Living rocks, known as lithops, are autotrophs. Examples of autotrophs include plants, algae and some types of bacteria. Autotrophs use sunlight, water and carbon dioxide to form sugar, which they use in cellular respiration. The conversion of sunlight, water and carbon dioxide into sugar is known as photosynthesis.
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.