Insular biogeography

Insular biogeography is a specialized branch of biogeography. It examines the factors that influence species richness and diversification in isolated natural communities. While the name suggests oceanic islands, the field applies to any ecosystem surrounded by unsuitable habitat. This includes mountain peaks, seamounts, oases, and fragmented forests. It even applies to natural habitats isolated by human development, such as a patch of grassland surrounded by highways. In this field, an "island" is defined as any suitable habitat area surrounded by an expanse where that specific ecosystem cannot exist.

The core theory of insular biogeography proposes that the number of species in an undisturbed island is determined by two opposing forces: immigration and extinction. Immigration is the process of new species arriving at the island. Extinction is the process of species dying out within that environment. Over time, these two forces reach a balance known as an equilibrium level of species richness. This balance helps scientists predict how many different types of organisms will live in a specific isolated area.

Two main physical factors influence these forces: distance and size. The distance effect describes how isolation affects immigration. Islands located near a mainland or other islands receive more immigrants. Islands that are more isolated are less likely to receive new arrivals. This is because the distance makes it harder for organisms to travel. The species-area effect describes how island size affects extinction. Larger islands provide more habitat area and more diverse types of habitats, which is called habitat heterogeneity. This increased variety and space reduces the chance of species going extinct due to random events.

There are also specific modifications to these rules. The rescue effect occurs when populations on less isolated islands are saved from extinction by new immigrants. These newcomers bolster the existing population. Additionally, the target effect suggests that island size can influence immigration. Some species may actively target larger islands because they offer more resources and available niches. Larger islands might also accumulate more species simply by chance because of their size.

The field was established in the 1960s by ecologists Robert H. MacArthur and E. O. Wilson. They aimed to predict the number of species on newly created islands. They tested their theories in the Florida Keys using mangrove islands. Wilson and his student Daniel Simberloff used methyl bromide to clear arthropod communities from small islands. They monitored how quickly species returned. They found that islands closer to the mainland recovered faster, supporting the distance effect. This research helped prove that immigration and extinction patterns follow predictable rules.

Scientists also use the species-area relationship to understand these patterns. This concept shows that species richness is directly proportional to the area being studied. As the area of an island or ecosystem increases, the number of species typically increases as well. This relationship can be expressed mathematically through an equation where the number of species depends on the area. While this rule works well on oceanic islands, it also applies to Island Like Systems (ILS). An ILS is an ecosystem, like a pond, that is isolated within a different landscape. In these systems, the "mainland" is the source of new species, and the surrounding area is the "matrix."

These principles are vital for conservation biology. When humans build roads or housing, they cause habitat fragmentation. This turns national parks and reserves into islands. Scientists worry about "ecosystem decay," where these small islands lose species as they reach a new, lower equilibrium. This led to a famous debate called SLOSS, which means "single large or several small." Some ecologists, including Jared Diamond, argue that one large reserve is better than several small ones. This is because larger areas can hold more species and provide more stable environments for wildlife.