While the number of farmed animals now exceeds the number of wild animals, that is likely because wild populations are now much reduced and their habitat much reduced in scale. So my question is this. Would there have been more animals on the earth in the past before humans appeared, say prior to 300,000 years ago, than there are farmed animals now? I mean to include all kinds of animals such as insects, fish, crabs and other sea animals, reptiles, amphibians, mammals, and birds.
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You included insects, which breaks the question a bit.
Estimates for all domestic and wild mammals, birds, fish, reptiles, and amphibians, using the most generous numbers here https://reducing-suffering.org/how-many-wild-animals-are-there/#Summary_table comes to about 2 quadrillion. A little less but I rounded up. The number of insects today is estimated at 10 quintillion. https://www.si.edu/spotlight/buginfo/bugnos
So 0.2% of all insects outnumbers everything else. Have insect populations declined since humans arrived on the scene by at least 0.2%? Yes. So the number is now smaller.
https://en.m.wikipedia.org/wiki/Decline_in_insect_populations
I wrote a far longer comment but accidentally navigated away from it on my phone and lost it. But if you ignore insects that’s still the case. Every category of animal has declined recently, with the possible exception of mammals. It is possible the number of mammals today outnumber mammals from 300k years ago. Since most mammals today are domesticated and are fed intensively grown crops, the carrying capacity of a unit of land has been artificially raised by a lot. Not sustainably I might add, but that wasn’t the question. But mammals themselves have populations orders of magnitude smaller than amphibians or birds, which have declined. And fish outnumber amphibians and birds and have also suffered massive population loss.
The (overwhelming) majority of (non-human) *mammalian* biomass is livestock. But the number (and biomass) of wild animals still exceeds the number (and biomass) of farmed animals.
Livestock (0.1 Gt carbon) and humans (0.06 Gt C) together comprise a small fraction of global animal biomass (~2 Gt C) (Bar-On et al., 2018, see Table S23 in supplement for the breakdown of global biomass.) Cnidaria (e.g., jellyfish, coral) represent a comparable amount of biomass to livestock. Most animal biomass is either arthropods or fish (marine arthropods being ~1 Gt C, and fish ~0.7 Gt C according to Bar-On et al. (2018)). Rosenberg et al. (2023), estimate that the total dry biomass of terrestrial arthropods is between 0.1 and 0.5 Gt, likely ~0.3 Gt. That would be ~0.15 Gt carbon**, comparable to humans and livestock combined.
Going by number of individuals instead of biomass, arthropods outnumber livestock to a far greater degree. Even a chicken is orders of magnitude more massive than most individual arthropods. Rosenberg et al. (2023) estimate that there are ~10^(19) soil arthropods globally (with a factor of two uncertainty). That’s on the order hundreds of millions of soil arthropods for every single domestic chicken and mammal. (And wild honeybees still outnumber farmed/managed honeybees.)
Similarly, while there are some truly massive fish, most fish aren’t very big. Furthermore, the 0.7 Gt carbon (thus, ~1.4 Gt total dry biomass) fish biomass estimate from Bar-On et al. (2018) may be quite conservative. Bianchi et al. (2021) and souces cited therein indicate several Gt dry biomass of fish, with their estimate for commercially exploited fish alone being ~1.1+/-0.1 Gt, comparable to the total fish mass based on Bar-On et al. (2018).
** Keep in mind that when comparing biomass: (1) Estimates vary, and fish and arthropod biomass especially aren’t that well constrained compared to mammalian biomass. (2) Some sources give biomass in terms of dry mass, while others consider the biomass in terms of just carbon, which comprises roughly half the dry mass of animals.