The verb "decimate" comes from the Latin decimus, meaning "tenth." It meant to take a tenth from: tithe.
In its more bellicose application (bellicose, from bellum, meaning war), "decimate" meant to select by lot and kill every tenth man. Roman officers would do this to a cohort (ca. 480 soldiers) if they didn't fight bravely.
In modern usage, like the one below, it just means to destroy a large part of.
The scientist ascribed the ------- of the park’s remaining trees to the ------- of the same termite species that had damaged homes throughout the city.
a) decimation . . prevalence
b) survival . . presence
c) growth . . mutation
d) reduction . . disappearance
e) study . . hatching