Here's why:

* Interdependence: Plants and animals are interconnected within an ecosystem. Plants provide food and oxygen for animals, while animals help pollinate plants and disperse seeds.

* Energy Flow: Plants are the primary producers, converting sunlight into energy. This energy is then passed on to animals, creating a complex food web.

* Balance: The presence of both plants and animals helps maintain a healthy and balanced ecosystem.

Think of an ecosystem like a puzzle - plants and animals are essential pieces that work together to create a complete picture.