Biological Soil Crusts

Points covered:

• What lives in soil and leaf litter?
• What do soil and leaf litter organisms like to eat?
• What do they do?

Words to know:

• GONDWANA - the ancient southern supercontinent that included Australia, New Guinea South America, Africa, India and Antarctica.
• FUNGI - decomposer organisms, such as mushrooms and mildew.
• PROTISTS - single-celled organisms, such as protozoans and algae.
• PARASITES - organisms that live in or on others.
• ORGANIC DETRITUS - disintegrated animal or plant remains.
• ARTHROPOD - joint-legged animals, such as slaters, insects and spiders.
• SPRINGTAIL - a type of primitive insect.
• LICHENS - associations of alga and fungi

The soil and leaf litter world is teeming with a great variety and number of living creatures. It rivals coral reefs as one of the most biodiverse places on Earth. It is a realm of darkness with tiny caverns, tunnels and crevices. It houses heavily armoured eight-legged mites that clamber about like miniature tanks, elongated earthworms that burrow through the soil, and false scorpions that stalk their prey with their massive pincers outstretched. The soil and litter world is also home to velvet worms, survivors of ancient Gondwanan forest faunas, lichens and fungi with strange fruiting, bodies and countless microorganisms, such as protists and bacteria.

Many of the animals living in soil and leaf litter are both tiny and remarkably abundant. For example, microscopic nematodes or roundworms are so common that if everything else were to disappear, the Earth's outline could still be recognisable as a continuous sheet of roundworms. There is a great diversity among the species and lifestyles of these tiny organisms: roundworms may be parasites on plant roots, suckers of organic detritus particles and bacteria, or free-living predators with fearsome teeth.

The most visible inhabitants of forest litter are arthropods, such as ants, beetles, millipedes, slaters and spiders. Many of these are important decomposers of plant matter while others, like spiders and some ants and beetles, are predators. The most important soil arthropods that decompose organic matter are springtails and mites. While numerous, they are less obvious because they are small. Their food preferences range from decaying plant material to carcasses and faeces with associated fungi and bacteria. The interactions within the community of soil and leaf litter organisms help maintain soil fertility and structure. Nutrients locked up in dead organic matter, like leaf litter, are released via a complex food chain. This starts with leaf litter being broken down by decomposer organisms (animals, fungi, protozoa and bacteria). The excreta and carcasses of both the decomposers and their predators provide organic material which micro-organisms actually turn into the basic nutrient materials needed by plants.

Mike Gray
Arachnology
Australian Museum