What is the main contribution of autotrophs to ecosystems?

Autotrophs which are also called producers are vital to any ecosystem. They are the basis of the food chains that operate in an ecosystem. Autotrophs are green plants or algae, capable of photosynthesis. 
Autotrophs contain the green pigment chlorophyll which allows them to capture light energy from the sun and use this energy to power the photosynthesis reaction. Carbon dioxide and water are converted into chemical energy in the form of glucose molecules. The glucose can later be consumed by organisms like herbivores (plant eaters) or indirectly by carnivores (meat eaters) which consume other animals. Autotrophs will use the glucose they produce to provide energy for their own needs. The chemical energy in food like glucose sugar is potential energy that can be used for cellular respiration, growth and any other work that cells perform.
To summarize, autotrophs are the bottom of the food chain. Without their contribution of producing organic molecules like glucose, from the inorganic molecules-- carbon dioxide and water, the entire ecosystem would collapse. There would be no available energy for consumers in the ecosystem to carry out their life functions.
 
https://www.nationalgeographic.org/encyclopedia/autotroph/