What would most likely happen to the food web if the amount of grass drastically decreased?

The grass is the producer, so if it died the consumers that feed on it - rabbits, insects and slugs - would have no food. They would starve and die unless they could move to another habitat. All the other animals in the food web would die too, because their food supplies would have died out. The populations of the consumers would fall as the population of the producer fell.

If there is no grass, the consumers that feed on it will also die. The food web would decrease when it comes to most of the meat we consume.