Servane Gillet

Jean-François Ponge

Are acid-tolerant collembolan communities able to colonise metal-polluted soil?

Keywords: Soil fauna, Acidophily, Colonisation, Pollution, Heavy metals, Microcosms

A short-term microcosm experiment was carried out to determine whether acidtolerant collembolan communities are able to colonise metal-polluted soil. Polystyrene boxes were divided in two compartments by a perforated wall allowing free passage of soil fauna and preventing physical contact between soil substrates. All compartments were filled either with an acid dysmoder (pH 4.4) collected in a beech forest from Belgian Ardennes (Willerzie, Belgium) or with one of three neutral polluted soils (P1, P2, P3) collected in the Bois des Asturies, along a metal-pollution gradient downwind of a zinc smelter (Auby, France).

Different combinations were established, with five replicates each, and were incubated during three weeks at 15°C, in darkness. Afterwards, Collembola were extracted and determined to the species level. It appeared that populations from the acid soil colonised the neutral soil polluted by heavy metals. Within three weeks, the number of species increased in compartments filled with the most heavily polluted soils (P2, P3) when they were in contact with the acid soil. However, colonisation was effected by only a few individuals. At the species level, the onychiurid Protaphorura eichhorni and the isotomid Folsomia quadrioculata colonised the polluted soil. For Protaphorura eichhorni the migration rate was highest when the soil was the least polluted (P1), while Folsomia quadrioculata showed a higher rate of dispersion to the medium polluted soil (P2) compared with the least polluted (P1). This behaviour could be explained by the humus form which was a mull in P1, more compact than the mor observed in P2 and P3, the physical structure of which approaches that of the dysmoder. Our observations point out that both species are more affected by soil structure than by pollution.

Introduction

Under acid conditions, availability and mobility of metal ions are high due to the chemical form in which they are present in the soil solution [START_REF] Berggren | Metal solubility and pathways in acidified forest ecosystems of south Sweden[END_REF][START_REF] Reddy | Solubility and mobility of copper, zinc and lead in acidic environments[END_REF]. At low pH heavy metals are dominated by mobile ionic forms (Zn 2+ , Pb 2+ ) followed by soluble salts (ZnSO 4 , PbSO 4 ) and most aluminium exists as Al 3+ [START_REF] Nair | Calculations of equilibrium concentration of mono-and polynucear hydroxyaluminium species at different pH and total aluminium concentrations[END_REF].

The accumulation of humified organic matter which characterises acid soils [START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a montain spruce forest[END_REF][START_REF] Bernier | Altitudinal changes in humus form dynamics in a spruce forest at the montane level[END_REF][START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF] and the presence of high amounts of undissociated phenolic and aliphatic acids in the organic matter also increase the solubility of aluminium in the soil solution [START_REF] Duchaufour | Pedologie[END_REF]. Plant secondary metabolites like phenolics play also a role in chemical interference [START_REF] Ponge | The forest regeneration puzzle[END_REF]. The toxicity of tannins to soil fauna is well documented, even at low concentrations [START_REF] Poinsot-Balaguer | Effects of tannin compounds on two species of Collembola[END_REF]. [START_REF] Verdier | Étude de l'atmosphère du sol. Éléments de comparaison et signification écologique de l'atmosphère d'un sol brun calcaire et d'un sol lessivé podzolique[END_REF] and Sextone and Mains (1990) also noted in the atmosphere of acid soils an increase in carbon dioxide and toxic gases such as methane.

Several parallels can be traced between conditions prevailing in acid and polluted soils. Like acid soils, polluted soils are characterised by an accumulation of organic matter in the topsoil, but in the latter case organic matter is poorly humified [START_REF] Bengtsson | The Gusum case: a brass mill and the distribution of soil Collembola[END_REF][START_REF] Coughtrey | Litter accumulation in woodlands contaminated by Pb, Zn, Cd, and Cu[END_REF][START_REF] Gillet | Humus forms and metal pollution in soil[END_REF]. In polluted soils, the high mobility of metals can be explained by [START_REF] Pédogénèse | [END_REF] an excess of metal load as it was observed near petroleum [START_REF] Adeniyi | Determination of total petroleum hydrocarbons and heavy metals in soils within the vicinity of facilities handling refined petroleum products in Lagos metropolis[END_REF] and smelting complexes [START_REF] Denaix | Transferts de Zn, Pb et Cd par voie soluble ou colloïdale[END_REF], and (2) soil acidification by nitrogen and sulphur deposition [START_REF] Tomlinson | Air pollutants and forest decline[END_REF]. The last parallel lies in the amount of phenolics in the litter since polluted sites are often reclaimed by planting poplar [START_REF] Gillet | Humus forms and metal pollution in soil[END_REF], a tree which is well-known for the high tannin content of its foliage [START_REF] Lindroth | Effects of genotype and nutrient availability on phytochemistry of trembling aspen (Populus tremuloides Michx.) during leaf senescence[END_REF].

In spite of the toxicity of acid soils to a lot of invertebrates [START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF], no decrease in density and local diversity was observed in collembolan communities [START_REF] Loranger | Does soil acidity explain altitudinal sequences in collembolan communities?[END_REF]. This observation can be attributed to physiological adaptation to acid conditions which could be inherited from Palaeozoic times (Ponge, 2000b). Indeed, to live in an acid environment, animals and plants have developed some resistance mechanisms like, for instance, cytoplasmic sites where aluminium may be harmlessly accumulated in the case of plants [START_REF] Clarkson | Metabolic aspects of aluminium toxicity and some possible mechanisms for resistance[END_REF], or excreted by periodically renewing the midgut epithelium in the case of Collembola [START_REF] Joosse | Uptake and excretion of lead by litter-dwelling Collembola[END_REF][START_REF] Hopkin | Deficiency and excess of essential and non-essential metals in terrestrial insects[END_REF]. Some authors have described adaptation and resistance of collembolan communities submitted to long-term contamination by heavy-metals [START_REF] Joosse | Lead tolerance in Collembola[END_REF][START_REF] Posthuma | Life-history patterns in metal-adapted Collembola[END_REF][START_REF] Tranvik | Relative abundance and resistance traits of two Collembola species under metal stress[END_REF]).

However, a decrease in biodiversity [START_REF] Hågvar | Microarthropoda and Enchytraeidae (Oligochaeta) in naturally lead-contaminated soil: a gradient study[END_REF][START_REF] Filser | Experimental studies on the reactions of Collembola to copper contamination[END_REF] and a change in feeding habits [START_REF] Gillet | Changes in species assemblages and diets of Collembola along a gradient of metal pollution[END_REF] have also been observed under the infuence of metal-pollution. Those observations allow us to hypothesise that acidtolerant fauna could also be tolerant to conditions prevailing in polluted soils [START_REF] Chauvat | Colonization of heavy metal-polluted soils by Collembola: preliminary experiments in compartmented boxes[END_REF]. This work follows a preliminary study by [START_REF] Chauvat | Colonization of heavy metal-polluted soils by Collembola: preliminary experiments in compartmented boxes[END_REF] which showed erratic colonisation of lead acetate treated mull humus by acid-tolerant species like Willemia anophthalma, Proisotoma minima and Xenylla tullbergi. The aim of the present work was to simulate in the laboratory, under controlled light and temperature conditions, the inoculation of an acid soil, with its complete acid-tolerant fauna, into a metal-contaminated site. We asked whether fauna accustomed to the presence of free metals like iron and aluminium ions in the soil solution of acid soils was able to colonise a neutral soil polluted by heavy metals (Cd, Zn, Pb) in spite of a pH increase. To follow colonisation of polluted soils by acid-tolerant soil fauna, we selected Collembola as an abundant and diversified faunal group, present in polluted as well as acid and neutral soils [START_REF] Hågvar | Microarthropoda and Enchytraeidae (Oligochaeta) in naturally lead-contaminated soil: a gradient study[END_REF][START_REF] Bengtsson | The Gusum case: a brass mill and the distribution of soil Collembola[END_REF].

Materials and methods

Microcosm experiment

In order to study attraction/repulsion behaviour rather than reproductive success of Collembola and to avoid food shortage, a short-term experiment in the laboratory was done.

The polluted soil used in our experiment was collected in June 2001 from the Bois des Asturies at Auby (Nord, France). This poplar plantation, located near to and downwind of a zinc smelter, suffers from heavy pollution by zinc, lead and cadmium. The topsoil was collected from plots P1, P2 and P3, which have been already described by [START_REF] Gillet | Humus forms and metal pollution in soil[END_REF]. Plot P1, 490 m from the smelter, is characterised by a mull humus form and the presence of macro-invertebrates (earthworms, millipedes). Plots P2 and P3, 340 m and 235 m from the smelter, respectively, were characterised by a mor humus form [START_REF] Ponge | Comment distinguer dysmoder et mor? L'exemple de la forêt domaniale de Perche-Trappe (Orne)[END_REF] with a well-developed organic horizon and a low faunal activity, with microinvertebrates only. The total amount of zinc decreases from P3 to P1 whereas the total amounts of Pb and Cd are similar at P2 and P3 and lower at P1. On plot P1 we used the top 10 cm of the carbon-rich organo-mineral A horizon and on plots P2 and P3 we used the top 10 cm of the organic OM horizon [START_REF] Ponge | Comment distinguer dysmoder et mor? L'exemple de la forêt domaniale de Perche-Trappe (Orne)[END_REF].

The acid humus was collected at the same time in a mature beech forest (Fagus sylvatica) at Willerzie (western Ardennes, Belgium), at 445 m altitude. This forested site, which is located 130 km from the polluted site, has been described and studied by [START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF] and [START_REF] Ponge | Horizon and humus forms in beech forests of Belgian Ardennes[END_REF]Ponge ( , 2000a) ) as Site N°16. The humus form is a dysmoder [START_REF] Brêthes | Classification of forest humus forms: a french proposal[END_REF], with an OH horizon more than 1 cm depth. We used a mixture of holorganic OF and OH horizons from the dysmoder after the OL horizon had been discarded.

The experiment was conducted in 65 mm high polystyrene boxes measuring 175x115 mm divided in two compartments by a wall pierced with 2 mm diameter holes at a rate of 400 holes per wall. The division allowed free passage by micro-invertebrates but prevented physical contact between soils in adjacent compartments. On the same day the two compartments of each box were filled with intact topsoil horizons with their original fauna, one coming from the acid beech forest and the other from one of the three polluted plots.

Different combinations were established with five replicates each (Table 1). Microcosms were incubated at 15°C in darkness during three weeks. At the end of the experimental period, Collembola were extracted by the dry-funnel method [START_REF] Edwards | A comparison of extraction methods for terrestrial arthropods[END_REF] and identified to the species level under a light microscope at 400x magnification using [START_REF] Gisin | Collembolenfauna Europas[END_REF], [START_REF] Stach | The apterygotan fauna of Poland in relation to the world-fauna of this group of insects. Tribe : Orchesellini[END_REF][START_REF] Stach | The apterygotan fauna of Poland in relation to the world-fauna of this group of insects. Tribe : Entomobryini[END_REF], [START_REF] Deharveng | À propos des Folsomia du groupe quadrioculata Tullberg[END_REF]), Fjellberg (1992) and Hopkin (in prep.).

Chemical analyses

Five soil cores 5 cm diameter and 10 cm depth, litter included, were collected with a core sampler in May 2002 at each of the three polluted plots (P1, P2, P3). They were airdried at 25°C in an air-forced chamber then stored in plastic bags before analysis. Previous to extraction of heavy metals, polluted soils were sieved through a 2 mm mesh. To estimate forms of zinc which can be extracted from the soil by living organisms, also called bioavailable zinc [START_REF] Suter | Ecological Risk Assessment for Contaminated Sites[END_REF], 50 ml of an equal mixture of 0.1 M Na 2 EDTA.2H 2 O and 1 M NH 4 Ac were added to 5 g of soil, shaken for two hours, filtered and diluted ten times to avoid particulate metal deposition. All filtrates were kept at 4°C until analysis. Zinc was detected by Inductively Coupled Plasma Atomic Emission Spectroscopy (ICP-AES) at =213.856 nm according to the method by [START_REF] Weissenhorn | Arbuscular mycorrhizal contribution to heavy metal uptake by maize (Zea mays L.) in pot culture with contaminated soil[END_REF]. For analysis of exchangeable zinc 50 ml of a 1 M NH 4 NO 3 solution was added to 5 g of soil, shaken for one hour, filtered and diluted to 1/10. Then the filtrate was analysed for zinc by the above mentioned method. Zinc extracted by the EDTA-acetate method includes exchangeable zinc since in both procedures the ammonium ion displaces the metal from sorption sites [START_REF] Tan | Principles of Soil Chemistry[END_REF]. The amount of zinc bound to organic matter and metal hydroxides was determined by subtracting exchangeable zinc from EDTA-acetate zinc. The water-soluble zinc was analysed after shaking 50 ml of deionised water with 5 g of soil for one hour and filtered through 0.45m. The water-soluble fraction was subtracted from exchangeable zinc measured as above, in order to give a more reliable estimate of zinc reversibly sorbed to mineral and organic matter.

The pH of the dysmoder was measured in water and in potassium chloride according to ISO 10390 [START_REF] Afnor | Qualité des Sols[END_REF]. The soil was suspended in deionized H 2 O and 1 M KCl (1:5 soil:water in volume) for pH H 2 O and pH KCl, respectively. Each suspension was shaken for five minutes, then pH was measured in the supernatant after sedimentation for 2 h with a glass electrode.

Statistical analyses

Data were analysed by correspondence analysis, a multivariate method using the chisquare distance [START_REF] Greenacre | Theory and Applications of Correspondence Analysis[END_REF]. Active variables (species, represented by the number of individuals in microcosm compartments) and passive variables (combinations, coded as 1 or 0) were projected simultaneously in a space formed by the first factorial axes (those explaining the highest global variation). The introduction of passive variables is helpful in interpreting factorial axes when they are well-correlated to these axes. Passive variables were not used in the calculation of eigenvalues, thus they did not influence the formation of factorial axes. The projection of passive variables (combinations) is a point in the vicinity of the species (active variables) which characterise this variable the best. To give the same weight to all variables (discrete as well as continuous), they were transformed into: X = (x-m)/s + 20, where x is the original value, m the average value of the variable and s its standard deviation. The addition to each standardised variable of a constant 20 allows all values to remain positive. After such a transformation, factorial coordinates of variables can be interpreted directly in terms of their contribution to factorial axes: the further a variable is projected from the origin along a given axis the more it contributes to this axis [START_REF] Loranger | Does soil acidity explain altitudinal sequences in collembolan communities?[END_REF]. Each active variable (species) was doubled with a conjugated X' = 40-X since some variables may have a dual nature if their absence from experimental compartments is as significant as their presence [START_REF] Greenacre | Theory and Applications of Correspondence Analysis[END_REF].

To test for the significance of correlations and observed effects, Spearman correlation coefficients and one-way analyses of variance (ANOVAs) followed by Newman-Keuls tests were used at the 0.05 level of probablility for rejection of null hypothesis. Non-parametric Mann-Whitney tests were also used for comparisons between groups.

Results

Chemical analyses

The dysmoder was characterised by a pH H 2 O of 4.4 and a pH KCl of 3.3 (Table 2).

The difference (delta pH) of 1.1, compared to 0.5 only in the polluted Auby soils, points out that the acidity linked to soil particles was more important in the acid soil than in the polluted soil. The pH H 2 O (6.8) and the pH KCl (6.3) were far higher in the polluted soil than in the dysmoder, thus potential colonists from the acid soil have to struggle against a pH increase.

Zinc was by far the most abundant heavy metal in the Bois des Asturies, which explains why measurements of heavy metals were done only on this metal. The maximum amount of bio-available zinc found in our samples was 22 600 mg kg -1 . The amount of EDTAacetate zinc was higher at P2 and P3 than at P1 (Table 3). Zinc linked to organic matter and oxides was the dominant form of EDTA-acetate zinc, amounting 94% at P1, 84% at P2 and 91% at P3, followed by exchangeable zinc (5.7% at P1, 15.1% at P2 and 8.1% at P3) and a little content of water-soluble zinc (0.5% at P1, 0.6% at P2 and 0.5% at P3).

Microcosm experiment

Table 4 lists the collembolan species found at the end of the experiment, classified according to the sites (Auby and Willerzie) from which they are issued. Eleven species were common to both sites, 21 were only present in the Ardennes and 10 were only present in the Bois des Asturies. The total species richness in the compartments with polluted soils (22 species) compared well with that of the field community (21 species) as described by [START_REF] Gillet | Changes in species assemblages and diets of Collembola along a gradient of metal pollution[END_REF] on the same number of replicates.

Correspondence analysis showed that after three weeks the species composition of collembolan communities in compartments filled with polluted soils was influenced by the presence of an adjacent compartment filled with an acid soil, as ascertained by the addition of new species (Fig. 1). Axis 1 (11.9% of the total variance) was correlated with the number of individuals (r = 0.71; p<0.05). It indicated an increase in the number of individuals in compartments filled with P2 and P3, when adjacent to the acid soil. However this increase was not significant (Fig. 2). Axis 2 (10.8% of the total variance) was correlated with the number of species (r = 0.48; p<0.05). It indicated an increase in the number of species in compartments filled with P1, P2 and P3, when adjacent to the acid soil (P1/A, P2/A and P3/A). However, this addition was significant only for P2 and P3 (Fig. 3). Axis 1 was correlated with the density of species from Willerzie (acid soil, unpolluted), like Folsomia quadrioculata (r = 0.377; p<0.05), Mesaphorura tenuisensillata (r = 0.32; p<0.05), Lipothrix lubbocki (r = 0.41; p<0.05) and Pseudisotoma sensibilis (r = 0.36; p<0.05). Axis 2 was correlated with the density of Willerzie species such as Protaphorura eichhorni (r = 0.59; p<0.05), Folsomia quadrioculata (r = 0.61; p<0.05), Lipothrix lubbocki (r = 0.32; p<0.05) and Pseudisotoma sensibilis (r = 0.35; p<0.05). This indicated that acid-tolerant species penetrated in P1, P2 and P3. However, this colonisation concerned only few individuals (Fig. 2).

Multivariate analysis (Fig. 1) depicted differences in the fate of collembolan communities from polluted soils when faced to an adjacent acid soil. The addition of new (acid-tolerant) species differed according to the distance to the smelter. Protaphorura eichhorni characterises the acid-tolerant collembolan community which colonised the least polluted soil P1 while Folsomia quadrioculata, Lipothrix lubbocki, Pseudisotoma sensibilis and Schaefferia emucronata characterised the community which colonised the most polluted soils P2 and P3.

In the acid compartment the number of species and individuals was not affected by the presence of a heavy metal polluted soil in the adjacent compartment (Figs. 2 and3, Table 5).

At the species level, Folsomia quadrioculata from the Ardennes colonised compartments filled with polluted soils (Fig. 4). When in contact with the three polluted soils collected in the Bois des Asturies, it colonised P2 to a greater extent than P1. The behaviour of Protaphorura eichhorni was somewhat different (Fig 5). The number of animals passing to the polluted compartment decreased from P1 to P3. Other acid-tolerant species showed a significant passage from the unpolluted acid soil to the polluted soils, namely Lipothrix lubbocki, Pseudisotoma sensibilis and Mesaphorura tenuisensillata, while others, such as Pseudosinella mauli and Willemia anophthalma showed only a weak, insignificant, dispersal rate to the polluted compartments (Table 5).

Discussion

In the microcosm experiment, the absence of a negative impact of a compartment heavily contaminated with metals on the population of the adjacent acid soil could be explained by i) the absence of transfer of pollutants through the perforated plastic wall, ii) in the case some transfer occurred, most probably through animal activity, by the tolerance of acid-tolerant fauna to pollution.

Like in the study by [START_REF] Chauvat | Colonization of heavy metal-polluted soils by Collembola: preliminary experiments in compartmented boxes[END_REF], who used lead acetate as a contaminant, we observed colonisation of soils heavily polluted with heavy metals by acidtolerant Collembola. [START_REF] Chauvat | Colonization of heavy metal-polluted soils by Collembola: preliminary experiments in compartmented boxes[END_REF] found that Folsomia manolachei, a closely related species [START_REF] Deharveng | À propos des Folsomia du groupe quadrioculata Tullberg[END_REF] coming from a coniferous forest, colonised the polluted soil. [START_REF] Filser | Experimental studies on the reactions of Collembola to copper contamination[END_REF] demonstrated that Folsomia manolachei and Folsomia quadrioculata (although insignificantly in the latter case), colonised a copper-polluted soil more than an unpolluted neutral control. In a parallel experiment, they showed a greater abundance of the strongly acidophilic species Willemia anophthalma in a soil directly treated by copper salts than in an untreated (and unpolluted) control. In a field study, [START_REF] Hågvar | Microarthropoda and Enchytraeidae (Oligochaeta) in naturally lead-contaminated soil: a gradient study[END_REF] classified Folsomia quadrioculata among species the most tolerant to lead pollution, although [START_REF] Sjögren | Dispersal rates of Collembola in metal polluted soil[END_REF] observed that this species was affected by the presence of heavy metals. The bulk of these observations allow to think that acid-tolerant Collembola are more tolerant than other species to life in metal-polluted soils. However, the absence of Folsomia quadrioculata in the most polluted site studied by [START_REF] Bengtsson | The Gusum case: a brass mill and the distribution of soil Collembola[END_REF] and the fact that in our experiment the number of migrants found in polluted compartments was low point out that we need more knowledge on the genetic variation within species such as Folsomia quadrioculata sensu lato. This group of species is known to live in acid as well as in neutral soils [START_REF] Ponge | Biocenoses of Collembola in atlantic temperate grass-woodland ecosystems[END_REF], thus we can speculate that it exhibits a wide variation in its adaptation to metal pollution as this has been demonstrated in Orchesella cincta [START_REF] Posthuma | Life-history patterns in metal-adapted Collembola[END_REF].

Our study showed a great variability between collembolan species in their rate of colonisation of polluted soils. According to [START_REF] Johnson | Dispersal of collembolan, Folsomia candida Willem, as a function of age[END_REF] the colonisation of a new habitat is performed by "dispersers", which are individuals feeling obliged to disperse.

They explained this behaviour and the variability in dispersion rates by the age of individuals.

Younger Collembola tend to stay in the vicinity of the oviposition site, while older individuals tend to go away, spreading their eggs over a wide area. Consequently, it was probable that, at the time we collected our experimental soil, some species were represented by a majority of adults (dispersers) and some others by a majority of juveniles with a poor colonisation capability. Moreover, it exists a large between-species variation in the dispersion rate, which depends on both biological and environmental conditions [START_REF] Joosse | Relationship between saturation deficit and the survival and locomotory activity of surface dwelling Collembola[END_REF][START_REF] Hertzberg | Migration of Collembola in a patchy environment[END_REF].

Multivariate analysis indicated that the colonisation of the polluted soil by acid-tolerant

Collembola differed for the soils taken at different distances to the smelter, and that Protaphorura eichhorni better colonised the less polluted site P1, while the contrary was observed in Pseudisotoma sensibilis, Lipothrix lubbocki and Folsomia quadrioculata. Other site factors than the direct effect of heavy metal concentration would probably explain such discrepancies. Differences in humus form correlated with the degree of pollution [START_REF] Gillet | Humus forms and metal pollution in soil[END_REF] explain the observed variation better. The humus form at P1 was a fine, carbon-rich mull more compact than the mor present at P2 and P3, the structure of which approaches the dysmoder from the beech forest. The influence of pore size distribution on the recolonisation of defaunated soils by Collembola has been demonstrated by [START_REF] Vannier | Etude in situ du retour des microarthropodes sur des fractions de sol de granulométrie différente[END_REF]. In our experiment Protaphorura eichhorni, an onychiurid species, is well-adapted to life in deep soil, even when compact, compared to other congeneric species [START_REF] Didden | Reactions of Onychiurus fimatus (Collembole) to loose and compact soil. Methods and first results[END_REF][START_REF] Ponge | Biocenoses of Collembola in atlantic temperate grass-woodland ecosystems[END_REF]. In contrast, species such as Pseudisotoma sensibilis, Lipothrix lubbocki and Folsomia quadrioculata, live in well-aerated environments such as litter and mosses [START_REF] Ponge | Biocenoses of Collembola in atlantic temperate grass-woodland ecosystems[END_REF], and could thus be poorly adapted to the compact P1 fine mull. Thus, strongly acidophilic (Protaphorura eichhorni, Pseudisotoma sensibilis, Lipothrix lubbocki) or acid-tolerant (Folsomia quadrioculata) species were all able to colonise metal-polluted soils but they also seemed affected by the physical structure of the topsoil thus by the humus form.

The influence of other soil fauna groups, like predator-prey interactions, on the dispersion of Collembola cannot be discarded in our experiments. For instance, some collembolan species could escape the acid dysmoder towards the polluted soil, because of a reduced impact of predation in an adjacent polluted compartment. However, this kind of interaction is unlikely, because another study on the same polluted site [START_REF] Gillet | Changes in species assemblages and diets of Collembola along a gradient of metal pollution[END_REF] revealed that predators of Collembola were abundant in the three plots P1, P2 and P3.

Our study did not demonstrate any attraction of acido-tolerant or acidophilic species to the polluted soil. Species from the Belgian Ardennes were found in much lower densities in adjacent comparments filled with polluted soils (Table 5), which showed that they preferred the acid soil from which they originated. However, at the end of the three-week experiment, these species increased the species richness of the polluted soils, which more than doubled at the medium level of contamination P2 (Fig. 3). Thus the answer to the title question is yes, at least in the short-term.

Another question is whether tolerance to heavy metal toxicity and tolerance to acidity are physiologically related. We did not demonstrate such relationship, since acidophilic species such as Protaphorura eichhorni, Pseudisotoma sensibilis and Lipothrix lubbocki did not disperse better than ubiquitous species such as Folsomia quadrioculata. Some tiny, poorly motile acidophilic species such as Mesaphorura tenuisensillata, M. jevanica and Willemia anophthalma dispersed poorly if not at all in polluted compartments. Thus we cannot rule out that random movements of animals from the Ardennes (unpolluted acid soil) in adjacent compartments with neutral polluted soil could result in similar patterns. Other experiments, allowing separation between dispersion and tolerance to heavy metals, are therefore needed before reaching a straighforward conclusion.

= 0 = 0 = 0 0.1 = 0 = 0 = 0 0.1 = 0 = 0 = Lipothrix lubbocki 0.7 = 0.8 = 0 = 0 0.7 = 1.6 = 0.2 = 0 0.7 = 1.2 = 0.4 > Mesaphorura jevanica 5.7 = 3.2 > 0 = 0 5.7 = 3.4 > 0 = 0 5.7 = 5.8 > 0.2 = Mesaphorura tenuisensillata 6.4 = 5.8 > 0 = 0 6.4 = 8 > 0.4 > 0 6.4 = 5.6 > 0.2 = Micranurida granulata 0.1 = 0 = 0 = 0 0.1 = 0 = 0 = 0 0.1 = 0 = 0 = Paratullbergia callipygos 0.1 = 0.2 = 0 = 0 0.1 = 0.2 = 0 = 0 0.1 = 0 = 0 = Pogonognathellus flavescens 1 = 1.6 = 0.2 = 0 1 = 1.2 > 0 = 0 1 = 0.4 = 0 =

Figure legends Fig 1 .

 legends1 Figure legends

Fig 2 .Fig 3 .

 23 Fig 2. Total numbers of collembolan individuals in compartments A in boxes A/Px (grey

Fig 4 .Fig 5 .

 45 Fig 4. Total numbers of Folsomia quadrioculata in microcosm compartments. Otherwise as

  Fig.1

Table 1 N° com bination Com partm ent 1 Com partm ent 2

 1 values of the topsoil from the acid beech forest and the three polluted plots. Data are means of five replicates (fifteen for average P1+P2+P3) follow ed by standard errors

	Combinations realised in microcosms (A = acid soil,
	Willerzie; P1, P2, P3 = polluted soil, Auby)
	1	P1	P1
	2	P2	P2
	3	P3	P3
	4	A	P1
	5	A	P2
	6	A	P3
	7	A	A

pH

  Concentration of different forms of zinc in the polluted soils (mg/kg). Data are means of five replicates follow ed by standard errors. Note that forms linked to organic matter and oxides, as w ell as exchangeable and w ater-soluble forms are included in EDTA-acetate zinc

		Table 3
			A in box		A in box		P1 in box	P1 in box	A in box		A in box	P2 in box		P2 in box	A in box	A in box		P3 in box	P3 in box
			A/A		A/P1		A/P1	P1/P1	A/A		A/P2		A/P2		P2/P2	A/A		A/P3		A/P3	P3/P3
		Arrhopalites spinosus	0.5	=	0.4	=	0		=	0	0.5	=	1.2	=	0	=	0	0.5	=	0.8	=	0	=
		Dicyrtomina minuta	0.2	=	0.4	=	0		=	0	0.2	=	0	=	0	=	0	0.2	=	0.2	=	0	=
		Folsomia quadrioculata Isotoma viridis	137.9 = 176.4 > 0.1	5.6	P1	>	0	P2 137.9 = 198.4 >	12.6	>	0	137.9 = P3	151	>	13.6	>
		EDTA-acetate extracted				6200 ± 1100 b		20000 ± 1200 a			19000 ± 1100 a
		linked to OM and oxides				5800 ± 980 b		16900 ± 940 a			17000 ± 1400 a
		exchangeable					350 ± 88 c				3000 ± 270 a			1500 ± 330 b
		w ater-soluble					32 ± 3.5 b				110 ± 15 a			92 ± 7.2 a
	1	Significant differences between sites (P <0.05) are indicated by different letters (A NOVA ).
	2

Acknowledgements

We thank the Centre National de la Recherche Scientifique (Programme Environnement, Vie et Sociétés) for financial support within a research project directed by Pr. D. Petit (Lille, France), and the Centre de Pédologie Biologique (Ing. G. Belgy, Vandoeuvrelès-Nancy, France) for zinc measurement in extracts.


C Kounda-Kiki
email: c.kounda-kiki@laposte.net

A Vaçulik

J F Ponge

C Sarthou

Soil arthropods in a developmental succession on the Nouragues inselberg (French Guiana)

Keywords:

Tropical inselbergs are isolated rock outcrops with a special type of vegetation surrounded by rain forest. They are exposed to harsh climate (alternation of heavy rain and severe drought) and provide few nutrients for plant growth. The aim of our study was to investigate a possible correlation between primary plant succession, size and diversity of soil arthropods. The study site was the Nouragues inselberg, in French Guiana (South America). Nine soil samples (three samples in each vegetation type) were taken for study of soil arthropod communities and their food habits in three habitat types: Pitcairnia geyskesii (Bromeliaceae), Clusia minor (Clusiaceae) and Myrcia saxatilis (Myrtaceae) represent three stages in a primary plant succession on this inselberg. Soil arthropods were classified into morphospecies under the dissecting microscope and their food habits were characterized by examining their gut contents under the light microscope. A variation in food habits was observed, cyanobacteria being found in arthropod guts only during Pitcairnia stage, being replaced by plant material at Myrcia stage. Carnivory was prominent in oribatid mites, contrary to temperate records. All our samples contained large numbers of microarthropods, principally mites and collembolans.

At Myrcia stage arthropod density was significantly higher than at the two other stages.

Macro-invertebrates are present only at late and intermediate successional stages. The number 2 of macro-predators increased by a factor of 10 in species richness and 100 in abundance along the succession. These results suggest that abundance and diversity of soil arthropod increased throughout plant succession and show the importance of organic matter as an explanation for the observed phenomenon.

Introduction

The term inselberg (island hill in German), was proposed by [START_REF] Bornhart | Trophic diversity, niche breadth, and generation times of exopterygote insects in a secondary succession[END_REF] to characterize isolated rock outcrops which rise abruptly from the surrounding landscape. They occur in many regions of the tropics and subtropics, as well as in temperate biomes. Inselbergs exhibit very particular soil and microclimate conditions, which only few organisms can tolerate.

French Guiana inselbergs consist of Precambrian granites [START_REF] Choubert | Les Précambriens des Guyanes[END_REF][START_REF] Hurault | Recherches sur les inselbergs granitiques nus en Guyane française[END_REF][START_REF] Hurault | L'érosion régressive dans les régions tropicales humides et la genèse des inselbergs granitiques[END_REF] and they support a discontinuous, xeric and low vegetation, commonly called rock savanna, which is well adapted to granite exposed directly to the sun. Cyanobacterial films cover nearly all surfaces not occupied by rock savanna. These prokaryotic organisms fix atmospheric nitrogen, and they are the first colonizers of the granite, accelerating mineral weathering and liberating nutrients for the initiation of primary plant succession [START_REF] Sarthou | Cyanophycées de l'inselberg des Nouragues (Guyane Française)[END_REF].

Studies of plant communities have indicated a gradual increase in species richness during early stages of primary succession [START_REF] Drury | Succession[END_REF][START_REF] Myster | Dynamics of associations between plants in ten old fields during 31 years of succession[END_REF] while this process is reversed at late, forested stages [START_REF] Christensen | Changes in structure, pattern, and diversity associated with climat forest maturation in piedmont, North Carolina[END_REF][START_REF] Howard | Temporal patterns of vascular plant diversity in southeastern New Hampshire forests[END_REF]. The few studies which have been carried out on arthropods in successions indicate a parallel increase [START_REF] Bornhart | Trophic diversity, niche breadth, and generation times of exopterygote insects in a secondary succession[END_REF][START_REF] Steffan-Dewenter | Early succession of butterfly and plant communities on set-aside fields[END_REF] or decrease [START_REF] Southwood | The relationships of plant and insect diversities in succession[END_REF][START_REF] Paquin | Comparison of macro-arthropod communities associated with three succession stages of the Canadian boreal forest[END_REF]) in species richness.

There have been many studies of the vegetation of the Nouragues inselberg, located in French Guiana (South America). [START_REF] Sarthou | Dynamique de la végétation pionnière sur un inselberg en Guyane Française[END_REF] described three stages of the plant succession on well-drained medium slopes, which were identified by their homogeneous physiognomy and structure: a herbaceous community dominated by the bromeliad Pitcairnia geyskesii and two shrub communities dominated by Clusiaceae (Clusia minor) and Myrtaceae (Myrcia saxatilis), respectively. They form, in this order, the three main stages of the development of the rock savanna ecosystem. However, until recently there have been no studies of the soil fauna and their microenvironment [START_REF] Vaçulik | Soil invertebrate activity in biological crust on tropical inselbergs[END_REF]. In particular we want to test the hypothesis of an increase in animal size and diversity of functional groups during the development of ecosystems which had been postulated by [START_REF] Odum | The Strategy of ecosystem development[END_REF] on theoretical bases and applied to soil fauna [START_REF] Rusek | Pedozootische Sukzessionen während der Entwicklung von Ökosystemen[END_REF][START_REF] Goralczyk | Nematodes in a coastal dune succession: Indicators of soil properties?[END_REF][START_REF] Goralczyk | Bodengenese als Standortfaktor für Mikrofauna-Ciliaten und Nematoden in Dünenböden[END_REF][START_REF] Verhoeven | Ciliates in coastal dune soils of different stages of development[END_REF]. To test this hypothesis, we studied the distribution of soil arthropods and their food habits at each successional stage.

Materials and methods

Study area

The field work was carried out at the Nouragues inselberg (411 m above sea level), which is located in the Nouragues natural reservation (4°5'N and 52°42'W). The inselberg is composed of a tabular outcrop of Caribbean granite, of pinkish monzonitic-type, containing 27% potassium-feldspar (orthoclase) and 37% plagioclase, along with 33% quartz as coarsegrained crystals and 2% accessory minerals (pyroxene, corundum, apatite) [START_REF] Grimaldi | Nouragues: dynamics and plant-animal interactions in a neotropical rainforest[END_REF]. The whole-rock chemical composition [START_REF] Sarthou | Dynamique de la végétation pionnière sur un inselberg en Guyane Française[END_REF] shows that the granite is highly siliceous (76.4% SiO 2 ) and rich in alkalis (4.6% K 2 O, 4.2% Na 2 O). The climate is tropical humid, and is characterized by a dry season (from July to November) and a wet season (from December to June) interrupted by a very short dry season in March. Mean annual precipitation is 3000-3250 mm. The daily temperature ranges between 18-55°C and the daily air humidity between 20-100% [START_REF] Sarthou | Dynamique de la végétation pionnière sur un inselberg en Guyane Française[END_REF]. The temperature of the bare rock surface may reach 75°C in the dry season. Most of the surface of the granitic outcrop is covered by cyanobacteria [START_REF] Sarthou | Cyanophycées de l'inselberg des Nouragues (Guyane Française)[END_REF]. The bromeliad Pitcairnia geyskesii, the prevailing plant of the inselberg, is an epilithic species (30-50 cm tall) which always forms dense carpets of varying size (decimeters to decameters in diameter), covering low to medium slopes as well as gullies and shallow depressions. The Clusia minor community is also widespread. It represents the shrub vegetation unit of the rock savanna, forming dense thickets, 2-8 m tall. This community occurs both in depressions and slopes, on sandy and organic soils, respectively. Myrtaceae (Myrcia saxatilis) occur as both shrubs and trees. There was no obvious sign of human activity on the study area.

Soil sampling procedure

Samples were collected in April 2002 during a rainy period at each successional stage by forcing an aluminum cylinder 15 cm diameter and 10 cm height into the topsoil until the granite was reached. In soil thicker than 10 cm only the top 10 cm were sampled. Three samples were taken in each vegetation type, but at different places within the inselberg, in order to ensure representativity. For Pitcairnia carpets, sampling was done in the organic matter accumulated under cyanobacterial crusts which border them upslope, given that soil and organic matter are absent at the inside of epilithic carpets, due to intense run-off.

For the Pitcairnia stage, Pit1 was taken in a very small (60 cm diameter), Pit2 in a medium (3-5 m) and Pit3 in a large (10-15 m) carpet of Pitcairnia geyskesii. For the Clusia stage, Clu1 was taken at the inside of a shrub thicket of Clusia minor surrounded by a ring of Pitcairnia geyskesii, Clu2 was taken in the downslope layering part of another shrub thicket, the upslope part of which was already colonized by Myrcia saxatilis, while Clu3 was taken in the upslope layering of another shrub thicket, not surrounded by Pitcairnia geyskesii. For the Myrcia stage, Myr1 was taken at the centre of an old tree thicket of Myrcia saxatilis, surrounded by a ring of Clusia minor with dead and starving remnants of Pitcairnia geyskesii, while Myr2 and Myr3 were taken at the inside of two younger thickets of Myrcia saxatilis, surrounded by Clusia minor. Dead stems of Clusia. minor were observed on the ground in Myr3.

Fauna extraction and analysis

Soil fauna samples were sealed in plastic bags and transported to the laboratory (Brunoy, France) within two days for extracting arthropods using Berlese-Tullgren funnels [START_REF] Edwards | A comparison of extraction methods for terrestrial arthropods[END_REF]. Animals were preserved into 95% ethanol. Care was taken that ethanol vapour did not enter extraction funnels, by placing the collecting tubes far under the funnel mouth. Arthropods were separated into morphospecies, on the basis of characters observable under a dissecting microscope. Biodiversity indices were calculated using morphospecies following [START_REF] Oliver | Designing a cost-effective invertebrate survey: a test of methods for rapid assessment of biodiversity[END_REF].

The size of adults of all morphospecies (overall length of body, appendices excluded) was measured to the nearest 0.04 mm, using an eye piece reticle. Morphospecies were grouped into three size classes: class 1 (<0.4 mm), class 2 (0.4-1 mm), and class 3 (>1 mm).

Soil invertebrates were mounted in chloral-lactophenol (25 ml lactic acid, 50g chloral hydrate, 25 ml phenol) for examination of their gut contents under phase contrast at x 400 magnification [START_REF] Ponge | Food resources and diets of soil animals in a small area of Scots pine litter[END_REF]. Gut contents were divided into nine categories: plant material, cyanobacteria, fungal material (spores and hyphae), humified organic matter, bacteria, mineral matter, pollen, animal matter and empty guts. The only visible structures were those which have not already been solubilized during digestion. Any dissolved or assimilated material could not be seen under the microscope.

Morphospecies were classified into four categories, according to the presence or absence of animal prey, plant and microbial material, and current knowledge on the taxa.

Micropredators included gamasid and trombidid mites, and a few oribatid mites with animal prey in their gut. Macropredators included carnivorous macro-arachnids (spiders, pseudoscorpions), centipedes and carnivorous beetles. Microsaprophytophages included most oribatid mites, uropodid mites, Ellipura (springtails and Protura), and insect larvae (no one was found with prey in the intestine). Macrosaprophytophages included millipedes, woodlice, adult flies, non-carnivorous beetles and other insects.

Data analysis

Abundance of animals and numbers of morphospecies were used for the calculation of total population size of arthropods and species richness, respectively. To compare the species richness of samples on the same volume or abundance basis, we calculated the theoretical number of morphospecies per unit volume (one cm thick humus) and per 200 individuals by a jackknife method [START_REF] Legendre | Numerical ecology[END_REF]. The jackknife was replicated ten times, allowing to compute an average value for each sample. We found that ten replicates were enough to give a confidence interval (standard error of the mean times Student's t) of less than 10% of the mean. This procedure allowed non-biased estimates of biodiversity when comparing samples of varying size, which was the case here, due to variable depth sampled.

Data were analyzed by correspondence analysis [START_REF] Greenacre | Theory and applications of correspondence analysis[END_REF]. Active variables (animals groups, percentages of food items, arthropod densities, number of morphospecies, raw or estimated per cm or per 200 individuals, body size) as rows and samples as columns were simultaneously projected on the first factorial axis, i.e. that explaining best the global variation.

All variables were transformed into X=(x-m)/s+20, where x is the original value, m is the mean of a given variable, and s is its standard deviation. The addition to each standardized variable of a constant factor of 20 allows all values to be positive, correspondence analysis dealing only with positive numbers. Following this transformation, factorial coordinates of variables can be interpreted directly in term of their contribution to the factorial axes: the farther a variable is projected from the origin of the axes (barycentre) along a given direction (along a factorial axis) the more it contributes to this axis. Variables were doubled in order to allow for the dual nature of most parameters (low values are as important as high values). To each variable X was thus associated a twin X' varying in an opposite sense (X'= 40-X). Such a doubling proved useful when dealing with ecological gradients [START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF] or when it was judged interesting to classify samples according to their bulk abundance, besides changes in species composition [START_REF] Loranger | Impact of earthworms on the diversity of microarthropods in a vertisol (Martinique)[END_REF].

The Mann-Whitney non-parametric procedure was used to establish significant differences between pairs of successional stages, using 5% as the significance level.

Results

The composition of soil fauna

The total number of arthropods counted was 5680. They were classified into 239 morphospecies which included 9 higher taxa: Acarida (mites), other Arachnida (spiders and pseudoscorpions), Myriapoda (millipedes and centipedes), Isopoda (woodlice), Ellipura (mostly Collembola but including also some Protura), Coleoptera (beetles), Diptera (flies), other Insecta, other Arthropoda (Table 1). Among arthropods, mites were the most diverse and abundant group, followed by springtails (Collembola).

Table 1 shows that the mean number of morphospecies per sample and the thickness of humus profiles increased (Mann-Whitney test, P<0.05) from Pitcairnia to Clusia and Myrcia stages. When the number of morphospecies was weighted by the sampling volume, the only increase was from Clusia to Myrcia stages (x 1.6). The number of arthropods at the Clusia stage was half that observed at the Myrcia stage, Pitcairnia being intermediate. When weighted by thickness, Pitcairnia was by far the most populated environment, while the ratio Clusia:Myrcia (1:2) did not change.

The size of soil arthropods was affected by the plant succession (Table 1). Although the mean size did not change, either calculated per morphospecies or per individual, the balance between the three size classes varied to a great extent. More small arthropods (<0.4 mm) and fewer big arthropods (>1 mm) were found in Pitcairnia, when compared with the two later stages. Both total abundance and diversity of Myriapoda and Arachnida other than Acarida (mostly spiders and pseudoscorpions) increased dramatically from Pitcairnia to Clusia and Myrcia. The species richness of mites (Acarida) and springtails (Ellipura) increased similarly, but this was not reflected in their population size (Table 1). The density of Ellipura was halved in Clusia compared with the two other stages, while the number of morphospecies was as higher in Clusia than in Myrcia and three-fold that in Pitcairnia. An increase in the abundance of beetles (Coleoptera) and termites and ants (other Insecta) was observed from Clusia to Myrcia.

Turnover of species

Morphospecies were classified according to their occurrence in one or several steps of the plant succession. The distribution of these classes according to the total number of morphospecies and of individuals is shown in Figure 1. A group of 20 species is common to the three stages studied, thus it is the basic arthropod community of the inselberg. It represents more than a third of the total richness at the pioneering Pitcairnia stage. However, it comprised a lesser part of the richness at the two other stages. These common species include mites (for more than half), springtails and insects (termites included).

Many more species were common to Clusia and Myrcia stages than to Pitcairnia and one or the other stages (Fig. 1). This holds both for morphospecies and individuals. This indicates that the transition from herbaceous to woody stages is accompanied by a high turnover of arthropod species. Fewer changes occurred during the transition from Clusia to Myrcia stages, despite the presence of species typical of each stage. These specialized species are numerous (almost half the total richness) but their population size is small (no more than one fifth of the total arthropod abundance).

Diets

Percentages of the food categories found in intestines varied across successional stages (Table 1). Overall microbial material (mostly fungal hyphae and bacteria) was heavily consumed, followed by plant material, humus, animal prey and at lowest frequency mineral matter (Table 1). The presence of animal prey in five oribatid mite morphospecies was unexpected, as was the absence of animal prey in all but one beetle morphospecies. Cyanobacteria, although abundant at the Pitcairnia stage, were consumed only by adult insects. Oribatid mites did not consume them, although they ingested higher plant and fungal material. Pollen and mineral matter were more frequently encountered at the Myrcia stage. More species consuming fungal hyphae were found at the Clusia stage than other stages.

There was a dramatic increase in the contribution of macropredators to the arthropod community, both in species and individuals, across the succession. The increase was by 10 in species richness, and by 100 in population size (Table 1). Conversely, the contribution of macrosaprophytophages decreased in richness, from a third to a quarter of the total number of species, but this decrease was not reflected in the population size. Thus the increase in biodiversity we observed along the successional trend (mostly from Pitcairnia to Clusia stages) was mainly due to macropredators.

Synthesis of results by correspondence analysis

Correspondence analysis, made on all population and gut content variables describing the arthropod community, revealed differences among stages along the successional gradient.

Coordinates along Axis 1 discriminated the three vegetation stages (Table 1). Only the first axis of correspondence analysis, explaining 34 % of the total variance, was interpreted, and the graphical presentation of the data (samples and variables) is given in Figures 2, 3, and4.

The projection of the nine samples reflected their position throughout the plant succession. A clear distinction can be made between Pitcairnia and the two other vegetation types. The Pitcairnia stage, on the negative side of Axis 1, is characterized by low thickness of organic matter, low diversity of morphospecies and dominance of small-sized organisms (Fig. 2).

Among the three samples taken along Pitcairnia carpets, Pit 3 had higher Axis 1 coordinates, indicating a higher developmental stage of the arthropod community. At the opposite side of Axis 1, Myrcia samples, and among them more particularly Myr 1, were characterized by a high thickness of organic matter, a higher diversity of arthropod species and the dominance of bigger forms. Clusia samples were projected on the positive side of Axis 1, but near the origin, thus indicating that the abovementioned features were not so much pronounced at this stage of the plant succession. The examination of the scaling of samples along Axis 1 revealed that Clusia and Myrcia samples differed less from each other than they differed from Pitcairnia samples. The total number of arthropods was roughly centered around the origin, thus indicating that it did not contribute to Axis1 to a great extent.

The projection of group taxa (Fig. 3) showed that all arthropod groups but Diptera (flies) exhibited a higher diversity (comprised a higher number of morphospecies) at late stages of the plant succession (positive side of Axis 1). This global trend was not reflected in the abundance of individuals (population size) per animal group.

The successional gradient depicted by Axis 1 concerned also alimentary categories which were well spread over this axis (Fig. 4). The Pitcairnia stage was associated with the abundance of cyanobacteria and fungal spores, while the Myrcia stage, and to a lesser extent the Clusia stage, was characterized by pollen grains, mineral matter and humus. Bacteria, when scored on the basis of individuals, characterized the Myrcia stage, while they did not characterize any stage when scored on the basis of morphospecies, which indicated that they were found in some species with a high population size under Myrcia. Fungal spores and fungal hyphae were not projected together, the second food category being near the origin on the positive side of Axis 1. This could indicate a decrease in the sporulation rate of soil fungi along the successional gradient. Empty guts were more often found at the Pitcairnia stage when scored on individuals. As above, this indicated that some abundant arthropod species with a fluid regime were found at this early stage, or that most individuals were in a resting stage.

Discussion

The cyanobacterial crusts associated with the pioneering Pitcairnia carpets had high arthropod populations but low diversity compared with woody stages of the plant succession (Clusia, Myrcia). At the intermediate Clusia stage, there were fewer individuals, but higher diversity when species richness was weighted by the population size. The last stage, Myrcia, was characterized by both high individual and species richness according to the importance of organic matter. This last observation is in accordance with results found by [START_REF] Goralczyk | Bodengenese als Standortfaktor für Mikrofauna-Ciliaten und Nematoden in Dünenböden[END_REF] and by [START_REF] Verhoeven | Ciliates in coastal dune soils of different stages of development[END_REF], where soil animals become more diverse with soil development.

The size of arthropods is lower in Pitcairnia than in the two other vegetation types, as this has been observed with plants along successional gradients. The size of species should vary according to the stability of habitats [START_REF] Peters | The ecological implications of body size[END_REF][START_REF] Ebenman | Size-structured populations[END_REF]. More macroarthropods, especially macropredators, were present at the two later stages, which were similar except for the abovementioned decrease in total arthropod abundance under Clusia. It might be argued that the absence of macropredators and the fixation of atmospheric nitrogen into organic matter may contribute to favour animal life in cyanobacterial crusts, in spite of restricted space. At the Myrcia stage, the development of a thick humus cover (30 cm to 1 m, [START_REF] Sarthou | Mécanismes de colonisation par la végétation d'un inselberg granitique en Guyane Française[END_REF]) may result in an increase in deep refuges for microarthropods, thus preventing predation from decreasing their population size.

Feeding habits were shown to vary along the successional gradient. Pollen, mineral and humified matter characterized late successional stages. This accompanied the development of vegetation (higher plants) and the associated building of thicker organic soil profiles. Although soils on tropical inselbergs are superficial and mostly organic [START_REF] Sarthou | Mécanismes de colonisation par la végétation d'un inselberg granitique en Guyane Française[END_REF], we observed the presence of earthworm casts in the organic matter accumulated under Myrcia, and the anchorage of the root system of trees at this stage, contrary to earlier developmental stages. This may account for the presence of mineral grains at the top of humus profiles. Cyanobacteria, which are the main component of humus profiles at the Pitcairnia stage [START_REF] Vaçulik | Soil invertebrate activity in biological crust on tropical inselbergs[END_REF], were not heavily consumed by most arthropods.

However, their presence in some animal guts at both Pitcairnia and Clusia stages indicated that, despite the weak occurrence of these autotrophic organisms under the shade of Clusia, they constituted a relic component of the ecosystem, inherited from earlier, better illuminated stages. Cyanobacteria were almost absent under Myrcia, at least according to our census of arthropod guts.

Leaf litter was present at Clusia and Myrcia stages, while there was practically no litter at the Pitcairnia stage, except a thin cyanobacterial crust [START_REF] Vaçulik | Soil invertebrate activity in biological crust on tropical inselbergs[END_REF]). Thus the development of vegetation was accompanied by an increase in organic matter production, enabling more organisms to live in the ecosystem, according to the scheme suggested by [START_REF] Odum | The Strategy of ecosystem development[END_REF], and as applied to soil development by [START_REF] Rusek | Pedozootische Sukzessionen während der Entwicklung von Ökosystemen[END_REF]. The observed increase in arthropod diversity per unit surface, which affected all taxa, may be explained by the increase in food availability and habitat size. Within this global trend, the particular increase in large arthropods can be explained by more pore space and refuges under thick litter [START_REF] Garay | Study of a mixed forest litter of hornbean (Carpinus betulus L.) and oak (Quercus sessiliflora Smith). III. Organization of the edaphic macro-arthropod community as a function of litter quantity[END_REF]. [START_REF] Hooper | Interactions between aboveground and belowground biodiversity in terrestrial ecosystems: patterns, mechanisms, and feedbacks[END_REF] demonstrated that the diversity of soil fauna increased together with aboveground production which is prone to occur along our successional gradient from prostrated (Pitcairnia) then to shrub (Clusia) and to thicket (Myrcia) vegetation.

In addition to these observations, other details revealed interesting patterns in soil animal communities of tropical inselbergs. Mite guts exhibited more fungal than plant material. Other researchers found similar results in temperate soils [START_REF] Daniel R P | Fungi as source of food for soil mites[END_REF][START_REF] Luxton | Food and energy processing by oribatid mites[END_REF]). However, we observed several oribatid mite species which presented an omnivore diet, including animal prey, as has been found in temperate peat moss bogs [START_REF] Behan-Pelletier | Feeding habits of sixteen species of Oribatei (Acari) from an acid peat bog, Glenamoy, Ireland[END_REF]. This might be explained by a partial predatory behaviour in an environment poor in nitrogen. The high percentage of empty guts at the Pitcairnia stage should be also noted. There are several possible explanations for this phenomenon. It could be due to nutritional shortage but also to the physiological stage of the animals [START_REF] Joosse | The role of food in the population dynamics of Orchesella cincta (Linné) (Collembola)[END_REF] or to sucking feeding habits.

We have shown that different successional stages have different characteristics with respect to density of soil arthropods, total number of species present, and mean number of species per sample, and that this could be explained by vegetation dynamics and accompanying changes in food and space availability. Another hypothesis, based upon [START_REF] Connell | Mechanism of succession in natural communities and their role in community stability and organisation[END_REF] "facilitation"model, is that soil arthropods themselves may alter the environment so as to cause changes in the composition of plant communities. Both hypotheses are not mutually exclusive, as has been demonstrated in temperate forest soils [START_REF] Bernier | Humus form dynamics during the sylvogenetic cycle in a mountain spruce forest[END_REF].

Figure legends Figure 1

 legends1 Figure legends

Figure 2

 2 Figure 2 Correspondence analysis. Projection of samples and a selection of active variables

Figure 3

 3 Figure 3 Correspondence analysis. Projection of samples and other active variables (higher

Figure 4

 4 Figure 4 Correspondence analysis. Projection of samples and other active variables (gut

  Fig. 1

  Fig.3

Table 1

 1 Mean values (with standard error) of the mains variables featuring soil arthropod communities in the three studied successional stages (3 replicates each). Results of calculations based both on morphospecies diversity and abundance. Significant differences between pairs of successional stages (Mann-Withney U test) are indicated by different letters, combining results from three tests

		Pitcairnia	Clusia	Myrcia
	Thickness of litter/humus sampled (cm)	3±0.6b	9.3±0.7a	10±0a
	Number of morphospecies per sample	28.7±8.2b	67.7±4.1a	89±12.9a
	Number of morphospecies per cm	22.3±6.4b	22±1.7b	35.7±6.6a
	Number of morphospecies per 200 individuals 21±4b	53.1±1.3a	56.6±6.2a
	Number of arthropods per m 2	42232±23235ab 20955±3039b 43952±10661a
	Number of arthropods per m 2 /humus depth (cm)12885±4940a	2224±181b	4414±1064a
	Various diversity measures
	Mean size of adults (mm)	1±0.1	1.1±0.1	1.1±0.04
	% class 1 (<0.4 mm)	40.4±5	38.4±1.2	34.2±2.3
	% class 2 ( 0.4-1 mm)	34.7±8.9	37.7±2.5	39±3
	% class 3 (>1 mm)	24.2±5.5	24.8±3	26.4±2
	Total Acarida	14.3±3.5b	33.3±2.7a	46.3±9.3a
	Total other Arachnida	0.3±0.3b	5.3±2a	6.7±1.3a
	Total Myriapoda	0b	3.3±0.3a	4±1a
	Total Isopoda	0	0	0.7±0.7
	Total other Arthropoda	0	0.3±0.3	1±1
	Total Ellipura	4±1.1b	13±0.6a	15.7±2.7a
	Total Coleoptera	2.7±2.2	4±1.5	6.3±0.9
	Total Diptera	3.7±0.3	3.3±0.3	2.3±0.7
	Total other Insecta	3.7±1.2	5±1.1	6±2
	% micropredators	30±4.1	32±2	31±3
	% macropredators	0.7±0.7b	8.6±2.9a	9.2±0.8a
	% microsaprophytophages	35.6±2.2	38±2.2	40.3±4.3
	% macrosaprophytophages	33.8±2.7a	22.2±3.7b	20±3.8b
	% plant material	13.5±0.2	14.6±2.6	12.1±1.2
	% cyanobacteria	0.3±0.1a	0.2±0.1a	0.02±0.02b
	% fungal material (spores)	1.7±1	0.3±0.1	0.6±0.2
	% fungal material (hyphae)	11.3±1b	15.5±0.7a	15.5±2.4ab
	% humified organic matter	7.2±1.1	9±0.6	8.4±0.4
	% bacteria	5.2±1.1	6.4±0.5	4.4±0.5
	% mineral matter	0.08±0.01b	0.2±0.03a	0.2±0.04a
	% pollen	0b	0.03±0.03ab	0.06±0.01a
	% animal matter	1.4±0.8	2±0.7	2.2±0.6
	% empty guts	59.2±2.3a	51.8±1.4b	56.3±2.7ab
	Various abundance measures
	Mean size of adults (mm)	0.7±0.1	0.8±0.06	0.79±0.04
	% class 1 (<0.4 mm)	65.2±3.4a	49.6±7.7b	44.1±3.1b
	% class 2 ( 0.4-1 mm)	29.6±3.9	38.5±6.5	39.3±3.3
	% class 3 (>1 mm)	5.2b	11.9±1.2a	16.6±2.7a
	Total Acarida	473.7±306.7	284.3±49	516.7±165
	Total other Arachnida	0.7±0.7b	21±7.5a	22.3±9.6a
	Total Myriapoda	0b	10±5.1a	23±9a
	Total Isopoda	0	0	10±10
	Total other Arthropoda	0	1.7±1.7	2±2
	Total Ellipura	158±13.5a	75.3±2.6b	173±25a
	Total Coleoptera	25.7±25.2ab	4.7±1.9b	38.3±11.4a
	Total Diptera	16.7±8.6	5.7±0.7	3.7±1.8
	Total other Insecta	90±83.5ab	14±5.5b	59.7±19.2a
	% micropredators	32.4±10	18.6±4.1	16.3±4.8
	% macropredators	0.04±0.04b	5.6±1.5a	3±0.9a
	% microsaprophphytophages	56.4±12.8	66.4±3.1	63.6±2.6
	% macrosaprophytophages	11.2±6.3	9.5±1.8	17±4.5
	% plant material	9.2±1.7	15.9±4.8	10.7±0.9
	% cyanobacteria	0.1±0.05ab	0.05±0.01a	0.002±0.002b
	% fungal material (spores)	1.6±1.4	0.3±0.2	0.4±0.2
	% fungal material (hyphae)	15.7±3.4	22.1±7	22.7±1.9
	% humified organic matter	5±3.7	16.1±7	15±1.7
	% bacteria	2.7±1.4b	7.7±1.6ab	7±0.4a
	% mineral matter	0.03±0.01b	0.1±0.01ab	0.2±0.03a
	% pollen	0b	0.008±0.008a 0.02±0.004a
	% animal matter	5.5±5.4a	2±1.6a	1.5±0.4b
	% empty guts	60.3±7.4a	35.6±1.1c	42.5±0.5b
	Axis 1 coordinates (correspondence analysis) -0.035±0.06c	0.008±0.003b 0.03±0.008a

Acknowledgements

We wish to thank the Centre National de la Recherche Scientifique for financial support and commodities, in particular P. Charles-Dominique and his staff at the Nouragues Field Station.


Sébastien Garnier

Jean-François Ponge

Acid-tolerant Collembola cannot colonize metal-polluted soils at neutral pH

Keywords: Heavy metals, Collembola, Acid-tolerance, Inoculation

A microcosm experiment was performed to test the hypothesis that Collembola living in an acid soil (pH 4) were able to colonize a heavy metal-polluted soil of pH 7.

After 6-month incubation, the added fauna were not recovered, except for a few individuals, while the original fauna were still as abundant as at the beginning of the experiment. It was concluded that, despite similarities between the chemical environment of acid soils and that of metal-polluted soils, differences in biotic and abiotic factors prevent acid-tolerant populations to survive and reproduce in a polluted site.

Introduction

The use of acid-tolerant organisms for the bioremediation of polluted soils has been proposed, on the basis of affinities between the chemical environment of acid and polluted soils, in particular those polluted with heavy metals [START_REF] Chauvat | Colonization of heavy metal-polluted soils by collembola: preliminary experiments in compartmented boxes[END_REF]Gillet and Ponge, submitted). The lack of metal specificity in detoxication mechanisms of soil invertebrates [START_REF] Vandenbulcke | Implication of the midgut of the centipede Lithobius forficatus in the heavy metal detoxification process[END_REF][START_REF] Köhler | Localization of metals in cells of saprophagous soil arthropods (Isopoda, Diplopoda, Collembola)[END_REF], and the presence of free (ionic) forms of aluminum, iron, manganese and other metals at low pH [START_REF] Tan | Principles of Soil Chemistry[END_REF], suggested that tolerance to acidity could allow tolerance to a broad range of heavy metals, too. However, neutral pH, excess of calcium and dramatic changes in food resources and habitat could affect survival and reproductive rate upon inoculation to metal-polluted sites [START_REF] Kreutzer | Effects of liming on soil processes[END_REF]Gillet andPonge, 2002, 2003). The objective of this study was to initiate a self-reinforcing process by adding more, better adapted fauna, in order to i) stimulate humification [START_REF] Bernier | Earthworm feeding activity and development of the humus profile[END_REF][START_REF] Ponge | Horizons and humus forms in beech forests of the Belgian Ardennes[END_REF][START_REF] Davidson | Impacts of fauna in an upland grassland soil as determined by micromorphological analysis[END_REF], ii) increase the number of micro-sites able to fix heavy metals [START_REF] Senesi | Copper (II) and iron (III) complexation by humic acid-like polymers (melanins) from soil fungi[END_REF][START_REF] Dupuy | Infrared and chemometrics study of the interaction between heavy metals and organic matter in soils[END_REF], thus preventing metals from circulating through the ecosystem.

Several experiments were performed to check whether acid-tolerant Collembola were able to colonize heavy-metal polluted soils. Short-term microcosm experiments using compartmented boxes showed that some species dispersed from an acid source (a dysmoder humus of pH 4) to a neutral soil heavily polluted with metals such as lead, zinc, or cadmium [START_REF] Chauvat | Colonization of heavy metal-polluted soils by collembola: preliminary experiments in compartmented boxes[END_REF]Gillet and Ponge, submitted). However, these dispersal experiments were of a too short duration for allowing reproduction and survival success to be assessed. A field site polluted with zinc, cadmium and lead was inoculated with microarthropods but resampling of the site after 7 and 14 months failed to reveal any significant dispersal from the inoculum, which kept its original population even after 14 months (Gillet, unpublished data). We suspected that the animals did not disperse to a great extent because they were not forced to leave their original habitat under environmental pressure [START_REF] Tranvik | On the advantage of Folsomia fimetarioides over Isotomiella minor (Collembola) in a metal polluted soil[END_REF]. As a consequence, it was decided to perfom laboratory experiments, into which a known amount of acidtolerant fauna could be introduced and monitored over six months, thus allowing the assessment of survival and reproduction of the introduced population. This time lapse was within a range between one generation time (1 to 2 months) and maximum longevity (1 to 2 years) for most Collembola [START_REF] Siepel | Life-history tactics in soil microarthropods[END_REF].

Materials and methods

The acid soil used for inoculating acid-tolerant fauna was a dysmoder humus [START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF] of pH H 2 0 4.4 and pH KCl 3.3, originating from a beech forest at Willerzie, Belgium [START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF]Gillet and Ponge, submitted). The metalpolluted soil was obtained from a field downwind a zinc smelter at Auby, France. It corresponded to the most polluted plot P1, as described in Gillet andPonge (2002, 2003). The top 10 cm of the soil, which was a mor humus [START_REF] Ponge | Comment distinguer dysmoder et mor? L'exemple de la forêt domaniale de Perche-Trappe (Orne)[END_REF] of pH H 2 0 6.9 and pH KCl 6.5, contained 35,000 mg/kg of Zn [START_REF] Gillet | Changes in species assemblages and diets of Collembola along a gradient of metal pollution[END_REF], half of which was in a bioavailable form (Gillet and Ponge, submitted). The two soils were collected on the same day (2002.09.22). They included only ectorganic horizons, which were homogenized by hand in a large plastic sheet after discarding fresh litter and ground vegetation. After homogenization, samples were directly put in microcosms before transport to the laboratory. Experimental microcosms were 11 x 8 x 4 cm (L x l x h) polystyrene boxes with lids, which were filled with soil, leaving enough overhead space for free movement of surface-active invertebrates. Lids were pierced at their centre with a 2 cm diameter hole, covered with nylon gauze, to allow gas exchange with the surrounding atmosphere. Ten replicates with the polluted soil (300 cm 3 ) were kept closed without any further treatment. Ten other replicates were placed under Berlese funnels used for the extraction of microarthropods [START_REF] Edwards | A comparison of extraction methods for terrestrial arthropods[END_REF]) and were inoculated with fauna from 300 cm 3 dysmoder humus. In parallel, microarthropods from 300 cm3 of both substrates were extracted using the same method. During the extraction, which took 10 days, uninoculated boxes were also incubated in the extractor. Once the inoculation was completed, all microcosms (10 inoculated, 10 uninoculated) were incubated in an air-forced chamber at constant temperature (15°C), in darkness.

Incubation lasted for 6 months, after which time microarthropods were extracted in the same way.

The microcosms were kept at field moisture, their weight being kept constant by adding deionized water each fortnight.

Extracted animals were collected and preserved in 95% ethyl alcohol. They were sorted under a dissecting microscope and mounted in chloral-lactophenol for examination in phase contrast microscopy. Several keys were used for the identification of Collembolan species. In particular the (still experimental) key by Hopkin (in prep.) was judged very handy. It was completed by more detailed monographs by [START_REF] Gisin | Collembolenfauna Europas[END_REF]), Zimdars & Dunger (1994), [START_REF] Jordana | Fauna Iberica. VIII. Collembola Poduromorpha[END_REF], [START_REF] Fjellberg | The Collembola of Fennoscandia and Denmark. I. Poduromorpha[END_REF][START_REF] Bretfeld | Synopses on palaearctic Collembola[END_REF] and [START_REF] Potapov | Synopses on Palaearctic Collembola[END_REF].

A special attention was apid to the tullbergiid species Mesaphorura macrochaeta Rusek. The sex ratio of adult specimens was determined after pooling all animals from the same treatment (Dysmoder, Original Mor, Incubated Mor, Inoculatedincubated Mor). We have noted before that this parthenogenetic species shifted to sexual reproduction under environmental stress, including pollution by heavy metals [START_REF] Niklasson | Environmental stress and reproductive mode in Mesaphorura macrochaeta (Tullbergiinae, Collembola)[END_REF][START_REF] Gillet | Changes in species assemblages and diets of Collembola along a gradient of metal pollution[END_REF]. The gut contents of all specimens of M. macrochaeta were examined under phase contrast and classified into 6 categories (Empty guts, Holorganic humus, Hemorganic humus, Fungi, Bacteria, Exuviae), since it has been shown that the same species changed its food diet when submitted to a higher dose of heavy metals [START_REF] Gillet | Changes in species assemblages and diets of Collembola along a gradient of metal pollution[END_REF]. Gut contents were characterized for each individual. Most intestines were void or displayed only one food category. In cases where several categories were present in the same gut, each category was scored to the nearest 0.1, the sum of the scores being fixed to 1 for each individual.

Densities of the different species, as well as total abundance of Collembola and number of species per sample were compared between treatments using Mann-Whitney procedure at .05 significance theshold [START_REF] Glantz | Primer of Biostatistics[END_REF].

Results

The species composition of the dysmoder humus largely differed from that of the polluted soil (Table 1, Fig. 1). The dysmoder humus exhibited more than ten times the total abundance of the mor (polluted) humus, and more than three times its number of species. Some species were present in the acid soil but were totally lacking in the neutral (polluted) soil, the most abundant ones being Protaphorura eichhorni (Gisin), Folsomia quadrioculata (Tullberg), Friesea truncata Cassagnau, Isotomiella minor (Schäffer), Mesaphorura tenuisensillata Rusek and Ceratophysella denticulata (Bagnall). They were classified as dysmoder species. Some others were present in the neutral soil, but were totally lacking in the acid soil, the most abundant ones being Protaphorura armata (Tullberg) and Lepidocyrtus cyaneus Tullberg. They were classified as mor species. Finally, some species were present in both substrates, such as M. macrochaeta, Parisotoma notabilis (Schäffer) and Sphaeridia pumilis (Krausbauer). They were classified as common species.

No males were recorded in M. macrochaeta from the acid soil (337 ind.), while a quarter of the adults were males in the population from the neutral soil (189 ind.). Prominent differences were observed between dysmoder and mor in the distribution of gut contents (Fig. 2). Holorganic humus was dominant in M. macrochaeta from the beech forest, while empty guts, followed by hemorganic humus, were dominant in the polluted soil before incubation. Fungi were also notable in acid soil guts. On the contrary fungi, together with bacteria and exuviae, were scarcely observed in the neutral, polluted soil.

After incubation, no decrease was observed in the number of individuals living in the polluted soil (Table 1), but the number of species decreased to a large extent (Table 1, Fig. 1). The collapse in species richness was due to the disappearance of epigeic species such as Isotoma viridis Bourlet, L. cyaneus, Sminthurinus elegans (Fitch), S. pumilis and Willowsia nigromaculata (Lubbock). However, two hemiedaphic species were also observed to disappear at the end of the incubation period, namely P. notabilis and Micranurida pygmaea Boerner. The dominant species M. macrochaeta and P. armata, which comprised 80% of the total population, were not affected at all by incubation conditions (Table 1).

Food habits of M. macrochaeta were not affected to a great extent by incubation, apart from a small increase in the proportion of empty guts (Fig. 2). After 6 months, hemorganic humus was still dominant in the food bolus, although mineral matter was practically absent in the holorganic humus used for the experiment, as ascertained by the composition of humus profiles [START_REF] Gillet | Humus forms and metal pollution in soil[END_REF]. The sex ratio of M. macrochaeta changed little during the experiment, the proportion of males passing from a quarter to a third of the adult population (Table 1).

Inoculation with dysmoder fauna doubled the number of species of mor humus when compared with the incubated, uninoculated substrate (Table 1, Fig. 1), but it did not increase the number of individuals (Table 1). Thus, most introduced animals were not recovered at the end of the experiment. Only four dysmoder species were found after incubation, namely F. truncata, P. eichhorni, Pseudosinella alba (Packard) and Pseudosinella mauli Stomp, but only in very low densities (less than one specimen per microcosm in average). The abundance of M. macrochaeta, which was dominant in mor humus but was also present in dysmoder, was not affected by the inoculation procedure (Table 1), thus it can be concluded at first sight that most introduced specimens of M. macrochaeta died during incubation. On the contrary, the other dominant species in the polluted soil, P. armata, decreased in abundance when acidtolerant fauna was inoculated, while it did not in the absence of inoculation (Table 1).

The examination of gut contents of M. macrochaeta at the end of the incubation period (Fig. 2) showed that inoculation of dysmoder fauna caused a small increase in empty guts, a strong decrease in the fraction of hemorganic humus and a strong increase in holorganic humus and fungi. The sex ratio of M. macrochaeta was not affected by inoculation (Table 1), suggesting that the population recovered at the end of the experiment was the original population of the mor humus.

Discussion

From our experimental results, it can be concluded that most inoculated specimens died within 6 months, despite an increase in species richness due to a trace population and thus that most acid-tolerant microarthropods were unable to colonize heavily polluted soil used for the experiment. Interestingly, introduced species like M. macrochaeta, which were abundant in the mor (polluted, neutral) humus, but were also abundant in the dysmoder (unpolluted, acidic) humus, died too. Only the original population of the polluted site can be considered tolerant to the environmental conditions (chemical, biotic) prevailing in mor humus. We ruled out the possibility of a mixing of M. macrochaeta specimens from the two sites in inoculated microcosms at the end of the experiment. Even though the examination of gut contents (more fungi, more holorganic humus, compared to uninoculated microcosms) could lead us to this conclusion, the absence of any change in the sex ratio clearly indicated that no parthenogenetic females from the dysmoder were added to the original population of the mor humus. The constant density of M. macrochaeta following inoculation indicated that no juveniles were added, too. The observed changes in gut contents could be caused by i) the fall of some debris which accompanied the escape by animals of the drying substrate in Berlese funnels (although most of them were retrieved and sorted out by hand after inoculation), ii) the use of cadavers of introduced specimens by the original fauna of mor humus. The second cause is hardly probable, since arthropod exoskeletons were absent from M. macrochaeta intestines in inoculated microcosms, although some were observed in the absence of inoculation (Fig. 2).

The duration of the experiment was enough to allow reproduction to occur, given the generation time (from egg to egg) of most Collembola [START_REF] Siepel | Life-history tactics in soil microarthropods[END_REF]. In the presence of predators, the absence of any significant change in the collembolan population (apart from the disappearance of rare species, which is expected in microcosms) was indicative of an equilibrium condition. The disappearance of most introduced specimens could be due to saturation of the original population [START_REF] Longstaff | The dynamics of collembolan populations: competitive relationships in an experimental system[END_REF][START_REF] Bernier | Lumbricus terrestris L. distribution within an experimental humus mosaic in a mountain spruce forest[END_REF][START_REF] Winkler | Local and regional species richness in communities of surface-dwelling grassland Collembola: indication of species saturation[END_REF] or to unfavourable food and environmental conditions. The first cause can be ruled out, given the abundance of food (decaying roots, fungi, humus) present in the sampled holorganic profiles, as ascertained from direct observation and from quantitative analyses [START_REF] Gillet | Humus forms and metal pollution in soil[END_REF]. However, poor food quality could be suspected, since the food habits of introduced specimens of M. macrochaeta clearly differed from that of conspecific specimens living in the polluted soil. It is remarkable that mineral matter, which was nearly absent in the sampled profiles [START_REF] Gillet | Humus forms and metal pollution in soil[END_REF], was retrieved in high amount in the gut contents of M. macrochaeta, even after 6 months.

This species was thus able to search for fine mineral particles (supposed to have detoxifying properties) in an environment in which these were nearly absent. It can be hypothesized that the population from the beech forest, where holorganic humus was the main food source but could be consumed without any danger, was unable to mix it with mineral matter in the polluted mor humus. The litter of the polluted mor was partly made of plant debris from hyper-accumulating plants such as Arabidopsis halleri (L.)

Figure captions Fig. 1 .

 captions1 Figure captions

Fig. 2 .Table 1

 21 Fig. 2. Distribution of gut contents of Mesaphorura macrochaeta according to the four

Acknowledgements

We thank the Centre National de la Recherche Scientifique (Programme Environnement, Vie et Sociétés) for financial support within a research project directed by Pr. Daniel Petit (Lille).

O'Kane & Al-Shehbaz, which accumulates zinc in its above-ground parts [START_REF] Sarret | Forms of zinc accumulated in the hyperaccumulator Arabidopsis halleri[END_REF]. The inadaptation of added specimens of M. macrochaeta to toxic food or soil solution is a possible cause for the observed phenomenon. Ecotoxicological tests on the isotomid springtail Folsomia candida have shown that reproduction and growth of Collembola were affected by heavy metals, in particular Cd and Zn, at doses far under those found in our polluted soil [START_REF] Sandifer | Effects of pH on the toxicity of cadmium, copper,lead and zinc to Folsomia candida Willem, 1902 (Collembola) in a standard laboratory test system[END_REF][START_REF] Smit | Development of zinc bioavailability and toxicity for the springtail Folsomia candida in an experimentally contaminated field plot[END_REF][START_REF] Crouau | The use of Folsomia candida (Collembola, Isotomidae) for the bioassay of xenobiotic substances and soil pollutants[END_REF], thus toxic effects were not unexpected. However, it has been demonstrated that tolerance to heavy metals varies between populations of the same species and that contamination by trace elements may select better-adapted genotypes [START_REF] Posthuma | Genetic differentiation between populations of Orchesella cincta (Collembola) from heavy metal contaminated sites[END_REF][START_REF] Chenon | Genetic polymorphism in nine clones of a parthenogenetic collembolan used in ecotoxicological testing[END_REF]. In the case of the two populations we compared, that of Auby, living in a soil with 35,000 mg/kg Zn, 190 mg/kg Cd and 6000 mg/kg Pb [START_REF] Gillet | Changes in species assemblages and diets of Collembola along a gradient of metal pollution[END_REF] can be considered strongly adapted to heavy metals, either by a better selection of its food [START_REF] Fountain | Continuous monitoring of Folsomia candida (Insecta: Collembola) in a metal exposure test[END_REF][START_REF] Tranvik | On the advantage of Folsomia fimetarioides over Isotomiella minor (Collembola) in a metal polluted soil[END_REF]present results) or by physiogical adaptation such as increased metal excretion rate or shortening of the juvenile period [START_REF] Posthuma | Adaptation to soil pollution by cadmium excretion in natural populations of Orchesella cincta (L.) (Collembola)[END_REF][START_REF] Posthuma | Life-history patterns in metal-adapted Collembola[END_REF] or by shifting from parthenogenesis to sexual reproduction [START_REF] Niklasson | Environmental stress and reproductive mode in Mesaphorura macrochaeta (Tullbergiinae, Collembola)[END_REF][START_REF] Gillet | Changes in species assemblages and diets of Collembola along a gradient of metal pollution[END_REF]present results). At last, a shock effect caused by a sharp increase in pH when animals living at pH 4 were introduced into a soil of pH 7 can be another possible cause for the failure of colonization [START_REF] Crouau | The use of Folsomia candida (Collembola, Isotomidae) for the bioassay of xenobiotic substances and soil pollutants[END_REF], even for species know to live over a wide range of pH conditions such as M. macrochaeta [START_REF] Ponge | Biocenoses of Collembola in atlantic temperate grass-woodland ecosystems[END_REF].