Some natural recycling is obvious to humans. Fallen tree leaves decompose and create a richer soil for other plants. Bacteria and fungi are food sources for worms and insects, which in turn are food for animals. Microbes break down organic materials and create the basic elements of soil. Earthworms create tunnels that let air and water into the ground. Lichens emit chemicals that break down rocks and wood. Holiday trees recycled in the woods create shelter for small animals; those deposited in lakes or reservoirs become habitat for fish, or they can be chopped and turned into mulch. Most people are familiar with composting, where food waste is saved and used as fertilizer for gardens. The entire food chain could be said to rely on various forms of recycling.
Water is also recycled in nature. This is known as the water cycle. Water on earth evaporates into the atmosphere from the heat of the sun and from rain and snow. Air currents move that atmospheric water, creating clouds, which move around the globe and create precipitation. "When precipitation falls over the land surface, it follows various routes. Some of it evaporates, returning to the atmosphere, and some seeps into the ground (as soil moisture or groundwater). . . . The rest of the water runs off into rivers and streams, and almost all of this water eventually flows into the oceans or other bodies of water, where the cycle begins anew (or, more accurately, continues)." Approximately one-third of the water that falls on land drains into oceans.
Recycling is understood by scientists in areas far less accessible to the layperson, even regarding the science of other planets. In the journal Science, author Elizabeth Pennisi says, "Once again, microbes are proving just how versatile they can be. Key players nearly everywhere—from deep-sea vents to termite guts, and perhaps even on the Red Planet—microbes carry out biochemical reactions that help recycle the elements of life, such as carbon, nitrogen, and oxygen." Mathematical models are used to study recycling in nature: "The salutary adaptive capacity of nature often claimed can no longer keep up the