Most enzymes do in fact work on a single substrate in most cases, this is because of the enzyme - substrate specificity. Certain reactant molecules acting as substrates can only fit in the "lock" of particular enzymes and undergo a conformational change and result in Unique products to be released after the reaction has occurred.
Well the niche of a squirrel mostly depends on the niche of a temperate rainforest.The trees need water and sunlight to make a nut.Then the squirrel finds the nut and stores it for the winter