To me, it is insects. And maybe their close relatives in the oceans and lakes.
> The heavyweight champion is that furtive denizen of parks, meadows, and forests throughout the Americas, the white-tailed deer. It accounts for almost 10% of the total biomass of wild land mammals.
https://www.vox.com/science-and-health/2018/5/29/17386112/al...
That is not how I read your source. It seems to be plants, then bacteria, then everything else.