Insular biogeography, also called island biogeography, is a branch of biogeography that studies what causes different numbers of species and how species change over time in areas that are separated from other areas. This theory was first created to explain how the number of species relates to the size of islands in the ocean. Today, the term is used to describe any area, whether it exists now or in the past, that is separated by different types of environments. This includes places like mountain peaks, underwater mountains, desert oases, broken-up forests, and natural areas that have been cut off by human activities. The field began in the 1960s with ecologists Robert H. MacArthur and E. O. Wilson, who introduced the term "island biogeography" in their first work for Princeton's Monograph in Population Biology series. Their study aimed to predict how many species would live on a newly formed island.
Definitions
For the study of life and where it lives, an insular environment or "island" is any area of habitat that supports a specific ecosystem and is surrounded by a large area that is not suitable for that ecosystem. This can be a traditional island, which is a piece of land surrounded by water. However, the term can also describe other areas that act like islands, such as mountain peaks, isolated springs or lakes, and woodlands that are not connected to other similar areas. This concept is often used to describe natural habitats that are surrounded by areas changed by humans, such as grasslands near highways or neighborhoods, and national parks. Also, an area that is insular for one type of organism might not be insular for another. For example, some animals found on mountain tops may also live in nearby valleys, while others are only found on the peaks.
Theory
The theory of island biogeography explains that the number of species living on an undisturbed island depends on two main factors: immigration (when species move to the island) and extinction (when species disappear from the island). Islands that are isolated may develop unique species over time, as seen in Darwin’s study of finches in the Galapagos Islands. Immigration and extinction rates are influenced by how far an island is from a source of new species, such as the mainland or other islands. Islands farther away are less likely to receive new species than those closer to the source.
The chance that a species will go extinct after reaching an island depends on the island’s size. This is called the species-area effect. Larger islands have more habitat types and resources, which lowers the risk of extinction caused by random events. More varied habitats also increase the number of species that can survive after arriving.
Over time, the balance between immigration and extinction leads to a stable number of species on the island.
Isolation also affects extinction rates. Islands that are less isolated are safer for species because individuals from other populations can move in and help prevent extinction. This is called the rescue effect.
Island size can also influence immigration rates. Some species may prefer larger islands because they offer more resources and living spaces. Larger islands may also naturally host more species due to their size, a concept called the target effect.
Factors that influence species diversity on islands include:
– How far the island is from other land (neighbors or the mainland)
– How long the island has been isolated
– The island’s size (larger islands usually support more species)
– Habitat quality, which includes climate (such as tropical vs. arctic, humid vs. dry), and the types of plants and animals present
– The island’s location relative to ocean currents (which affect the movement of nutrients, fish, birds, and seeds)
– The island’s position relative to dust carried by wind (which brings nutrients)
– Chance events, such as unexpected arrivals of species
– Human activities
Species-area relationships
Species-area relationships describe how the size of an area affects the number of species found in that area. This idea comes from the study of island biogeography, which is easier to observe on islands because they are separated from other land areas. On islands, it is simpler to track which species arrive and which species disappear because fewer species can move in or out. Generally, the number of species increases as the area becomes larger. For example, larger islands tend to have more types of plants and other living things. It is important to note that the way species-area relationships work on islands may differ from how they work on large land areas, but studying both can still help scientists understand ecosystems better.
The species-area relationship can be written as a mathematical equation: S = cA^z. In this equation, S represents the number of species in an area, c is a constant number that helps define the starting point of the relationship, A is the size of the area being studied, and z is a number that shows how steeply the number of species increases with area.
This equation can also be rewritten using logarithms: log(S) = log(c) + z log(A). This form allows scientists to graph the relationship as a straight line, but the basic idea remains the same: the size of the area influences how many species are present.
Historical record
The theory can be studied through fossils, which show a record of life on Earth. About 300 million years ago, Europe and North America were located near the equator and had thick, wet rainforests. Climate changes during the Carboniferous Period damaged these rainforests. As the climate became drier, the rainforests broke into smaller pieces. These small forest areas were too hard for amphibians to live in but were good for reptiles. Reptiles became more varied and started eating different kinds of food as the environment changed. This event, called the Carboniferous rainforest collapse, caused a quick increase in the number and variety of reptiles.
Research experiments
The theory of island biogeography was tested by E. O. Wilson and his student Daniel Simberloff in the mangrove islands of the Florida Keys. Scientists studied the number of species on several small mangrove islands. They treated the islands with methyl bromide to remove their arthropod communities. After treatment, researchers observed how species returned to the islands. Within one year, the islands had recovered to levels similar to before the treatment. However, Simberloff and Wilson noted that the final number of species was changing slightly, maintaining a balance. Islands closer to the mainland recovered faster, as predicted by the Theory of Island Biogeography. The effect of island size was not studied because all islands were nearly the same size.
Research at the rainforest research station on Barro Colorado Island has produced many scientific papers about ecological changes after islands formed, such as the disappearance of large predators and the resulting changes in prey populations.
Applications to Island Like Systems (ILS)
The theory of island biogeography was first used to study oceanic islands, but these ideas can also be used in other areas of study. The way species move and interact in island-like systems (ILS) helps scientists understand how life changes in these areas. An ILS is not always a real island but is instead defined by how separated it is from the rest of the ecosystem. For a real island, the "matrix" is the water around it, and the "mainland" is the nearest land. In an ILS, the "mainland" is where species come from, but the "matrix" can be many different types of environments. By comparing ecosystems like a pond surrounded by land to real islands, scientists can apply ideas from island biogeography to ILS. However, the patterns of species arriving and disappearing that are seen on islands also happen between ecosystems on the mainland.
The theory of island biogeography includes ideas about the size of an island and how far it is from the mainland. These same ideas apply to ILS. The main difference is how size and separation change. For example, an ILS might change in size over time due to seasons, which affects how separated it is. The availability of resources, like food and water, also influences conditions in ILS. This is different from real islands, where resources are often less available.
The relationship between the size of an ecosystem and the number of species it holds can be studied in ILS as well. Usually, larger ecosystems have more species. However, the numbers used in these calculations (called z-values and c-values) are often lower for ILS than for real islands. These numbers also vary among different types of ILS.
Applications in conservation biology
After the theory was published, scientists quickly recognized its importance for conservation biology and began discussing its ideas in ecological studies. The theory suggested that reserves and national parks act like islands within human-altered landscapes, a process called habitat fragmentation. These areas might lose species as they adjust to their new conditions, a process called ecosystem decay. This was especially concerning for larger animals, which often need more space. A study by William Newmark, published in Nature and reported in The New York Times, showed that larger U.S. National Parks tend to support more mammal species.
This led to a debate called SLOSS, described by David Quammen as "ecology's own polite but intense disagreement." After Wilson and Simberloff's research, scientists found more evidence supporting the species-area relationship, which suggested that one large reserve could protect more species than several smaller ones. This idea was supported by Jared Diamond. However, other scientists, like Dan Simberloff, argued that this view was too simple and might harm conservation efforts. They believed that habitat diversity was as important as size in protecting species.
Island biogeography theory also inspired the creation of wildlife corridors, which connect fragmented habitats. These corridors help animals move between areas, increasing the number of species that can survive. However, they can also spread diseases between populations, making the benefits of connectivity more complex.
In terms of species diversity, island biogeography explains allopatric speciation, where new species develop in isolated groups due to natural selection. It also helps understand sympatric speciation, where new species arise from the same ancestor in the same area. While interbreeding between species can prevent speciation, some species have shown sympatric speciation. Island ecosystems often face challenges like limited resources, less environmental variety, and fewer predators or competitors. These conditions can cause changes in species' traits as they adapt to their new environments.