21.4: Island Biogeography

Biogeography is the study of the distribution of species and ecosystems in geographic space and through geological time. Organisms and biological communities often vary in a regular fashion along geographic gradients of latitude, elevation, isolation, and habitat area. The latter pattern, often called the species-area relationship, has long been a fascination for biogeographers. Figure \(\PageIndex{1}\) demonstrates this species-area relationship for amphibians and reptiles in the West Indies. A study from around 1957 was included in Robert H. MacArthur and Edward O. Wilson's famous book "The Theory of Island Biogeography", and showed that among seven islands studied, the largest, Cuba, had the most species, and the smallest, Redonda, had the fewest. This general pattern has been found in many different islands groups, and across various different taxa.

Figure \(\PageIndex{1}\): Species-area relationship for amphibians and reptilians on seven different islands in the West Indies. An original study (Darlington c. 1957), published in "The Theory of Island biogeography", showed a strong positive correlation between island size and number of species (blue line). A more recent study by Gao and Perry (2016) included data for many additional small islands (smaller than Redonda) and found that while the pattern holds, the relationship is flatter than in the original study--indicated by the pink line. Figure drawn by Andy Wilson, based on Figure 4 in Gao and Perry (2016).

While studying these species-area relationships for their seminal book, MacArthur and Wilson were drawn to another of the four geographic gradients--that of isolation. They observed that some large Islands had fewer species than would be expected for their size, while conversely, some small islands had more species than expected.

While studying patterns of species richness on islands, two ecologists, Robert H. MacArthur and Edward O. Wilson, noted some exceptions to the species-area relationship. For example, some large islands had fewer species than expected due to their size, while some small islands had more species than expected. These patterns were explained by MacArthur and Wilson's Equilibrium Theory of Island Biogeography, which takes into account the fact that islands are colonized by species from elsewhere, and natural extinctions will occur on those islands, over long periods of time. 

Imagine there are two islands (a and b) located off the coast of the mainland. Although the two islands are about the same size, island a is located much farther away than island b. If you are a bird that lives on the mainland, which island are you most likely to find? The answer is generally island b. This means immigration (or colonization) is influenced by the distance of an island from the mainland (a source of colonists). Therefore, islands that are closer to the island are more likely to receive immigrants than islands that are further away. 

Figure \(\PageIndex{2}\): Two offshore islands--the Theory of Island Biogeography suggests that fewer species will colonize the more distant island a, in comparison to island b (credit: Andy Wilson, redrawn from Hdelucalowell15 (CC BY-SA 4.0))

Once a species manages to reach and colonize an island, the rate of extinction is largely influenced by size of the island. This is because smaller islands tend to hold smaller populations (which are more likely to experience extinction due to stochastic effects like genetic drift). Larger habitat size reduces the probability of extinction of the colonized species due to chance events. Smaller islands are also likely to hold fewer populations in general because they have fewer resources and less diversity of resources. Larger islands have larger and more habitat areas, which typically leads to more differences in habitat or habitat heterogeneity. Higher heterogeneity means that there are more opportunities for a variety of species to find their suitable niches. Habitat heterogeneity also helps increase the number of species to successfully colonize after immigration.

Now let's consider the situation if we had two different islands sizes in our offshore archipelago, with c and d, much bigger than a and b Figure \(\PageIndex{3}\). We would expect extinction rates to be much lower on islands c and d than on the two smaller islands.

Figure \(\PageIndex{3}\): Four offshore islands--the Theory of Island Biogeography suggests that fewer species will go extinct on the larger islands c and d, in comparison to islands a and b (credit: Andy Wilson, redrawn from Wikipedia (CC BY-SA 4.0))

We could plot both immigration and extinction relationships on a single image, as in Figure \(\PageIndex{4}\). Note the y-axis is the number of species (either colonizing or going extinct). Now we see that of the four islands in Figure \(\PageIndex{3}\), island a (small and far) would likely have the fewest species, island d (large and close), would have the most, and islands b and c would fall between the two extremes.

 

Figure \(\PageIndex{4}\): Equilibrium Theory of Island Biogeogaphy (credit: Andy Wilson, redrawn from Wikipedia (CC BY-SA 4.0))

This basic graph makes a lot of assumptions but also offers a lot of insight. In this graph, immigration rates (blue lines) depend on proximity to mainland. Immigration rates also decline with species richness. That's becasuse its' easiest to immigrate to an island when it is empty because all the resources on the island are available. As the islands get more and more full, colonizing the island becomes more difficult.

Extinction rates (orange lines), as noted above, depend on island size. We also see that extinction rates tend to increase with the number of species. This should make sense: If there are no species on an island, extinction is impossible, but as more and more species arrive (and compete!) extinction becomes more likely.

Figure \(\PageIndex{2}\) shows the basic Equilibrium Theory of Island Biogeography. It suggests that islands will reach an equilibrium, or stable, number of species when immigration and extinction rates are equal! Note this model could be modified in multiple ways. Size of an island, for example, likely also impacts immigration rate (larger islands are easier to hit!), and islands that are close together may also share individuals (the Rescue effect!), but even this simple conceptualization has proven useful for understanding global species richness patterns.

One interesting point to note is that an equilibrium number of species is reached when immigration and extinction rates are equal, not when those processes stop! This means islands may consistently be changing species composition but should maintain fairly consistent levels of species richness. As odd as this sounds, early tests of the Equilibrium Theory of Island Biogeography supported these assumptions. When mangrove islands off the coast of Florida were fully cleared of their invertebrate (insect and arachnid) communities and allowed to recolonize, islands eventually stabilized with communities of about the same richness as they had before disturbance.

Conservation in Preserves as an application of Island Biogeography

The island biogeography model has crucial applications for wildlife management because wildlife reserves or patches of habitat can be considered “islands” of habitat in “an ocean” of an inhabitable area. For this reason, the Theory Island Biogeography has become central to our understanding of how habitat fragmentation leads to biodiversity loss. For a more detailed description of how habitat fragmentation leads to biodiversity loss, see the section on "Threats to Biodiversity" in the chapter on Conservation Biology from this book or see the section on "The Scramble for Space" in Conservation Biology in Sub-Saharan Africa (Wilson and Primack 2019).

Establishment of wildlife and ecosystem preserves is one of the key tools in conservation efforts (Figure \(\PageIndex{5}\)). A preserve is an area of land set aside with varying degrees of protection for the organisms that exist within the boundaries of the preserve. There has been extensive research into optimal preserve designs for maintaining biodiversity. Conservation preserves can be seen as “islands” of habitat within “an ocean” of non-habitat. In general, large preserves are better because they support more species, including species with large home ranges; they have more core area of optimal habitat for individual species; they have more niches to support more species; and they attract more species because they can be found and reached more easily. One large preserve is better than the same area of several smaller preserves because there is more core habitat unaffected by less hospitable ecosystems outside the preserve boundary. For this same reason, preserves in the shape of a square or circle will be better than a preserve with many thin “arms.” If preserves must be smaller, then providing wildlife corridors (narrow strips of protected land) between two preserves is important so that species and their genes can move between them. All of these factors are taken into consideration when planning the nature of a preserve before the land is set aside.  

Figure \(\PageIndex{5}\): Mequon Nature Preserve in Wisconsin. Image by Jennifer Tomaloff (CC-BY-NC-SA).

Contributors and Attributions    

Modified by Andy Wilson (Gettysburgh College), Kyle Whittinghill (University of Vermont) and Natasha Gownaris (Gettysburg College) from the following sources:

• https://sites.google.com/view/env10031004/lab/module-library/simulation-and-data-based-learning-activities/island-biogeography-simulation-simulation
• Preserving Biodiversity by OpenStax is licensed under CC BY 4.0 by Connie Rye (East Mississippi Community College), Robert Wise (University of Wisconsin, Oshkosh), Vladimir Jurukovski (Suffolk County Community College), Jean DeSaix (University of North Carolina at Chapel Hill), Jung Choi (Georgia Institute of Technology), Yael Avissar (Rhode Island College) among other contributing authors. Original content by OpenStax (CC BY 4.0; Download for free at http://cnx.org/contents/185cbf87-c72...f21b5eabd@9.87).
• Threatened and Endangered Species by California Department of Fish and Wildlife (public domain)
• Marine Mammal Protection by NOAA Fisheries (public domain)
• Ecotourism from Life Sciences Grade 10 by Siyavula (licensed under CC-BY)
• Issues and Opinions, Biological and Land from AP Environmental Science by University of California College Prep, University of California (licensed under CC-BY).  Download for free at CNX.