For a chemical reaction to occur, activation energy is required. Using the activation energy, reactants form products by breaking bonds which require energy. This type of reaction is an endothermic reaction. When products are formed, bonds are formed back by releasing energy. This type of reaction is an exothermic reaction. When activation energy is used for a reaction, a catalyst is not required. Adding a catalyst to the reaction causes the rate of reaction to increase.
Answer:
I believe it is telophase where animal cells begin to pinch in :)
Overgrazing could be one,also could be deforestation.