What factors will cause the size of population to decrease

A lack of food, warmth, water, and rest.

Explanation:

According to Maslow's Hierachy of Needs, you need these 4 things to survive. So, a lack of any of these materials will lead to a downfall in population.

The two factors that decrease the size of a population are mortality, which is the number of individual deaths in a population over a period of time, and emigration, which is the migration of an individual from a place

Explanation:

well, you see. the answer to your question is quite simple. the thing that is big, powerful, and over 9,000 is your mom.