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Eco represents various types of plant life. The energy value of soylent on a tile with some form of eco living on it is greatly increased. A tower with all of its soylent tiles covered by eco will output 5 times as much energy as a tower with no eco in its area.
If creeper contacts eco, it will reduce it to stumps, but it will slowly regrow if it's in a soylent area once the creeper is removed.
This image shows the stumps on the left, and the 4 types of eco on the right.