Final answer:
It is true that in salt marshes, only a small portion of primary production is directly consumed by herbivores. The net primary productivity available to these consumers is limited by factors like detrital food webs and the physical defenses of plants.
Step-by-step explanation:
The statement 'In salt marshes, only a small portion of primary production is consumed by herbivores' is true. Salt marsh ecosystems are characterized by highly productive primary producers, such as salt marsh grasses, which capture energy through photosynthesis. However, not all of this primary production is consumed directly by herbivores due to several factors.
For example, some of the primary production goes into the detrital food web, where decomposers break down the dead organic matter. This contributes to the matter and energy available in the ecosystem, but is not directly consumed by herbivores. Additionally, environmental stressors or physical defenses of the plants may limit the amount of primary production herbivores can consume. Consequently, the net primary productivity that is available to herbivores might be less than the gross primary productivity due to losses from respiration and other factors.