Island biogeography
Encyclopedia
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. It has since been extended to mountains surrounded by deserts, lakes surrounded by dry land, fragmented forest and even natural habitats surrounded by human-altered landscapes. Now it is used in reference to any ecosystem surrounded by unlike ecosystems. The field was started in the 1960s by the ecologists Robert MacArthur
and E.O. Wilson, who coined the term theory of island biogeography, as this theory attempted to predict the number of species that would exist on a newly created island.
and extinction
. And further, that the isolated populations may follow different evolutionary routes, as shown by Darwin's observation of finches in the Galapagos Islands
. Immigration and emigration are affected by the distance of an island from a source of colonists (distance effect). Usually this source is the mainland, but it can also be other islands. Islands that are more isolated are less likely to receive immigrants than islands that are less isolated.
The rate of extinction once a species manages to colonize an island is affected by island size (area effect or the species-area curve
). Larger islands contain larger habitat areas and opportunities for more different varieties of habitat. Larger habitat size reduces the probability of extinction due to chance events. Habitat
heterogeneity increases the number of species that will be successful after immigration.
Over time, the countervailing forces of extinction and immigration result in an equilibrium level of species richness.
In addition to having an effect on extinction, island size can also affect immigration rates. Species may actively target larger islands for their greater number of resources and available niches; or, larger islands may accumulate more species by chance just because they are larger (target effect).
in the mangrove
islands in the Florida Keys
. Species richness on several small mangroves islands were surveyed. The islands were fumigated with methyl bromide to clear their arthropod
communities. Following fumigation the immigration of species onto the islands was monitored. Within a year the islands had been recolonised. Islands closer to the mainland recovered faster as predicted by the Theory of Island Biogeography. The effect of island size was not tested, since all islands were of approximately equal size.
Research conducted at the rainforest research station on Barro Colorado Island
has yielded a large number of publications concerning the ecological changes following the formation of islands, such as the local extinction of large predators and the subsequent changes in prey populations.
had been realised and was being vigorously debated in ecological
circles. The realisation that reserves and national park
s formed islands inside human-altered landscapes (habitat fragmentation), and that these reserves could lose species as they 'relaxed towards equilibrium' (that is they would lose species as they achieved their new equilibrium number, known as ecosystem decay) caused a great deal of concern. This is particularly true when conserving larger species which tend to have larger ranges. A study by William Newmark, published in the journal Nature
and reported in the New York Times, showed a strong correlation
between the size of a protected U.S.
National Park and the number of species of mammals.
This led to the debate known as single large or several small
(SLOSS), described by writer David Quammen
in The Song Of The Dodo as "ecology's own genteel version of trench warfare". In the years after the publication of Wilson and Simberloff's papers ecologists had found more examples of the species-area relationship, and conservation planning was taking the view that the one large reserve could hold more species than several smaller reserves, and that larger reserves should be the norm in reserve design
. This view was in particular championed by Jared Diamond
. This led to concern by other ecologists, including Dan Simberloff, who considered this to be an unproven over-simplification that would damage conservation efforts. Habitat diversity was as or more important than size in determining the number of species protected.
Island biogeography theory also led to the development of habitat corridor
s as a conservation tool to increase connectivity between habitat islands. Habitat corridors can increase the movement of species between parks and reserves and therefore increase the number of species that can be supported.
In species diversity, island biogeography most describes allopatric speciation
. Allopatric speciation is where new gene pools arise out of natural selection in isolated gene pools. Island Biogeography is also useful in considering sympatric speciation, the idea of different species arising from one ancestral species in the same area. Interbreeding between the two differently adapted species would prevent speciation, but in some species, sympatric speciation
appears to have occurred.
. Princeton, N.J.: Princeton University Press.
Biogeography
Biogeography is the study of the distribution of species , organisms, and ecosystems in space and through geological time. Organisms and biological communities vary in a highly regular fashion along geographic gradients of latitude, elevation, isolation and habitat area...
that attempts to establish and explain the factors that affect the species richness
Species richness
Species richness is the number of different species in a given area. It is represented in equation form as S.Species richness is the fundamental unit in which to assess the homogeneity of an environment. Typically, species richness is used in conservation studies to determine the sensitivity of...
of natural communities. The theory was developed to explain species richness of actual islands. It has since been extended to mountains surrounded by deserts, lakes surrounded by dry land, fragmented forest and even natural habitats surrounded by human-altered landscapes. Now it is used in reference to any ecosystem surrounded by unlike ecosystems. The field was started in the 1960s by the ecologists Robert MacArthur
Robert MacArthur
Robert Helmer MacArthur was an American ecologist who made a major impact on many areas of community and population ecology....
and E.O. Wilson, who coined the term theory of island biogeography, as this theory attempted to predict the number of species that would exist on a newly created island.
Definitions
For biogeographical purposes, an "island" is any area of suitable habitat surrounded by an expanse of unsuitable habitat. While this may be a traditional island—a mass of land surrounded by water—the term may also be applied to many untraditional "islands", such as the peaks of mountains, isolated springs in the desert, or expanses of grassland surrounded by highways or housing tracts. Additionally, what is an island for one organism may not be an island for another: some organisms located on mountaintops may also be found in the valleys, while others may be restricted to the peaks.Theory
The theory of island biogeography proposes that the number of species found on an undisturbed island is determined by immigrationImmigration
Immigration is the act of foreigners passing or coming into a country for the purpose of permanent residence...
and 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...
. And further, that the isolated populations may follow different evolutionary routes, as shown by Darwin's observation of finches 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...
. Immigration and emigration are affected by the distance of an island from a source of colonists (distance effect). Usually this source is the mainland, but it can also be other islands. Islands that are more isolated are less likely to receive immigrants than islands that are less isolated.
The rate of extinction once a species manages to colonize an island is affected by island size (area effect or the species-area curve
Species-area curve
In ecology, a species-area curve is a relationship between the area of a habitat, or of part of a habitat, and the number of species found within that area. Larger areas tend to contain larger numbers of species, and empirically, the relative numbers seem to follow systematic mathematical...
). Larger islands contain larger habitat areas and opportunities for more different varieties of habitat. Larger habitat size reduces the probability of extinction due to chance events. Habitat
Habitat
* Habitat , a place where a species lives and grows*Human habitat, a place where humans live, work or play** Space habitat, a space station intended as a permanent settlement...
heterogeneity increases the number of species that will be successful after immigration.
Over time, the countervailing forces of extinction and immigration result in an equilibrium level of species richness.
Modifications
In addition to having an effect on immigration rates, isolation can also affect extinction rates. Populations on islands that are less isolated are less likely to go extinct because individuals from the source population and other islands can immigrate and “rescue” the population from extinction (rescue effect).In addition to having an effect on extinction, island size can also affect immigration rates. Species may actively target larger islands for their greater number of resources and available niches; or, larger islands may accumulate more species by chance just because they are larger (target effect).
Influencing factors
- Degree of isolation (distance to nearest neighbour, and mainland)
- Length of isolation (time)
- Size of island (larger area usually facilitates greater diversity)
- The habitat suitability which includes:
- Climate (tropical versus arctic, humid versus arid, etc.)
- Initial plant and animal composition if previously attached to a larger land mass (e.g. marsupials, primates)
- The current species composition
- Location relative to ocean currents (influences nutrient, fish, bird, and seed flow patterns)
- Serendipity (the impacts of chance arrivals)
- Human activity
Historical record
The theory can be studied through the fossils, which provide a record of life on Earth. 300 million years ago, Europe and North America lay on the equator and were covered by steamy tropical rainforests. Climate change devastated these tropical rainforests during the Carboniferous Period and as the climate grew drier, rainforests fragmented. Shrunken islands of forest were uninhabitable for amphibians but were well suited to reptiles, which became more diverse and even varied their diet in the rapidly changing environment; this event triggered an evolutionary burst among reptiles.Research experiments
The theory of island biogeography was experimentally tested by E. O. Wilson and his student Daniel SimberloffDaniel Simberloff
Daniel Simberloff is a biologist and ecologist who earned his Ph.D. from Harvard University in 1969.Simberloff started his studies in ecology as a student of the biologist E. O. Wilson, one of the co-authors of the theory of Island biogeography . For his Ph.D...
in the mangrove
Mangrove
Mangroves are various kinds of trees up to medium height and shrubs that grow in saline coastal sediment habitats in the tropics and subtropics – mainly between latitudes N and S...
islands in the Florida Keys
Florida Keys
The Florida Keys are a coral archipelago in southeast United States. They begin at the southeastern tip of the Florida peninsula, about south of Miami, and extend in a gentle arc south-southwest and then westward to Key West, the westernmost of the inhabited islands, and on to the uninhabited Dry...
. Species richness on several small mangroves islands were surveyed. The islands were fumigated with methyl bromide to clear their arthropod
Arthropod
An arthropod is an invertebrate animal having an exoskeleton , a segmented body, and jointed appendages. Arthropods are members of the phylum Arthropoda , and include the insects, arachnids, crustaceans, and others...
communities. Following fumigation the immigration of species onto the islands was monitored. Within a year the islands had been recolonised. Islands closer to the mainland recovered faster as predicted by the Theory of Island Biogeography. The effect of island size was not tested, since all islands were of approximately equal size.
Research conducted at the rainforest research station on Barro Colorado Island
Barro Colorado Island
Barro Colorado Island is located in the man-made Gatun Lake in the middle of the Panama Canal. The island was formed when the waters of the Chagres River were dammed to form the lake. When the waters rose, they covered a significant part of the existing rainforest, and the hilltops remained as...
has yielded a large number of publications concerning the ecological changes following the formation of islands, such as the local extinction of large predators and the subsequent changes in prey populations.
Applications in conservation biology
Within a few years of the publishing of the theory its application to the field of conservation biologyConservation 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...
had been realised and was being vigorously debated in ecological
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...
circles. The realisation that reserves and national park
National park
A national park is a reserve of natural, semi-natural, or developed land that a sovereign state declares or owns. Although individual nations designate their own national parks differently A national park is a reserve of natural, semi-natural, or developed land that a sovereign state declares or...
s formed islands inside human-altered landscapes (habitat fragmentation), and that these reserves could lose species as they 'relaxed towards equilibrium' (that is they would lose species as they achieved their new equilibrium number, known as ecosystem decay) caused a great deal of concern. This is particularly true when conserving larger species which tend to have larger ranges. A study by William Newmark, published in the journal Nature
Nature (journal)
Nature, first published on 4 November 1869, is ranked the world's most cited interdisciplinary scientific journal by the Science Edition of the 2010 Journal Citation Reports...
and reported in the New York Times, showed a strong correlation
Correlation
In statistics, dependence refers to any statistical relationship between two random variables or two sets of data. Correlation refers to any of a broad class of statistical relationships involving dependence....
between the size of a protected U.S.
United States
The United States of America is a federal constitutional republic comprising fifty states and a federal district...
National Park and the number of species of mammals.
This led to the debate known as single large or several small
SLOSS Debate
The SLOSS Debate was a debate in ecology and conservation biology during the 1970s and 1980s as to whether a single large or several small reserves were a superior means of conserving biodiversity in a fragmented habitat....
(SLOSS), described by writer David Quammen
David Quammen
David Quammen is a science, nature and travel writer whose work has appeared in publications such as National Geographic, Outside, Harper's, Rolling Stone, and The New York Times Book Review....
in The Song Of The Dodo as "ecology's own genteel version of trench warfare". In the years after the publication of Wilson and Simberloff's papers ecologists had found more examples of the species-area relationship, and conservation planning was taking the view that the one large reserve could hold more species than several smaller reserves, and that larger reserves should be the norm in reserve design
Reserve design
Reserve Design is the process of planning and creating a nature reserve in a way that effectively accomplishes the goal of the reserve.Reserves have a variety of goals and many different factors need to be taken into account in order for a reserve to be successful. These factors include habitat...
. This view was in particular championed by Jared Diamond
Jared Diamond
Jared Mason Diamond is an American scientist and author whose work draws from a variety of fields. He is currently Professor of Geography and Physiology at UCLA...
. This led to concern by other ecologists, including Dan Simberloff, who considered this to be an unproven over-simplification that would damage conservation efforts. Habitat diversity was as or more important than size in determining the number of species protected.
Island biogeography theory also led to the development of habitat corridor
Habitat corridor
A habitat corridor is a strip of land that aids in the movement of species between disconnected areas of their natural habitat. An animal’s natural habitat would typically include a number of areas necessary to thrive, such as wetlands, burrowing sites, food, and breeding grounds...
s as a conservation tool to increase connectivity between habitat islands. Habitat corridors can increase the movement of species between parks and reserves and therefore increase the number of species that can be supported.
In species diversity, island biogeography most describes allopatric speciation
Allopatric speciation
Allopatric speciation or geographic speciation is speciation that occurs when biological populations of the same species become isolated due to geographical changes such as mountain building or social changes such as emigration...
. Allopatric speciation is where new gene pools arise out of natural selection in isolated gene pools. Island Biogeography is also useful in considering sympatric speciation, the idea of different species arising from one ancestral species in the same area. Interbreeding between the two differently adapted species would prevent speciation, but in some species, sympatric speciation
Sympatric speciation
Sympatric speciation is the process through which new species evolve from a single ancestral species while inhabiting the same geographic region. In evolutionary biology and biogeography, sympatric and sympatry are terms referring to organisms whose ranges overlap or are even identical, so that...
appears to have occurred.
See also
- DisturbanceDisturbanceIn ecology, a disturbance is a temporary change in average environmental conditions that causes a pronounced change in an ecosystem. Outside disturbance forces often act quickly and with great effect, sometimes resulting in the removal of large amounts of biomass...
- Island ecologyIsland EcologyIsland 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...
- Mammals of the CaribbeanMammals of the CaribbeanA unique and diverse albeit phylogenetically restricted mammal fauna is known from the Caribbean region. The region—specifically, all islands in the Caribbean Sea and the Bahamas, Turks and Caicos Islands, and Barbados, which are not in the Caribbean Sea but biogeographically belong to the same...
- Patch dynamicsPatch dynamicsPatch dynamics is a conceptual approach to ecosystem and habitat analysis that emphasizes dynamics of heterogeneity within a system ....
- Distance decayDistance decayDistance decay is a geographical term which describes the effect of distance on cultural or spatial interactions. The distance decay effect states that the interaction between two locales declines as the distance between them increases...
- Sky islandSky islandSky islands are mountains that are isolated by surrounding lowlands of a dramatically different environment, a situation which, in combination with the altitudinal zonation of ecosystems, has significant implications for natural habitats. Endemism, vertical migration, and relict populations are...
Further reading
MacArthur, R. H. and Wilson, E. O. 1967. The Theory of Island BiogeographyThe Theory of Island Biogeography
The Theory of Island Biogeography is a 1967 book by Edward O. Wilson and Robert MacArthur which laid the foundations for the study of island biogeography. An edition with a new preface by Edward O. Wilson was published in 2001 ....
. Princeton, N.J.: Princeton University Press.
Other References
- Newmark, W. D., A land-bridge island perspective on mammalian extinctions in western North American parks, Nature, 325, 430 - 432 (29 January 1987)
- David Quammen. 1997. The Song of the Dodo: Island Biogeography in an Age of Extinctions. Scribner. ISBN 0-684-82712-3
- Allan A. Schoenherr, C. Robert Feldmeth, Michael J. Emerson. 2003. Natural History of the Islands of California. University of California Press.