Original articleSoil invertebrates and ecosystem services
Introduction
Soils are essential sources of a wide diversity of ecosystem services defined as the goods and ecosystem functions that provide benefit to human populations [30], [86]. They support most agro-sylvo-pastoral production systems (production services) through the beneficial services that they mediate: soil formation, nutrient cycling and primary production. Soils also participate in the provision of regulation services (climate regulation by controlling greenhouse gas fluxes and C sequestration; flood control, detoxification, protection of plants against pests) through their influences on organic matter dynamics and the wide-ranging effects on soil physical properties. Soils finally contribute to cultural services although to a rather minor degree given the surprisingly widespread lack of interest of many societies in the sustainable use of this key resource. These services are provided by a large range of organisms whose effects are still relatively poorly explored, especially for the smaller body-sized taxonomic groups ([19], [38], this volume).
Soil invertebrates are enormously diverse. According to recent estimations, soil animals may represent as much as 23% of the total diversity of living organisms that has been described to date [38]. Their sizes range across three orders of magnitude. The smallest Nematodes and Protozoa (protists) of the microfauna less than 200 μm on average live in the water-filled porosity. Microarthropods, Enchytraeidae and the many groups of the mesofauna (0.2–2 mm) live in the air filled soil porosity. The largest arthropods, Mollusca, Annelida and Crustacea comprise the macrofauna that lives in the surface litter or in nests and burrows that they create in the soil [70]. In some places, vertebrates of the megafauna may become conspicuous elements of the soil fauna. This is the case for example for small rodents in deserts, pocket gophers in prairie ecosystems, moles and wild pigs in temperate grasslands and forests and other still poorly studied soil vertebrates in a large number of ecosystems [85], [98].
In some ecosystems, the local diversity of soil fauna may be enormous: far above that of groups of above-ground plants or animals. For example, Schaefer and Schauermann [103] found 1000 invertebrate species in a temperate climate forest in Germany. In most sites of tropical or temperate areas of the world, a standard sampling of soil macrofauna (= invertebrates visible at the naked eye) in an area limited to a watershed of a few km2 may yield 100–400 species [6], [84], [90]. This biodiversity is highly sensitive to any disturbance since the soil environment is their habitat and the source of all the resources they need ([11], [25], [56], [58], this volume).
In this paper we first present a general conceptual overview of biotic interactions in soils to explain the intimate links among invertebrates and other soil organisms and their importance for the continued functioning of the soil environment. The concept of self organization is used to describe these links within and across scales and emphasize the role of soil invertebrates in this apparently complex web of interactions. We then present a synthetic review of the ecosystem services that are affected by invertebrate activities and broadly explain the mechanisms involved. We finally address the practical consequences of these findings for sustainable management of soils and in monitoring soil quality.
Section snippets
Evidence for self organization in soils
Soil ecosystem services are emergent properties—at the plot or landscape scale—resulting from the wide range of processes operating at much smaller scales, in which invertebrates are involved. These processes are mediated by biological interaction systems that develop at a limited number of discrete scales [73]. These interaction systems have the properties of self organized systems strictly following the definitions given by Kauffman [66] and Perry [95].
Discrete scales in soil function
Five relevant scales have been identified in soil function, and invertebrates are major actors at three of them. At each scale, interactions among organisms of one or several groups develop within the boundaries of such structures as bio films, micro aggregates or the functional domains of invertebrate ecosystem engineers [73].
Invertebrates, the engineers of self organized systems in soils
Soil invertebrates are key mediators of soil function for the diversity of ecosystem engineering processes in which they partake. The comminution and incorporation of litter into soil, the building and maintenance of structural porosity and aggregation in soils through burrowing, casting and nesting activities, the control of microbial communities and activities, plant protection against some pests and diseases, acceleration of plant successions are among the many effects they have on other
Invertebrates as promoters and indicators of the provision of ecosystem services
The engineering activities developed by invertebrates contribute significantly to the production and delivery of soil ecosystem services in many ways (Table 1).
Conclusion
Despite many decades of intensive research, initiated by such great scientists as Gilbert White or Charles Darwin, invertebrates are still poorly acknowledged as mediators of soil function and the delivery of ecosystem services. The recent publication of a number of specific textbooks and synthesis papers in journals with large overall scientific impact may however mark the start of a significant change in thought and practice [3], [27], [76], [114], [115],140]. Traditional textbooks of Soil
Acknowledgements
We are greatly indebted to Alister Spain for language editing.
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