Available online at www.sciencedirecl.com --" EUROPEAN JOURNAL Of -.;- ScienceDirect SOll BIOLOGY ELSEVIER European Journal of Soil Biology 42 (2006) S23-838 http://france.elsevier.comldirectlejsobi Original article The values of sail animaIs for conservation biology a b c b T. Decaëns ,*, II Jiménez , C. Gioia , G.J. Measeyb, P. Lavelle •Laboratoire d'écologie-ECOD/V, UPRES EA /293, université de Rouen, 76821 Mt Saint Aignan cedex, France b Laboratoire d'écologie des sols tropicaux, lRD, 32, Avenue Henri-Varagnat, 93143 Bondy cedex, France C Bureau d'élude AL/SE. 228, ZAC de la Forge-Seret, 76520 Boos, France Available online 21 July 2006 Abstract It has taken time for the international community to accept the idea of biodiversity values, a concept which had previously been restricted to the limited aesthetic and touristic aspects ofwildlife. This situation changed following the International Convention on Biodiversity in Rio de Janeiro (1992), which focussed on "the forgotten environmental problem" ofbiodiversity erosion and made the first clear reference to the values of living species. Biodiversity values refer to direct or indirect, economic or non-economic interest, a given species or ecosystem may represent for human populations. These values are generally split into intrinsic and instrumental (use) values, the last category itself being divided into direct and indirect economic values. Obviously, each of these values cames different weights, and cannot be considered as being weighted equally in terms of justification for species or ecosystem conservation. Soil is probably one ofthe most species-rich habitats of terrestrial ecosystems, especially if the defmition is extended to related habitats like vertebrate faeces, decaying wood, and humus ofhollow trees (i.e. epiphytic soils). The diversity ofsoil communities (sensu lato) thus probably encompasses a large part of terrestrial animais. This highly speciose fauna has been the subject ofrecent research efforts, and current trends in soil fauna studies include aspects ofbiology, autecology, ecotoxicology, or functional ecology. During the past 20 years, recognition of the importance of soil fauna in the functioning of soils and by extension of terrestrial ecosystems has been continuously growing, ending in sorne important applications in agronomy. However, despite the general agreement about the ecological importance of soil fauna and its economic consequences, the absence of concem about this group from conservationists in their studies is conspicuous. This paper aims at presenting soil fauna within the scope of conservation biology concepts, trying to identify the different values of soil fauna and how they participe to the provisioning of key ecosystem services. Finally, the enormous gap between their recognised usefulness and their consideration in protection policies is discussed. © 2006 Elsevier Masson SAS. Ali rights reserved. Keywords: Soil animaIs; Soil biodiversity; Biodiversity values; Ecosystem goods and services; Conservation biology; Concepts 1. Introduction touristic aspects of wildlife [29]. This situation has changed, mainly since the International Convention on Tt has taken time for the international community to Biodiversity in Rio de Janeiro (1992), which focussed accept the idea of biodiversity values, a concept pre­ on "the forgotten environrnental problem" of biodiver­ viously associated only with the limited aesthetic and sity erosion and made the frrst clear reference to the values of living species [60]. Biodiversity was at this occasion defined as "the variability among living • Corresponding author. organisms from a11 sources including terrestrial, marine E-mail address:[email protected] (T. Decaëns). and other aquatic ecosystems and the ecological com- 1164-5563/$ - see front matter © 2006 Elsevier Masson SAS. Ali rights reserved. doi: 10.1 016/j.ejsobi.2006.07.001 S24 T Decaëns el al. / European Journa/ ofSail Bi%gy 42 (2006) S23-S38 plexes of which they are a part; this includes diversity tify the different values of soil fauna and how they rely within and between species, and of ecosystems" [63]. on key ecosystem services. The enormous gap between Biodiversity values thus refer to the direct or indirect, their recognised usefulness and their consideration in economic or non-economic interest a given species or protection policies is finally discussed. ecosystem may represent for human populations. Iden­ tifying species values cornes down to answering the 2. How many species in the soil? question: What is a species worth [l3]? These values are generally split into intrinsic and instrumental (use) Estimating the diversity of soil animal communities values, the last category being itself divided into direct first requires a definition of a "soil animal". Wolters and indirect economic values [29,52,66,106]. [129] distinguishes between the "full-time inhabitants" Obviously, each of these values carries different (many micro- and mesoarthropods, earthworms and weights, and may not be considered equally powerful macroinvertebrates) and the "part-time inhabitants" of in terms of justification for species or ecosystem con­ soil (Iike many vertebrates, soil dwelling insect larvae servation. or mound-building insects). But what about species that Soil is probably one of the more species-rich habitats mainly live in/on the litter layer like for example Car­ of terrestrial ecosystems [129]. If extending the defini­ abidae predators? Where does soil began and end? tion of soils to related habitats like vertebrate faeces, What about vertebrate dung, decaying wood, rocks, decaying wood, humus of hollow trees, the diversity hollow trees and other "epiphytic" soils; ail habitats of soil communities (sensu lato) probably encompass termed "soil annexes" by Gobat et al. [54]. In this the majority of terrestrial animal species. This highly paper we adopt a sensu lato definition of soil animais speciose fauna has been the subject of recent investiga­ and will take into consideration examples from these tion efforts, with current trends including biology, aute­ habitats highly connected to the soil sensu strito. cology, ecotoxicology and functional ecology. During Soils probably represent a necessary substrate for a the past 20 years, the recognition of the importance of large part of global biodiversity. For example, the soil fauna in the functioning of soils and by extension majority of animais in terrestrial habitats are soil inha­ of terrestrial ecosystems has been continuously grow­ bitants for at least one stage of their life cycle [7,129]. ing, ending in sorne important applications in agronomy A rapid survey of invertebrate and vertebrate groups (see e.g. [82]). reveals that at least 1/4 of described living species are Despite the general agreement about the ecological strictly soil or litter dwellers, the main part of which is importance of soil fauna and its economic conse­ insects and arachnids (Fig. 1). This prodigious below­ quences, the absence of concem about this group from ground diversity le<! to the proposai of the "enigma of conservationists in their studies is conspicuous [129]. soil diversity" [2], demanding why biotic mechanisms This paper ai ms at presenting soil fauna within the that normally reduce diversity (e.g. the competitive scope of conservation biology concepts, trying to iden- exclusion process) have allowed such a large number Vertebrata Annelida <1% Microinv 1% 2% Bacteria and Fungi 4% virus <1% Otber Arthropoda 5% Other Insects AnimaIs 80% 55% SoU animaIs 23% (i.e. - 360,000) Total number of described living species: - 1,500,000 Fig. 1. Importance ofsoit animaIs for the global biodiversity (a) and relative importance of major taxa wilhin soil communities worldwide (b) (data from [27,68,104,120,124]). T. Decaëns el al. / European Jal/mal ofSoil Biology 42 (2006) S23-S38 825 of coexisting soil dwelling species [127]. A set of con­ microarthropods. AdditionaIly, even for higher body­ cepts and ecological theories are proposed to explain sized groups for which the taxonomic knowledge may this pattern [33,85,118], involving local processes shap­ be higher, basic species biology, ecology and distribu­ ing the interactions between individuals and popula­ tion patterns remain unknown (see e.g. [50] for earth­ tions, such as trophic niche partitioning, spatial and worms). temporal segregation, relaxation of density dependant To explain this major drawback in soil biodiversity regulations by adverse soil conditions, and a high diver­ knowledge, authors generally stress the lack of taxo­ sity of micro-habitats [53,129]. AIl these rnechanisrns nomic expertise [4,11,23,53,124] and of standard sam­ are probably favoured by the nature of the physical pling methods and protocols [5,23,93]. AIl this deserves environment, as the three-dimensional structure of the much effort in methodological and taxonomic knowl­ soil, the wide range of pores, surface types and micro­ edge to allow meaningful estimation of species richness cIimates, may reduce competitive exclusion and pro­ in soil communities, its main patterns and how it affects mote greater species coexistence through resource par­ soil processes and ecosystem functioning [5,23]. titioning. Despite its supposedly critical contribution to global biodiversity, soil fauna has received compara­ 3. The intrinsic values of species tively iittle taxonomic attention when compared with other groups such as higher plants and vertebrates. To Many conservation biologists believe that