Where does all of the energy in an ecosystem ultimately come from?

Question

Where does all of the energy in an ecosystem ultimately come from?

Answer 1

The sun - however plants and other organisms are needed to capture that energy into a form that is readily available to herbivores and omnivores.  They serve as a transfer mechanism for the suns energy into organic materials.

Answer 2

Most of it comes from the sun through photosynthetic organisms.  There are significant populations of chemosynthetic organisms in some environments (deep oceanic volcanic vents) that provides additional energy sources where light is not present.