My learning co op made these super cool ecosystems last week. There are many versions all over the web, but I think I simplified it.
What you need:
2 pop bottles
garden soil and tiny plants/weeds
pond water/with some little creatures and plants
little piece of material
Basically, you cut the top off one pop bottle and invert another pop bottle into it. You will also have to cut the bottom off the inverted bottle to put in the dirt. Then you duct tape (the duct tape allows you to reopen your ecosystem easily in case you need to make changes) it all back together. That is it! Simple.
In the bottom, we put pond water, minnows, water bugs, water plants, a ghost shrimp and some snails. The snails and shrimp will keep the water clean, the plants will provide oxygen and a bit of food (along with micro-organisms) to the fish. We put enough water to barely touch the spout of the inverted bottle.
The top layer is the earth. We used some very open weave fabric rubber banded at the spout (think tulle, cheese cloth, panty hose - what ever you have around). Then a few handfuls of gravel and some rich garden dirt. We added a few weeds, some moss, pill bugs, a worm, a piece of rotting wood and a little layer of leaves.
We used a piece of cotton string to hang from the dirt into the water. I am not so sure if is is necessary - but I guess the idea is that it aids in the transfer of water and micro-nurtients.
Ours has been going strong for a over a week, complete with condensation and rain, and new things growing and moving.
We were originally inspired by this post and several others on the web.