by Kevin Baldwin
Islands have always been fascinating places. The old story-tellers, wishing to recount a prodigy, almost invariably fixed the scene on an island — Faery and Avalon, Atlantis and Cipango, all golden islands just over the horizon where anything at all might happen. And in the old days at least it was rather difficult to check up on them. Perhaps this quality of potential prodigy still lives on in our attitude towards islands.
— John Steinbeck, from The Log from the Sea of Cortez, 1941
In addition to providing great settings for stories, islands have also been a source of fascination and inspiration to biologists. They have had an influence on biology, ecology, and conservation that is far greater than their small areas would suggest. Because they frequently occur in groups called archipelagos, they provide separate but similar environments that have in effect, acted as replicated natural experiments for both nature and the scientists who study it. In the 19th century, Darwin and Wallace's explorations of the Galapagos Islands and Malay Archipelago clearly demonstrated patterns in nature that begged for explanation. It is doubtful that the they would have made their intellectual leaps to the elucidation of natural selection without having experienced those sites first-hand. Islands are like conceptual models: They offer simplified versions of reality. Smaller and less diverse than continents, patterns on islands were easier to see and comprehend.
I. Island Biogeography
In the 20th century, islands were important in advancing our understanding the origin and maintenance of diversity of species. In 1967, Robert MacArthur and Edward Wilson published a book entitled “The Theory of Island Biogeography” that revolutionized the study of ecology and biogeography. MacArthur and Wilson's approach was radical in that it deliberately avoided historical explanations for species diversity and sought to identify and explain more general patterns based upon current organisms' attributes and their relationships to current environments. It also refocused ecological inquiry from simply describing patterns to generating and testing theories that could account for those patterns.
The three island patterns that were linked together by a common theory were:
1. Species-area relationships: Larger islands have more species than smaller ones (there are more places to live, and species are less likely to go extinct if there are more individuals spread over a large area).
2. Isolation: Islands that are farther away from the mainland have fewer species than ones close to land.
3. Species turnover: The number of species on an island tends to remain constant although the identity of the species may change through a process called species turnover.
This last pattern was well documented on the island of Krakatoa, which blew up in 1883. Expeditions to the island following the explosion documented a steadily increasing number of species until 1920, after which diversity remained constant, with species extinctions being balanced by new colonizations.
MacArthur and Wilson's model of island biogeography was presented elegantly and graphically (see Figure), with extinction and colonization rate curves intersecting at an equilibrium (i.e., constant) number of species. In the figure, the low colonization rate curve as would be observed on an isolated (far) island crosses the high extinction rate curve, as would be observed on a small island, at a low number of species (where two heavy lines cross). Similarly, the intersection of low extinction (large island) and high colonization (near island) is at a higher number of species. Combinations of low and high colonization rates yield intermediate equilibrium species numbers.
This model ushered in a new era of ecology by setting out a general theoretical framework with which to interpret the world rather than just noting which species were located where. The use of mathematics in the model led many investigators to think more clearly about ecological problems and to identify which variables needed to be measured and which could be safely ignored. One of Ed Wilson's students (Dan Simberloff) tested the model by fumigating mangrove islands of different sizes and distances from the Florida Keys and then monitoring rates of recolonization, extinction, and equilibrium species numbers. It worked as predicted, and the theory was supported.
Today, island biogeographic theory is providing valuable insight into some of the problems facing conservation biology. It is no secret that increasing human land use has had a detrimental effect on species diversity around the globe. Island biogeography informs us as to how and why it is occurring and how we may best preserve what is still left.
If we think of large patches of undeveloped habitat as large islands, then we can understand that initially they should support a high diversity of species. As development occurs within large plots, they will be effectively divided ad isolated into smaller and smaller islands. What is an ordinary road to us can present an impassable obstacle to some species. A superhighway could be a barrier to all except birds. Even some forest-dwelling birds will not cross open areas. Something as seemingly innocuous as a lawn may be as forbidding as a paved parking lot to some species.
The equilibrium number of species on an island is a balance between colonization and extinction. Smaller islands of habitat will have higher rates of extinction because they will support smaller populations that are more likely to go extinct due to chance. They also are likely to be structurally simpler with fewer habitat types that can support fewer species. Small islands also have more perimeter relative to their area, and this increased edge allows more incursions by predators and parasites. As development continues, the habitat patches will get smaller and more isolated from one another. Isolation makes it less likely that new colonization will make up for higher extinction rates. Habitat fragmentation can continue in this manner until only small patches of habitat with few species remain. Small species may live out their entire lives within one patch and thus be less likely to suffer these effects. Large ones may not be so lucky. Fragmentation is one reason why large predators like bears, panthers and wolves are especially susceptible to extinction.
One partial solution to the fragmentation problem may be to connect island reserves to one another with corridors of habitat. However, some worry that narrow corridors may increase mortality (by increasing “edge”) and/or act as corridors for disease as well. Currently, most people think that large reserves are the best bet for preserving species diversity because as large “islands” they intrinsically have lower extinction rates. Having large reserves near or connected to one another could increase colonization rates. Buffer zones of less dense development around reserves may also increase their effective size and connectedness.
If we are interested in preserving biodiversity, and add climate change to the mix, it is harder to remain optimistic. Imagine you are living in an isolated patch of habitat and then the temperature increases. In an ideal, “whole” world you can imagine moving up mountains or towards the poles to remain in your preferred or even required temperature zone. This is not so easy in a fragmented world.
II. Volcanic Islands
Islands are exemplars of the immense creative and destructive powers of geology and time. They are reminders that: “mankind inhabits this earth subject to geological consent—which can be withdrawn at any time” (Winchester 2011).
Many islands are either volcanic in origin or result from tectonic activity. If Tim Burton were to design a baseball, it might resemble our earth with the stitched seams corresponding to mid-ocean ridges and subduction zones that delineate the tectonic plates. Where the plates spread at divergent boundaries, hot spots of lava can force their way up to form islands like Iceland and Ascension (in the south Atlantic). Where plates collide and one dives beneath the other, the subducted plate melts as it is forced deeper. The resulting magma rises and forms chains of volcanos arrayed along arcs on the edge of the opposing plate, like the Aleutian and Japanese Islands.
Another type of island chain can form as a plate is pushed over a hot spot and magma periodically bubbles through and cools. The Hawaiian Islands are an example of a hot spot chain. Kaui is about 5 million years old, while the Big Island is only about 1 million years old. If you are looking for a very long term real estate investment, another island is beginning to rise to its southeast,…
As a child, I remember being intrigued by descriptions of Surtsey, a volcanic island that emerged from the north Atlantic Ocean near Iceland in 1963 (coincidentally, the year I was born). The primal nature of newly ejected hot lava, cooling and eventually making new habitat for many life forms was compelling and full of possibility.
Later I learned of the 1883 explosion of Krakatoa and its subsequent recolonization by life and began to further appreciate not only islands' potential for rebirth but also their fragility and their potential to bring about climate change.
Whether through explosions or mere eruptions, volcanic islands have played a big role in planetary and human affairs by altering weather and climates for extended periods. The explosion of Santorini, (about 100 km. north of Crete) in 1600BC, was 100 times larger than Krakatoa and is thought to have given rise to the stories about the destruction of Atlantis and/or may have triggered the unusual events chronicled in Exodus. The eruption of Hekla 3 in Iceland during 1150BC led to ashes raining down in China and corresponded to a 90% population decrease in the British Isles. The eruption of Mt. Etna on Sicily in 42BC, was well documented by the Romans. In 1783, Laki erupted in Iceland and ejected enough sulfur to choke victims in Europe, perhaps leading to the death of 20,000 people in Britain (de Castella 2010). There was a major eruption of Mt. Asama in Japan in 1784. Together, these two eruptions led to unusual weather that may have precipitated the French Revolution. The explosion of Tambora in Indonesia in 1816 caused “the year with no summer.” Krakatoa brightened sunsets for years after its explosion. More recently, Mt. Pinatubo's eruption in 1991, led to a global cooling of about 0.5 degrees for a couple years. The April 2010 eruption of Iceland's Eyjafjallajokul volcano closed European airspace for nearly a week.
Volcanic eruptions are of course natural events, but tell us much about the effects of lofting millions of tons of particulate matter and gases into the upper atmosphere, much as we are doing by burning fossil fuels.
Not surprisingly, one of the most remote islands on the planet used to be nearly barren. Ascension Island's utility as a British strategic naval base in the south Atlantic was restricted by its limited fresh water supply. In a little known story, Charles Darwin and his friend Botanist Joseph Hooker, Kew Gardens, and the Royal Navy worked together to fashion Ascension into a more productive ecosystem (Falcon-Long 2010). Under the scientists' guidance, the Navy planted many different species of trees from the garden and as they took root and grew they began to capture rain, while reducing evaporation. In effect, they dramatically boosted the colonization rate. Like the terraforming Genesis device in Star Trek III: The Search for Spock, the project created a self-perpetuating ecosystem. Today, Ascension is home to an artificial cloud forest that was assembled from a pan-global selection of plants over just a few decades.
From one perspective Darwin and Hooker's plan could be seen as the height of imperial hubris. From another perspective, this island story is quite literally life-affirming. No matter how badly we mess up our island, with a little encouragement, life will somehow find a way to come back.
Like it or not, we humans as a species have become a biogeophysical force. We started small by deforesting islands like Easter island. Later we caused extinctions on islands by over-harvesting (e.g., the Dodo bird on Mauritius), or introducing invasive or predatory species to them (the introduced brown tree snake on Guam is responsible for the extinction or twelve bird species). We seem to be excelling at turning once continuous habitats into isolated, fragmentary, islands of habitat. As they wink out due to warmer temperatures, real oceanic islands may disappear under the waves of higher sea levels as glaciers and icepacks melt and large storms increase in magnitude and frequency. Islands formed from coral reefs may begin to dissolve as increasing levels of carbon dioxide begin to acidify the oceans. Zoological and botanical collections from these islands will remain in museums, and like the legend of Atlantis, be reminders of both their possibility and fragility. We should use the first two (cautionary) tales offered by islands, namely the hazards of fragmentation and atmospheric modification, to avoid having to resort to the measures of the third.
Tom de Castella. 2010. The eruption that changed Iceland forever. BBC News. 16 April. http://news.bbc.co.uk/go/pr/fr/-/2/hi/uk_news/magazine/8624791.stm
Howard Falcon-Lang. 2010. Charles Darwin's ecological experiment on Ascension isle. BBC News. 1 September. http://www.bbc.co.uk/news/science-environment-11137903
Al Gore. 1992. Earth in the Balance: Ecology and the Human Spirit. Houghton-Mifflin, Boston, MA.
Robert H. MacArthur and Edward O. Wilson. 1967. The Theory of Island Biogeography. Monographs in Population Biology. Princeton University Press. Princeton, NJ.
John Steinbeck. 1995. The Log from the Sea of Cortez. Penguin Books. New York, NY.
Simon Winchester. 2011. The Scariest Earthquake is Yet to Come. Newsweek 13 March.