Tropical Islands: Earth’s Evolutionary Guards
Isolation and remoteness offer extraordinary circumstances for species to evolve
This article was written two decades ago during a deep dive into the journeys of Charles Darwin and Alfred R. Wallace. It’s an homage to the work of scientists, past and present, who explore the faraway and hard to reach niches of the planet.
Islands are old and young, large and small, remote and nearby. For the planet's wild plant and animal species, these seemingly simple attributes play a major role in the progression of evolution and potential extinction. Formed by the upheavals of tectonic shifting and the rise and fall of water levels over time since the Ice Ages, islands are where the evolutionary action is.
Scattered throughout the oceans in isolated pockets of protection, islands are biological laboratories and experiment stations. Arising from the ocean floor as sea mounts or separated from the continental mainland, islands became refuges for stranded wildlife and places where wild creatures happen to make landfall. Limited habitats and few competitors for food resources formed idyllic conditions for some strange and unusual natural adaptations.
Some species became giants, others dwarfs. Some lost their ability to fly. These are all traits that evolved when certain factors came together, allowing each to be expressed. But the main feature that jump-starts all of these odd and peculiar transformations is insularity. Away from large continental land masses, where animals freely come and go to find new niches, island living demands quicker changes for survival.
Islands not only stand out on the map, they make evolutionary change stand out as well. This distinctive quality is brought on due to their isolation and limited space. Borneo, Java and Sumatra were once connected to the Malay Peninsula, and the rest of Asia, during glacial periods when sea levels fell to expose the underlying Sunda Shelf. Mammals and other creatures, taking advantage of land bridges, dispersed across the wide plains of Sundaland. Later on as temperatures got warmer, sea levels rose to once again divide Sundaland into the Malay Peninsula and the assortment of islands of present day.
Evidence from fossil remains shows the effects of this geographical isolation over two million years ago. Early on, Sundaland was home to a number of elephant-like animals - mammoths, mastodons and stegodonts - as well as hippopotami, antelope and the sabre-toothed cat. Stegodonts were much larger than modern elephants and their tusks reached out twice as long. Later, dwarf species evolved among the elephants on the islands of Suluwesi, Timor and Flores. And imagine finding a 2.5 metre long giant pangolin crawling around the forest.
As the cooler periods thawed out, warmer climates and high water meant that island populations were unable to recolonize and became more vulnerable. Plants and animals either adapted to the new conditions or died out. In Borneo, the Javan rhinoceros, the Malayan tapir and a wild dog species all succumbed to changes in the environment resulting from increased isolation. One survivor, the Sumatran rhinoceros, evolved from a long-legged species into a stockier animal with shorter limbs, signalling a possible adaptation to closed rainforests from more open savannah.
Island Biogeography -- Evolutionary Pioneers: Darwin and Wallace
Most of what we know about the patterns of species distribution and the development of evolutionary theory derives from studying life on islands. Biogeography tries to separate out the facts of how plants and animals got to where they are or why they didn't make it to a particular place. Island biogeography is what Charles Darwin was doing in the Galapagos Islands off Ecuador; it's also what Alfred Russel Wallace was doing in the Malay Archipelago, both in the 1800s. By spending a few years collecting specimens and observing birds, tortoises and other wildlife in island habitats, these two researchers started a sequence of events that led each to the theory of natural selection.
It's not a coincidence that both Darwin and Wallace chose to go to islands. Most people are aware of Darwin's trek to the Galapagos Islands, from 1832 to 1836, on board the H.M.S. Beagle as the resident naturalist. Intended as a minor stopover on a five-year voyage, Darwin made the most out of his expedition by collecting a boatload of bird skins, insects, pickled reptiles and plants. Three years later he published an account of his travels to the delight of a wanderlust audience.
Not as well connected and funded as the aristocratic Darwin, Wallace was forced to pay his own way by shipping back numerous specimens, collected during his voyages, to a broker in England. His first attempt to observe and collect, an excursion to the Amazon from 1848 to 1852, ended in utter failure. Having stockpiled most of his specimens for one large shipment on his return home, he watched in horror as the vessel caught fire and sank in the Atlantic Ocean with four years worth of cargo.
With only a few drawings and maps remaining from the shipwreck, Wallace was left to ponder many of nature's questions without any raw biological data. So unlike Darwin, who stayed home to study his collections after returning from the Galapagos, Wallace needed to find another destination to start all over again. Considering both the scientific and market value of his next choice, Wallace set off for the tropical islands of the Malay Archipelago in 1854. Covering a territory as large as Europe to Central Asia, Wallace travelled nearly fourteen thousand miles and amassed a collection of over 125,000 specimens of beetles, butterflies, birds, shells, reptiles and mammals.
Darwin was a methodical, careful thinker and not one to rush off and write a manuscript without full attention to all the facts. His colleagues urged him to write an abstract of his theories before someone beat him to the publishing punch. That someone was Wallace. During his eight years (1854-1862) travelling throughout the Malay Archipelago, Wallace posted several letters to Darwin describing his own findings and observations. While Wallace was far away island hopping, Darwin seized on the opportunity to publish his seminal work, The Origin of Species, in 1859, noting in the introduction that
... I have been urged to publish this Abstract. I have more especially been induced to do this, as Mr. Wallace, who is now studying the natural history of the Malay Archipelago, has arrived at almost exactly the same general conclusions that I have on the origin of species.
Wallace returned to England a few years later and didn't publish his travelogue narrative until 1869 -- a bit too late to enjoy the adulation as the co-founder of the theory of natural selection. Nonetheless, Wallace was gracious enough to dedicate his classic work, The Malay Archipelago, to Darwin: "I dedicate this book not only as a token of personal esteem and friendship but also to express my deep admiration for his genius and his works."
Darwin knew that all the toil and trouble of documenting the various life forms existing on islands would some day come to fruition. But, despite his keen scientific skills, he failed to separate and tag his specimen collections according to each island visited. This proved to be somewhat disastrous later when he realized the importance of sorting out each species by geographic locale to understand their adaptations. Luckily, the captain of the H.M.S. Beagle, who made his own collections, properly tagged his specimens, allowing Darwin to backtrack and correctly pinpoint some of his own samples.
Wallace couldn't afford to be that sloppy with his work. He carefully noted where specimens came from so that he could find them again if necessary. As a professional collector, Wallace financed his trips by sending back species that were in demand in Europe. This meticulous attention to detail and geography also became a distinctive quality of Wallace's thinking. While waiting out the monsoon season in Sarawak in 1855, Wallace wrote a scientific paper on his thoughts on species distribution.
Darwin was alerted to Wallace's Sarawak paper and slowly began to realize that he was headed on the right path to his own thoughts on evolution. David Quammen summed up the historical situation in his masterful book, The Song of the Dodo: "Wallace's Sarawak paper was an overture. It hinted toward evolution but stopped short of explaining how the process might work. At that point in time, he had no theory to offer. Charles Darwin did have a theory but wasn't yet ready to offer it."
One must remember that in the mid-1800s, the prevailing explanation for how animals came to be and where they were found was something called "special creation." Progressive scientists, such as Darwin, were confounded by the Victorian mindset that God played the major role in species origins and distribution. It's no wonder that Darwin was hesitant to lay out his natural selection theories against the Almighty. [It's interesting to note that the modern day equivalent to "special creation" is the notion of "intelligent design." Again this implies that the Creator was shrewd enough to design each animal and plant for their particular habitat, even for those occurring more recently.]
Before we head back out to the islands, let's add a final note about Darwin's Origin of Species. A century after its publication and after all the critical brickbats, the biologist Julian Huxley tried to put this monumental work into perspective:
Why is The Origin of Species such a great book? First of all, because it convincingly demonstrates the fact of evolution: it provides a vast and well-chosen body of evidence showing that existing animals and plants cannot have been separately created in their present forms, but must have evolved from earlier forms by slow transformation. And secondly, because the theory of natural selection, which the Origin so fully and so lucidly expounds, provides a mechanism by which such transformations could and would automatically be produced. Natural selection rendered evolution scientifically intelligible.
Island Oddities
The intelligence gathering for natural selection occurred mainly on island habitats. Starting with Darwin and Wallace and followed up by a multitude of biologists, ecologists, zoologists and other curious researchers, life forms found on islands were visibly different from those found on continental mainlands. For example, what could explain the larger size of the Komodo Dragon of Indonesia? How did the famous, but now extinct, Dodo bird of Mauritius end up flightless? What factors made it possible for giant tortoises and sea-diving iguanas to evolve on the Galapagos Islands? Why are some island species unafraid of humans or other potential enemies?
Both evolutionary and ecological forces are in play in the process of change. Most changes occur at the species level, where evolution leads to differences in size, mobility, colour, and location. In contrast, ecological changes affect community dynamics with both positive and negative results. For various biological reasons, island mammals tend towards dwarfism, while island reptiles lean towards gigantism. Other species, such as birds and beetles, have arrived by flight to island habitats, only to evolve wings of little use down the evolutionary road. Many island species are endemic, living and dying in restricted localities; while others are relics, cut off from populations gone extinct on the mainland.
Evolution is slow; extinction is rapid. It has taken thousands of years for many animals to evolve and survive the onslaught of natural disasters, species invaders and climatic and habitat changes. Scientists have only just begun to understand the complex dynamics and interrelationships of species distribution and adaptation. It's a painstakingly slow process that must be proven with each new discovery or theory presented. For many species time has already run out, but for others the lessons learned from island studies may yet prove to be beneficial.
On the enchanting Indonesian island of Bali, a smaller sub-specie of tiger once prowled its finite forests. Not much was really known about the Bali tiger because it became extinct just 30 years after it was first described in the early 1900s. And there wasn't much that could be done for the sub-species anyway. Coupled with escalating habitat destruction and hunting, the Bali tiger was doomed to die out without any room to move or an influx of new members. The Javan tiger suffered a similar fate. Although it lasted fifty years longer, the demand for skins and conflicts with villagers bolstered hunters to kill them by the hundreds. Sumatra is now the only island left in Indonesia where tigers still remain.
So why do researchers fuss over extinctions of small populations on small islands? Are there not tigers in other parts of the world, such as on the larger continent of Asia? Again the study of island biogeography leads us to some answers and, of course, more questions. In the last few hundred years, island birds were studied quite extensively. Or at least more data is available for avian fauna than for mammals and reptiles.
As pointed out by Quammen, one scientist tallied up the losses since the 1600s and found that 171 species and sub-species of bird are no longer counted among the planet's biodiversity. Out of the total number of extinctions, over ninety percent (155 species) lived on islands. That's an astonishing number. To make matters worse, only twenty percent of the world's bird species are restricted to islands. This means that most extinction occurred in habitats housing a limited number of species. And to complete this dire scenario, it was found that seventy-five percent of island extinctions occurred on smaller, rather than larger islands.
The Emergence of Terrestrial Islands
Darwin and Wallace made the first forays to study the world of islands, now islands are popping up on the mainlands of every continent. Small patches of primary forests, isolated mountain habitats and other fragmented landscapes are all too common. In the past two hundred years, researchers have shown us how animals adapt and die out in isolated ecosystems. Cut off from larger populations and left with limited resources for survival, the rate of extinction increases with every new highway or agricultural plantation that separates species into smaller and smaller areas. In effect, human activities have created terrestrial islands amid a sea of disturbed land; and the consequences for these isolated landlocked habitats will not be all to different from those islands surrounded by seawater.
Primate diversity is a unique and fascinating aspect of mammals on Borneo Island. Among the menagerie are long-limbed gibbons, tiny bug-eyed tarsiers, big-nosed proboscis monkeys and the “man of the forest” or the orang-utan. All of these species are endemic to the rainforests of Borneo. But the orang-utan is in trouble. The only great ape in Asia may not survive into the next century, at least not in the wild. Logging continues to eat away at precious swamp and hill forests, only to be replaced by a stagnant sea of oil palm plantations. Some plantation owners hire hunters to shoot-to-kill these large primates if found nibbling on oil palm saplings. And periodic fires in peat lands cause damage to forests and create havoc and death for wildlife.
For the orang-utan the rainforest is getting smaller, its food scarcer and the risk of local extinction increases with each new logging permit. Several attempts are being made to delay its pending demise, including the translocation of animals to wildlife refuges and reserves. But it remains to be seen if this conservation measure will provide large enough land areas for orang-utans to thrive. Other researchers are studying the adaptability of orang-utans to exist in restricted habitats adjacent to plantations. If only the humans would cooperate, maybe it’s possible for our primate cousins to hazard a go into the disturbed land seas.
Similar scenarios are found for tigers and rhinos. These larger species require more land to maintain viable populations. But again the encroachment of agriculture and development activities renders smaller and smaller parcels of land to save for wildlife concerns. Without sufficient size, animals tend to become rare and that can eventually lead to extinction.
Conservationists are keenly aware of the problems of limited space and are looking into ways to connect these fragmented landscapes. With the aim of creating larger areas, wildlife managers try to puzzle together the pieces represented by parks, reserves, rivers and sanctuaries. Biological corridors are just one approach taken to find slivers of forests or other ecosystem byways that will allow species to move from one area to another. In an island context, this is an attempt to secure land bridges so that animal populations will not be cut off from critical habitats or reduce the genetic pool to unhealthy levels.
From the global wanderings of two bald and bearded British naturalists to the concept of biological passageways, the impact of the lessons of island biogeography on modern conservation theories and practices is as enormous as the oceans surrounding the isles. And today it still remains a wild journey of discovery and investigation.
The entire planet is shrinking into a scattered mass of isolated land and water domains, and only a return to the study of islands will help us sort out what will hang on and what will be nonexistent in the future.
READING SUGGESTIONS
1. Darwin, Charles, 1859. The Origin of Species By Means of Natural Selection or the Preservation of Favoured Races in the Struggle for Life. A Mentor Book, New American Library, 1958 edition, New York.
2. Quammen, David. 1996. The Song of the Dodo: Island Biogeography in an Age of Extinction. A Touchstone Book. Simon & Schuster Publishers, New York.
My appreciation to you for taking the time to read these selections and essays. I hope you find some enjoyment and insights about the world we live in. Thanks for supporting and sharing Continental Drift.
— Rick Scobi (@rickscobi)