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.
When a volcano erupts, it creates an ash cloud that darkens and cools a region for a few days. It causes long-term environmental damage and harm the health of people and animals. The mineral-rich ash has spoiled lakes, rivers and lagoons, coated plants in a dense layer of grey, and altered the sensitive habitat of animals.
There are five gases that are produced by volcanic activity. All of these gases are harmful except for water vapor. These five gases are water vapor, carbon dioxide, sulfur dioxide, fluorine, and chlorine. Carbon dioxide is one of the main causes of the Greenhouse effect. Chlorine is emitted as hydrochloric acid into the atmosphere. Chlorine destroys the ozone layer that protects the DNA of plants and animals (Fisher). There are many hazards from volcanic eruptions. The different kinds of hazards can be lava flows, pyroclastic fall deposits, volcanic gases, tsunamis, and many more.
Volcanic ash produces a covering over plants chloroplasts. Without the exposure of the chloroplasts to the sunlight, they are unable to absorb light. Absorption of light is necessary as it is the first stage in the process of converting the sun's energy into food for the plant.
Another manner in which Volcanic ash can disrupt the cycle is hat it may make the air so dark as it blows through-out it. It darkens the air because it blocks the sun from seeping through. By creating a shadow and not releasing the sunlight the plant can not access the light which it needs to complete the process.
Depending on the type of ash fluorine poison can occur within the plant. Rainfall may result in the plant absorbing the ash through its leaves and roots. The ash combined with the plants effort to create energy results in an unbalanced reaction that leads to the poison. The plants may continue to grow or die.
The depths of the ash are also a component in the plants response. A thin layer of ash may blow away or be washed away quickly allowing for the plant to recover. The thicker the ash the less likely the recovery of the plant.
the cloud blocks the sun so plants cant photosynthisis and the animals that eat the plants wont get any neutrients form them
Some of the responses reference older beliefs/expectations of how Volcano's affect plants. Recently however the opposite has been found to be true. The amount of ash in the atmosphere causes light to be scattered and more diffuse. Apparently this affect allows plants to process CO2 more efficiently. This article explains it all:
As the question points, volcanic ashes are very much dangerous to the flora and fauna near by. During volcanism molten magma rises toward the surface and the gases held in the melt begin to form tiny bubbles. The increasing volume taken up by gas bubbles makes the magma less dense than the surrounding rock, which may allow the magma to continue its upward journey. Closer to the surface, the bubbles increase in number and size so that the gas volume may exceed the melt volume in the magma, creating a magma foam. The rapidly expanding gas bubbles of the foam can lead to explosive eruptions in which the melt is fragmented into pieces of volcanic rock, known as tephra. If the molten rock is not fragmented by explosive activity, a lava flow will be generated.
Together with the tephra and entrained air, volcanic gases can rise tens of kilometers into Earth's atmosphere during large explosive eruptions. Once airborne, the prevailing winds may blow the eruption cloud hundreds to thousands of kilometers from a volcano. The gases spread from an erupting vent primarily as acid aerosols (tiny acid droplets), compounds attached to tephra particles, and microscopic salt particles. This acid/chemical rain also destroys the plants and animals nearby.
The most abundant gas typically released into the atmosphere from volcanic systems is water vapor (H20), carbon dioxide (C02) and sulfur dioxide (S02). Volcanoes also release smaller amounts of others gases, including hydrogen sulfide (H2S), hydrogen (H2), carbon monoxide (CO), hydrogen chloride (HCL), hydrogen fluoride (HF), and helium (He). These gases when pumped in huge amount has its own adverse effect on the near by biological system