Agriculture Reference
In-Depth Information
EARTHWORMS AS INDICATORS OF THE SUSTAINABILITY OF
AGRICULTURE: FROM THE FARMER'S PERSPECTIVE
Large populations of earthworms are popularly believed to reflect soil health. They often make up
a large proportion of the biomass of the soil fauna, and they respond positively to many agricultural
management practices. Earthworm populations and biomass are also correlated with a range of
edaphic variables. It is therefore not surprising that earthworms have been suggested as potential
indicators of the sustainability of agricultural practices that farmers might use (Oades and Walters
1994; Buckerfield et al. 1997).
A useful indicator of soil sustainability must be attractive to farmers so that they will understand
and adopt it, should be easy to measure reliably, and should be responsive to environmental change
in a timely fashion. Farmers know that earthworms are generally beneficial. However, few are
aware of the species they have on their land, and, like scientists, they are unclear just how many
earthworms they need in their soil (i.e., what are the abundance thresholds they need to seek). It
is important that farmers recognize the species of earthworms they have on their farms, realize the
varying abilities of different species to influence soil properties and plant production, and note the
strengths and weaknesses of their earthworm resource. Simple keys have been devised that farmers
might use for the common earthworm species in Australia (e.g., Baker and Barrett 1994; Mele and
Hollier 1995), but these are now out of print and should be revised and reprinted.
Sampling for earthworms, whichever method is used, is notoriously labor intensive and fraught
with inaccuracies (Baker and Lee 1992); for this reason (and others), some authors (Doube and
Schmidt 1997; Lobry de Bruyn 1997; Baker 1999) have questioned the practicality of using earth-
worms as biological indicators. Soil moisture can vary within short periods of time and affect the
numbers of earthworms collected (Baker et al. 1993c). Earthworms are usually distributed in patches
within fields and vary in populations from year to year when no overt changes in management practices
occur (Baker et al. 1992a, 1993b; Baker 1999). Spatial distribution patterns vary among species (Baker
1999). Farmers are busy people, but they must take care to collect sufficient earthworm samples to
make their data meaningful and to enable detection of differences in populations through time and
space. Some earthworms have a large reproductive potential (e.g., epigeic species), but the species of
earthworms that predominate in Australian agricultural fields probably do not (Lee 1985). Although
drastic physical or chemical disturbance might quickly reduce earthworm populations, recovery may
well take several years, and farmers need to be appreciative of this phenomenon.
The fact that the earthworm fauna of Australian agricultural habitats is dominated by introduced
species raises an immediate question: How far have these species spread to occupy sites that are suitable
for them? The answer is unknown, but it seems likely that there are many potential sites yet to be
occupied by exotic earthworms. More than 40% of pastures in one region of western Victoria lack
A.
trapezoides
, but there seems to be no good reason other than lack of opportunity to colonize that can
be given for its absence from these pastures. Although the presence of large populations of a diverse
community of earthworms is usually a healthy sign, there is a strong risk that low populations, or indeed
absence, of earthworms might be misinterpreted as a ÑproblemÒ at a particular site when the real
problem is not with the soil
per se,
but the chance of earthworm dispersal to it. Some seemingly
ÑhealthyÒ soils in Australia lack earthworms (e.g., some kraznozems). That is not to say that these soils
would not be more productive with the arrival of appropriate species of earthworms.
CONCLUSIONS
Much progress has been made in earthworm ecology in Australian pastures in recent years. However,
at least four major gaps are obvious in current knowledge:
1.
Hardly anything is known of the earthworm fauna in subtropical and tropical systems
and its ability to influence soil properties and pasture production there.
