ONCE IN A BLUE MOON


The statistics are certainly alarming. Fifty to one hundred acres of tropical rain forest are disappearing from the face of the planet every minute, cleared at the hands of a single, primate-descended species. So far, there is no end in sight. Scientists, politicians, and world leaders may quibble at the exact rate of deforestation but the principal argument of international concern is centered on the lasting consequences of the destruction. Even if the chainsaws and bulldozers were to stop instantaneously today, no one knows the effects on the remaining forest or how the cleared areas will recover; if in fact, they do recover.

Predictions from the scientific community, like steel nails at a hardware store, are varied and a dime a dozen. Governments, businessmen, and logging companies have always seen this as a weakness in the contentions of conservationists. If scientists cannot agree among themselves, the argument goes, how are we to believe any of their forecasts of the rain forest's future? More devastating still is the lack of direct evidence in the recovery of a rain forest ecosystem. Some predictions have been based on educated guesses or even emotions. Most predictions, however, are based on anecdotal data acquired by returning, usually years later, to a previously cleared area of rain forest and noting the extent of habitat regeneration. Others are based on the fossil pollen record, accumulated from ancient rain forest systems of eras long ago. The studies are far from perfect. Surveys are often incomplete and one-time events, from which little may be inferred about the ongoing process of regrowth and succession. More importantly, the areas are generally subject to continued human disturbance, biasing any results and overestimating the absolute rates of recovery.

While researchers and decision-makers continue to debate the effects of destroying the world's rain forests, a scientist at the Ohio State University is studying a system in which the rain forest and every living creature has already been completely exterminated. Mark Bush, an assistant professor of zoology, has documented the regrowth and return of a rain forest on the island of Krakatau in the South Pacific. Though it has been over a hundred years since an enormous volcanic eruption rocked Krakatau and the surrounding areas, the island has been left remarkably undisturbed and uninhabited, a perfect "natural experiment". It is currently covered with what may seem, on cursory inspection, like a lush tropical rain forest, including a dense canopy of trees which has closed up after only forty years. The regeneration of the rain forest is assumed to be quite rapid and complete. Bush, however, has discovered that the Krakatau system is extremely poor in species diversity. Compared to a similar area of non-disturbed tropical rain forest that may house up to 800-1000 species of trees, a "recovered" area on Krakatau may have only 80 species. This suggests that the recovery of rain forests after destruction may be measured in millennia rather than decades or centuries, as previously thought by rain forest conservationists. The wholesale clearing of rain forests around the globe has resulted in small, left-over fragments of undisturbed forest amongst vast regions of destroyed habitat- as isolated as remote islands in the middle of endless seas. The ensuing loss in biodiversity for disturbed rain forest patches may be a long-lasting, if not permanent consequence.

Bush, concerned with the conservation and recovery of rain forests around the globe, is studying the miniature system of Krakatau to model the effects of worldwide deforestation. "Islands are simple ecosystems, models for other, more complex ecosystems. By looking at Krakatau, we may stand a good chance of understanding the larger ecosystems and as a result, answer some of the major questions facing rain forests conservationists today. How many species will an area of tropical rain forest hold? How long will it take for a rain forest to recover after disturbance? In what order will species return? And underlying it all, just how do rain forests work?"

In August of 1883, a volcanic eruption on the island of Krakatau, ten times the magnitude of Mt. St. Helens and 2,000 times the force of the largest nuclear explosion yet known, shook the world - literally. After four hours of continual shaking and a heavy rain of ash, two-thirds of the original island had disappeared. Three miles to the north, new islands of steaming ash and pumice stood where there had previously been ocean over a hundred feet deep.

The sound of the explosion was heard as far off as Rodriguez Island, an incredible 2,700 miles away, as well as over 1/13th. of the entire earth's surface. A tsunami (a giant tidal wave) caused by the eruption hurtled toward the densely populated island of Java, wiping out everything in its path. Over 36,000 people were killed by a wave reaching heights of 120 feet and enormous chunks of coral reef were hurled ashore, some weighing well over 600 tons. A witness described the scene as something he could not have "dreamed in a nightmare."

Volcanic ash rose in a cloud the size of Nebraska, blotting out the sun and creating an instant night for an area over two million square miles. The dust veil created by the ash lowered global temperatures as much as 1 degree F. and they did not return to normal until five years later. The dust in the atmosphere also created fantastic, red sunset afterglows in Europe, half-way around the world. They continued for three years after the eruption and were cause for several fire company emergencies, called out in search of the apparent blazing inferno visible on the horizon. The dust even gave a strange, blue-like tinge to the moon, giving rise to the popular phrase, "Once in a blue moon".

Scientists have now been given the opportunity to study a natural experiment that only does come around "once in a blue moon". Nothing on the island survived the blast and with the additional creation of new islands, researchers have been given a clean slate to observe. Beginning in 1884, a Dutch survey documented the re-introduction of the tropical flora and fauna, continuing through until the 1930s. It is one of the most detailed studies of species done for a rain forest ecosystem. Every plant and animal species was identified and the date of introduction was noted. It is one of the few cases in the world where scientists are able to describe the succession of plants and animals in a system by direct observation, rather than by piecing together static evidence or fossil clues of the past. The island group is also the last remaining vestige of Java's lowland rain forest.

Bush and a group of scientists from England have continued the work started by the Dutch team and have since tagged every tree on large sections of the island, allowing them to note the growth or death of an individual plant. With these data, Bush can tell which types of plant species are prone to extinction and which are capable of growing to dominate the forest canopy. His results show striking new patterns, contrary to accepted theory.

The theory of "island biogeography", postulated by Robert MacArthur of Princeton University and E.O. Wilson of Harvard University, states that the species diversity of an island is directly dependent on the size of that island. The larger the island, the greater the number of species. It also states that the colonization of an island should proceed with an ever-decreasing rate of introduction of new species countered by an ever-increasing rate of extinction. Where immigration and extinction rates meet, an equilibrium is established and the species number remains stable. Mature forests are ones in which the equilibrium has been reached. Bush has found "that the models and concepts of the theory cannot be applied to floral systems." The plant species he has observed respond very specifically to their habitat requirements. "If something in their requirements is not there, then they obviously can't use it to survive," Bush said. The number of species present on an island is then dependent on what the island's habitat is like, rather than simply its size. The plant species are also not going extinct as predicted by the models. The immigration of new species is following a decreasing pattern, but the expected exponential increase in extinction of species has simply not occurred. Extinction rates have stabilized, remaining relatively rare events. The species continue to accumulate and a great number of the island's colonists stand a good chance of long-term survival. It is the first direct evidence that refutes the long-held theory of MacArthur and Wilson.

The process of floral colonization on the island is happening in "spurts" - rapid pulses of immigration and extinction followed by periods of slower change. Succession of plant communities has long been considered to be a slow, gradual accumulation and displacement of colonizing species - changing from simple to complex systems. The process can be seen in an abandoned plot of land or even a backyard left to its own devices. The progressive reclaiming of the land plods tirelessly along; as grasslands slowly give way to thorny bushes and small trees, which in turn are imperceptibly usurped by the larger, forest trees. This gradualistic view was also very prevalent in evolutionary thinking. Evolution was presumed to be a long, continuous process of slowly accumulating change, until it was challenged in 1972 by the "punctuated equilibrium" theory of paleontologists Stephen J. Gould and Niles Eldridge. Punctuated equilibrium theory contends that evolution occurs as long periods of relative stasis punctuated by abrupt episodes of relatively rapid change. Bush now proposes that the process of colonization follows the same pattern as the evolutionary theory of Gould and Eldridge. "Punctuated succession" proceeds as a series of locally catastrophic events, producing rapid episodes of floral extinctions as one habitat type is replaced by another.

On Krakatau, the succession from grassland to woodland results in the extinction of those species that are utterly dependent on the grassland community for survival. The grassland species are pushed into smaller and smaller areas by the encroaching trees, all with little reduction in the number of species. When the grassland community is finally limited to isolated pockets, the enclosure of the leafy tree canopy above the grasses ends in catastrophic losses of species. These floral and faunal extinctions are followed by an influx of newer species characteristic of a maturing rain forest.

Bush has also observed the appearance and disappearance of the assortment of insects and animals that make their way to the island, in particular, butterflies. He has found that the colonization and turnover of these organisms is directly linked with the community dynamics of the trees present, again, not the island's size. There are open ground species of butterflies on the island which are normally found along coasts or grasslands. The composition of the coastal butterfly community has not changed in one hundred years, but has simply been a continual accumulation of species and those that arrived in the beginning are still thriving. At last count there were over sixty coastal species. However, the butterflies that were dependent on the grasslands were lost in a punctuated event when the forest canopy closed over and eliminated their habitat. At that time there were seven documented extinctions, occurring as rapidly as the extinction of the grassland. Since then, there have only been two butterfly extinctions - a span of over fifty years.

Even more remarkable is the scarcity of butterflies in the inland forest. Only two species have managed to colonize the vast area of rain forest in the island's interior. "Although you have gained the forest," notes Bush, "you haven't gained the butterflies that go along with the forest. Normally you would expect to find over a hundred species." If the butterfly colonization followed the pattern as predicted by the models of MacArthur and Wilson, the forest would be filled with butterfly species. The paucity of species is something unexpected.

The explanation lies in the life histories of the butterflies. The lack of butterfly species is directly related to the island's plant composition. The diets of caterpillar larvae are very restricted, often to a single species of plant. A butterfly without its food plant has no hope of colonizing a new habitat. Species accumulation proceeds then in a hierarchical succession, rather than a monotonic fashion as previously thought. Even if a butterfly species succeeds in making the long journey out to the island, this is no guarantee of success. If the plants needed by the caterpillar aren't there in the first place, the offspring of the arriving butterfly will not have a chance to survive. The plants are determining, to a degree, the insects that will follow.

The plant- animal interconnection goes even further. Not only is the immigration of animals determined by the plant communities but the immigration of plants is determined by the arrival of animal species. Initially, the bare island was not an attraction to animals. The first colonizers were plants, whose seeds were brought in by the wind or sea. Once the plants were established, they attracted birds and bats, which use the trees on islands as roost sites or safe havens. Along with their arrival came something even more important to the developing rain forest - their droppings. The droppings of fruit-eating birds and bats are laden with seeds of fruit-bearing trees. As those trees grow, mature, and bear fruit, a new wave of fruit-loving animals comes, bringing even more seeds. The end result of this snowballing is a diverse and species-rich rain forest. In the beginning all of the colonizing plants were entirely wind or sea dispersed. Now, 80% of the newly arrived tree species are bat or bird dispersed. The animals themselves are creating the forest.

Finally, Bush's results suggest that rain forest formed by first colonization of bare habitat is very low in species diversity. Though tropical rain forests will "green up" and a tree canopy can form after only forty years, the forest is nothing like the original. "After a hundred years, there are trees on Krakatau that are 14 feet in circumference and 140 feet tall, but there aren't many different types of trees around. To the layman, it may look like a rain forest has bounced back to its original state, but in reality, this is not a complete forest," Bush noted. "In a typical mainland rain forest, you can find a vast number of tree species within a hectare plot and when you tally them up, there are only 1-10 individuals of each species. On Krakatau, you find 300-400 individuals of a single species and a few rare species thrown in for good measure. Most of the rain forest on the island is dominated by vast stands of three tree species and little else - a pattern typical of more temperate climate forests like those in the United States. This suggests that Krakatau's rain forest is only in the early stages of succession, though it is over a hundred years old. If rain forests always need such enormous amounts of time to recover, the question then arises as to how stable they actually are? Probably none are truly in equilibrium, since all rain forest is subject to continual change - induced by landslides, floods, volcanic eruptions, etc."

His results may force scientists to reevaluate their expectations of a "stable and mature rain forest" and to reconstruct time-tables that have been modeled for the recolonization of deforested areas. If people desire the original diversity present in a tropical ecosystem that has been destroyed, they may have a long wait.


This article originally appeared in The World & I .



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Copyright © 1995 Dr. Nicholas B. Carter.
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