By Steve

Humans believe that they are the dominant species on this planet given how much of the planet we have taken over and how little we are at threat from other species. But is this actually the case? The human population hit 7 billion in 2011. So how do we compare to other species, when ranked by population?

By far and away the most populous species on the planet are bacteria with numbers in the vicinity of 4 quadrillion quadrillion (4×10^30) easily outnumbering all other animal populations combined. In the oceans there are an estimated 500 trillion krill, and while that is still an impressive number, their biomass is only around 50% of that of the global human population.

If we then move onto larger beings there are an estimated 10 billion billion ants in the world- (who counted them all?), with a combined biomass of almost 10 times that of humans. And who would have thought there are more than 20 billion domestic chickens wandering around providing humans with a significant source of their daily nutritional requirements?

So it should come as no surprise then that the top ten mammalian populations are held exclusively by humans and their domesticated friends. As modern civilisation has allowed human populations to expand well beyond those of subsistence level, the associated agricultural production of livestock has seen populations of cattle (1.4 billion) and sheep (1.1 billion) far exceed what would be expected in wild ecological systems. Likewise, domesticated pets like cats and dogs are estimated to have populations of around 500 million each, due to their close association with humans.

But it becomes highly controversial as to who holds the crown when populations of over 7 billion are mentioned. Although there is no real way of knowing, there is an over-riding body of evidence that would suggest that humans might be third on this list after mice and rats, which although not domesticated strictly speaking, have a liking for human’s domesticated lifestyles.

By comparison populations of non- domesticated large mammals struggle to get into the tens of millions and although kangaroos are highly successful at 60 million- most wild mammal populations are in the single millions or lower due to human encroachment on their habitat. Make no mistake, that when you see the population sizes of large terrestrial mammals falling (500 000 elephants, 200 000 chimpanzees, 120 000 for all big cat species and 20 000 polar bears), the growing human population is directly responsible for their decline.

So when it comes to numerical superiority humans are right up there, and we probably should be celebrating our success. But rather the tone this week has been somewhat circumspect. In general there seems to be some concern with the size of our (over) population and the challenges we now face on a global scale. Not the least of which is providing on going food security, not for the 7 billion people alive today, but the 9 billion people expected to inhabit the planet by 2050.

So while humans may well be the most dominant species on the planet thanks to their ability to explore and survive in so many different environments, ultimately they are still part of that environment. In fact within the biosphere; the part of the planet in which living organisms inhabit, all species are connected through their environmental surrounds. The air, water and elemental cycles that give rise to life, and life again after death are intimately associated by the laws of biology, chemistry and physics. Should any one species contributing to global biomass (be it bacteria, ants or humans) disappear suddenly the balance of interconnected dominance would shift for those that remain. A reminder that life on this planet is greater than the sum of its parts.