For example, in a trout stream with organic pollution, microbial metabolism of the pollutants can deplete the dissolved oxygen. The trout, who require a high concentration of oxygen, will leave, die, or linger in only a few spots where reaeration of the stream is good, but carp that thrive at lower dissolved oxygen may replace them. The stream may not change in appearance, but it is no longer a joy to trout fishermen. This ecosystem is rugged in the sense that it continues to function by readjusting its population. Chemical elements take different forms in the environment.
Inorganic compounds form organic compounds that can break down once again to inorganic compounds.
For example, carbon, oxygen, nitrogen, sulfur, and other elements combine in various oxidized or reduced states. The processes of life incorporate them into organic molecules. With death and breakdown of organic materials, these elements escape back to the air, land, or water for potential recycle. We can also consider food chains, although food web is perhaps a better term, in which tiny animals feed on tiny plants, only to be consumed by bigger ones. Those that die provide nutrients for other plants and animals or decompose and enter the chemical cycles mentioned earlier.