弱弱的求问population的复数问题

Mandoline

2. (24511-!-item-!-188;#058&001020)
Organized in 1966 by the Fish and Wildlife Service, the Breeding Bird Survey uses annual
roadside counts along established routes for monitoring of population changes of as many as,
or of more than 250 bird species, including 180 songbirds.
(A) for monitoring of population changes of as many as, or of
(B) to monitor population changes of as many, or
(C) to monitor changes in the populations of
(D) that monitors population changes of
(E) that monitors changes in populations of as many as, or

这道题最后选C 这里population可以加s是为什么啊？在线愚钝浅薄，望高人指点？（此题目源于prep语法精讲第二题）

靠谱的飘忽

这里的population指的是鸟的种类吧,种类的时候是可以用复数的