CBSE Class 10 Science 2017 · Paper 31/1/2 · Question 3

A forest is considered a natural ecosystem because it has come into being and maintains itself by natural processes, without being created or looked after by human beings. Its producers, consumers and decomposers interact with sunlight, air, water and soil on their own, so nutrients are recycled and the forest keeps itself going; nobody sows, waters, manures or weeds it. Gardens and crop-fields, by contrast, are made and maintained by people and are artificial ecosystems.