Glycogen is a polysaccharide made of long chains of glucose. glucose is a

Glycogen is a polysaccharide made of long chains of glucose. Glucose on the other hand is a monosaccharide. A monosaccharide is the common base unit of all carbohydrates. They are also called simple sugars.

Other examples of common monosaccharides are fructose (fruit sugar) and galactose. 

Other organisms can change their behavior to take the place of the plants in the ecosystem.