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ECOLOGICAL ECONOMICS ELSEVIER

Ecological Economics 23 (1997) 167-178

The status of soil macrofauna as indicators of soil health to monitor the sustainability of Australian agricultural soils Lisa Alexandra Lobry de Bruyn * Department of Ecosystem Management, University of New England, Armidale NSW, 2351, Australia Received 4 June 1996; accepted 10 March 1997

Abstract Australian farmers are searching for reliable, easily measured indicators of soil health to monitor sustainability of their enterprises. Over the past 5 years earthworms have been promoted as indicators of soil health by some researchers. Others have been reluctant to accept soil macrofauna in general as soil health indicators. Their reluctance is based on the difficulty of interpreting biological data in relation to soil health as there is no clear understanding of the links between soil macrofauna and soil health. The problem is further compounded by the inherent difficulty in studying soil biota, inadequate experimental design, and the lack of long-term commitment to funding such studies. This paper reflects on current research, and maps out conditions and directions for future research if the role of soil macrofauna in soil health is to be better understood. The compilation of adequate baseline data, the appropriate delineation of experimental plots, attention to the totality of environmental conditions including land management practices, the consideration of impact by macrofauna other than earthworms, are some of the directions are outlined. The challenge in the future will be to shift the emphasis of soil macrofauna research towards understanding their function in soil processes essential to ecosystem functioning. Without this sort of experimental evidence scientists cannot indicate to the farmer whether the soil resource is declining in quality, is stable or in a process of renewal based on the presence or absence of certain macrofauna. © 1997 Elsevier Science B.V.

Keywords: Sustainability; Bioindicators; Soil health; Soil macrofauna; Ecosystem function

I. Introduction

1.I. The need f o r soil sustainability indicators and their desirable characteristics * Tel.: + 61 67 733119; fax: +61 67 732769; e-mail: 1 1 o [email protected]

G o v e r n m e n t s a r o u n d the world ( S t a n d i n g C o m m i t t e e o n Agriculture, 1991; A c t o n a n d Gre-

0921-8009/97/$17.00 © 1997 Elsevier Science B.V. All rights reserved. PII S092 1-8009(97)00052-9

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gorich, 1995; Munasinghe and Shearer, 1995) have identified three areas of agricultural sustainability which need to be monitored to appreciate if the agricultural management practices that are being advocated are creating more sustainable farming systems. These are the natural resource base, economic viability of agricultural production, and other ecosystems which are influenced by agricultural activities. The protection of the natural resource b a s e - - s o i l - - a n d the prevention of further degradation is critical to the development of sustainable agriculture. The maintenance of soil health is vital if s o i l - - a nonrenewable resource -is to continue to produce food and fibre (Acton and Gregorich, 1995). Soil health (also often referred to in the literature as soil quality) can be defined as: 'The capacity of the soil to function within ecosystem boundaries to sustain biological productivity, maintain environmental quality, and promote plant and animal health' (Doran and Parkin, 1994). The interaction of soil health along with soil stability and soil resilience contribute to the sustainable use of the soil resource (Lal, 1993). Soil health or quality is measured by chemical, biological or physical indicators (such as organic carbon, microbial biomass or hydraulic conductivity) which are describing an attribute (such as organic matter, labile organic matter and permeability of the soil, respectively). In turn these attributes describe the soil's capacity to perform ecosystem processes such as accepting, holding and releasing water or energy. It has been highlighted in several international workshops on sustainable agriculture (Doran et al., 1994; Hawksworth, 1991) that soil macrofauna can be used as an integrative measure of soil health, assuming their importance in regulating soil processes which are vital to the continued formation of soil and protection against soil degradation. The focus of this paper is to assess the status of soil macrofauna, in the Australian environment, as indicators of soil health to monitor the sustainability of the resource base at the farm level. Soil macrofauna in this paper will refer to earthworms, ants, and termites. Fauna can be categorised in terms of their interaction with the soil, into three

groups: epigeic (those which process organic matter on or near the soil surface), endogeic (those living in the soil) and anecic (those which transfer materials between soil and litter habitats) (Anderson, 1995). Those soil fauna which are classified as anecic are considered to be more active in their interactions with the soil than the other two groups (epigeic and endogeic), There have been supporters of soil macrofauna as bioindicators (Majer, 1983) because they believe that they conform to most of the following attributes which are considered desirable qualities of an indicator. The indicator should be: • closely related to one or more of the assessment goals, • important to the overall structure and function of the agroecosystem, • responsive to a range of environmental stresses, • easily measured and quantifiable, • easily interpreted, • holistic and • have integrative effects over time (Pankhurst, 1994). Others have been more cautious in their endorsement of bioindicators of soil health and suggest much more basic research is required (Paoletti and Bressen, 1996). There are three principal reasons for examining soil macrofauna and their relationship with soil health and soil sustainability. Firstly, recent government reports (Hamblin, 1992; Standing Committee on Agriculture and Resource Management, 1993) have identified their potential as bioindicators of soil sustainability at the farm level, though at this stage there has been little rigorous experimentation to test their validity in Australian agroecosystems (Pankhurst et al., 1995). Bioindicators are required to monitor changes in soil health and to provide early warning of adverse trends and identification of problem areas (Pieri et al., 1995). Secondly, farmers need indicators of soil health which they can easily and reliably use to monitor their soil sustainability. Ease and reliability are significant properties as farmers are unlikely to adopt the soil sustainability indicators derived by scientists, if they require too much technical expertise, are expensive and timely to conduct, and the results are difficult to interpret (Baker and Dalby, 1994). Thirdly, farmers have

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been slow to adopt sustainable management practices because they cannot see the benefits of the new techniques and perceive a higher risk and uncertainty with them (Vanclay, 1992). At the same time there is a strong desire by farmers to monitor their farm goals and determine if changes in farm management are leading towards a more sustainable farming system. A simple monitoring tool which illustrates trends in soil health may act as a persuader to change or affirm management practices. There is also a need to reconcile the land manager's short-term goals, and decisions made for continued economic survival of an agricultural enterprise with the short- to long-term requirements of the ecosystem to ensure true sustainability. To achieve a balance between shortterm economic considerations and long-term resource conservation there has been a development of natural resource accounting which incorporates soil health, regional environmental impacts, farm profitability, and government policy to evaluate agricultural sustainability (Faeth et al., 1991 after D o r a n et al., 1996). The objectives of this paper are threefold. They are: 1. to identify the trends and gaps in current research on soil macrofauna as indicators of soil health to monitor soil sustainability, 2. to suggest directions for future soil macrofauna research in the area of soil health, and 3. to discuss whether the focus on soil macrofauna as indicators of soil health, and hence soil sustainability, holds promise.

2. Methods This paper has followed two approaches in order to resolve the issues outlined above. One avenue was to ask eminent researchers in the discipline of soil ecology in agricultural soils questions pertaining to the direction of soil macrofauna research, the trends and gaps in current research, and the importance of soil macrofauna as an indicator of soil sustainability. A mail survey was sent to 31 scientists, with 22 actively researching soil macrofauna and nine with past or peripheral interest in the area. Sixty-three precent

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of the active researchers responded to the questionnaire. The other avenue was to examine Current Contents T M over 1993-1996 on published Australian research in this area, as well as drawing from some review articles which summarised research activity prior to 1993. I will illustrate some of the points with specific examples from current research, but the focus will not be on particular research findings except where they have a bearing on the status of soil macrofauna as indicators of soil health. This review is divided into five sections: (1) the need for soil sustainability indicators and their desirable characteristics, (2) the underlying assumptions made as a precursor to soil macrofauna research, (3) the trends and gaps in current research focus, (4) future directions in soil macrofauna research and (5) the nature of the research required to appreciate fully the value of soil macrofauna as soil health indicators to monitor the soil resource. In the last three sections I will discuss the responses to the questionnaire.

3. The assumptions required to conduct soil macrofauna research Certain assumptions have been made concerning the worth of soil m a c r o f a u n a as indicators of sustainability which influence our ability to formulate an objective assessment of their role in soil health. One is the assumption (supported by some research in A u s t r a l i a - - H u m p h r e y s , 1985; Lobry de Bruyn, 1990; Mitchell, 1986, 1988) that soil macrofauna are influential in modulating the flow of energy, water, air and nutrients as well as in maintaining or enhancing soil health. Recent review papers demonstrate the impact of soil biota on soil processes which include nutrient cycling, soil bioturbation and soil structure formation (Anderson, 1995; Curry and Good, 1992; Hole, 1981; Lee and Foster, 1991), but are largely based on research conducted outside Australia. On this premise it is then understood that soil macrofauna m a y act as a surrogate or integrative measure for soil processes such as nutrient cycling and soil structure maintenance. The biological activities which are considered important in these soil processes are (Hole, 1981):

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1. Mixing of organic and mineral particles, 2. Redistribution of organic matter and microorganisms, 3. Creation of biopores for movement of air, water and nutrients, 4. Promotion of humification, 5. Production of faecal pellets, 6. Interaction with other soil biota, 7. Fragmention of plant residues, 8. Stimulation of microbial activity. The other assumption is our anthropocentric view that we can indeed manipulate soil macrofauna for our purposes and that we label certain groups as beneficial or winners which discounts the possibility of others holding this position. The following quotes are given as examples: 'Insight into the need for, and approaches to, the development of land use practices that capture and exploit the beneficial activities of soil biota and soil biotic processes.' (Pankhurst et al., 1994), and '... how soil biota can be manipulated to make agriculture less dependent on non-renewable resources.' (Pankhurst et al., 1994). There is also a belief that those farming systems which include soil conservation practices such as zero or minimum tillage are more sustainable than conventional practices, but to prove this unequivocally would require 20-30 years of comparative analysis. Current thinking regarding what constitutes a sustainable farming system needs to be challenged as the changes which have been made so far to existing farming systems are marginal. Researchers and practitioners have not been prepared to vigorously challenge the foundations upon which sustainable farming systems have been built, but have been merely content to make cosmetic alterations to the structure. In order to draw some valid conclusions in this matter consideration needs to be given to determining the characteristics which define a sustainable farming system, and how they are to be measured. Doran et al. (1996), amongst others, suggests that the predominant agricultural research emphasis which has concentrated on increasing the economic efficiency and technical prowess of agricultural production by taking a reductionist approach needs to shift towards a paradigm which acknowledges that an integrated,

holistic, multi-disciplinary approach is needed to solve the problems of agriculture. A corresponding shift in research paradigm has been experienced in economics which is shifting towards a transdisciplinary approach underpinned by an ecocentric philosophy (Gill, 1996). 4. The trends and gaps in current research into soil macrofauna and soil health

Invariably, soil macrofauna research in Australia has concentrated on aspects of biodiversity, especially of introduced earthworm species, either at the species or community level, largely ignoring functional diversity. There is an important impediment in using biodiversity (measured simply by species richness) as an indicator of a healthy soil. Firstly there is a need to understand and be able to identify which species or groups of species have key functions in the maintenance of energy and material flows through an ecosystem (Silver et al., 1996). It has been assumed that a soil ecosystem with low biodiversity is less resilient, more vulnerable to perturbations, and ultimately not as able to function as well as a soil ecosystem with high biodiveristy. However, little is known about the contribution of individual species or groups of species to ecosystem functioning and the effect of their removal from the soil ecosystem (Collins and Benning, 1996). Establishing who are the important macrofauna in terms of soil health requires an understanding and quantification of their impact on the soil profile, and their association with certain soil types. The initial research into introduced earthworm species in Australia has focused on large surveys of different soils and agricultural activities and collected information on composition, abundance, and activity (Abbott and Parker, 1980; Baker et al., 1992b, 1994; Kingston and Temple-Smith, 1989; Mele, 1991), but the information from each site can rarely be directly compared to each other due to differences in soil type, rainfall and intensity of agricultural activity. This information is useful as a reconnaissance of what we have and in what quantities, but interpreting this information to compose trends in soil health for particular soil types is difficult.


Another dominant area of macrofauna research in Australia has been in establishing the life cycle patterns of earthworms under cereal or pasture in Western Australia, South Australia, western Victoria and the midlands and northern areas of Tasmania (Baker et al., 1992a, 1993a,b; Buckerfield, 1992; Garnsey, 1994a; Kingston, 1989; McCredie et al., 1992). Documenting periods of earthworm activity, peaks and troughs in earthworm abundance, and how these attributes vary over the seasons is crucial to identifying optimum sampling opportunities, understanding the impacts of land management practices on earthworms and formulating links with other soil properties. The environmental factors which control earthworm distributions were most highly correlated to rainfall, percentage nitrogen and carbon, and soil texture (Baker, 1994). The link between soil macrofauna and soil processes has received less attention by Australian researchers. Recent work by Hirth et al. (1995a,b) and Hindell et al. (1994a,b) have looked more closely at the influence of introduced earthworms on changes in soil structure. For parts of semiarid Australia, under natural vegetation, there have been studies to assess the influence of ant nests on bioturbation and water infiltration (E1dridge, 1993, 1994; Eldridge and Pickard, 1994). My own research (Lobry de Bruyn, 1993a,b; Lobry de Bruyn and Conacher, 1994a,b, 1995) in the wheat-belt of Western Australia has examined the relationship between the soil engineers (ants and termites), and soil properties, and found that they exert a significant influence on soil physical properties. There are few researchers in Australia working on mound-building or subterranean termites and their influence on soil properties (Holt et al., 1995; Lobry de Bruyn and Conacher, 1995; Park et al., 1994). Overall, there is a lack of data on the relationship between ants or subterranean termites and soil processes (Lobry de Bruyn and Conacher, 1990) in agricultural soils in Australia. A large proportion of the Australian research is also more concerned with the impacts of agricultural practices on soil biodiversity or population/ community dynamics, usually of introduced earthworms, under various tillage and stubble management systems (Doube et al., 1994; Haines

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and Uren, 1990; Holt et al., 1993; Lobry de Bruyn, 1993a,b,c; Robertson et al., 1994a; Radford et al., 1995; Rovira et al., 1987; Tisdall, 1985). The general consensus from the above studies is that soil macrofauna biodiversity and abundance increases under minimum tillage and stubble retention. King (1994) has reviewed the impact of pasture management on soil biota under Australian conditions. Soil macrofauna research has chiefly concentrated on introduced earthworms, with the strongest period of research activity and funding support from industry sources occurring in the mid 1980s to early 1990s. The majority of researchers surveyed considered that earthworms were the most examined soil macrofauna. However, upon closer examination the majority of earthworm research, especially of exotics, is confined to the south east of Australia. There has been some work in Northern Queensland on earthworms in cropping systems (Robertson et al., 1994b) and their influence on pasture production in South-East Queensland (Blakemore, 1994). There was a negative response to this question from a small sector of respondents, who felt that earthworm research had received too much attention, at the expense of more worthwhile macrofauna (such as ants) or more easily interpreted bioindicators (e.g. nematodes and microbial activity) of soil sustainability. The survey respondents identified several gaps in research knowledge. The two areas mentioned most frequently by respondents were, firstly, our poor understanding of the role soil macrofauna have in soil processes and plant growth, especially the mechanisms by which soil macrofauna exert their influence on soil processes, in particular nutrient cycling and soil hydraulic properties. There have been some attempts to quantify the relationship between earthworm abundance and/ or type with pasture production (Garnsey, 1994b; Temple-Smith et al., 1993) or with cereal yields (Stephens et al., 1994a). The other area mentioned in relation to research gaps was the exclusion of soil macrofauna in natural ecosystems, and not enough emphasis on the biology and ecology of native earthworms. Other research areas requiring attention were the development of accessible and

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reliable identification or taxonomic tools, although there have been several attempts to produce keys for non-specialists to identify introduced earthworms (Baker and Kilpin, 1992: Mele et al., 1994; Parlevliet, 1992), as well as a better understanding of population dynamics, species and community interactions (Lobry de Bruyn and Mele, 1996). The least frequently mentioned area of needy research was the conservation of rare or beneficial soil macrofauna, and the impacts of agricultural practices on soil macrofauna.

5. Direction for future research

The respondents to the questionnaire gave a distinct impression that there was no united strategy amongst researchers for soil macrofauna research with several respondents suggesting there was no focus or sense of direction, From their comments I suggest a removal of the competitive/ territorial barriers that exist between scientists and research organisations to allow for the development of a collaborative research ethos which fosters open communication and cooperation amongst researchers. There was also a perception that the priorities o f the researchers and the funding agencies were not in synchrony due to the difficulty of promoting soil macrofauna research which is essentially a long-term investment over short-term objectives. A recent paper by Park and Seaton (1996) supports the need for integrated research which suggests a greater correlation between political and decision-making time scales and research and funding 'cycles'. The other area of internal conflict was between high technological research such as molecular/genetic biology and low technological investigations which relied on surveys and field research. There was also a call by the respondents to the survey for the quantification of soil macrofauna and their role in soil processes and pest control, and an elucidation of their function in ecosystem processes. Similarly a greater need for integration of research purpose with other related areas, and multidisciplinary projects was deemed necessary by most respondents.

The respondents were equally divided on the value of soil macrofauna as indicators of soil health to monitor the sustainability of the soil. The arguments against soil macrofauna as indicators of soil health were that they did not respond quickly enough to environmental change due to low fecundity, data were difficult to interpret because of their patchy and non-ubiquitous distributions, as well as not enough was known about the environmental factors that controlled their distributions. Additional problems in using soil macrofauna as indicators of soil health are that they require significant commitment of resources (time, people and money) to quantify and the resulting data is difficult to relate directly to productivity. Data on soil macrofauna need to be part of a broader system of ecosystem process indicators, with a need for groups that non-specialists are able to identify. There were also problems in identifying which groups were best reflecting ecosystem processes, and hence required further investigation. These arguments were also supported by Baker and Dalby (1994) who gave examples from current earthworm research in southern Australia. My own experience has shown that sampling earthworms at inappropriate times can give misleading results (Lobry de Bruyn, 1993c). I also discovered that different species of earthworms can respond very differently to the same management practice (Lobry de Bruyn, 1993c). Some of the respondents were not convinced that ants and earthworms were useful indicators of soil sustainability, or the evidence was conclusive. Nevertheless, Andersen (1990, 1993) has conducted various studies in mine-site rehabilitation using ant communities as indicators of successful habitat restoration. Wylie (1994) saw earthworm abundance as a poor indicator of sustainability and noted that earthworm numbers can be low in what is considered a 'healthy', well-structured soil. This observation is not unique and soil macrofauna indicators would need to be tailored to particular soil types, land use types and climatic regions. There would be little point in using national or regional soil macrofauna indicator of soil health because of their patchy distributions, but if combined with other soil measurements, they could be useful indicators at the farm scale.


This leads on to the issue of scale which is well documented by Anderson (1995) who considers why soil macrofauna, although important in plant growth, cannot be linked reliably to plant productivity as defined by yield. His four main reasons were: (1) that agricultural management practices such as tillage, fertiliser usage and irrigation transcend the effects of the biological processes on plant growth; (2) unless soil properties affected by soil macrofauna are limiting plant growth, their effects may not be quantified by yield measurements (Clements et al., 1991); (3) the effects of soil macrofauna, which are active for short periods of time, are often obliterated by processes operating over longer time scales; and (4) finally, yield is measured at a spatial and temporal scale (over 1 ha and yearly) inappropriate to the activity of soil macrofauna which operate over several metres and in short-term events (weeks to months) (Anderson, 1995). To reliably link soil macrofauna with soil processes it requires researchers to define the temporal and spatial scales at which soil macrofauna operate. 6. Nature of research needed

There are a number of requirements that need to be met if soil macrofauna are to be used as indicators of soil health (Linden et al., 1994). These are summarised below. • The identification of cause-effect mechanisms. It is very difficult to directly link soil biological activity with plant production, but relationships could be formed with other vital components of soil health--soil structure and nutrient cycling; • The need to identify easily visible soil macrofauna that respond quickly to changes in food resources, habitat or microclimate conditions; • The need to identify soil quality indices which can be sampled, sorted and identified easily; • The need for a baseline or reference points to compare changes in indicator parameters which must be monitored through time, probably for at least seven years (Douglas, 1987); • The need to differentiate between long- and short-term management effects on soil biota; and

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• The need for a universal protocol for measuring soil biota which would standardise results and make comparisons possible between experiments. There is also a need for a greater recognition by funding organisations of the importance of soil biota in categorising soil health, which has already been acknowledged by farmers. An analysis of the top 50 responses of farmer-identified soil health properties in the United States of America indicated that organic matter (1) and earthworms (3) rated more highly than yield (10) and cost of production and profit (30) (Romig et al., 1995) as indicators of soil health. Unfortunately the intention of the question 'what type of research is needed' was misinterpreted by the respondents as 'what research areas require further investigation?' The respondents identified the need for quantification of the role of soil macrofauna in soil processes and to what extent they improve or maintain soil fertility. I was after the nature of investigations required to answer such questions as: how do we establish definitive links between soil macrofauna and soil processes, especially considering some of the difficulties outlined above. One of the obstacles has been that the majority of research projects have been observational and there has been little manipulation of macrofauna such as in experiments carried out by Bohlen et al. (1995) who have manipulated earthworm populations in large-scale field experiments in agroecosystems. There needs to be funding initiatives to support long-term, innovative, high-risk fundamental research which has applied implications for stakeholder groups. Another limitation of soil macrofauna research has been that funding bodies are concerned with agricultural land use only and have a three to five-year life span for projects. This prevents work being done on native fauna in naturally vegetated regions or transcending boundaries between agricultural and naturally vegetated land. The funding time frame of three to five years also puts considerable constraints on research outcomes and actively discourages researchers from establishing long-term projects/field sites. This type of disincentive precludes important research which

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needs to be over several seasons to be certain of experimental trends vs. seasonal variation, and also to be aware of the appropriate spatial and temporal sampling scales. The results from a year long study can be misleading or inconclusive, especially if the year records a below or well above average rainfall or soil temperature. In Australia there have been only two studies which have sampled earthworm populations for more than 1 year (3 years, Kingston, 1989; Hutchinson and King, 1980). Often conclusions have been based on one sampling (Doube et al., 1994), sometimes with no indication whether soil moisture conditions were optimal. The following examples, taken from earthworm research around Australia, illustrate the clear need for a standardised sampling protocol which would allow comparisons to be made between research projects (Lobry de Bruyn and Mele, 1996). For instance, studies into earthworm life cycles have no consistency in sampling number or design, usually with no replication of conditions, and varying in sampling intensity from monthly to twice over an earthworm season (May-November) or a year (Lobry de Bruyn and Mele, 1996). All these factors try to reduce soil heterogeneity, instead of accommodating the known variability of the soil environment. If there are time and cost constraints, earthworm sampling could be restricted to strategic points in the year, and to facilitate comparisons over a number of years it is important to sample at similar levels of soil moisture and temperature rather than at the same time each year. There needs to be a forum where researchers can compare and contrast sampling protocols and decide what is the optimal strategy for examing soil macrofauna and their role in soil health. The sudden commitment in the early 1990s to evaluate the earthworm resource in Australian agricultural soils has meant that instead of setting up completely new field experiments, researchers have often utilised well-established experimental sites which were initiated for other purposes. There are two obvious problems with this. Firstly there is no baseline survey of the macrofauna, and hence no concept of the patchiness of the population prior to experimentation, and secondly, the

experimental design may not be appropriate for examining biota which are mobile, such as ants or termites. Often the experimental plots are long and narrow (6 x 40 m) and this limits the sampling procedure to in situ types such as soil cores or hand sorting sods of soil. Under these circumstances pitfall traps, which are commonly used to sample surface active invertebrates, will not necessarily be sampling fauna which reside within the sample plot. To test the importance of soil macrofauna in nutrient cycling and soil structure maintenance we must be able to compare soils where their actions take place to soils without soil macrofauna. This is a complex issue since the soil would have been influenced by soil biota, at any time, during its formation. The following examples illustrate the type of strategies available which attempt to act as controls against soil macrofauna active areas. For controls, Baker (1994) has used PVC cylindrical enclosures in field conditions, with and without earthworms; Stephens et al. (1994b) has used pot experiments, with and without earthworms, under glasshouse conditions; and others have used areas which show obvious signs of soil macrofauna activity, such as an ant nest, and compared it with areas where there is none (Holt et al., 1995; Lobry de Bruyn, 1990). Considerable emphasis has been placed on earthworm species and their role in the maintenance of soil quality in both Australia and at international conferences (Lee, 1985; Kretzschmar, 1992; Brussaard and Kooistra, 1993; Pankhurst et al., 1994). However, for a large proportion of Australia (with highly variable rainfall distributions and less than 600 mm per annum) introduced earthworms are unable to survive and proliferate. We cannot be sure about the opportunities for dispersal and the rate of spread of the introduced earthworms in Australian agricultural soils. More consideration needs to be given to other soil biota (such as ants) which are abundant, widespread and diverse in many agro-ecological systems of Australia. Also a greater understanding of soil macrofauna's habitat needs, which are ultimately controlled by our treatment of the soil ecosystem, is required. A challenging aspect for the future of soil macro-


f a u n a research is to resolve the universal p r o b l e m e x p e r i e n c e d b y all soil b i o p e d o l o g i c a l e x p e r i m e n t s in t h a t o b s e r v a t i o n s o f p o p u l a t i o n c h a n g e s o r species c o m p o s i t i o n fluxes m a y be a result o f factors o t h e r t h a n f a r m m a n a g e m e n t . C o n f o u n d ing factors are n o t a l w a y s c o n s i d e r e d , such as g e o g r a p h i c l o c a t i o n o r t i m i n g o f sampling, a n d c a n h a v e a significant i m p a c t on results. T o u n d e r s t a n d the limits o f s u s t a i n a b l e l a n d use, we m u s t d e t e r m i n e w h a t are the critical biological i n d i c a t o r s o f soil q u a l i t y a n d their t h r e s h o l d s a n d hence o f soil sustainability. Soil resilience is an i m p o r t a n t a t t r i b u t e in u n d e r s t a n d ing the soil's response to d e g r a d a t i v e processes, a n d has been h i g h l i g h t e d in the i n t e r n a t i o n a l scientific a r e n a as a n issue o f g l o b a l concern, especially u n d e r c u r r e n t a g r i c u l t u r a l l a n d use practices (Lal, 1993; G r e e n l a n d a n d Szabolcs, 1994). T h e r e is a need to test theories in the a r e a o f soil resilience t h a t include the critical limits o f a p r o p erty f r o m which the soil c a n recover to the initial state, a n d the rate o f t h a t recovery. W e need to l o o k b e y o n d the t r a d i t i o n a l soil b i o t a e x p e r i m e n t s t h a t have largely e x a m i n e d aspects o f b i o d i v e r s i t y at species l e v e l - - b y q u a n t i f y i n g c o m p o s i t i o n , a b u n d a n c e o r activity v a r i a t i o n with l a n d m a n a g e m e n t practices. W e need to d e v e l o p r i g o r o u s e x p e r i m e n t s t h a t m e a s u r e soil b i o t a at v a r i o u s levels ( p o p u l a t i o n o r c o m m u n i t y o r b i o l o g i c a l process level), a n d relate these changes to soil q u a l i t y / s o i l health. W i t h s o u n d e x p e r i m e n t a l evidence we c a n indicate to the f a r m e r w h e t h e r the soil resource t h e y rely o n is declining in q u a l i t y / c o n d i t i o n , is stable o r in a process o f renewal. By e x a m i n i n g the r e l a t i o n s h i p between soil m a c r o f a u n a a n d soil structure, a n d levels a n d t y p e s o f o r g a n i c c a r b o n , we c o u l d use soil m a c r o f a u n a as a s u r r o g a t e / i n t e g r a t i v e m e a s u r e o f soil processes essential to e c o s y s t e m f u n c t i o n i n g - - n u t r i e n t cycling ( A n d e r s e n a n d Sparling, 1995), a n d to t r a n s p o r t ing nutrients, air a n d water. T h e r e is a need for c o m m i t m e n t to research t h a t is p r e p a r e d to t r a n s c e n d b o u n d a r i e s a n d t a k e a holistic a p p r o a c h in e x a m i n i n g the f u n c t i o n o f soil m a c r o f a u n a in h e a l t h y a n d d e g r a d e d soils, r a t h e r t h a n the c o m p a r t m e n t a l i s e d / r e d u c t i o n i s t a p p r o a c h which never revisits the big picture. Finally, there is a need to d e v e l o p p r a c t i c a l m a n -

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a g e m e n t strategies which d u p l i c a t e / r e c r e a t e the processes which o c c u r in h e a l t h y soils so t h a t we m a y be closer to o u r u l t i m a t e a i m o f d e v e l o p i n g a s u s t a i n a b l e f a r m i n g system.

Acknowledgements I w o u l d like to t h a n k m y colleagues w h o res p o n d e d to the q u e s t i o n n a i r e , a n d were p r e p a r e d to share their reflections on the status o f soil m a c r o f a u n a research. I a p p r e c i a t e their o p i n i o n s a n d o p e n n e s s o n the subject.

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