Answer: d. the energy in trees is less available to herbivores.
Explanation:
A grassland ecosystem is an ecosystem that is dominated by the grasses as the producers. It is also called as rangeland because this can support a huge diversity of herbivores because of the abundance of grasses. The forests ecosystems exhibit large number of trees the herbivores are incapable of obtaining leaves as food from long trees.
On the basis of above explanation, it can be concluded that a grassland ecosystem support more herbivores than a forest ecosystem can because the energy in trees is less available to herbivores.