We humans tend to assume we rule the Earth. With our advanced tool making, language, problem solving and social skills, and our top predator status, we like to think of ourselves as the dominant life form on the planet.
But are we?
There are organisms that are significantly more numerous, cover more of the Earth’s surface and make up more of its living biomass than us. We are certainly having major impacts in most corners of the globe and on its other inhabitants.
But are there are other living things that are quietly having greater, more significant influences? Who or what is really in charge?
If world domination is a numbers game, few can compare with tiny six-legged, shrimp-like springtails, or Collembola. Ranging from 0.25-10mm in length, there are typically around 10,000 per square metre of soil, rising to as many as 200,000 per square metre in some places. The 6,000 known species of these wingless arthropods can be found in all manner of habitats all over the world, from beaches and cliffs to the Antarctic and the highest mountain ranges on Earth.
“On tarmac you might need to go down a few inches, but anywhere you go on a land surface I would put money that there are springtails just under your feet,” says Dr Peter Shaw, a zoologist at the University of Roehampton, UK, and the UK Recorder for Collembola.
Ants control every millimetre of the Earth’s surface
Springtails are so named because those that live on surfaces have a springing organ called a furca on the undersides of their abdomens. Flicking this organ allows them to jump up to 10cm to escape predators. Despite sharing the same name, soil-dwelling springtails don’t have furcas. The group’s defining feature is that they all have a tube on their abdomens that they use to suck up water and from which a sticky substance can be exuded to help them stick to surfaces.
Alongside fungi, springtails speed the recycling of dead plants into reusable nutrients. Their importance in this process varies widely according to habitats and the presence or otherwise of other decomposers such as earthworms. But some estimates suggest they are responsible for up to 20% of litter fall decomposition in some places.
Springtails used to be described as the most abundant insects on Earth. However, DNA analysis carried out around 15 years ago found they are actually relatives of insects.
Ants do pretty well in the numbers game too, with estimates of their global population ranging from 10,000 trillion to a quadrillion (a million trillion). While counting ants is difficult and these estimates could be out by a good few zeros, it’s pretty safe to say ants are the most numerous insects in the world.
Despite being outnumbered by springtails, they have far greater and more varied powers to influence the environments in which they live.
“Ants control every millimetre of the Earth’s surface wherever they live, which is most places,” says Mark Moffett, an entomologist at the Smithsonian Institute in Washington DC, US, who in 2011 published a book called Adventures Among Ants. “These territories are basically micromanaged by ants, altering or removing things even at a microbial level to their benefit.”
The biomass of plants on land has been estimated to be around 1,000 times that of animals
Ants exert their control in a wide range of ingenious ways, from moving more earth about than earthworms, clearing away their dead to reduce the spread of disease and waging war. Leaf cutter ants farm fungi as a food source and use a bacterial pesticide related to penicillin to improve the productivity of their farms, while herder ants keep herds of aphids so they can milk them for a sugary substance called honeydew.
Of the 14,000 or so known ant species, the most domineering, aggressive ones are those that are so well adapted that they are capable of moving freely around the world forming giant colonies of billions of individuals, allowing them to take on and beat much bigger enemies.
Beetles are the most dominant, species-rich group of organisms
One such species, the Argentine ant, has spread from its South American origins to every continent except Antarctica. They can grow especially quickly because queens tolerate fertile princesses acting as extra breeders. They deploy brute force of numbers, ruthlessness and advanced war strategies to over-run rivals, other animal species and native plants, and have established super-colonies that stretch up to 6,000km along the coastlines of the Mediterranean, California in the US, and western Japan.
But then perhaps a few large things can dominate lots of small things in less obvious but more fundamental ways.
Leaving aside bacteria, the biomass of plants on land has been estimated to be around 1,000 times that of animals. And while other life forms may be more numerous individually, more obviously assertive or more diverse, the vast majority couldn’t exist without the oxygen that plants supply through photosynthesis.
Angiosperms, or flowering plants, make up around 90% of all plant species. They cover a large proportion of the Earth’s land, account for much more biomass than terrestrial animals and provide the structural canvass of the vast majority of land-based eco-systems.
“The way a desert is structured differently to a tropical rainforest or your local park is down to the way flowering plants partition up those particular spaces,” says Sandy Knapp, Head of Plants, at the Natural History Museum in London, UK. “They provide places for insects to go, and the space in which other things evolve and change.”
Then again, maybe world domination is more a matter of diversity and specialisation.
Scientists have so far named some 400,000 species of beetle, meaning they make up between one in five and one in three of all types of described life form, depending which of the various figures for the total species count you believe. They have become successful by evolving to take on highly specific roles, such as pollinating particular trees or feeding on the dung of specific animals.
Wolbachia are extremely widespread and devious
“Beetles are the most dominant, species-rich group of organisms in terrestrial eco-systems,” says Max Barclay, beetle collection manager at the Natural History Museum in London. “They have divided the world up into very small pieces to specialise in their different jobs, managing to co-exist without competing with each other.”
It is not just their adaptability and diversity that gets beetles on the shortlist. They also have pivotal roles in most eco-systems, releasing nutrients that are then available to other life forms, by breaking down wood and dung, for example. If insects - of which 40% of species are beetles - were not about, for example, most plants would not get pollinated and so would not be about to generate oxygen.
Weevils are a particularly good example of the importance, and some would say dominance, of beetles.
With their mouths on the ends of long snouts, they can drill holes in plants, into which they deposit their eggs through a special ovipositor, or egg-laying tube. This protects their larvae and gives them a separate food source from adults so they are not in competition. They are tightly associated with specific plants, giving them especially important roles within eco-systems. With some 60,000 species in a number of families, they are also highly diverse and specialised, even for a family of beetles.
So far so human-centric. Were he alive today and reading this article, the American scientist and popular science author Stephen Jay Gould would probably protest that we have so far missed a form of life that has proved even more adaptable, is indestructible and astonishingly diverse.
We are living, wrote Gould, in the Age of Bacteria.
Wolbachia provide a particularly good example of the below-the-radar dominance of bacteria. Extremely widespread and devious, they live within the cells of around two-thirds of insects and other arthropods, such as spiders and mites. They can pass between species.
However their main method of transmission is through the eggs of host females.
Nothing competes with them in terms of their dominance
And they exert their dominance by messing with the reproduction of almost every animal they infect, causing some species to change sex, killing off males, and altering their sperm. In doing so, they have in turn affected the survival and evolution of thousands of other species.
Usually parasitic, their extraordinary range of ways to manipulate their hosts, usually to favour females over males for their advantage, has led some scientists to dub them the “Herod Bug”, after the biblical king with the blood of thousands of male children on his hands.
For starters some Wolbachia can induce changes to turn male butterflies, woodlice and crustaceans into females, thereby doubling their chances of being passed on. For the same reason, they can also trigger chromosome changes that allow females of some bees, wasps and ants to make clones of themselves, reproducing without the need for males, and fertilisation by sperm.
Then there are their male-killing abilities. Research by Greg Hurst, Professor of Evolutionary Biology at the University of Liverpool, UK, has established that Wolbachia can trigger the death of some male ladybird and butterfly embryos in species in which there is strong competition for resources among young siblings. The females become stronger, and by eating their dead brothers they are better able to help spread the bacteria.
Wolbachia has yet another cunning ability – it can modify the sperm of infected males. This means an infected male mosquito, for example, can only have viable offspring if it mates with a female infected with the same Wolbachia strain.
Cyanobacteria are the most important and successful microorganisms on Earth
On top of this, insects and other arthropods can pick up genes from the bacteria, potentially speeding up the process of the emergence of new species, through lateral gene transfer.
“Wolbachia can, from the way they manipulate and alter their hosts, be drivers of evolutionary change in many species,” says John Werren, Professor of Biology at Rochester University, New York, US.
Their presence in so many insects and other arthropods, and their abilities to manipulate their hosts to their advantage, in ways that may have created many thousands of new species, makes Wolbachia are a leading candidate for the world’s most dominant life-form.
“I’m fairly comfortable in saying as far as intracellular bacteria go, and as far as terrestrial bacteria go, nothing competes with them in terms of their dominance,” adds Werren.
But of course there is more to the Earth than what takes place on land. And not everything that makes oxygen is a plant.
In fact, before cyanobacteria evolved as the first photosynthetic organisms over 2.5 billion years ago, the atmosphere contained very little oxygen. This change to an oxygen-rich atmosphere laid the foundations for the biodiversity we see on Earth today.
If you look up and down the sizes of living things, microbes dominate their scale, humans dominate their scale, ants tend to dominate things in between
Cyanobacteria form motile strings of cells that can break away from their colonies to form new ones. They can be found in almost all aquatic and terrestrial habitats, living within lichens, plants and animals, as well as forming giant visible blue-green blooms in the oceans.
Apart from generating oxygen, their other pivotal role comes from their ability to convert atmospheric nitrogen into organic nitrate or ammonia, which plants need to get from soil to grow.
These roles in nitrogen fixing and early photosynthesis, as well as their ubiquity across habitats, have led scientists such as Ian Stewart of the University of Queensland, Australia, and Ian Falconer of the University of Adelaide, Australia, to argue that cyanobacteria such as trichodesmium are the most important and successful microorganisms on Earth.
Even this cursory look at a handful of life forms from disparate corners of the tree of life reveals that it is easier to talk about organisms being more dominant or having greater impacts at different physical scales.
“If you look up and down the sizes of living things, microbes dominate their scale, humans dominate their scale, ants tend to dominate things in between,” says Moffett.
Beyond counting individual numbers, weight and surface area cover, the definition of dominance as impact on other life forms and their environment varies according to the priorities of those defining the terms. “How good a given measure is depends on what question you’re asking,” says Knapp.
Ants may look pretty dominant if they have just wrecked or destroyed your crops, for example, but they wouldn’t get far without the oxygen that plants provide. Plants wouldn’t have been able to colonise land as they did some 470 million years ago without the fungi that help enhance their photosynthetic carbon uptake and make it easier for them to reproduce.
Fungi meanwhile would never have gained their pivotal roles in most of the world’s eco-systems without the many and varied symbiotic relationships they form with animals, plants and microbes.
And so on.
“It’s a little like trying to work out whether a famous soccer player or a basketball player is more dominant,” says Werren.
While efforts to claim top dog status for any single life form will always founder on questions of definitions, what such discussions surely highlight is the complex interdependency that exists between the millions of different species of life on Earth.
“Asking which group of organisms is the most important is a bit like asking which of four pillars holding up a house is most important,” adds Knapp. “If you took any of them away the whole thing would fall over.”