A team of scientists mined previous studies for estimates of the total mass of carbon found in each group of organisms on Earth as a way to measure relative biomass.
Links:
[1] http://admin.indiaenvironmentportal.org.in/news/biomass-study-finds-people-are-wiping-out-wild-mammals
[2] http://admin.indiaenvironmentportal.org.in/category/newspaper/mongabay
[3] http://admin.indiaenvironmentportal.org.in/category/thesaurus/wildlife
[4] http://admin.indiaenvironmentportal.org.in/category/thesaurus/mammals
[5] http://admin.indiaenvironmentportal.org.in/category/thesaurus/endangered-species
[6] http://admin.indiaenvironmentportal.org.in/category/thesaurus/extinct-species