Here's why:

* Vastness: The oceans cover approximately 71% of the Earth's surface, making them the largest ecosystem on the planet.

* Depth: Oceans have a vast range of depths, from the shallow coastal areas to the deepest trenches, supporting a wide variety of life.

* Biodiversity: The marine ecosystem boasts an incredibly diverse array of life forms, from microscopic plankton to massive whales.

While other ecosystems like forests and grasslands are also important, the sheer size and depth of the oceans make them the dominant ecosystem on Earth.