Living creatures interact with one another and their physical surroundings in an ecosystem, a functional unit of nature.
The ecosystem can range in size from a tiny pond to something as large as a forest or a sea.
There are two types of ecosystems: terrestrial and aquatic.
Terrestrial ecosystems include forests, grasslands, and deserts.
Aquatic ecosystems include ponds, lakes, wetlands, rivers, and estuaries.
Ecosystems can be categorized as either natural or man-made.
Self-sustaining natural systems include ecosystems found in forests and other natural settings.
Artificial ecosystems, such as those seen in aquariums and agricultural settings, are restricted and unable to sustain themselves without human intervention.