How important are soil invertebrates? A new paper in Science Advances may have the answer. Researchers have estimated the abundance of all land arthropods (animals like insects, crabs, spiders and millipedes). It turns out that two-thirds to three-quarters of all these lifeforms live in our soils.
Soil is the by-product of animal life on Earth. It’s the deposition of millions of years of waste from plants and larger animals. Animals turn this into an ecosystem. In that way, soil is the largest ecosystem on Earth. (Similar to how our skin is the largest organ in the human body). Like a skin, it offers both protection and a way to carry life-supporting nutrientsEnergy and nutrients are the same thing. Plants capture energy from the Sun and store it in chemicals, via the process of photosynthesis. The excess greenery and waste that plants create, contain chemicals that animals can eat, in order to build their own bodies and reproduce. When a chemical is used this way, we call it a nutrient. As we More.
How much does all this soil invertebrate life add up to?
The measure that scientists use can appear a bit arbitrary in our general thinking. Scientists use theoretical ‘dry weight’, which is a measure of biomassThe weight of living organisms. Biomass can be measured in relation to the amount of carbon, the dry weight (with all moisture removed) or living weight. In general it can be used to describe the volume of energy that is contained inside systems, as the size of animals relates to their metabolism and therefore, how much energy they contain and More without water. This gives a more meaningful comparison of the mass of different groups. Since it relates more directly to carbon content.
In theory, animal biomass should vary in direct proportion. That means any order of magnitude (10x) increase in biomass is associated with a 10x increase in abundance. There are more invertebrates than mammals, therefore, their biomass should be a lot higher.
In theory therefore, arthropods alone should have a dry biomass about ten times larger than wild mammals. However, wild mammals have been largely replaced by people and livestock. This now amounts to 96% of all mammal biomass on Earth.
Estimates of total biomass of arthropods are now similar to the weight of all humans and livestock on Earth. This means one of two things (or both). There are too many of us, or too few of them. In contrast, earthworms and nematodes are similar sizes and abundance to arthropods. So, their combined masses are about equal, as they should be.
On a side note – you may be wondering whether humans are too abundant. This is a subject I will come back to at some point. It comes down to whether we are out-of-balance with other animals. It seems obvious that we are. The consequence? Well, there is an infinitely powerful level of inertia in ecosystems. This will inevitably push us back into order at some stage, irrespective of how much we try to do to stop it. More on that later.
Why are soil invertebrates important?
As the authors of this study point out, soil arthropods are still largely ‘wildlife’, which makes this system relatively intact. Without them existing ‘wildly’, and in huge abundance, our farming systems would have collapsed long ago.
As I’ve discussed in numerous other articles on this blog, the existence of soil (and its invertebrates) also depends on larger-bodied animals. Some of the greatest success stories given in Isabella Tree’s book Wilding wouldn’t have occurred unless ants had recolonised the land first. And this couldn’t have happened before reintroduction of mammals.
Rebalancing livestock and other wild mammals will be the only way to reverse horrendous rates of global soil erosion. Here is an amazing video that shows the work being done by Odonata Foundation where reintroduction mammals into farmed grassland is replenishing soil condition rapidly.
The importance of micro-predators among soil invertebrates
The scientists also found that termites made up a staggering 40% of all soil animals in the world. Termites are a bit like the farmers of an ecosystem. Their abundance enables them to use large quantities of plant waste. But left alone, this wouldn’t be enough. Even at the soil-scale, predators are essential.
Spiders, mites and scorpions were 14% of biomass. Spiders are the megafaunaThe largest animals that represent the top of the trophic pyramid. These are the final building blocks in ecosystem structures for maximum entropy production. Megafauna can be measured at any spatial scale. The largest animal that ever lived on Earth is the Blue Whale. In a grassland, spiders could be considered megafauna The term is generally reserved for animals larger More of the soil world and as predators, responsible for keeping those systems intact. As was reported recently, ecosystems with predatory spiders retain far more soil carbon than those without. This relationship between predators and ecosystemsHow ecosystems function An ecosystem is a community of lifeforms that interact in such an optimal way that how ecosystems function best, is when all components (including humans and other animals) can persist and live alongside each other for the longest time possible. Ecosystems are fuelled by the energy created by plants (primary producers) that convert the Sun's heat energy More is really important! Without predators, the food chainA single thread in a food web illustrating the chain of animals that eat each other. At the base of the food chain are small high-energy (fast metabolism) animals and at the other end large low metabolism animals. An example would be whales eating krill that eat plankton that eat algae. Or lions that eat gazelles that eat grass. More pyramids that support ecosystems collapse like a house of cards.
(A) Biomass distribution between the different taxonomic groups of soil arthropods. (B) A different comparison between the biomass of alternative groups of soil arthropods.
Rebalancing the Earth’s soil
Simply by looking at the abundance of wildlife, including soil arthropods, mammals and birds, we can immediately tell whether an ecosystem is healthy.
So, rather than looking to try to modify what we have left, there is a better way. We can simply ask: ‘What’s missing?
If you are a farmer and you want to know if you’re likely to go bankrupt, you’d better start looking at how many termites, ants, spiders and springtails are in your soil. If there aren’t enough, then you’re simply going to need more native animals.
