Ash from a massive volcanic eruption would certainly decrease the energy intake of nearby plants and animals.
When it comes to plants, the ash from the eruption would disrupt their ability to photosynthesize. The ash would cover many of their leaves and prevent them from getting the exposure to sunlight that they need.
This, in turn, would reduce the energy intake of animals. Animals that eat the plants would no longer be able to get as much food because the plants would be dying or at least not growing as well as they did before the eruption.