What's Earth's dominant species by mass?
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By biomass, Earth's dominant species is likely a tree: black spruce or Dahurian larch.
Humans biomass is ~350 million metric tons, but cattle top animals at ~650 million tons. Plants dominate: sugar cane harvests are ~2 billion tons annually. The true winners are boreal trees like black spruce (over 11 billion tons in Canada) and Dahurian larch (at least 8.4 billion tons in Russia). Their carbon mass alone is ~3 times that of all animals combined.
What commenters are saying
Commenters appreciated the human-written feel. A top comment noted that anaerobic bacteria underground exist in cubic volume and could surpass these estimates if counted as one species, though others countered they are not a single species and their density is extremely low. Another highlight: over 2/3 of avian biomass is poultry. Several referenced xkcd's land mammal biomass visualization.