It is often the smallest things that get overlooked, and life in the soil is probably the most neglected habitat of all. Tilling the soil or weeding the garden puts us in touch with a few members of the soil environment, and we might notice a spider, snail, beetle, or worm now and again. But underneath the surface there is a dynamic interplay between predators, prey, producers, consumers, and decomposers that we rarely witness. And their interactions are as complex and diverse as those found in the African plains, the tropical forest, or the rocky intertidal.
However, their habitat is measured in millimeters, not acres. The interactions between these organisms are frequently on the microscopic tips of root hairs, or in the thinnest layer of water film encasing a particle of soil, or in the miniscule air space between roots and organic debris. All this activity takes place within the top three inches of soil, but its diversity and productivity supports the rest of life on Earth.
Earthworms, of course, are the mighty tillers of the soil, and great lengths have been taken to promote them, nurture them, and train people to raise them. They are the champions of soil excavation. They burrow through the soil, feeding on dead and decaying organic material and dragging dead leaves and grasses down from the surface. The extensive network of tunnels they create helps water penetrate into the soil, allows air to circulate through it, and improves soil “texture” by binding it into aggregates. Their wastes are deposited as nutrient-packed, ammonium-laden “castings” that serve as a prime plant fertilizer. These tiny heaps of droppings can often be seen littering the ground after a rain, dotting the landscape like hundreds of miniature volcanic eruptions.
Earthworms are extremely valuable, and they have a great marketing department, but there are many other species that collectively interact to make soil what it is.
Mites, nematodes, fungi, bacteria, and protozoans are the workhorses of the subsurface world, but due to their microscopic size they go mostly unnoticed. Their work frees up the components in complex molecules that plants can use directly. For example, earthworm castings are valuable packets of nutrients—including ammonium, or NH4+, a vital molecule for plant growth—but plants are able to access those nutrients only when they are released by the bacteria and fungi that feed on the castings.
As Earth’s atmosphere surrounds our planet in a protective, life-sustaining envelope, so the rhizosphere surrounds plant roots with a film of water and soil particles. Bathed in this lubricating sheath, root hairs grow through the soil seeking nutrients, oxygen, and more water. As the root hair makes its way, old damaged cells are sloughed off like dandruff to be replaced with new cells. Bacteria congregate just below the tips of the root hairs to feed on these dead plant cells. Bacteria also feed on the wastes of other animals, degrade and neutralize pollutants, and improve the soil texture, or tilth, allowing air and water to reach the root zones of plants. Some bacteria are known to inhibit the hatching of parasitic nematode eggs, thus acting as a subterranean natural pesticide.
Certain fungi form a symbiotic relationship with plant roots called mycorrhizal associations. The fungus body is made up of many strands of filaments called hyphae. Grab a handful of leaf litter and you can see the expansive network of fine, white, hairlike threads of hyphae coating the underside. Fungi cannot photosynthesize, so they have to get their sugars and starches another way: The hyphae penetrate the outer wall of the plant root and transfer nutrients, especially phosphorous, from the soil to the plant while receiving carbohydrates from the plant in exchange. The long, trailing hyphal strands also serve as extensions of the plants roots, providing more surface area for nutrient and water uptake. Plants can extract phosphorus from the soil on their own, but it is a slow process. Mycorrhizal associations speed up this process because hyphae are more efficient at transporting phosphorus.
Several species of protozoans live in the soil, feeding on bacteria, fungal strands, and other organic wastes. These single-celled organisms also inhabit the air spaces between soil particles and the rhizosphere surrounding roots. They move along by beating cilia or long filaments called flagella. Some are the amoeba familiar from our freshman biology class. Sliding along on extensible pods, they engulf bacteria, absorb them, and digest them. Protozoans excrete more ammonium than they use, benefiting plants that take up this natural fertilizer.
Cruising through this damp and dark habitat are the nematodes, microscopic nonsegmented worms with voracious appetites. Some specialize in feeding on bacteria, while others seek fungal hyphae to bore into them and suck out the contents of their cells. There are also predatory nematodes that target protozoans and other nematodes. Once again, their need for ammonium is less than what they ingest, so the excess is excreted as waste, giving plants quick access to it.
Root nematodes have been the subject of much research because they damage plant roots and transmit soil diseases. However, like insects, most nematodes are beneficial; they are more numerous than their disease-causing kin and are an integral component of healthy soil.
Soil arthropods are fewer in number than the protozoans and nematodes, but we at least have a chance of seeing them without a microscope. Characterized by articulating joints and a hard outer exoskeleton made of chitin, arthropods comprise more than 75 percent of life on the planet. Soil arthropods such as mites, millipedes, springtails, ants, termites and centipedes play a vital role in building complex, diverse, and healthy soils.
Mites are one of the more numerous of the soil macroinvertebrates. They are arachnids, eight-legged members of the arthropod group that includes spiders and scorpions. Measuring a robust millimeter or so in diameter, mites perform many functions in the soil. Some are involved in skeletonizing leaf material; others prey on nematodes or scavenge fungal spores and bacteria. Several hundred species of mites, and more than 15,000 individuals, can be found in a single shovelful of soil.
Millipedes are like the professional tree trimmers of the underground neighborhood. They are able to eat and digest the bigger, bulkier leaf material and reduce it in size for smaller soil organisms to work on. They, too, leave tunnels and burrows near the surface where they have been grazing, enhancing aeration and water penetration. Since they do not have a waxy waterproof coating on their exoskeleton, we encounter them during the cool, wet seasons of the year. Their cylindrical bodies have two legs per segment; as grazers they are slow-moving creatures that amble through the garden as if they had all the time in the world and no place special to go. When threatened or disturbed they will coil into a protective spiral and exude noxious chemicals from pores lining both sides of the body. Bitter and acrid, these chemicals are powerful enough to kill some insects and discourage other animals from trying to eat them.
Centipedes might look something like a millipede but they have much longer legs—one pair per body segment—and are fast runners. The first pair of legs has been modified into a pair of poison-injecting fangs, allowing centipedes to be active predators that hunt for larger soil arthropods such as beetles, crickets, or spiders. Most range in size from one to three inches. One of our local natives is brown and another is a vivid orange-red. Their bodies are flattened from top to bottom, allowing them to be at home scrambling through loose soil or mats of leaf litter. As with millipedes, their burrows serve to aerate the soil. And, as predators, centipedes help keep other organisms in check.
Sow bugs and pill bugs are familiar soil-dwelling crustaceans that are also important for breaking down larger elements of organic debris. Armed with chopping mouthparts, they make quick work of heavy fibrous plant material, rendering it more easily digested by the smaller denizens of the decomposition food web. Since they evolved from marine crustacean ancestors, their respiratory organs are better suited to a wet environment. Unlike their gill-breathing aquatic relatives, they can breathe atmospheric oxygen, but they don’t have insects’ more efficient tracheal systems. Given their preference for humid habitats, they are well equipped for their role of recycling detritus in damp, cool leaf litter. The body is covered with overlapping plates, a protection against predation and desiccation. Females carry eggs in a pouch under the body called a marsupium where they are kept cool and moist until hatching. Pill bugs can keep themselves from drying out by rolling into a ball. Although they are detritivores and valuable as decomposers, both species can be troublesome to gardeners due to their fondness for seedlings.
Ants are significant insects of the soil. Harvester species collect the seeds of grasses, annuals, and perennials, ferrying them underground to be stored and eaten when needed. Other species feed on fruits, nectar, or animals and in so doing recycle this organic material into the ground. Underground colonies house thousands of workers that regularly excavate mineral particles to the surface during expansion of their homes. The exchange of mineral soil for organic humus improves soil quality, while extensive networks of galleries and tunnels provide passageways for both water and air.
The Jerusalem cricket, or potato bug, is a huge burrowing invertebrate that, while infrequently encountered, causes great surprise when unearthed. These formidable-looking animals, easily identified by their striped, bulbous abdomen, shiny head, and spine-covered legs, can be as large as your big toe. Armed with a set of powerful jaws and the ability to run quickly, they appear intimidating but bite only if grabbed. They feed on roots, bulbs, tubers, soft-bodied insects, and occasionally, dead organisms. Although their habitat is primarily underground, they will come to the surface at night to forage. They are solitary and territorial, coming together only during the breeding season. They communicate by stridulating: scraping spines on the rear legs against roughened bristles on the side of the abdomen. Females lay their eggs in a small, excavated chamber and provide maternal care for them. Like earthworms, Jerusalem crickets are beneficial organisms, loosening the soil, and recycling organic debris.
Some animals use the soil as a temporary nesting site. Bumblebees frequently adopt abandoned mouse nests or gopher holes in which to raise their young. Many native solitary bees are ground nesters and deposit eggs in an excavated chamber stored with a bolus of pollen and nectar or a paralyzed caterpillar as food for the soon-to-hatch larvae. These legless grubs remain in the burrow dining on the provisioned meal until they metamorphose into the adult winged form, usually emerging the following spring. As pollinators they split their time between flowers growing aboveground and the nest site underground. They depend on healthy soil for both their feeding and nesting activities.
Also inhabiting this interface of subterranean and aboveground worlds is a variety of opportunistic hunters. Wolf spiders actively pursue their prey—crickets, earwigs, beetles, ants and sow bugs—by chasing them down and overpowering them, relying on keen eyesight and swiftness instead of aerial webs. Predaceous beetles are common in this leaf litter and topsoil zone; one local species in the genus Scaphinotus specializes in feeding on native snails. Its head, mouth, and thorax are extremely narrow, allowing it to enter the small opening of the fingernail-sized snail. The snail itself is not a significant pest since it occurs in small numbers and feeds on organic debris rather than garden plants. The introduced European snail, however—a behemoth by comparison—is the source of many a gardeners’ frustration, owing to its preference for leafy green vegetation and flowers. Unfortunately, it is much too large for the Scaphinotus beetle to control.
All these organisms are not just present in soil; they are active agents in making the soil productive, and even become essential components of the soil when they die. By contributing and breaking down organic debris, they make carbon and nutrients available to the plants, which then transport those components aboveground, where they become available for humans and other wildlife. So when you tread on the soil, do so lightly, and in appreciation of all the work that is going on below.
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Islais Creek Park is the first official San Francisco site on the San Francisco Bay Area Water Trail.