Compost Ecology – A Complex Ecosystem of Composting Organisms

May 23, 2010
Mike Bown

Compost - USDA: Wikimedia Commons

A compost heap supports a complex and dynamic ecological food-web of microorganisms which break down organic matter and release nutrients to create humus.

A compost heap is home to a complex and dynamic ecosystem of bacteria, moulds, fungi, other micro-organisms and invertebrates. Most are invisible to the human eye, but they are present throughout the composting process. Others that are large enough to see usually appear during the later cooler stages of decomposition.

The Compost Ecosystem

Within the compost heap compost organisms form an ecological food-web in which each organism has a particular part to play in turning the organic waste into dark, crumbly, finished compost. Each has its own preferred nutrient and temperature requirements so that the type, number and distribution of organisms in the compost may vary with the materials being composted and changing chemical and physical conditions within the compost heap, particularly, moisture, temperature, aeration and pH.

This food-web can be thought of as consisting of three levels of consumers: primary, secondary and tertiary consumers.

Primary Consumers

These are organisms that take their nutrients directly from the raw organic matter in the compost. Primary consumers are mainly bacteria, actinomycetes and fungi which secrete enzymes to break down organic residues and absorb the nutrients released.

Bacteria are normally predominant in the early stages of compost breakdown where, if conditions are suitable, the temperature of the heap will rise very quickly, possibly up to 55°C and above. The types and distribution of bacteria in the heap will change as the temperature rises. Hot composting has the added advantage of accelerating decomposition of organic materials and releasing nutrients for further bacterial activity as the heap cools down later.

Fungi and actinomycetes proliferate as the compost heap cools and are responsible for breaking down some of the more resistant plant chemicals, such as cellulose and lignin, into smaller molecules which can be used by other composting organisms.

Primary consumers also include invertebrates such as millipedes, worms, slaters, snails, slugs and beetles which shred, consume and digest the plant materials internally. Their droppings may contain shredded but undigested plant residues which provide increased surface area for further action by micro-organisms.

Secondary Consumers

Secondary consumers are usually multicellular organisms which consume the micro-organism primary consumers (particularly bacteria). These organisms do not consume the raw organic materials in the compost. They include protozoa and rotifers, nematodes, flatworms, springtails and mites. Their excreta or droppings are returned to the compost for further recycling by the primary consumers. Secondary consumers usually proliferate as the heap cools down after the hot phase.

Tertiary Consumers

Tertiary consumers are usually larger, active predatory invertebrates which consume and digest the secondary consumers. These include centipedes, beetles, ants, earwigs, spiders and pseudoscorpions. Like secondary consumers their excreta and the undigested remains of their prey are recycled into the compost ecosystem. They are usually the last organisms to colonise the heap as it cools and their prey proliferate and become established.

Support Compost Ecology

To make good compost it is important to ensure from the start that conditions for composting are just right to provide the nutrients required for the succession of composting organisms. This means providing the right combination of compostable raw materials to create a suitable habitat for the composting ecosystem to become established and run its course. If this natural succession of organisms is not established or interrupted then composting will be slow and inefficient resulting in poor quality compost.