Agriculture Reference
In-Depth Information
F
S
I
IELD
TUDIES
OF
NTERACTIONS
There are two major field approaches for studying the impacts of earthworms on the other soil
biota. One approach involves the partial or, preferably, total elimination of earthworms from
replicate field study plots for subsequent comparisons with plots in which no earthworm elimination
has occurred or alternative earthworms have been additionally inoculated. There are a number of
areas where this situation can be studied, such as areas where the partial or total elimination of
earthworms is normally a chronic, unplanned side effect of human activities and areas where
earthworm elimination is a direct, acute result of planned human activities or experiments.
Examples of the former can be seen in areas subjected to various agricultural practices or to
various forms of soil pollution. However, in many of these examples, there may be severe difficulties
in experimental design and data interpretation (e.g., often it is difficult to establish appropriate,
analogous unimpacted control areas); often, it is difficult to determine the real cause of observed
changes in the soil biota (earthworm absence or physical/chemical impacts of disturbance itself).
Another example of the former is areas where land-use changes have been or are being made.
Yeates (1988) showed that, with a change in land use from grassland to
plantations
of different densities, earthworm populations disappeared from soils under high tree densities.
Yeates (1991) also reviewed the effects of historical land-use changes on the soil fauna in New
Zealand. Muys et al. (1992) showed, as part of a larger study, that 18 years after changing a pasture
to a
Pinus radiata
plantation, there was a decrease in earthworm biomass. However, neither
the study by Yeates (1988) nor that of Muys et al. (1992) aimed to investigate the effects of changed
earthworm populations on other groups of the soil biota.
In all the foregoing examples, there was the problem of the timing of the experiments on the
effects of changed earthworm population decline or elimination on other organisms. Lavelle (1988)
posed the question ÑWhen earthworms disappear, how long does it take before significant changes,
if any, occur in the soil structure and nutrient cycling?Ò
This may well be extended to include changes in the communities of other soil organisms. In
the situation of planned elimination of earthworms, two approaches have been taken: application
of biocides and electroshocking. Bohlen et al. (1995) reviewed these briefly, and, to avoid problems
in the use of biocides (e.g., their impacts on nontarget organisms, etc.), they used electroshocking.
However, if this type of approach is to be effective in soil community studies, then it must be long
term to allow the soil to ÑequilibrateÒ to a zero earthworm population state (i.e., again, the timing
of experiments becomes important).
The second major field approach is to study previously earthworm-free soils into which earth-
worms have been introduced purposely or into which unplanned invasions or colonization is
Quercus palustris
occurring (see
Chapter 5
, this volume). Stockdill (1982) showed that lumbricid earthworms (
A.
caliginosa
) introduced into pastures in New Zealand improved their soil qualities. Yeates (1981)
showed that the introduction of these earthworms into pasture soils led to a 50% reduction in
populations of soil nematodes plus a significant change from bacterial to fungal feeding species of
nematodes. Unfortunately, the impacts of this change on the microbial community structure are
unknown. Baker (see
Chapter 14
, this volume) provides a detailed account of the various impacts
of earthworm introductions into Australian soils. For instance, Hoogerkamp et al. (1983) showed
beneficial changes following the introduction of various mixtures of earthworm species into pas-
tures, developing on reclaimed polder soils. However, they did not study the impacts of these
earthworm introductions on the other soil biota, Marinissen and Bok (1988) showed that effects of
earthworms on the structure of polder soils resulted in increased size, populations, and distribution
of Collembola in those soils.
Fortunately, there are still areas that have been earthworm free for centuries and where colo-
nization by earthworms is beginning, presumably as an indirect result of human activities. Earth-
worm invasions of forests in the Kananaskis Valley (southern Alberta, Canada) have begun, and
the impacts of this are discussed in detail in the following section.
