Introduction
The theory of island biogeography is a scientific theory that explains how the number of species on an island is determined by the balance between two opposing forces: immigration and extinction. The theory was developed by Robert MacArthur and E.O. Wilson in the 1960s and has since become an important concept in ecology and conservation biology.
The theory proposes that the rate of immigration of species to an island is influenced by the distance of the island from the nearest mainland and the size of the island. Generally, larger islands tend to have more species than smaller islands, and islands closer to the mainland tend to have more species than those further away.
The rate of extinction of species on an island is influenced by factors such as island size and the number of species present. Small islands with few species are more likely to experience extinctions than larger islands with more species.
The theory of island biogeography also suggests that there is a balance between immigration and extinction that determines the number of species that can be supported by an island. If immigration rates are high and extinction rates are low, the number of species on the island will tend to increase until it reaches a point where immigration and extinction rates are equal. This point is called the island’s species equilibrium.
The theory has important implications for conservation biology, as it suggests that preserving larger areas of habitat is important for maintaining biodiversity and that connecting isolated habitats through corridors can help reduce extinction rates. The theory of island biogeography has been widely accepted and has been used to guide conservation efforts in island ecosystems around the world.
Definitions of Island Biogeography
The study of the distribution and diversity of species on islands, and the factors that influence these patterns. This includes examining the effects of island size, distance from mainland, and habitat heterogeneity on the number and types of species found on an island.
For biogeographical purposes, an insular environment or “island” is any area of habitat suitable for a specific ecosystem, surrounded by an expanse of unsuitable habitat.
Island Biogeography
The Theory of Island Biogeography is a 1967 book by the ecologist Robert MacArthur and the biologist Edward O. Wilson. Islands represent small areas, isolated from other habitats.
The theory of island biogeography is a scientific theory that aims to explain the distribution of species on islands. Developed by Robert MacArthur and E.O. Wilson in the 1960s, the theory suggests that the number of species on an island is determined by two main factors: immigration and extinction.
According to the theory, the rate of immigration of species to an island depends on the distance between the island and the nearest mainland, as well as the size of the island. Larger islands generally have more species than smaller islands, and islands closer to the mainland generally have more species than islands that are further away.
The rate of extinction of species on an island is influenced by factors such as island size and the number of species present. Small islands with few species are more likely to experience extinctions than larger islands with more species.
The theory of island biogeography has been supported by numerous studies and has important implications for conservation biology. It suggests that preserving larger areas of habitat is important for maintaining biodiversity and that connecting isolated habitats through corridors can help reduce extinction rates.
The theory of island biogeography also suggests that the balance between immigration and extinction determines the equilibrium number of species on an island. If immigration rates are high and extinction rates are low, the number of species on an island will tend to increase until it reaches a point where immigration and extinction rates are equal. This point is called the island’s species equilibrium.
The theory also proposes that islands with similar characteristics (e.g. size, distance from mainland) will tend to have similar species compositions. This concept is known as the “species-area relationship” and has been observed in many different types of habitats, not just islands.
However, the theory of island biogeography has its limitations. It assumes that all species have equal colonization and extinction rates, which may not be the case in reality. The theory also does not take into account other important factors that can affect species diversity on islands, such as competition, predation, and environmental factors.
Overall, the theory of island biogeography has been an influential and widely accepted framework for understanding the distribution of species on islands. It has helped shape our understanding of how species respond to habitat fragmentation and loss, and has been used to guide conservation efforts in island ecosystems.
Islands are used as microcosms for studying evolutionary and ecological problems. Due to their isolation, evolutionary processes work at different rates. Serve as natural experimental plots that offer differences in sizes and number of species. Interaction is much less complex than in mainland habitats.
The model also predicts that the extinction rate and immigration rate are affected by two important features of the island.
- Size
- Distance from the nearest mainland
Small islands generally have lower immigration rates and higher extinction rates.
- Small islands are smaller targets for immigration and have less niches.
- Large islands generally have higher immigration rates and lower extinction rates.
- Larger islands are larger targets for immigration and have more niches.
Larger islands and closer islands support a greater number of species than smaller or more distant islands. These relationships can be described as ‘species-area’ or species-distance’. These patterns not only hold for true oceanic islands, but also mountain tops, lakes, and other habitat islands. large islands close to mainland’s have greater biodiversity than small, isolated islands
Island biogeography (also called insular biogeography) provides some of the best evidence in support of natural selection and the theory of evolution. The term describes an ecosystem that is isolated by being surrounded by different ecosystems. For the purposes of this theory, an island is defined as more than just a piece of land surrounded by water. It includes mountain peaks, a lake surrounded by a desert, a patch of woodland, or even a national park. The theory provides a model to explain the richness and uniqueness of species, both plants and animals, found in an isolated area.
The two events that determine how many species are found in an isolated ecosystem are immigration and extinction. Research has shown that how big the island is and how far it is from the mainland greatly influence the number (richness) of species found there (see image below). Once species have established themselves on an island, the rate at which they will go extinct depends on the size of the island, with there being less likelihood of extinction on larger islands. This is called the species-area relationship. This relationship is not just observable, but it can also be predicted mathematically. By the same theory, the farther an island is from the mainland, the fewer species it tends to have. This is referred to as the species-distance relationship.
The image above shows how the size of an island and its distance from the mainland interact with immigration (colonization, the orange lines) and extinction events (green lines) to influence the richness of species.
Robert H. MacArthur (1930-1972)
The Theory of Island Biogeography is a 1967 book by the ecologist Robert MacArthur and the biologist Edward O. Wilson.
Robert H. MacArthur was an American ecologist born in New York City in 1930. He received his undergraduate degree from Swarthmore College and his PhD from Yale University. MacArthur was known for his research on the theory of island biogeography, as well as his work on competition and niche theory.
In the 1960s, MacArthur and his colleague E.O. Wilson developed the theory of island biogeography, which explained how the number and types of species found on an island were influenced by factors such as island size and distance from the mainland. The theory has had a profound impact on the field of ecology and has been used to guide conservation efforts in island ecosystems around the world.
MacArthur also made significant contributions to the study of competition and niche theory, which explore how different species interact and compete for resources in ecosystems. His work helped to establish a quantitative framework for understanding the processes of competition and coexistence in ecological communities.
Tragically, MacArthur died at the young age of 42 in 1972, from complications related to bone cancer. However, his legacy continues to inspire ecologists and conservationists around the world.
Applications
MacArthur and Wilson’s theory of island biogeography has been widely applied outside of island ecosystems. For microbiota, the theory has been applied to the distribution of ectomycorrhizal fungi on trees, the distribution of bacteria in water-filled treeholes, and the distribution of fungi among shrubs. While for flora and fauna, the theory’s predictions have been realized with the species richness of plants on mountains and with the species richness of aquatic snails in bodies of water.
Novel applications looked at plants as islands for insect species and the dependence of the species richness of mites on the areas of the host ranges of rodent species. MacArthur and Wilson’s work has been used as a basis in other ecological theories, notably the unified neutral theory of biodiversity, and has been foundational for the fields of landscape ecology, invasion biology, and conservation biology.
Edward O. Wilson (1929 – 2021)
Edward Osborne Wilson (June 10, 1929 – December 26, 2021) was an American biologist, naturalist, and writer. His specialty was myrmecology, the study of ants.
Wilson has been called “the father of sociobiology” and “the father of biodiversity” for his environmental advocacy, and his secular-humanist and deist ideas pertaining to religious and ethical matters. Among his contributions to ecological theory is the theory of island biogeography (developed in collaboration with the mathematical ecologist Robert MacArthur)
Edward O. Wilson was an American biologist and naturalist born in Birmingham, Alabama in 1929. He was one of the most influential scientists of his time, known for his research on ants and his contributions to the fields of biogeography, biodiversity, and conservation biology.
Wilson was a prolific writer, having authored over 30 books and hundreds of scientific articles. One of his most influential works was the theory of island biogeography, which he developed with his colleague Robert MacArthur in the 1960s. This theory explained how the number and types of species found on an island were influenced by factors such as island size and distance from the mainland. Wilson’s work on island biogeography helped to establish the field of conservation biology, which seeks to understand and protect the natural world.
Wilson also made significant contributions to the study of ants, and is considered one of the world’s foremost experts on the subject. He discovered new species of ants and was the first to describe the social behavior of many ant species. He also helped to establish the field of sociobiology, which explores the social behavior of animals, including humans.
Wilson was the recipient of numerous awards and honors throughout his career, including two Pulitzer Prizes for his books “The Ants” and “On Human Nature”. He was also a member of the National Academy of Sciences and the American Philosophical Society. Wilson passed away in 2021 at the age of 92, but his legacy as a pioneering scientist and advocate for conservation continues to inspire future generations.
Perhaps the most significant event in ecological biogeography was the publication of a monograph on island biogeography by E.O. Wilson and Robert MacArthur. In this seminal work, they proposed theories to explain the number of species found on islands.
Biogeography was stuck in a “natural history phase” dominated by the collection of data, the young Princeton biologists Robert H. MacArthur and Edward O. Wilson argued in 1967. In this book, the authors developed a general theory to explain the facts of island biogeography. The theory builds on the first principles of population ecology and genetics to explain how distance and area combine to regulate the balance between immigration and extinction in island populations. The authors then test the theory against data. The Theory of Island Biogeography was never intended as the last word on the subject. Instead, MacArthur and Wilson sought to stimulate new forms of theoretical and empirical studies, which will lead in turn to a stronger general theory. Even a third of a century since its publication, the book continues to serve that purpose well. From popular books like David Quammen’s Song of the Dodo to arguments in professional literature, The Theory of Island Biogeography remains at the center of discussions about the geographic distribution of species.
Willi Hennig
Willi Hennig (20 April 1913 – 5 November 1976) was a German biologist who is considered the founder of phylogenetic systematics, also known as cladistics.
Willi Hennig was a German entomologist and evolutionary biologist born in 1913 and passed away in 1976. He is best known for his contributions to the development of phylogenetic systematics, also known as cladistics.
Hennig believed that the evolutionary history of organisms could be accurately represented through the construction of a branching tree-like diagram, or a cladogram, which would show the relationships between different species based on shared characteristics. He argued that only shared derived characters, or synapomorphies, should be used to classify organisms, rather than the overall similarity of traits. This approach allowed for a more accurate representation of evolutionary relationships, as it focused on evolutionary novelties that arose in specific lineages.
Hennig’s ideas were controversial at the time, as they challenged the traditional classification system, which relied heavily on overall similarity. However, his approach has since become widely accepted and is now a fundamental concept in evolutionary biology.
Hennig’s contributions to cladistics have had a significant impact on the field of biology, particularly in the study of biodiversity and the reconstruction of evolutionary history. He was a prolific writer and published several influential books, including “Phylogenetic Systematics” in 1966. Today, Hennig is remembered as one of the most important evolutionary biologists of the 20th century, and his work continues to inspire scientists and students around the world.
Chronological Method
- Study a group and determine derived characters
- Produce a phylogeny
- Examine the phylogeny of a monophyletic group With respect to the distribution of its
members - Reexamine the phylogeny using information from the Distribution and perhaps
make changes in the previously determine derived characters
Leon Croizat
Leon Croizat was a French-born biologist and biogeographer who made significant contributions to the study of biogeography, or the distribution of species on Earth. He was born in 1894 in Paris and passed away in 1982 in Caracas, Venezuela.
Croizat’s work was heavily influenced by his belief in the unity of life, which he believed was best represented by a single branching tree of life. He rejected the idea of multiple independent centers of origin, or polyphyletic groups, and instead advocated for a monophyletic view of evolution, which emphasized the shared ancestry of all life on Earth.
Croizat developed a new approach to biogeography, which he called panbiogeography, that sought to identify areas of endemism, or regions with high levels of unique species. He also developed a new method for constructing biogeographic maps, which he called track analysis, that involved tracing the distribution of taxa along their evolutionary paths.
Croizat was a prolific writer and published several influential books, including “Panbiogeography” in 1958 and “Space, Time, Form: The Biological Synthesis” in 1962. He was a controversial figure in the field of biogeography, as his ideas challenged the traditional views of the time. However, his contributions to the field have had a significant impact on the way we understand the distribution of life on Earth. Today, Croizat is remembered as one of the pioneers of modern biogeography, and his work continues to inspire scientists around the world.
The Panbiogeographical Method
- Map the ranges of the species of a given group
- Connect the ranges with a line to form a track, these would be a minimum line, connecting all localities with the shortest possible path
- Do this over and over again for different taxa
- If tracks follow the same routes, they form a generalized track
- These generalized tracks are an empirical phenomenon
- Unite continental areas that together are an estimate of the distribution of an ancestral biota
Conclusion
In conclusion, Robert H. MacArthur, Edward O. Wilson, Willi Hennig, and Leon Croizat were all influential biologists who made significant contributions to the fields of ecology, biogeography, and evolutionary biology.
MacArthur and Wilson developed the theory of island biogeography, which explained how the number and types of species found on an island were influenced by factors such as island size and distance from the mainland. Their work helped to establish the field of conservation biology.
Hennig contributed to the development of phylogenetic systematics, which focused on the construction of a branching tree-like diagram to show the relationships between different species based on shared characteristics. His approach challenged the traditional classification system and is now widely accepted in the field of evolutionary biology.
Croizat developed panbiogeography, a new approach to biogeography that sought to identify areas of endemism, or regions with high levels of unique species. He also developed a new method for constructing biogeographic maps, called track analysis, that traced the distribution of taxa along their evolutionary paths.
References
- Dr. M.B.POTDAR (Professor Shivaji University Kolhapur)