Some ponds have different plants and animals while others have none. The amount of oxygen in a pond determines how much life can exist there. Reaction to sunlight causes water plants to produce oxygen, which they consume at night. A pond choked with plants becomes starved of oxygen each night and cannot support many larger species such as fish. Warmer shallow ponds contain less oxygen than deep ponds.
Warm water holds less dissolved oxygen than cold water does.
In every pond a balance develops between oxygen producers and consumers and the physical condition of the water itself. The smaller the pond, the more easily the balance is upset. As common species decline, the importance of keeping ponds as wildlife habitats becomes more apparent. A backyard pond, however small, provides an important breeding site for insects, amphibians, and even fish or birds. Even ponds as small as three feet across are valuable wildlife refuges.