21
Biotic Manipulations Involving
Belowground Animals
Diana H. Wall and O. James Reichman
Introduction
The experimental approach for the study of soil
biota in ecosystems has evolved rapidly with the
increase in our knowledge. From the mid 1960s to
mid 1990s, the influence of soil biota on ecosystem
processes in natural settings was determined primarily by manipulations with selected biocides.
These were used to exclude specific groups of soil
biota. More recently (mid-1990s to present), soil
ecologists have assessed those experiments and
found sufficient evidence to suggest that other experimental approaches are more appropriate for
evaluating the relation between soil community
structure and ecosystem processes. However, laboratory microcosms continue to be an additional
experimental means for relating soil biota to ecosystem processes. Because of the constraints of the
soil habitat and the complexity of biotic interactions, the approaches used for analyzing soil biota
and ecosystem processes differ somewhat from the
biotic manipulations in aquatic and aboveground
ecosystems.
In this chapter, we first introduce the soil biota
and the problems inherent in evaluating their role
in ecosystem processes. We then discuss manipulations that exclude organisms and the more recent
approaches using natural gradients as treatments
and manipulations of the belowground habitat. Finally, we briefly discuss the experimental approach
of combining field experiments with laboratory microcosms, which has proved invaluable for relating
soil organisms to ecosystem processes. In all cases,
we caution that the question being asked, the hypothesis being tested, and the organisms being stud318
ied will determine the approach to be used. We
therefore have not described differences in experimental design, since these can differ significantly
with organism size, abundance and habitat scale.
Where possible, we indicate references for sampling and extraction methods and for determination
of biomass for the different organisms.
Soil Biota
Experiments to determine the significance of biota
in ecosystem functioning have most often involved
aboveground organisms. Generally, aboveground
species are more obvious and easier to manipulate
than those that live below ground. Yet, in many systems more biomass occurs belowground, therefore
knowledge of the soil organisms (including roots)
and how they interact with aboveground biota to
affect ecosystems is crucial for understanding overall ecosystem function.
The soil itself is a complex matrix with wide
variation in physical and chemical properties. For
example, in the United States, there are more than
2000 soil types, all varying in physical and chemical properties and above- and belowground biota.
A further complicating aspect of below ground studies is the rich diversity of organisms that live in the
soil (Dindal 1990; Coleman and Crossley 1996).
The animate component within soils includes plant
roots, vertebrates (from small snakes and salamanders to large mammals), invertebrates (including
some of the most speciose taxa such as nematodes,
mites, beetles, ants, and termites), algae, fungi, and
microbes. The connections of these groups to
Biotic Manipulations Involving
Belowground Animals
Diana H. Wall and O. James Reichman
Introduction
The experimental approach for the study of soil
biota in ecosystems has evolved rapidly with the
increase in our knowledge. From the mid 1960s to
mid 1990s, the influence of soil biota on ecosystem
processes in natural settings was determined primarily by manipulations with selected biocides.
These were used to exclude specific groups of soil
biota. More recently (mid-1990s to present), soil
ecologists have assessed those experiments and
found sufficient evidence to suggest that other experimental approaches are more appropriate for
evaluating the relation between soil community
structure and ecosystem processes. However, laboratory microcosms continue to be an additional
experimental means for relating soil biota to ecosystem processes. Because of the constraints of the
soil habitat and the complexity of biotic interactions, the approaches used for analyzing soil biota
and ecosystem processes differ somewhat from the
biotic manipulations in aquatic and aboveground
ecosystems.
In this chapter, we first introduce the soil biota
and the problems inherent in evaluating their role
in ecosystem processes. We then discuss manipulations that exclude organisms and the more recent
approaches using natural gradients as treatments
and manipulations of the belowground habitat. Finally, we briefly discuss the experimental approach
of combining field experiments with laboratory microcosms, which has proved invaluable for relating
soil organisms to ecosystem processes. In all cases,
we caution that the question being asked, the hypothesis being tested, and the organisms being stud318
ied will determine the approach to be used. We
therefore have not described differences in experimental design, since these can differ significantly
with organism size, abundance and habitat scale.
Where possible, we indicate references for sampling and extraction methods and for determination
of biomass for the different organisms.
Soil Biota
Experiments to determine the significance of biota
in ecosystem functioning have most often involved
aboveground organisms. Generally, aboveground
species are more obvious and easier to manipulate
than those that live below ground. Yet, in many systems more biomass occurs belowground, therefore
knowledge of the soil organisms (including roots)
and how they interact with aboveground biota to
affect ecosystems is crucial for understanding overall ecosystem function.
The soil itself is a complex matrix with wide
variation in physical and chemical properties. For
example, in the United States, there are more than
2000 soil types, all varying in physical and chemical properties and above- and belowground biota.
A further complicating aspect of below ground studies is the rich diversity of organisms that live in the
soil (Dindal 1990; Coleman and Crossley 1996).
The animate component within soils includes plant
roots, vertebrates (from small snakes and salamanders to large mammals), invertebrates (including
some of the most speciose taxa such as nematodes,
mites, beetles, ants, and termites), algae, fungi, and
microbes. The connections of these groups to