Here's why:

* Photosynthesis: Plants are the primary producers in most terrestrial ecosystems. They use sunlight, water, and carbon dioxide to create their own food through photosynthesis.

* Base of the food chain: Plants form the base of the food chain, providing energy and nutrients to herbivores, which in turn are eaten by carnivores and omnivores.

* Essential for ecosystem function: Plants provide oxygen, shelter, and habitat for various organisms. They also play a crucial role in regulating the climate and soil formation.

While there are some exceptions, such as ecosystems based around hydrothermal vents or chemosynthesis, the vast majority of terrestrial ecosystems rely on plants as their fundamental food source.