he size of the fox population will . . . a increase. An increase in the size of any population leads to an increase in the sizes of all other populations in an ecosystem. increase., An increase in the size of any population leads to an increase in the sizes of all other populations in an ecosystem. b stay the same. Possums and foxes do not eat each other, so the number of births and deaths in the fox population will stay the same. stay the same., Possums and foxes do not eat each other, so the number of births and deaths in the fox population will stay the same. c decrease. The larger opossum population will leave fewer energy storage molecules for all other populations in the ecosystem, so the fox population will reproduce less. This will lead to fewer births than deaths in the fox population.