单项选择题

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At 5:30 in the morning we are deep in a dark forest on an island in the middle of the Panama Canal. We’ve been out walking for only 15 minutes, but I’m already soaked in sweat.
As a colleague and I plod along, my head lamp picks out the occasional trail marker, but mainly the light seems to operate as a major local landmark for insects. Several mosquitoes have already discovered the delights of the soft parts of my ears, while others are slowly working their way between my socks and legs to be discovered later after much scratching. Suddenly a deranged roaring and barking starts 25m above my head and builds chaotically and intensity before slowly quieting after several minutes. Similar mad choruses respond from other areas of the forest. Hearing the dawn cacophony of howler monkeys always given me a deep sense of pleasure -- the joy of being back in the tropics. It may be a hot, humid place where insects, plants and fungi rule, but the phone and fax won’t find me here. I’m free to watch monkeys, collect data and try to tease out a tiny piece of the great puzzle of life’s diversity.
That diversity faces disaster, and every biologist has a horror story to tell. Each year many of us return to the field after a cold winter’s teaching to discover that-our research sites have been destroyed and our experiments and study organisms have disappeared. We can see with our own eyes the mass extermination of the world’s animal and plant life as forests, savannas and wetlands give way to farmland, housing developments and shopping malls. If current rates of habitat destruction continue, it is likely that we will condemn from a quarter to half the world’s currently living species to extinction within the next 100 years.
Nowhere is life more diverse than in tropical rain forests, and nowhere is the assault on life more tragic. Scientists are only beginning to understand the complex webs of interdependencies among various species. Increasingly, ecological research in the tropics in revealing how dependent humans are on forests for a wide variety of important services, particularly regulation of the earth’s atmosphere and climate. We may owe as much to the residents of the rain forests as we do our cattle, corn and wheat.
Much of our understanding of tropical-forest biology comes from research on Barro Colorado Island, a 1,600-hectare dot in the middle of the Panama Canal. B.C.I. , as the island is affectionately known to the biologists who work there, is covered with dense tropical forest, which was declared a nature reserve in 1923. The Smithsonian Tropical Research Institute facility on B. C. I. , established in 1946, is a Mecca for tropical biologists, who work to uncover the complex links between the large variety of species that live in forests and to demonstrate the importance of these woodlands as sources for medicines and other products of incalculable value to humans.
The atmosphere at the research station is probably similar to that at Los Alamos, New Mexico, in the 1940s when a group of the world’s top physicists were cloistered together trying to design the atom bomb. The justified the creation of a nuclear weapon by assuming it would provide the ultimate deterrent that could be used to reinforce peace in a democratic world. Similarly, the longer-term future of human civilization on earth is dependent on the earth’s forests, which act as its lungs, livers and kidneys. That is why scientists on B. C. I. are struggling to unravel the mysteries of the forests before they disappear.
At first the forest in Panama just looks like a wall of green. Then you start to notice differences between plant species, and the sheer diversity seems suddenly overwhelming. Variations between plants are often subtle and only apparent for the short period of time that a species bears flowers or fruit. Slowly you begin to identify specific types and family groups such as the palms, heliconias and fig trees. Yet each of these families contains many species, every one of which has a subtle variation on an evolutionary theme that has found a slightly different way of competing for limited light and nutrients, or escaping from predators and diseases.
The fig trees provide a spectacular example of the complex interaction of species that enables forests to function. Of more than 1,000 species of figs in the world, at least 20 are found in Panama. Most tree species on B. C.I. bear fruit only seasonally, producing an abundance of it at the beginning of the rainy season in May and June. This is all consumed by variety of birds, monkeys and bats, and by the end of the rainy season, in October through December, there is a major shortage of food in the forest. Saving the day are the fig trees, which may bear fruit at any time.
Why do fig trees follow a different fruiting strategy from that of other trees It turns out that figs are pollinated by tiny insects called fig wasps. The female wasp enters the fig flowers when they appear, lays her eggs and then dies. In the process she brings in fig pollen, which fertilizes the flower and spurs development of the fig fruit. Meanwhile the wasp’s eggs develop within the flower into larvae, which feed on some of the fruit before metamorphosing into adults and mating within the fruit. The males then die, while the females, by now covered in pollen, leave the fig in search of a new flower in which to lay their eggs, thus keeping the pollination going. Research by Allen Herre and colleagues at B. C.I. has shown that adult female fig wasps live for only two days and that each species of the fig wasp is specific to species of fig. Thus there always has to be a fig in flower while one is in fruit to ensure that the cycle continues. Since a fig-tree population must bear fruit all year because of the wasp’s short lifespan, the figs keep fruit-eating mammals alive during the fry season when other food sources are scarce. The fig tress are in turn dependent on these fruiteaters as dispersers of their seeds. Of course, the fruit-eaters also disperse the seeds of other plant species that produce fruit during the rainy season. This creates a long-term dependence of the other plants on the presence of figs.
All this suggests that a minimum number of fig trees is essential for a healthy tropical forest. Furthermore, studies from a variety of other habitats indicate that the disappearance of just one or two keystone species can lead to extinctions throughout the local community. In some cases it may take decades before trouble starts to show up.
Ultimately the forest has the vast potential of time to recover from almost anything we do to it. Like a large green heart, it has often expanded and contracted on a time scale of tens of thousands of years, while simultaneously fostering the diversity of species that inhabit the planet, including our own species. Unfortunately our human ability to understand our relationship to the forest and other habitats has to be mustered on a much faster time scale. Luckily scientists trying to demonstrate the dependence of humans on natural communities are making huge strides. History will show that their long-term contribution to the quality of life far exceeded those of other scientific endeavors such as the bombmaking at Los Alamos. It will also be far easier to justify their work to our grandchildren.
The author is primarily concerned with describing ______.