One season hands over to another. Some organisms thrive on decay, but most must make special preparations for winter and a life with little sun. Whole populations of animals are now forced to travel great distances in pursuit of food and warmth. 300,000 Baikal teal gather to escape from the Siberian winter by migrating south to Korea - the world's entire population in a single flock. But there are parts of the world that have no seasons. In the tropics the sun's rays strike the earth head on and their strength is more or less constant all year round. That is why the jungle grows so vigorously and supports so much life. This forest covers only 3% of the planet's surface, but it contains more than 50% of all its plants and animals. The canopy is particularly rich. There are monkeys, birds and millions of species of insects, exactly how many we have no idea. The character of the forest changes as we descend, becoming ever darker and damper, favouring different kinds of animals and plants.


季节更替，有些生物依靠枯枝败叶茁壮生长，但大部分物种必须为阳光极少的冬天做特别的准备。大批动物不得不一起长距离迁徙去寻找食物和温暖。30万只花脸鸭聚集在一起，向朝鲜半岛南迁以躲避西伯利亚的寒冬。这个迁徙的鸟群数量竟然如此众多！但地球上也有的地方没有四季。在热带，阳光是直射的，这里的气温一年到头变化不大。所以，这里的森林生长十分茂盛，能够供养如此多的生命。赤道森林只占地球表面的3%，但它的动植物却超过全球动植物总量的50%。树冠部分的动物物种异常丰富。有猴子，鸟类，还有数百万种昆虫，数量到底有多少我们无法统计。从树冠往下，情况会发生变化，它变得更暗，更潮湿，这正是某些动物和植物所喜欢的。