Which of the following is not a factor in determining population size? a the food supply
b. birth rate
C. available niches
d. detritivore activity

Detritivore activity. The decompositions of wood does not affect population size.

unicellular organisms, like bacteria, are able to perform all life functions within one single cell. multicellular organisms need many different types of cells to carry out the same life processes. each of these special types of cells has a different structure that it perform a specific function.