Many violent volcanoes spew a lot of pyroclasts and gases such as carbon dioxide and sulfur dioxide high into the atmosphere. The fine pyroclast would linger in the atmosphere for months and even years blocking out the sun. This would cause vegetation to die out and decrease. Photosynthetic organisms re usual producers and the entry point of energy from the sun into ecosystems. Their death would mean the collapse of ecosystems.
Settling pyroclast would also blanket vegetation in thick layers hence also denying them access to sunlight. There would be a definite drastic decline in population and communities and ecosystems, and even extinction akin to the extinction even to the dinosaurs during the meteorite strike.
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Check out a similarity with the extinction of dinosaurs;
