That’s really the only right answer. If we’re looking at the impact of a single species, as opposed to a genus, family, or order (like most of the other answers are doing, e.g., “spiders”), humans are the only single species whose absence would cause vast changes in the biosphere. We have no other close taxonomic relatives that could step into our “niche” and continue doing what we’re doing. Losing one species of mosquito (instead of the whole genus) or one species of plankton (instead of the entire… god, what, order? Clade?) wouldn’t produce any significant effect by itself.
That’s really the only right answer. If we’re looking at the impact of a single species, as opposed to a genus, family, or order (like most of the other answers are doing, e.g., “spiders”), humans are the only single species whose absence would cause vast changes in the biosphere. We have no other close taxonomic relatives that could step into our “niche” and continue doing what we’re doing. Losing one species of mosquito (instead of the whole genus) or one species of plankton (instead of the entire… god, what, order? Clade?) wouldn’t produce any significant effect by itself.