Where do things live?
Plants and animals are not distributed evenly around the world. Tropical forests, for example, are thought to contain almost half the world' s plant and animal species, around a third of its birds, and a large proportion of its insects and micro-organisms. This is partly because tropical rainforests grow in areas where rainfall and temperature are much the same throughout the year.
Icy Antarctica, on the other hand, has few land species, although its oceans provide food for the krill, a shrimp-like creature, which in turn feeds whales, seals, fish, squid, and penguins.
Coastal regions such as coral reefs, mangrove swamps, and estuaries support a vast array of marine plants and animals. Around two-thirds of all the fish caught by humans live, feed, and breed in these ecosystems.
Freshwater ecosystems also have a rich variety of species and many different kinds of fish will live together in the same waters. In Africa' s Lake Victoria, for example, more than 170 fish species have developed different feeding habits which allow them to co-exist. Some have strong, blunt teeth for eating molluscs; others use chisel-shaped incisors to feed on weeds, while a third group take eggs and young from other fish with their reduced teeth and wide-opening jaws. Meanwhile, the Amazon is home to 3,000 different fish - 15 times as many as all Europe' s freshwaters. It is thought that this great river still harbours many undiscovered species.
Millions of years ago, the continents were joined together in one "megacontinent". Gradually they drifted apart, taking with them early forms of the plants and animals we know today. Different species evolved in different places - polar bears in the Arctic, penguins in the Antarctic, grey squirrels in North America, red ones in Europe. Every island which split off from the mainland gradually developed its own ecosystem. Madagascar, for example, boasts 6,000 plants that are found nowhere else, and half of Papua New Guinea' s birds are unique to the island.
It is easy to forget that many areas, such as tropical rainforests, which we regard as "pristine wilderness" are actually lived in by people. In fact, 95% of the Earth' s land surface, including inhospitable deserts and remote moorlands, is used and inhabited by people.
Over the centuries, people have had an important influence on the distribution of species. The Romans planted vines all over their empire; 16th-century European explorers brought back potatoes and tomatoes from South America, while fruitgrowers from New Zealand, California, and Italy have recently started to cultivate the Chinese gooseberry or kiwi fruit.
And throughout history, farmers have experimented with their crops, growing them in different combinations and sequences, and cross-breeding species to develop new strains. A British orchard, for example, traditionally contained dozens of different apple trees, some bred to have large fruit, others small; some to be sweet, others sour; some to ripen early, others late.
Although many human activities - cutting down trees, draining marshes, and clearing wild areas to create farmland and towns - have had dramatic and often harmful results, people have also enhanced their environment. Much of Europe' s countryside has been shaped by people, and precious ecosystems - like lowland dry heath, which supports rare birds and lizards - only exist because humans have adapted landscapes to suit their own needs.
|