I asked my dad what one of the the merits of having a pond was. He said that it may draw beneficial organisms to the garden where we placed our ponds in half wine barrels. I also asked him if he felt the pond was stable. He thinks it may need some work because a crawdad got out and probably died in the hot sun.
The first step is to get the producers in there, the very bottom of the food chain. We chose duckweed, a fast spreading and growing plant. It was easy to collect because it floats on the surface of almost every pond or creek.
You then need the primary consumers: the creatures that eat the plants. We chose minnows, a very common algae eating fish that hides from predators in duckweed. We also got another algae lover: a bullfrog tadpole, which is a vegetarian until it grows up. When it does, we will have to release it back into the wild or it will eat everything in the pond.
The final step is to get a predator or a secondary consumer. We chose crawdads (one for each pond) because they will both hunt the minnows and eat them after they die. However, the day after we caught the crawfish, I found it floating on the top of the pond. I thought it had died, but it was just the exoskeleton. It had molted! Unfortunately though the a few days later one did get out and die.
We also got two final additions: our last surviving goldfish and some mosquito fish. Both of these will keep down mosquito population.
In addition to benefitting our garden in many ways creating the pond was a fun, worthwhile project.