While this material is simple to use (all you to have to do is match picture-to-picture), it still contains a lot of information. Preschoolers are capable of using this material, however they tend to create a tangled mess without help. Each packet covers a specific biome. Each card shows an organism and what it consumes. The students connect the plants and animals using colored string. I use different colored string for herbivore and carnivore relationships. Some teachers like to add a Sun card so the plants have something to consume. While each card does not show the complete diet of each organism, we tried to get its main foods.
I made the food web packets using Pages. The images are from Wikipedia. I printed them on photo paper and then laminated them. You can use string to connect the cards or use something better.
In the Upper Elementary, we do a project where the students have to add 10 more cards to a food web.