ecosystem involves natural beautiful creature such as fish,mammals,reptiles,amphibians etc.
when crude.oil is spilled as u know oil is less dense than water so it remains on top of water surface which causes deprivation in oxygen amounts due to inability of oxygen entering due to covered layer .......so aquamarine creatures like fish etc. can die due to suffocation which affects ecosystem as fish etc. are a part. of it........
HOPE IT HELPS !!!