Soil & Water Management Centre

Developing & improving your soils

 

Invertebrates determine the nature of soil

Three faunal types that have a complex role in soil processes are defined as micro, meso and macrofauna. Together their presence alters the physical and chemical structure of the soil and the rate and extent of soil processes.1 


Microfauna: less than 100 μm, e.g. nematodes are associated with water films on the  surface of soil and organic particles, and water filled pores.




Mesofauna: are of sufficient size (100 μm-2 mm) to overcome the surface tension of water on soil particles but are not large enough to disrupt the soil structure in their movement through soil pores e.g. Acari (mites), Collembola (springtails), enchytraeid worms, small Diplopoda (millipedes), and many small larval and adult insects. Many species are mycophagous and therefore affect fungal populations strongly.


Macrofauna: species large enough to disrupt the soil by their burrowing and feeding (2 mm to 20 mm wide). The most important taxa are Isopoda (woodlice), larger Diplopoda, earthworms, Coleoptera (beetles), Diptera (flies), ants, and molluscs. To download a technical guide on earthworm biology, how they help farmers and how you can promote them click here. Created by by L. Pfiffner, Agricultural Ecologist at the Research Institute of Organic Agriculture (FIBL)

Other: Mites are important in the soil as fungivores, bacteriovores and nematode predators2. Generalist fungivore mites may be important in nutrient mineralization from decaying roots3. They also are important in fragmenting litter, dispersing microbial spores, and stimulating microfloral activity. Of non-insect groups, both Isopoda and Diplopoda are important as vectors of vesicular arbuscular mycorrhizal (VAM) fungi4.

Perhaps the most important contribution of invertebrates to soil structure is their faeces. The fine structure of soils, and therefore many of its structural features that contribute to soil fertility, is largely determined directly (topsoil) or indirectly (mineral soil) by invertebrate faecal dynamics.

In general, the micro- and mesofaunas appear to enhance microbial activity5, accelerate decomposition67, and mediate transport processes in the soil8. Springtails, earthworms and nematodes are especially important in soil processes in the UK.

1 Stork & Eggleton 1992, Largerlof and Andren, 1988, Arnold and Potter, 1987,Rabatin and Stinner, 1988, 5 Wright et al, 1989, 6,7 Christiansen et al., 1989; Setala et al., 1988, 8 Anderson, 1988. 


To understand the effect of soil management practices such ploughing, we can assess the invertebrate communities in our soils as a proxy for soil quality. 

Soils that have poor hydrological, physical or chemical properties will inevitably have poor invertebrate communities. The loss of particular species may seem to have no direct impact on soil quality, but it may severely affect those species with more direct roles through food web interactions. But how practical is the elucidation of such communities, and can we get the information we require from simple, easily sampled parameters?

Biological parameters

Pros

Cons

Example


Biomass, density & total abundance

  • Represents simple variables that can be measured independent of the difficulties involved in recognizing factors at the species or microsite level (Anderson et al.,1985).
  • Can be variable and represent "snapshots" in time.
  • Tells us little about community structure and nothing about the likely course of future soil processes.


King et al. (1985) compared springtail populations in natural and improved pastures in Australia and showed that although abundance rose under improved pasture, the overall species richness dropped. In addition, species composition changed, with many more introduced than endemic springtail species present in the improved pasture.


Species richness

  • Provides a broad measure of the complexity of communities and perhaps their resilience to change.
  • Has the potential to tell us more about invertebrate communities and soil quality than straight biomass, density or abundance.
  • Practical difficulty of distinguishing invertebrate species and species interactions.


Trophic group diversity

  • Reflects the common resources that different species use: primary producers (green plants), herbivores, predators and decomposers.
  • Trophic group structure is increasingly analyzed in impact assessments of management practices on soil communities. A particularly useful method is to examine groups of species that use the same trophic level resource but in different ways. These have been termed functional groups or guilds.
  • In practice, trophic analysis of the soil fauna is very difficult because it requires knowledge of the biology of individual species or groups of similar species.
  • Studies of particular taxonomic groups tell us little, because of the convergence in trophic function of unrelated organisms and the divergence in trophic behaviour of related taxa.


The number of earthworm guilds (functional groups) fell from four in tropical forest to one in cultivated land in Peru (Lavelle, 1988a,b). In the temperate region, plowing buries leaf litter and sward, thus removing the food of one guild of earthworms (those that form permanent burrows and forage on the surface), and destroying the habitat of another (those that live in the leaf litter and sward). A third guild (those that live and feed entirely within the soil) increase in biomass and abundance (Rushton, 1988).


Keystone species and ecosystem engineers

  • "Keystone" species usually are very abundant or of considerable biomass (e.g., oak trees in an oak woodland). Alternatively, if they are small or less abundant, they play a critical role in the food chain.
  • Certain groups of organisms, sometimes even single species, may "engineer" environments for many other organisms and thus form the bedrock of communities.
  • No easy way to identify possible keystone species in the very complex soil environment.


In soil, groups that might be considered ecosystem engineers are earthworms as they make a major contribution to soil structure, and thus to soil community stability and soil quality.


International Measures of Soil Quality-Ranking Parameters

The need for a global assessment of land degradation was acted upon by the Food and Agriculture Organization, which established an international set of standards of soil types. Nigel Stork and Paul Eggleton (1992) presented a paper at a meeting on the Assessment and Monitoring of Soil Quality to investigate the possibility of establishing some simple measures of assessing soil quality.

As well as two or three each of hydrological, chemical, physical and microbiological characteristics of the soil, it was recommended by Stork & Eggleton that a simple/priority index of soil quality might include these three invertebrate measures 

1. Keystone species or ecosystem engineers. Where such groups can be identified unambiguously, their abundance, biomass and density may represent the best criteria for quickly and effectively assessing the invertebrate contribution to soil quality.

2. Taxonomic diversity at the group level. Assessment of the abundance, biomass and density of all soil invertebrates at the order/class level will provide a simple indication of the ecological complexity of the soil community.

3. Species richness of several dominant groups of invertebrates. Again, this will indicate the ecological complexity of the soil community. Inevitably, these three measures will provide only a simple guide to soil quality.



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