Final answer:
The standing crop will decrease if herbivores consume all of the net primary production, as this leads to no energy being available to support primary producers and can affect the ecosystem's health and balance.
Step-by-step explanation:
The standing crop or stock will decrease if all of the net primary production is consumed by herbivores. Net primary productivity (NPP) represents the energy that remains in primary producers after metabolic processes such as respiration and heat loss. These primary producers form the base of an ecosystem's energy pyramid and support various trophic levels above them. If herbivores consume the entire net primary production, there is no energy left to support the primary producers themselves, which can lead to their decline in the following season. The decrease in primary producers can then influence the entire ecosystem adversely, possibly causing herbivores to move to new areas in search of food or even leading to a collapse of the food web.
Herbivores that do not return after overgrazing would mean less net primary productivity in the ecosystem because the grasses, while they might grow back, would not be consumed again contributing back to the cycle. Conversely, if herbivores were to return, the overgrazing could result in depleted resources, leaving inadequate food availability for the primary producers and thus lower productivity in the following season. The overall health and continuity of an ecosystem depend on a balanced consumption and replenishment of its net primary production.