While primary production refers to the synthesis of plant material, secondary productivity refers to the synthesis of

The given blank can be filled with animal tissue.

Explanation:

Secondary productivity refers to the fixation of energy by the heterotrophic species or the frequency of biomass formation. These obtain all their vitality from photosynthetic plants or via other autotrophs, either indirectly or directly, and their productivity tells about the sizes of the food chains and the number of trophic levels within an ecosystem. Thus, secondary productivity can be considered as the synthesis of animal tissues.

"a.) well–developed vascular system."