Keystone species theories identify certain species that are keystones to a particular ecosystem. Typically the function of a keystone species is to limit the numbers of a species that will degrade the ecosystem if allowed to increase it’s population unchecked. It is clear that homo sapiens are one such destructive species. There is no question that homo sapiens downgrade ecosystems wherever they become numerous.
But homo sapiens are a complex species, and factors that have limited human population growth historically are numerous. However, if one were to identify a single group of organisms that have typically performed that function, that role would primarily fall to microorganisms, namely viruses and bacteria. These are the predators that have most commonly kept human populations in check, historically.
Human population has increased exponentially since the advent of vaccines and antibiotics, and this explosion in human population has been accompanied by what is now, very clearly, a mass extinction event, loss of biodiversity and a downgrading of ecosystem that is planet wide and rivals any of the mass extinctions in the history of Planet Earth.
Before anyone starts yelling and screaming, I am not pining for the days when the average human life span was between 30 and 40 years and childhood mortality was 46%, as was the case before the advent of effective vaccines and antibiotics. Humans have gifts that no other species on the planet have: the ability to store and pass on knowledge and the ability to create, use tools and create tools to make still more powerful tools. We have the ability to become our own keystone species, and it is something we need to do, if we are going to restore our own planetary ecosystem.
No I am not advocating genocide and mass murder. But I am advocating population limits and control. If we are truly going to restore our planetary ecosystem and make it an ecosystem that can sustain life for the long term, we need to come to terms with our sheer numbers. We need to understand that population control is our responsibility, not the responsibility of God or gods. Either that or we need to learn to live with a permanently downgraded ecosystem.