Soil nematodes are microscopically tiny roundworms, mostly less than 1/20th of an inch (less than 1 mm) long, that live in the soil. Most are beneficial to human crops while others are plant pests. A wide variety of nematodes live at different trophic levels. The trophic level of an organism is its position in a food chain. Some are predatory, and hunt other nematodes. Some become prey, and are themselves trapped by fungal hyphae (filaments).
Root-feeding nematodes are plant parasites. They are attracted to the roots of weak or diseased plants. Their mouth-part contains a tiny needle-like spear, called a stylet, that they use to pierce the cell walls of a plant root. They then drink the contents of the plant's cells. This causes significant damage to the plant, and is often fatal.
Healthy soil contains billions of bacteria per square inch, all of which play a role in the decomposition of dead plant material and the cycling of plant nutrients in the soil. Some nematodes feed on soil bacteria, which releases large quantities of nitrogen, a vital plant nutrient, because the bacteria contains more nitrogen than the nematode needs.
Healthy soil is laced and criss-crossed with fungal hyphae (microscopic filaments) that help decompose dead plant material and make vital nutrients available to plants. Some nematodes use their stylet to pierce the cell walls of the hyphae so they can drink the cell fluid. Like the bacterial feeders, this activity releases plant-available nutrients, including nitrogen.
This group feeds on other nematodes and similar-sized creatures. A few are omnivorous, and may feed on bacteria, fungi, algae and other animals including nematodes. These predators serve to keep all kinds of nematode populations in balance, but may themselves fall prey to predatory fungal hyphae or even other nematodes.
Nematodes as Prey
A group of soil fungi get their revenge on fungal-feeding and other nematodes by setting traps for them. When they detect nematode activity in their area, the fungi arrange their hyphae--tiny and sometimes sticky threads that weave and wander through the soil--into loops, which they then bait to attract nematodes. Nematodes that enter the trap are consumed by the fungal hyphae.