Role of invertebrate animals

Soil animals and soil microbes have different ways of decomposing organic detritus. Those soil animals which have mouthparts (e.g. arthropods) bite off bits of organic matter and fragment it into small pieces. Some earthworms select organic particles that are larger than their mouths, and can split them into smaller fragments. On passage through the invertebrate guts, further disintegration of the organic matter occurs.

Watch as the springtail eats dead plant litter and passes a faecal pellet out the other end. The springtail is head down, tail up! Springtails can deposit faecal pellets at the rate of once every 5 minutes. You can see how a large pile of dung has built up over the course of the 30 minutes as I watched it feed. The plant leaf is being fragmented as the springtail munches.

If a faecal pellet is examined under a microscope, you can see how the leaf is reduced to tiny fragments, increasing the surface area of the original piece of organic matter (e.g. a leaf) and increasing the number of sites for colonisation by microbes.

This physical fragmentation of organic matter is one of the main ways invertebrates help decomposition. Although soil animals digest organic matter internally, the variety of digestive enzymes is far fewer in number than those produced by soil microbes.

Invertebrate animals help microbes in other ways. Microbes hitch a ride either on the outside of the animals’ bodies or in their guts. Look at the left hand side of the faecal pellet of the springtail (see arrow) and you can see a characteristic fungal spore of the genus Alternaria. So microbes are picked up by the springtail in one spot and transported to another point of colonisation for the microbes, helping the microbes’ mobility throughout the soil.

In addition, invertebrate animals graze senescent microbial colonies and stimulate them into growth. Even the tiny springtails and mites can increase nutrient transfer from plant litter to soil by up to 50%.