What happens to the population size of each of the pond organisms when the perch population size is decreased? how many of the species in the food web are affected by a population change in a single species?

Many species would be affected by a decrease in perch(a fish). Most of the ponds organisms would populate and eventually out grow their living capacity.

the answer to your question is b

white blood cells or wbc s are mainly 2 types-

1.agranulocytes

2.granulocytes

the monocytes or macrophages of agranulocytes , dendritic cells which are accessory leucocytes, and b cells which is one type of lymphocytes   identify the antigens present on the pathogen's surface. these three cells are called antigen presenting cells(apc) which are phagocytes and destroy the antigens by secreting antitoxins. besides this leukocyte is always plays vital role in the immunity of the body.