English obscures it a little, but note that verbs in your sentence apply to "large parts of"; differently stated: "to destroy a great number or proportion of".

English obscures it a little, but note that verbs in your sentence apply to "large parts of"; differently stated: "to destroy a great number or proportion of". Decimate means quite specifically to remove or destroy a _portion_ of something, rather than simply to destroy something, even in most modern usage.