From the other current BioBulletin stories, you can see that various fish species are in real trouble. Consumer demand and destruction of aquatic habitat have seriously depleted many populations. If this concerns you, but you love to eat fish and shellfish, there is a way to make a difference: choose carefully when it comes to shopping at the fish market or picking an entrée at a seafood restaurant.           Earth's complex "web of life" protects humans from pests, pathogens (agents that cause disease), and parasites in complex and varied ways, most of which scientists are only beginning to understand. When habitats are damaged, the equilibrium between predator and prey or host and parasite can shift in various ways, sometimes with disastrous consequences. A key factor in the incidence of disease is human behavior. However, there are certain prevention methods to reduce the spread of infectious disease.   Named after Old Lyme, Connecticut, Lyme disease, a bacterial infection is transmitted by the bite of the deer tick--though it took many years to figure that out. Although deforestation is commonly linked to outbreaks of infectious disease, in the case of Lyme disease reforestation is the culprit. Deer populations have skyrocketed, and so have the ticks. Which wouldn't be a problem, since wild animal populations are the natural reservoirs for the Lyme disease bacteria, if man weren't encroaching on their habitat--but that's exactly what's happening in many suburban areas.           According to Ross MacPhee, Chairman of the Department of Mammalogy at the American Museum of Natural History, mammoths should still be around. "If you gave 'em a shave, they're very much like a modern elephant," MacPhee points out. "These guys are incredibly buffered against extinction." Yet die out they did, as part of a puzzling extinction event during the late Quaternary period which claimed hundred of other large mammal species, including the sabre-toothed tiger and the Irish giant deer. None of the possible causes satisfied MacPhee, until he was struck by a sudden insight: that the only thing capable of causing extinctions of this type and scale was a highly lethal infectious disease.   Spiders are the dominant terrestrial predators on the planet, and wherever they occur, spiders play a vital role in the terrestrial food chain. Without all those hungry, carnivorous spiders, insect populations would explode, food crops would be decimated, ecological balances would be ravaged, and humans would probably starve within a matter of months. Spiders also spin silk, which biotechnologists are trying hard to synthesize. Each species produces a unique venom. Spiders live almost everywhere; displaying an architectural talent, and a general adaptive genius.         Most of the world's butterflies, and certainly its largest and most beautiful ones, are found in tropical rain forests. Habitat destruction has greatly endangered a number of species, including the spectacular Queen Alexandra's Birdwing. Fortunately, butterflies can be farmed (though not quite like lettuce), providing a small-scale economic alternative to logging or oil palm plantations. A successful program in Papua New Guinea is ideal from a conservation point of view because the local ranchers become protectors of the forest.   Part of the Congo Basin rain forest, the Dzanga-Sangha Dense Forest Reserve is home to countless animal species. In the summer of 1998, eleven scientists from the Museum spent five weeks in the Dzanga-Sangha rain forest. They sampled its biodiversity, recording approximately thirty species new to the area, and worked with the park personnel who live in the reserve. One find, a robin with a brightly colored throat and belly, was recognized as a whole new species after ornithologists compared it to other specimens back in New York.           Otters are the most aquatic members of the weasel family. Most kinds of otters inhabit all types of fresh waters, as well as estuaries and marine coves. They can be found on every continent except Antarctica, and all thirteen species are listed as threatened or endangered. In 1995 the New York River Otter Project was formed to bring the river otter back to central and western New York State, where water quality had improved enough to sustain a sizable population.   An increasingly common way to manage rare or threatened populations is translocation--relocating members of the species, typically for release into the wild. Although this method is commonly referred to as "reintroduction," there are actually three distinct types of such translocations: augmentations, introductions, and "true" reintroductions. It does no good to reintroduce a species into a hostile environment. Many reintroductions don't succeed because the condition or pressures that caused the original population to decline or die out--human exploitation, habitat loss, or an exotic predator, for example--are still present.           The first discovery of the Asian Longhorn beetle in the United States, occurred during the summer of 1996 in Brooklyn, New York. The beetle, a major hardwood pest, has since infested many trees in Chicago during the summer of 1998, and scientists are worried that we may be on the brink of a potential ecological disaster. This beetle is one of thousands of alien species introduced to North America, some of which produce devastating results.   Chocolate comes from the seeds of the cacao plant, which is native to South America's tropical rain forest. Cacao plants, now grown all over the world, are pollinated exclusively by midges -- small, gnat-like flies -- which prefer moist rain forest to sunny plantations. As the rain forest rapidly disappears, it threatens to take along with it the key to chocolate production. Farmers today are starting to grow cacao like the original Mesoamericans farmers did -- in small patches -- preserving the forest ecosystem.           As one of the oldest types of living systems on earth, coral reefs are massive underwater structures formed by the limestone skeletons of tiny coral animals. Spectacularly beautiful, healthy reefs house an incredible variety of species. However, reefs are also very fragile, and because they grow very slowly and require clear, clean water in order to thrive, reefs today suffer worldwide from pollution and a serious decline in water quality.   Spring high tides trigger a massive migration of the Atlantic horseshoe crab, Limulus polyphemus onto sheltered Delaware and New Jersey beaches. Since 1990 however, the number of horseshoe crabs on the Delaware Bay have been reduced by half, while the population of migratory shorebirds that rely on the horseshoe crab eggs to fuel their journey, has also greatly declined.           This year NASA will launch the first of a series of satellites called the Earth Observing System, a long-term coordinated research effort to study the Earth as a global environment. Breath-takingly detailed photographs will reveal more about our planet than ever before.   The fires in Indonesia are a local tragedy with global consequences. When millions of hectares of the world's second largest rain forest burn, then the essential services they provide -- clean water, food, medicine and oxygen -- are also lost, to the people who live near them and to all of humanity. The need for fire fighting measures is immediate, but long-term solutions must be found that balance local and global needs and conserve this irreplaceable resource for future generations.           In January, 1998, Dr. Melanie Stiassny and an international team of researchers set out to learn about Gabonese fish and the rivers they live in. Gabon's pristine rain forest rivers hold an immense variety of fish. "These fish are our early-warning system" explains Dr. Stiassny. "Their health warns us of threats to the water and the environment."   According to the World Health Organization, as much as 80% of the world's human population relies on medicines made from natural ingredients, and on the traditional healers who dispense them.