With its vast area, diverse landscape and climate variations, China harbors a rich variety of wildlife. It is one of ten countries with the largest numbers of biological species in the world. In terms of temperature, the nation can be divided from south to north into equatorial, tropical, subtropical, warm-temperate, temperate and cold-temperate zones. In terms of moisture, it can also be divided from southeast to northwest into humid, semi-humid, semi-arid and arid zones. China's topography is varied and complicated, with towering mountains, different sized basins, undulating plateaus and fertile plains.