Question

What is the source of almost all of the energy in ecosystems?

Answer 1

Fundamentally, it is always the SUN!

Explanation:

Without the energy from sunlight, none of the other biological processes in most ecosystems would continue.

If you want to start at the first biological step of the food pyramid, then it would plants. The specific type will vary with the climate/region of the planet, so it cannot be reduced to a single type of plant.