In the past, if one pond dried up, another formed. Today natural ponds rarely occur in wet or marginal land that has been drained for agriculture. Larger, well-built artificial ponds tend to last longer, but often become too much trouble to keep up and are drained or allowed to dry out. Domestic waste, from tin cans to old tires, pollutes many ponds. Pesticides poison country ponds.
Fertilizer run-off causes excess growth of algae or weeds, which can choke the ponds.
No two ponds are alike. 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.