Which of the following is a natural ecosystem?

The correct answer is: Forest

• Forest ecosystem is the natural ecosystem.
• It is classified under terrestrial ecosystem.
• The forest ecosystem is not built due to human or animal interference. 
• The others like aquarium, crop fields and garden are made due to human activities which are made according to the interest of the humans.
• In a forest, the type of trees, the density, the various types of animals present are not dependent on any human decision
• Forest ecosystem can be classified as tropical evergreen forests, tropical deciduous forest, temperate evergreen forests, temperate deciduous forest and boreal