5.5 KiB
| title | chunk | source | category | tags | date_saved | instance |
|---|---|---|---|---|---|---|
| Island | 2/5 | https://en.wikipedia.org/wiki/Island | reference | science, encyclopedia | 2026-05-05T07:35:12.739706+00:00 | kb-cron |
=== Formation in freshwater === A fluvial island is an island that forms from the erosion and sedimentation of debris in rivers; almost all rivers have some form of fluvial islands. These islands may only be a few meters high, and are usually temporary. Changes in the flow speed, water level, and sediment content of the river may affect the rate of fluvial island formation and depletion. Permanent river islands also exist, the largest of which (that is completely inland) is Bananal Island in the Tocantins of Brazil, which has a maximum width of 55 kilometers. Lakes form for a variety of reasons, including glaciers, plate tectonics, and volcanism. Lake islands can form as part of these processes.
== Life on islands == The field of insular biogeography studies the ecological processes that take place on islands, with a focus on what factors effect the evolution, extinction, and richness of species. Scientists often study islands as an isolated model of how the process of natural selection takes place. Island ecology studies organisms on islands and their environment. It has yielded important insights for its parent field of ecology since the time of Charles Darwin.
=== Endemism ===
In biology, endemism is defined as the phenomenon where species or genus is only found in a certain geographical area. Islands isolate land organisms from others with water, and isolate aquatic organisms living on them with land. Island ecosystems have the highest rates of endemism globally. This means that islands contribute heavily to global biodiversity. Areas with high lives of biodiversity are a priority target of conservation efforts, to prevent the extinction of these species. Despite high levels of endemism, the total species richness, the total number of unique species in a region, is lower on islands than on mainlands. The level of species richness on islands is proportional to the area of that island, a phenomenon known as the species-area relationship. This is because larger areas have more resources and thus can support more organisms. Populations with a higher carrying capacity also have more genetic diversity, which promotes speciation.
=== Dispersal ===
Oceanic islands, ones that have never been connected to shore, are only populated by life that can cross the sea. This means that any animals present on the island had to have flown there, in the case of birds or bats, were carried by such animals, or were carried in a sea current in what is known as a "rafting event". This phenomenon is known as oceanic dispersal. Tropical cyclones have the capacity to transport species over great distances. Animals like tortoises can live for weeks without food or water, and are able to survive floating on debris in the sea. One case study showed that in 1995, fifteen iguanas survived a 300 km journey to Anguilla in the Caribbean, an island which no iguana had lived on previously. They survived floating on a mass of uprooted trees from a storm. Plant species are thought to be able to travel great distances of ocean. New Zealand and Australia share 200 native plant species, despite being separated by 1500 km. Continental islands, islands that were at one point connected to a continent, are expected to share a common history of plant and animal life up until the point that the island broke away from the continent. For example, the presence of freshwater fish on an island surrounded by ocean would indicate that it once was attached to a continent, since these fish cannot traverse the ocean on their own. Over the course of time, evolution and extinction changes the nature of animal life on a continental island, but only once it splits from the mainland. An example is that of the southern beech, a tree that is present in Australia, New Zealand, parts of South American, and New Guinea, places that today are geographically distant. A possible explanation for this phenomenon is that these landmasses were once all part of the continent Gondwana and separated by tectonic drift. However, there are competing theories that suggest this species may have reached faraway places by way of oceanic dispersal.
=== Evolution on island groups ===
Species that colonize island archipelagos exhibit a specific property known as adaptive radiation. In this process, a species that arrives on a group of islands rapidly becomes more diverse over time, splitting off into new species or subspecies. A species that reaches an island ecosystem may face little competition for resources, or may find that the resources that they found in their previous habitat are not available. These factors together result in individual evolutionary branches with different means of survival. The classical example of this is Darwin's finches, a group of up to fifteen tanager species that are endemic to the Galápagos Islands. These birds evolved different beaks in order to eat different kinds of food available on the islands. The large ground finch has a large bill used to crack seeds and eat fruit. The Genovesa cactus finch prefers cacti as a food source, and has a beak adapted for removing pulp and flowers from cacti. The green warbler-finch (in the habit of true warbler species) consumes spiders and insects that live on plants. Other examples of this phenomenon exist worldwide, including in Hawaii and Madagascar, and are not limited to island ecosystems.
==== The island rule ====