Interactions between Species

To soil ecologists, understanding the interactions between species is just as important as understanding the species themselves. One of the most important types of interactions between species is which species eat other species; this interaction is known as the food web. You are probably familiar with the food webs of above-ground species. In a food web for above-ground species, the source of energy is the sun. The sun helps grass and plants grow, providing food for herbivores (plant-eating animals) such as deer. Carnivores, such as wolves, eat these herbivores. When these animals die, decomposers (bacteria and other single-celled organisms) help to break down their bodies.

But the food web for soil species is different. The drawing below shows what a soil species food web looks like:

An example of a soil food web

The first major difference between the above-ground food web and the soil food web is that the soil food web has a different source of energy. In the above-ground food web, the energy source is the sun; in the soil food web, the energy source is decaying organic matter called detritus. Detritus provides food for decomposers such as bacteria and fungi, and when bacteria and fungi feed on detritus, they release nutrients back into the soil.

These bacteria and fungi are also food for larger soil organisms, such as mites, springtails (collembolans), eelworms (nematodes) and potworms (enchytraeids). Drawings of these species are shown below. Other soil species are herbivores; these organisms eat plant roots and have their own predators. All of these small animals are food for birds, salamanders or lizards.

Mite	Springtail (collembolan)	Eelworm (nematode)	Potworm (enchytraeid)

Erosion and Decomposition

Erosion is a natural process in the soil ecosystem that occurs when water washes away soil. If erosion were the only force affecting soil, soil would soon disappear completely. There must be some other force to balance erosion - soil needs a way to make more soil. That way is decomposition.

Decomposition happens when soil organisms such as bacteria and fungi break down organic matter. By feeding on leaf litter, decaying wood and other organic matter, soil bacteria and fungi "grind up" dead organic matter into small pieces, letting out the nutrients in the soil. These nutrients help to make humus, the dark, nutrient-rich part of the soil. Thus, decomposition not only provides food for decomposers, it also helps to create more soil.

Humus is "new soil," and so it is extremely important. It provides nutrients for the plants, shrubs and trees that grow on its surface. The new soil also helps support many of the animals that live underground, such as worms and ants, by providing these organisms with a place to live and a nutrient-rich food supply.

By making more soil through decomposition, soil helps to regenerate a living space for all the animals it supports. Decomposition also helps to prevent water shortages, ecosystem damage, and soil loss.

Soil food web diagram from NRCS Soil Biology Primer.