Earthworm Ecology in California
California’s oak savannas are home to many unique plants and animals. To the growing list of creatures for which oak woodlands are critical habitat, we’d like to add one more-the earthworm. Yes, California has its own, native earthworm fauna, with species found nowhere else in the world. We currently are studying these organisms in the grasslands and oak woodlands at the Hopland Research and Extension Station, in Mendocino County. In this article we would like to acquaint you with this little-known member of California’s wildlife and share some of our findings on the importance of earthworms to soil fertility.
Most of the earthworm species native to California began their evolution here over 100 million years ago, and are thus among California’s oldest residents. As climate and habitats have changed, earthworms have adapted accordingly, proving themselves to be hardy, resilient survivors. Today they are most abundant in oak savannas, but can be found in almost all habitats, from semi-desert chaparral to coastal forest. The only habitats in which native earthworms fail to thrive are those heavily affected by human activity, including irrigated croplands, orchards, and sub-urban areas. There, they have largely been displaced by earthworm species introduced from Europe, Asia, and Latin America. These exotic species tend to be very prolific and are well adapted to surviving in ecosystems dominated by human activity.
Our research at Hopland suggests this displacement of native by non-native earthworms in California is closely tied to land use (Figure 1). In undisturbed habitats, native earthworms predominate. As land use intensifies, the proportion of native relative to non-native species declines. In croplands, orchards, and irrigated pastures, native species are rare or absent, displaced by the more prolific non-native species. Thus, loss of natural habitat to development or conversion to cropland frequently is accompanied by an extirpation of endemic earthworm species.
Native earthworm species are extremely sensitive to soil disturbance and will quickly disappear from an area that has undergone tillage or tree harvest. They will not re-establish if the disturbance is continuous, as in most farming practices. This leaves an area free for colonization by exotic species, which are much more tolerant of human activity. However, exotic earthworms flourish only where conditions are optimal with respect to soil moisture and food resources. The irrigated, clover-amended pasture represents the closest thing to "worm heaven" for these species. The native species are able to get by with much lower quality food sources and can tolerate drier soils. Thus, although the exotic species are more prolific and will readily "take over" a suitable area, the native species hold their own in less-disturbed habitats. In moderately degraded pastures, native species can persist to a point. In severely degraded pastures and croplands, earthworms may be few or none at all. This is especially true in irrigated systems where salts have accumulated at the soil surface.
Regardless of whether the earthworms present in a given area are native or exotic, they influence soil fertility in three important ways: inter-action with soil microorganisms (bacteria and fungi); physical and chemical alteration of the soil through burrowing and casting activity; and serving as a food source to other animals. By breaking up plant residues on the soil surface and mixing them with the topsoil, earthworms bring these materials into contact with the soil microorganisms that carry out decomposition and the formation of soil organic matter. The microorganisms themselves are a nutrient-rich food source for earthworms, and are ingested along with soil and plant residues. The worms return these nutrients to the soil, primarily in their feces, known as casts, which are readily available for uptake by plants. Earthworms also facilitate the transport of microorganisms throughout the soil, and have been shown to promote the colonization of plants by symbiotic fungi (mycorrhizae) in agricultural and grassland systems.
Burrowing and casting activity increases the size and number of soil macropores, with an associated decrease in bulk density in the topsoil and an in-crease in both water infiltration and aeration capacities. Earthworm burrows also provide channels for root growth and enhance gas exchange for both roots and associated microorganisms, especially in compacted soils. Earthworm casts generally are higher in pH and have higher concentrations of carbon, nitrogen, phosphorus, and other nutrients than bulk soil. Depending on soil type, casts can persist as macroaggregates in the soil for long periods of time, serving as a form of slow release fertilizer.
Earthworms form an important component of both below- and above-ground food webs. As mentioned, earthworms graze on soil microorganisms. After death, their bodies serve as food for microbial and animal scavengers, and they are an important source of food for many predatory insects, birds, and mammals. As soil-dwelling animals, earthworms form a major link in the chain of bio-accumulation of pesticide and heavy metal residues. They are able to store relatively high concentrations of pesticides and heavy metals in their bodies, in some cases up to 10 times the concentration found in soil, and they pass these compounds right along to their predators. Birds are especially vulnerable to poisoning in this way. The potential for earthworms to concentrate and then transfer toxic compounds to birds and other animals raises serious questions about the wisdom of using municipal sludge contaminated with low concentrations of heavy metals or organic compounds as a soil amendment.
Because earthworms play such an important role in soil processes, any practice that creates a favorable environment for earthworms will pay off in the long run in enhanced soil fertility. In agroecosystems, those practices that pro-mote earthworm populations include the use of organic amendments such as com-posts and cover crops, and reduced tillage (which alos reduces compaction). In pastures, fertilization and the planting of high-quality forage will increase earthworm densities, but possibly at the expense of native plants and animals, including native earthworms.
The conservation of native bio-diversity is an important issue in its own right; however, additional reasons exist to think twice about adopting management strategies that displace native earthworms. Our studies, although preliminary, suggest that differences exist in the ways that native and non-native earthworms process soil nutrients. These differences may have long-term consequences for soil fertility in pastures. Both native and non-native earthworms enhance the availability of nitrogen and phosphorus, but native earthworms tend to be active at the soil surface for a longer period of time throughout the year. Thus, their net effect on soil fertility may be greater overall.
Much remains to be learned about the species diversity, life habits, and functional roles of California’s native earthworm fauna. Unfortunately, research on earthworm ecology in California has lagged far behind work done elsewhere in the United States and in Europe. Results from those studies in those regions are not readily applied to conditions here in California, and a real need remains for more research that addresses our unique climate, wildland ecosystems, and irrigated agriculture. This much is certain: earthworms are a significant component of both natural and managed ecosystems and, as such, are worthy of our best efforts at conservation. We may find, in time, that one of our most valuable and cost-effective tools in maintaining long-term soil productivity lies, literally, at our feet.
prepared and edited by Richard B. Standiford, Justin Vreeland, Bill Tietje