In nature the only source of energy is from the sun. So in ecological systems everything comes from the sun through the process of photosynthesis whereas now in human built environment our source of energy is from fossil fuels, renewable, wood energy or hydro-energy but it is not from the sun. So until we are able to operate and run a human built environment by imitating photosynthesis it will be a long while before we can have a true eco-system.