nation off the southeast coast of the African continent in the southwest Indian Ocean, about east of Madagascar...
. It evolved to become large, flightless and docile, and was subsequently driven to extinction by humans and introduced predators.
Habitat destruction
The depletion of natural resources can have dramatic effects on island ecology. On Easter IslandEaster Island
Easter Island is a Polynesian island in the southeastern Pacific Ocean, at the southeasternmost point of the Polynesian triangle. A special territory of Chile that was annexed in 1888, Easter Island is famous for its 887 extant monumental statues, called moai, created by the early Rapanui people...
, the depletion of the forest by humans not only resulted in widespread loss of species, but also the collapse of the island civilization. Today there are over 500 million people on islands, all dependent on local resources either directly (traditional use) or indirectly (ecotourism revenue). Population growth and development result in heavy deforestation
Deforestation
Deforestation is the removal of a forest or stand of trees where the land is thereafter converted to a nonforest use. Examples of deforestation include conversion of forestland to farms, ranches, or urban use....
, pollution
Pollution
Pollution is the introduction of contaminants into a natural environment that causes instability, disorder, harm or discomfort to the ecosystem i.e. physical systems or living organisms. Pollution can take the form of chemical substances or energy, such as noise, heat or light...
, and over-exploitation. Overharvesting of ocean fauna is particularly troubling as yields of coral reef fish species are an important food source for island populations.
Introduced species
Humans have contributed to globalization and decreased effective isolation of island communities, allowing for invasion of exotic species. This can have a profound effect on the native species. In GuamGuam
Guam is an organized, unincorporated territory of the United States located in the western Pacific Ocean. It is one of five U.S. territories with an established civilian government. Guam is listed as one of 16 Non-Self-Governing Territories by the Special Committee on Decolonization of the United...
, the introduced brown tree snake
Brown tree snake
The brown tree snake is an arboreal rear-fanged colubrid snake native to eastern and northern coastal Australia, Papua New Guinea, and a large number of islands in northwestern Melanesia....
ate nearly all of the native vertebrate species to extinction. Feral cats and dogs have also greatly diminished native vertebrate populations on islands, through both predation and disease. Introduced ungulate
Ungulate
Ungulates are several groups of mammals, most of which use the tips of their toes, usually hoofed, to sustain their whole body weight while moving. They make up several orders of mammals, of which six to eight survive...
s are another major threat, as they graze on native vegetation and can destroy entire forests. Exotic grasses can out-compete native understory
Understory
Understory is the term for the area of a forest which grows at the lowest height level below the forest canopy. Plants in the understory consist of a mixture of seedlings and saplings of canopy trees together with understory shrubs and herbs...
species and increase the risk of fire. Lastly, social insects such as ants also cause major problems.
Global warming
Global warming is emerging as a strong cause of species loss on islands. This can be due to sea level rise, intrusion of salt water into freshwater habitats, or species inability to adapt to increasing temperatures and extreme weather events. Plant species are particularly susceptible. In more isolated areas, such as the Southern Ocean Islands, indirect effects such as invasive species and global warming can play a greater role in influencing populations than overexploitation, pollution and habitat loss.Trophic cascades
Human activities and the introduction of non-native species often cause trophic cascadeTrophic cascade
Trophic cascades occur when predators in a food web suppress the abundance of their prey, thereby releasing the next lower trophic level from predation...
s, where direct effects on one species result in indirect effects on other species in the food web. An example is on Santa Cruz Island
Santa Cruz Island
Santa Cruz Island was the largest privately owned island off the continental United States, but is currently part-owned by the National Park service . The island, located off the coast of California, is long and from wide...
of the California Channel Islands, where DDT
DDT
DDT is one of the most well-known synthetic insecticides. It is a chemical with a long, unique, and controversial history....
poisoning reduced bald eagle
Bald Eagle
The Bald Eagle is a bird of prey found in North America. It is the national bird and symbol of the United States of America. This sea eagle has two known sub-species and forms a species pair with the White-tailed Eagle...
populations. This, along with an abundance of introduced feral pigs for prey, allowed golden eagle
Golden Eagle
The Golden Eagle is one of the best known birds of prey in the Northern Hemisphere. Like all eagles, it belongs to the family Accipitridae. Once widespread across the Holarctic, it has disappeared from many of the more heavily populated areas...
s to colonize the island and replace bald eagles. However, the golden eagles also ate native island fox
Island Fox
The island fox is a small fox that is native to six of the eight Channel Islands of California. There are six subspecies of the fox, each unique to the island it lives on, reflecting its evolutionary history...
es. Fox population levels decreased to near extinction, while skunk
Skunk
Skunks are mammals best known for their ability to secrete a liquid with a strong, foul odor. General appearance varies from species to species, from black-and-white to brown or cream colored. Skunks belong to the family Mephitidae and to the order Carnivora...
populations increased due to relaxed competition with foxes.
Conservation on islands
Since island ecosystems are self-contained, it should be possible to mitigate many of the threats to species. Ecologists and managers are working together to prioritize areas for conservation and to quickly design and implement action plans. Not everything can be put into a reserve, so it is important to first compile pertinent information and prioritize areas of concern. Once an area has been chosen, managers must then acquire ownership and gain support. Local experts and indigenous populations should also be involved in this process. Having clearly defined goals will facilitate the many necessary interactions between people and agencies. Once a reserve is in place, managers can then practice adaptive managementAdaptive management
-What is Adaptive Management ?:Adaptive management , also known as adaptive resource management , is a structured, iterative process of optimal decision making in the face of uncertainty, with an aim to reducing uncertainty over time via system monitoring...
and do continued community education.
On land, island conservation focuses on the protection of species and their habitat. In some cases, conservation can be integrated with agricultural production. For example, the Acacia koa plantations and wooded pastures in Hawaii
Hawaii
Hawaii is the newest of the 50 U.S. states , and is the only U.S. state made up entirely of islands. It is the northernmost island group in Polynesia, occupying most of an archipelago in the central Pacific Ocean, southwest of the continental United States, southeast of Japan, and northeast of...
are anthropogenically altered ecosystems, yet allow connectivity between forest fragments and thus maintain higher diversity than would open pasture. Other directions include habitat restoration, and eradication of introduced predators, ungulates, and exotic plants (via hunting, removal or biological control).
In marine ecosystems, there has been increasing establishment of “no-take” reserves. This allows for reestablishment of native species, and also augmentation of commercially harvested species. However, in both terrestrial and marine systems, these actions are expensive and do not always result in the desired outcomes. For example, some non-natives become keystone species
Keystone species
A keystone species is a species that has a disproportionately large effect on its environment relative to its abundance. Such species play a critical role in maintaining the structure of an ecological community, affecting many other organisms in an ecosystem and helping to determine the types and...
and their removal can cause more harm than good to the ecosystem. To be more effective, managers of island ecosystems should share information and learn from each other’s mistakes.
Island restoration
Island conservation tends to focus on preservation of individual species and their habitats. However, many ecologists caution that ecological and evolutionary processes should be conserved as well. The conservation of island communities as a whole is closely linked to restorationRestoration ecology
-Definition:Restoration ecology is the scientific study and practice of renewing and restoring degraded, damaged, or destroyed ecosystems and habitats in the environment by active human intervention and action, within a short time frame...
.
Active restoration on islands can be done for both animal species (translocations, induced breeding) and plant species (reforestation). Creating goals for restoration can be challenging because it is often impossible to restore the ecosystem to its “historic” or “normal” state, if that state can even be clearly defined. Restoration is never complete, as ecological communities are always in a state of change.