Africa teems with animal life of all shapes and sizes. The continent has thousands upon thousands of species of mammals, fish, reptiles, amphibians, birds, and insects. Many of these animals are linked in an intricate food web. For example, hippopotamuses deposit large amounts of nutrients in bodies of water where they rest and defecate; these nutrients support abundant growth of plants, insects, and other smaller creatures that in turn provide food for species higher in the food chain. Other animals are linked in symbiotic relationships, such as between big game animals and birds known as oxpeckers. These birds eat the ticks that pester the large animals. Animal life in Africa is often cut short by changing environmental conditions and illegal poaching due to either absence of conservation laws or inadequate implementation.