Where does all the food in an ecosystem ultimately come from?

Producers

Explanation:

Producers also known as autotrophs are the first level in the food chain. They are organisms that are capable of making their own foods. They utilize energy gotten from chemicals or the sun and in the presence of water converts the energy to carbohydrates like glucose.

In other words, they are capable of producing organic molecules (e. g. glucose) from inorganic sources of carbon (carbon dioxide). For example plants utilize energy from sunlight in the presence of water and carbon dioxide to produce glucose.

Examples of producers include most plants (a larger percentage are plants) found on land and also plankton and algae found in the water).

These producers provide food for consumers such as animals.