Passage of the Day
(Section) 36.
"Picture an island that's totally empty of life. It's just a mound of bare rock, surrounded by water.
Why would this island be empty? One possible reason: because it's volcanic and newborn, a mountain of lava lately rise from a vent on the ocean floor. Steaming and sterile, it might be a recent addition to the Hawaiian chain, forty or fifty miles southeast of Mauna Loa. It might be the island of Surtsey, freshly erected near Iceland in November of 1963. It might be one of the Galápagos group, which were young and uninhabited just a few million years ago.
There's another possibility: It might be Krakatau, an old island newly sterilized.
Krakatau is a shrine to island biogeographers because its ecosystem was obliterated and founded anew within scientific memory. This gave it the significance of a vast natural experiment on the dynamics of recolonization. Of course, Krakatau's recolonization wasn't so carefully controlled or so thoroughly monitored as experimentalism ideally demands. But since evolutionary biology and island biogeography are both descriptive sciences more than experimental ones, and since even descriptive scientists covet the hard validation that experimentalism seems to provide, the Krakatau case has been extremely valuable.
The cataclysm took place in a series of blasts during late August of 1883, throwing six cubic miles of igneous rubble into the sky above the Malay Archipelago. The crescendo came in a single stupendous explosion on the morning of August 27. They heard that one in Perth. The sky went dark, every barograph in the world winced, the sun appeared eerily filtered--looking green, then later blue--and thirty-six thousand people were killed, mainly by tidal waves hitting the coasts of Sumatra and Java. One wave was a hundred feet high, moving as fast as a train. Ships were pushed onto beaches in Ceylon, and a change in sea level reached Alaska. Fire engines were called out on false alarms as far away as New Haven and Poughkeepsie, and peculiar sunsets and other atmospheric effects went on for months afterward. the dust veil in the atmosphere cooled the planet, which didn't warm back to normal for five years.
When the smoke and the terror finally cleared at the site of Krakatau itself, thirty miles off the west coast of Java, a small crescent of cauterized rock remained where the island had been. That cauterized remnant was called Rakata. It was a truncation of the original name, K-rakata-u, a gentle etymological reminder of the ungentle geological truncation. Two other small islets, which stood nearby but hadn't been part of Krakatau itself, were also scorched. Although nobody can be certain, scientific opinion holds that not a single living thing on either Rakata or the other islets had survived the eruption--no plant, no animal, no egg, no seed, no spore. Nine months afterward, a French expedition to Rakata found nothing alive there except a single spider.
The spider of Rakata is emblematic of the fact that spiders in general are good dispersers. Devious beasts, they are wingless but still manage to fly. A thread of silk is paid out from the silk glands, it billows, it rises, it somehow attains purchase on an ascending column of air, and like a hang glider on a windy ridge, it lifts the spider away. This trick is dependent on forces that act at small scale. It wouldn't enable a full-grown tarantula to float through the skies of Arizona, thank God, but it does allow daintier spiders to go ballooning from one place to another. I've seen baby black widows, no bigger than poppy seeds, waft away on the thermals from a tensor lamp. The Rakata spider must have ridden a breeze out from Java.
The first botanical expedition, led by a Professor Treub, reached Rakata in 1886. Treub's team found mosses, blue-green algae, flowering plants, and eleven species of fern. The algae, consisting of slimy dark smears that coated the ground with a gelatinous matrix not unlike agar, had probably served as a welcoming mat for the spores of the ferns and the seeds of the flowering plants. The ferns were especially hearty and diverse. Among the flowering plants, four species belonged to the Compositae family (a group that includes dandelions, among other aggressive airborne dispersers) and two species were grasses. It's likely that the Compositae and the ferns had been delivered to Rakata by wind. There were also some species whose seeds would have washed in on the surf.
The arrival of other life forms was quick. By 1887 Rakata supported young trees as well as dense stands of grasses and an abundance of ferns. By 1889 it harbored not just spiders but butterflies, beetles, flies, and at least a single large monitor lizard of the speciesVaranus salvator, closely related to the Komodo dragon.
Varanus salvator, like the ferns and the Compositae, is notable for its ability to travel widely and colonize new habitat. It swims well, and on land it's a versatile opportunist, quick afoot, stealthy when it needs to be, capable of climbing and burrowing. Carnivorous but not fussy, it eats crabs, frogs, fish, rats, rotting meat, eggs, wild birds, and the occasional chicken from an unguarded coop. Flesh-eating animals tend to fare poorly on small islands and on new islands, where the pickings are slim, but V. salvator on Rakata enjoyed two advantages: It was a generalist, and it was a reptile. A generalist can eat less selectively, and a reptile can eat less often.
But even V. salvator depended on the presence of other animals, and those other animals depended on the presence of plants. By 1906, Rakata supported almost a hundred species of vascular plants, with a carpet of green on the summit and a grove of trees along the shore. The grove included the tamarisk-like species Casuarina equisetifolia, a good traveler across tropical oceans, as well as the coconut palm, Cocos nucifera, which turns up on virtually any balmy beach. Another beach-loving plant, the morning glory Ipomoea pes-caprae, had also appeared. Several years later there were fig trees and a few other species characteristic of secondary forest. The sun-loving ferns that had been so prevalent earlier were now retreating to high ground, forced out of the lowlands by the grasses and shade-casting trees.
In 1934, a half century after the new beginning, Rakata and its companion islets held 271 species of plants. One botanist has given us an informed guess as to how each of those species arrived. About forty percent came on the wind. Almost thirty percent floated across the sea. Most of the others had probably been carried by animals. They all possessed good dispersal ability, but the means were various.
Ferns do their traveling as spores, one-celled reproductive capsules that serve them in place of seeds. Spores are durable genetic packets, self-contained, resistant to drying, and tiny enough to be carried on a sneeze. With their spores riding the breezes in every direction, it's no wonder that ferns get around. Coconut palms achieve widespread dispersal because the coconut, at the opposite size extreme from a fern spore, is such a seaworthy seed. Some other plant species (such as the tropical vine Entada, also known as the sea bean) produce seeds with an air space between the embryo and the seed coat, suited for long-distance flotation. Darwin himself, during the years of work that led to The Origin of Species, did experiments to gauge the dispersal ability of various plant species. He put seeds, fruits, and sections of dried stems into seawater to see which species would float for how long and whether the seeds would retain their viability afterward. "To my surprise I found that out of 87 kinds, 64 germinated after an immersion of 28 days, and a few survived an immersion of 137 days." He also learned (and reported, in The Origin, with his usual half-barmy attention to detail) that ripe hazelnuts sank immediately and that asparagus floated much better if the plant was first dried.
The colonization of a new island isn't simply a matter of getting there. Dispersal is just the first of two crucial steps, the second being what ecologists call establishment. This distinction is especially germane for creatures dependent on sexual reproduction. Having hit the beach safely, a spider or monitor lizard still faces the problem of establishing a self-sustaining population. It needs to find food, protection, and (unless it's a pregnant female) a mate, all of which demand both adaptability and luck. If dispersal is difficult, establishment is difficulty squared. Among vertebrate animals, a reptile has the advantage of a relatively starvation-tolerant metabolism. And some reptile species (the gecko Lepidodactylus lugubris, for instance, widely distributed among small islands in the western Pacific) have even picked up the trick of parthenogenesis--single parenting taken to its logical ultimate. Parthenogenesis obviates the problem of finding a mate on every new island where a solitary pioneer might arrive.
Another mode of dispersal for terrestrial creatures is accidental transport on natural flotsam. The vehicle can be an old log, a newly uprooted tree, even a tangled mat of branches and vines washed out to sea from the mouth of a river or blown offshore by hurricane winds. Such flotsam can carry a colony of termites, an orchid bulb, a clutch of gecko eggs, a snake, maybe even a terrified rat. If the flotsam eventually washes ashore on some other coastline, the passengers have achieved dispersal. One biologist has called this sweepstakes dispersal, because the odds against success are so high. Over the great reaches of geological time, though, it seems to have happened often.
In rare cases the flotsam may even be massive and durable enough to support growing plants. A biogeographer named Elwood Zimmerman has collected testimony about 'floating islands' of vegetation washed out to sea and adrift in the blue wilderness between Sulawesi (Wallace knew it as Celebes) and Borneo. 'These mats of vegetation were lush and green, and palm trees of 20 to 30 feet high stood erect on floating masses. A survey of these rafts probably would reveal that numerous plants and animals were riding them.' Wallace himself, in Island Life, reported sightings of floating islands among the Moluccas. He added that, in the Philippines,
similar rafts with trees growing on them have been seen after
hurricanes; and it is easy to understand how, if the sea
were tolerably calm, such a raft might be carried along by a
current, aided by the wind acting on the trees, till after a
passage of several weeks it might arrive safely on the shores
of some land hundreds of miles away from its starting
point. Such small animals as squirrels and field-mice might
have been carried away on the trees which formed part of
such a raft, and might thus colonise a new island; though,
as it would require a pair of the same species to be thus
conveyed at the same time, such accidents would not doubt
be rare.
Occasionally there is even eyewitness evidence of animal transport. Wallace went on to mention the case of a large boa constrictor that rafted its way to the island of Saint Vincent in the West Indies, almost two hundred miles off the South American coast. The snake arrived 'twisted round the trunk of a cedar tree, and was so little injured by its voyage that it captured some sheep before it was killed'--an instance of successful dispersal followed by failed establishment.
And sometimes the natural flotsam might even be mineral, not vegetable--which brings us back to Krakatau. Among the various types of geological debris ejected during the explosions was pumice, a lightweight and sponge-structured volcanic glass. In its frothier form, pumice will float, and rafts of the stuff littered southern seas for as long as two years after the Krakatau eruption. Some of those pumice rafts drifted together into jams, clogging inlets on the coast of Sumatra; some washed ashore in South Africa, five thousand miles to the west; some floated eastward beyond Guam. One traveler described the concentration of floating pumice offshore from Java, with individual lumps clustered together over acres of ocean, each lump as big as a sack of coal. Another man, a ship's captain named Charles Reeves who encountered pumice on the Indian Ocean, lowered a boat for a closer look. 'It was curious and interesting to note how it had bee utilized by animals and low types of life as habitations and breeding places,' he reported. 'There were creeping thing innumerable on each piece.' Although Reeves confessed himself insufficiently learned to list them all by name, he did notice crabs and barnacles, and he saw small fish gathered underneath for feeding. Obviously the crabs, barnacles, and other 'low types of life' had climbed aboard after these lumps of ballistic pumice achieved splashdown; seeds, eggs, and adults of various terrestrial creatures had no doubt gotten onto them too, carried short distances by wind or wing or else picked up when the rafts made passing contact with a shoreline. A modern study of the Krakatau event suggests that similar eruptions over the centuries, tossing out huge quantities of floating pumice, have been important factors in the dispersal of species.
A more obvious mode of dispersal is long-distance flying. But even this isn't so straightforward as it might seem. Many species of bird and insect are reluctant to cross even modest stretches of sea. They will fly anywhere in a forest, but they won't commit themselves offshore. In the Solomon archipelago east of New Guinea, for instance, three species and several subspecies of white-eye (the genus is Zosterops) remain isolated from one another on closely neighboring islands. Likewise on Salawati and Batanta, two small islands off the western tip of New Guinea--they stand less than two miles apart, but the gap seems to have been wide enough to forestall the dispersal of seventeen bird species from one to the other. The gap between mainland New Guinea and the large island of New Britain is somewhat wider, forty-five miles. That has been distance enough to keep about 180 species of New Guinea birds from colonizing New Britain. And there's the case of Bali and Lombok, where Wallace noticed that some bird species had made the short crossing while many others had not.
It's not simply that the strong-flying species travel and the weak-flying species remain sedentary. Ecological or behavioral factors are also involved. Sea birds such as albatrosses, shearwaters, frigate birds, and pelicans make long ocean journeys, of course; since they can soar effortlessly for miles and rest on the water's surface, those species aren't much dependent on terra firma. Among land birds it's a trickier matter. Some species and groups of species are more inclined than others toward reckless or accidental ocean transits. High on the list of good travelers is the pigeon family.
The typical pigeon is a slightly plump bird with a small head and strong wings, adapted to a diet of seeds and fruit, for which it might be accustomed to make seasonal migrations. Those traits seems to predispose it toward transoceanic journeys. True pigeons and pigeon descendants are disproportionately well represented on some of the most remote islands. Sâo Thomé, a small nub of land offshore from West Africa, supports five species of pigeon. Anjouan, in the Indian Ocean north of Madagascar, also claims five different pigeons. Samoa has the tooth-billed pigeon and the white-throated pigeon. Palau has the Nicobar pigeon and the Palau ground-dove. New Guinea and its surrounding islands harbor forty-five pigeon species, roughly one-sixth of the world's total. And the Mascarene Islands have known their own generous share of pigeons and pigeon-like birds, among which the dodo is only the most famous. On Mauritius, a beautiful red-white-and-blue creature called the pigeon Hollandais (Alectroenas nitidissima) had followed the dodo into extinction by about 1835, and the pink pigeon (Nesoenas mayeri) is in jeopardy of extinction at this moment. The other Mascarenes,Réunion and Rodrigues, each harbored a large, flightless species of solitaire (Ornithaptera solitaria on Réunion, Pezophaps solitaria on Rodrigues) that, like the dodo, had pigeon affinities.
What's unusual about this roster of endemic pigeons is not just the breadth of dispersal but the breadth of diversity. The pigeon ancestors traveled commonly enough to colonize many islands--but they traveled rarely enough that, once they had colonized, they were likely to be sufficiently isolated for evolutionary divergence. In many cases, the divergence entailed loss of their ability to disperse.
The dodo itself stands as the best emblem of this general truth--that insular evolution often involves transforming an adventurous, high-flying ancestor species into a grounded descendant, no longer capable of going anywhere but extinct. It's our reminder that insular evolution, for all its wondrousness, tends to be a one-way tunnel toward doom.
- David Quammen,The Song of the Dodo:
Island Biogeography in an Age of Extinction (pgs. 141-147)
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