Autotrophs exist at the bottom of the food chain because they produce their own carbohydrates for cellular respiration. Lithops exist in the desert and resemble rocks, which prevents heterotrophs from eating them. Heterotrophs are unable to produce their own carbohydrates and must get them from other sources.
Another example of an autotroph is sulfur bacteria. Sulfur bacteria metabolize sulfur and produce sulfuric acid or sulfates, which plants need to survive.