Decimation is the near-total destruction of a group, like the decimation of your brother's fish when you accidentally overfeed them. Rest in peace, little fishies.
Decimation implies that nearly all of a population has been destroyed or killed. You might be angry about the decimation of the rainforest in a region of South America or the decimation of the coral in a reef where you'd hoped to snorkel some day. This noun comes from a Late Latin root, decimationem, which means "removal of a tenth." In ancient Rome, decimation was a military punishment that involved killing one out of every ten soldiers.