A system is objects co-relying on each other. No part of a system should break down. Every single part of a system must be in good working condition for the system to work well, they are dependent on each other. In this case it is organisms each having different functions that contribute to the system, each organism playing an important role.
EQ2: How humans and the environment are dependent on each other.
Some people in Vietnam live on houses that are located within the rivers. These people depend on the environment around them - the water from the river, the mangroves, the sea creatures etc - to survive and live. Humans make use of sea water to get salt to earn money or catch seafood such as cockels to sell and gain money as well. The environment depends on us to help to try to maintain it, an example is the planting of trees today.