Decomposers: The Creepy Crawly Critters of our Chesapeake Forests
It’s alive! The forest floor, that is. When walking through the woods we mostly see leaves, sticks, and other dead plant material on the ground. That layer of “duff” is teeming with life though, and that’s a very good thing. The tiny organisms of our forest floor are members of the ecosystem that are just as essential as trees are.
Most people are familiar with the terrestrial food web. There are primary producers (i.e. plants), who use nutrients (most importantly nitrogen and phosphorous from the soil and carbon from the CO2 in the air) and sunlight to produce organic matter. Then primary consumers like rabbits eat plants, converting the organic matter of the plants into energy to live and reproduce themselves. They get eaten by a secondary consumer, like a hawk, who uses the organic matter of the primary consumer to live and reproduce. When things die they get decomposed by fungi and microorganisms, which use the organic matter from the dead plants and animals to live and reproduce. The decomposers breathe out CO2 into the air and expel nutrients into the soil as waste, and plants use the recycled compounds to grow as the cycle continues. That’s usually the most we talk about decomposers. They are critical parts of the nutrient cycle that keeps all other organisms on Earth alive, but are hardly discussed. A whole world exists down on the forest floor though, with a complicated food web just within the duff layer. Let’s take a moment to look closely at these tiny members of our forests who we rely on, as creepy crawly as they may be.
Decomposers are often classified by size. The smallest are microbes, so small that they can’t be seen by the naked eye. Bacteria and fungi are the two main groups of microbes that live in forest ecosystems. There can be billions of microbes and thousands of different species found in just one gram of soil. Which is good, because microbes are primary nutrient cyclers of our forests. They utilize various chemical compounds and types of energy, such as carbon dioxide (CO2), light energy, inorganic compounds (NH4+, NO2-, Fe2+, S2-, etc.), simple sugars, and amino acids while consuming organic matter. Microbes are also experts at breaking down raw chemical compounds into forms that other organisms can use. They play a large role in the weathering of rocks and minerals, converting inert material to soil that plants can grow in. Fungi often live in symbiosis with plants, where the fungi provide the plant nutrients while the plant provides organic substances (sugars and amino acids) and space for the fungi to grow. These mycorrhizal fungi are used by 95% of plant families; without them most plants will be stunted or die.
Microfauna are the next largest group of decomposers; they don’t exceed 100 millimeters (mm) in length. The two main types of microfauna are protozoa and nematodes. Microfauna require organic carbon to function and mostly eat microbes and fine sediment particles. Some microfauna also live in the guts of larger organisms, where they play a critical role in decomposition of cellulosic material
Mesofauna, the next largest group of decomposers, range from 0.1 mm to 2 mm. Also known as “litter transformers,” these arthropods and nematodes either scavenge on partially decomposed detritus or eat microbes. No matter what they eat, all types of mesofauna contribute significantly towards increasing soil fertility by stimulating activity among the microorganisms and adding nutrients to the soil via their waste.
The most visible group of decomposers are the “ecosystem engineers”, or macrofauna. These organisms are larger than 2 mm, so their activities and movement throughout the forest floor have a large impact on the physical structure of the soil. Moles, slugs and snails, earthworms, and large arthropods are some of the many critters that dig through organic matter and build structures, altering and transforming their environment. Changing the porosity, density, and aggregation of the soil structure helps to stimulate activity of other organisms. For example, some macrofauna line their burrows with mucous, which provides more carbon and nitrogen for bacteria to consume. Physical changes to the soil are very important for plants; more soil structure allows better root growth and access to more nutrients, partly thanks to the improved habitat for microfauna. Macrofauna are also primary agents of bioturbation, the mixing of soil, sediments, and organic matter. This process is incredibly important; without it soil is far less fertile.
Although most of these soil organisms are invisible without a microscope, it’s easy to see that they are invaluable members of forest ecosystems. The nutrient cycle of the forest relies on decomposers to convert dead plant material to usable nutrients for future plant growth. Just like in our macroscopic world, there’s a complicated ecosystem living in our soil and duff layer, and every organism plays a role.