J F Ponge

Food resources and diets of soil animals in a small area of Scots pine litter

The fauna inhabiting a small area (ca. 5 cm  5 cm) were investigated in a Scots pine stand. After microstratification of the litter layers in the field and fixation in 95% ethyl alcohol, invertebrates, mainly mesofauna, were sorted under a dissecting microscope and mounted or dissected in order to study their intestinal guts. Faeces were mounted or sectioned to obtain information about the activity of other invertebrate groups not represented in the sample and to follow the fate of plant and microbial material after defaecation occurred.

Plant material, mainly from moss, bracken, pine needles and bark, was extensively consumed by enchytraeid and lumbricid worms, sciarid larvae and phthiracarid mites. Fungal material was ingested by all groups, either in combination with plant material or alone (camisiid and oppiid mites, some species of Collembola, sciarid and chironomid larvae). Isotomid springtails and chironomid larvae appeared to consume faecal material. The choice and the degree of comminution and digestion of the material differed greatly from one group to another, but without any indication of resource sharing.

INTRODUCTION

The role of fauna in forest soils has been the subject of many investigations. Despite their low contribution to total soil metabolism [START_REF] Macfadyen | The contribution of the microfauna to total soil metabolism[END_REF], invertebrates are known to influence microbial populations, and hence indirectly affect total metabolism, by regulating fungal growth [START_REF] Warnock | The influence of a springtail, Folsomia candida (Insecta, Collembola), on the mycorrhizal association of leek, Allium parum, and the vesicular-arbuscular mycorrhizal endophyte, Glomus fasciculatus[END_REF][START_REF] Ulber | Einfluss von Onychiurus fimatus Gisin (Collembola, Onychiuridae) und Folsomia fimetaria (L.) (Collembola, Isotomidae) auf Pythium ultimum Trow., einen Erregerdes Wurzelbrandes der Zuckerrübe[END_REF][START_REF] Gochenaur | Evidence suggests that grazing regulates ascospore density in soil[END_REF], disseminating fungal and bacterial propagules into new substrates [START_REF] Visser | Effects of collembolan grazing on nutrient release and respiration of a leaf litter inhabiting fungus[END_REF] or reactivating senescent microbial colonies [START_REF] Hanlon | Influence of grazing by Collembola on the activity of senescent fungal colonies grown on media of different nutrient concentration[END_REF]. The net effect of these activities appears to depend on the density of animals and conditions for the development of microflora [START_REF] Wolters | Effects of Mesenchytraeus glandulosus (Oligochaeta, Enchytraeidae) on decomposition processes[END_REF]. The importance of soil fauna in the development of soil structure can be seen as channels and faecal deposits throughout the humus profile [START_REF] Kubiëna | L'investigation microscopique de l'humus (Die mikroskopische Humusuntersuchung)[END_REF][START_REF] Kubiëna | Animal activity in soil as a decisive factor in establishment of humus forms[END_REF][START_REF] Zachariae | Spuren tierischer Tätigkeit im Boden des Buchenwaldes[END_REF][START_REF] Bal | Morphological investigation in two moder-humus profiles and the role of the soil fauna in their genesis[END_REF][START_REF] Babel | Micromorphology of soil organic matter[END_REF][START_REF] Bal | Zoological Ripening ofSoils[END_REF][START_REF] Kretzschmar | Caractérisation microscopique de l'activité des lombriciens endogés[END_REF].

Microscopic investigations in a small volume of soil [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume, premiers résultats. I. La couche L 1 d'un moder sous pin sylvestre[END_REF](Ponge, , 1985a(Ponge, ,b, 1988(Ponge, , 1990, in prep.) , in prep.) helped us to understand some functional relationships between soil animals, soil microflora and the living and dead plant material in the top centimeters of a moder humus. Results for fauna are presented here, and the ecology and ecological effects of soil and litter invertebrates are discussed.

MATERIAL AND METHODS

A unique sample was taken in August 1981 from a 35 yr old Scots pine (Pinus sylvestris L.) stand in the Orleans Forest (Loiret, France), which had not been thinned until the time of sampling. The ground flora mainly consisted of the moss Pseudoscleropodium purum (L.) and bracken (Pteridium aquilinum (L.). The humus was of the moder type [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume, premiers résultats. I. La couche L 1 d'un moder sous pin sylvestre[END_REF]. Microstratification of the surface horizons was made in the field on an area of ca. 5 cm  5 cm. Only the first three sub-layers were intensively studied, L 1 (entire brown needles, living mosses), L 2 (entire black needles, dead mosses) and F 1 (fragmented needles, roots, fungi and animal faeces).

These layers corresponded to the L n , L v and F r sub-Iayers (sensu [START_REF] Babel | Gliederung und Beschreibung des Humusprofils in mitteleuropäschen Wäldern[END_REF]. After dissecting the plant material out of the woodland floor it was immediately fixed in 95% ethyl alcohol. In the laboratory, plant fragments, animals and faeces were sorted under a dissecting microscope and appropriate techniques were used for their study [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume, premiers résultats. I. La couche L 1 d'un moder sous pin sylvestre[END_REF]. Most animals were mounted intact under a cover slide into chlorallactophenol (25 cm 3 lactic acid + 50 g chloral hydrate + 25 cm 3 phenol). Oribatid mites, which had a thicker tegument, were dissected and the cuticles discarded, but the small Oppia species were mounted whole. The volume of each individual was estimated by means of three measurements [length, width and thickness, see [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume, premiers résultats. I. La couche L 1 d'un moder sous pin sylvestre[END_REF] for further details].

All animal groups (mainly mesofauna and macrofauna) present at the time of sampling were collected.

Microfauna (protozoans, nematodes, rotifers) were poorly recovered, as a consequence of their small size and transparency. Animals that were living inside plant material (phthiracarid larvae, nematodes, amoebae) were also underinvestigated.

Unless otherwise stated, phase contrast microscopy was used to study gut contents. The presence of intact cytoplasm in the ingested cells was detected through its opacity [START_REF] Frankland | Importance of phase-contrast microscopy for estimation of total fungal biomass by the agar-film technique[END_REF].

VERTICAL DISTRIBUTION OF INVERTEBRATES

Figure 1 indicates the density and body volume of the five main mesofaunal groups recorded in the three litter layers. The sampled surface was approximately 0.25 dm 2 . The volume estimate which was used for the animals was V I [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume, premiers résultats. I. La couche L 1 d'un moder sous pin sylvestre[END_REF], i.e., the upper estimate (animals were compared to a parallelepipedic volume having the same dimensions). The true volume falls within a range from 0.25 V I to V I .

A marked increase in total density and bio-volume of fauna from the L to the F layers was observed.

This was mainly due to enchytraeid worms, mites and springtails (Collembola). Unlike enchytreids whose numbers regularly increased from L 1 , to F 1 , oribatid mites decreased from L 1 to L 2 then increased to F 1 , Collembola were numerous only in the F 1 layer. Within each group, the species composition of the layers differed except for enchytraeids represented by the single species Cognettia sphagnetorum (Vejd.). Oribatids were dominated by camisiid species in the L layers and by phthiracarid species in the F 1 layer [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume, premiers résultats. I. La couche L 1 d'un moder sous pin sylvestre[END_REF](Ponge, , 1985a[START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume. III. La couche F 1 d'un moder sous Pinus sylvestris[END_REF]. Springtails were dominated by isotomid species in all layers. Diptera larvae were dominated by a cecidomyid species in the L 1 layer, and by a sciarid species in the two other layers.

The groups which played a prominent role in the decomposition process of plant material were enchytraeids, phthiracarid mites, sciarid larvae and epigeous earthworms (Ponge, in prep.). The three former groups actively tunnelled through pine needles and pieces of bark (Figs. 2 and3), and the earthworms crushed the needles after ingestion of entire fragments.

FOOD RESOURCES AND THEIR FATE IN ANIMAL GUTS AND FAECES

Table 1 summarizes ingestion of the different food resources by the observed animal groups, and their fate in the guts or faeces. Digestion or transformation refers to cell walls, since cytoplasm was always digested.

Observations of intact plant and fungal material were made to facilitate the identification of material in guts and faeces. The treatise on plant anatomy by [START_REF] Esau | Plant Anatomy[END_REF] was used as a reference for nomenclature of higher plant tissues.

Plant tissues from pine needles were identified in the guts through the presence of lignified tissues such as tracheids from protoxylem (Fig. 4) and metaxylem. The transfusion tissue of pine was characterized by an accumulation of bordered pits (Fig. 5). Other hard structures, such as guard cells of stomata and epidermis, were used to identify material derived from pine needles. Pine needles were ingested by all groups except springtails and oribatid mites other than phthiracarids. Digestion occurred only in the intestine of sciarid larvae. In the postcolon of phthiracarid mites the plant cell walls were observed becoming brownish and their structure amorphous, especially at the center of the food pellets. The feeding activities of other animal groups was followed through their faecal pellets (Figs. 678).

The moss Pseudoscleropodium purum was consumed by animals both in the living state (L 1 layer) and after invasion by fungi (F 1 layer). Thus, in the L 2 layer, where moss was dead but relatively free of fungal hyphae, it was very rarely encountered in animal guts and faeces.

Pine resin was ingested by enchytraeids (Fig. 9), which were often found between bark and wood in fallen twigs and branches. This material was never observed in any other group and the degree to which the worms deliberately ingested resin, and were able to digest it, is unknown.

Pollen grains from pine were commonly encountered in the food bolus of many enchytraeids (Figs. 10 and 11) and Sciarids (Fig. 12) and in earthworm faeces. Lysis of the more resistant surface layers of pollen grains was observed in the gut of enchytraeid worms. In every case, pollen grains were ingested mixed with many other materials, since this food resource was finely dispersed throughout the litter.

Soil fungi were the resource the most widely selected by soil animals. Fungal material, predominantly hyphae, were observed in the guts of all observed animals except starved (moulting) individuals. Hyaline hyphae were the most abundant form in this soil volume. Most of them were produced by a mycorrhizal basidiomycete, belonging probably to the genus Hyphodontia [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume. III. La couche F 1 d'un moder sous Pinus sylvestris[END_REF]. It also colonized dead pine wood in our sample.

Hyphae of this fungus were observed to be connected to the pine root system (orange-brown coralloid mycorrhizae) and to penetrate the litter [START_REF] Ponge | Ecological study of a forest humus by observing a small volume. IV. Penetration of litter by mycorrhizal fungi[END_REF]. These hyphae were found in the guts of enchytraeid worms, where they appeared to be more or less digested (Fig. 13). Hyphae covered with oxalate crystals were egested as compacted masses once the chitinous walls had been fully digested. In oribatid mites, hyaline hyphae of Hyphodontia were observed in the genus Oppia, where digestion commonly occurred, and in several other species where digestion did, or did not, occur [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume. III. La couche F 1 d'un moder sous Pinus sylvestris[END_REF]. Some species of Collembola fed on this fungus, such as Pseudosinella terricola Gisin, 1967(Fig. 14), Willemia anophthalma Börner, 1901and Pogonognathellus flavescens (Tullberg, 1871). In every case digestion occurred, except for the oxalate crystals.

Sciarid larvae did not appear to be able to digest hyaline fungal walls, since these hyphae were always present without any change in their appearance (Fig. 15), but the opacity of the cytoplasm had disappeared (when observed in phase contrast microscopy), indicating that only the cell contents were used by these animals.

Hyaline hyphae were also ingested by members of the macrofauna, together with plant material, but unfortunately the feeding behaviour of these animals was observed only through their faeces. Since it was virtually impossible to discriminate between fungi colonizing faecal masses and those ingested with the original food material we could not reach a conclusion on this point. Dematiaceous (melanine stained) fungi were present mainly in the form of the dark mycelium of the sterile mycorrhizal ascomycete Cenococcum geophilum Fr. A broad spectrum of animal species was also feeding on this fungus (as specialized feeders or not), but, contrary to the aforementioned hyaline basidiomycete, the digestion of the hyphal walls of this fungus seemed difficult or even quite impossible for most groups.

Dematiaceous hyphal walls remained intact in enchytraeid worms (Figs. 16,13: compare to hyaline hyphae). In some cases, some signs of attack were visible, such as small holes in the thick walls of this fungus (Fig. 17), but this was probably due to the action of bacteria or amoebae prior to ingestion by the animal. More pronounced features were also observed, which might be due to the action of gut enzymes. In some other cases, where decay was still more pronounced, we hypothesize that the dematiaceous material had been already ingested by another animal and was present as faeces in the food bolus of enchytraeids. Dematiaceous hyphae and spores were present in the food bolus of oribatid mites, mainly camisiid species such as Platynothrus peltifer (Koch, 1839) and Nothrus sylvestris (Koch, 1839) and were also observed in faecal pellets (Fig. 18). Observation of faeces indicated that some transformation in plant tissue structure occurred, especially at the centre of the pellets, but this was not observed inside animal guts.

Filamentous cyanobacteria were found in enchytreid guts. Digestion was followed by comparing several parts of the same animal intestine: cells were separated then emptied (Fig. 19), with the cellulosic walls remaining untouched.

Unicellular algae were often found in the guts of enchytraeids (Fig. 20). Viability of the cells was recognized by opacity of their cytoplasm when observed in phase contrast. The presence of intact cells inside the intestine indicated that digestion of algae seemed to be somewhat difficult. Nevertheless digestion occurred with the help of intestinal microflora (Fig. 21): cells were (1) coated with bacteria, then (2) their cytoplasm disappeared, and (3) they collapsed. One species of Collembola, Lepidocyrtus lanuginosus (Gmelin, 1788) seemed to digest algae more easily, since no opacity was found in the cells (Fig. 22). Some cell walls were seen to be partially digested but, unfortunately, the few animals collected of this species made it impossible to conclude that digestion of the walls was always occurring. In this collembolan, death of the algae was only due to the action of the animal: no associated bacteria were found in the intestine of Collembola, contrary to other animal groups as will be seen below (and confirmed in transmission electron microscopy by [START_REF] Saur | Alimentary studies on the Collembolan Paratullbergia callipygos using transmission electron microscopy[END_REF].

Bacteria were present in a lot of plant fragments that were ingested both by macrofauna and mesofauna (especially pine needles in the L 2 layer, Ponge, 1985a), but in this case their fate was not easy to discern. Nevertheless it must be noticed that the food bolus of the collembolan species Megalothorax minimus (Willem, 1900) was always made of bacteria mixed with minute fungal and mineral partic1es (Fig. 23).

Faecal material was seemingly the main food of some species of Collembola belonging to the same family (Isotomidae), namely Folsomia manolachei Bagnall, 1839 ( = F. nana), Parisotoma notabilis (Schtiffer, 1896) ( = Isotoma notabilis) and Isotomiella minor (Schtiffer, 1896). Although the shape of ingested faeces had been lost, due to comminution by buccal parts, the ingested food is a mixture of different materials, always halfdigested and mixed with bacteria. When no comminution took place in the food bolus, as was the case in enchyraeid worms, entire faeces were recovered in the guts, especially the strongly compacted oribatid faeces (Fig. 24). This was also observed inside earthworm faeces [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume. III. La couche F 1 d'un moder sous Pinus sylvestris[END_REF]. Tunnelling of epigeic earthworm faeces composed of organic matter by phthiracarid mites (Rhysotritia duplicata) was also observed in the F 1 layer.

Animal remains on the contrary were commonly encountered, especially in enchytraeid guts. These were most often tests of testate amoebae. Digestion of the test of Trinema sp. or Phryganella acropodia (Hertwig and Lesser, 1874) Hopkinson, 1909, was observed. The case of P. acropodia deserved attention, because the test of this species is mainly made of aggregated mineral partic1es. Careful examination of all clusters of mineral particles found in the intestinal guts of enchytraeids suggested that they were derived from the disintegration of tests of this very common species.

INTESTINAL MICROFLORA

Association with bacteria presumed to be living in the intestine was commonly observed in all saprophagous groups, except Collembola. Intestinal microflora might be directly observed as bacterial clouds distinct from the food bolus (except in Fig. 21 where algal cells were in contact with them), without any lysis symptom, and was present even in starved animals. These bacteria were commonly observed in nematoda, rotifera, enchytraeidae, sciarid larvae, oribatid mites (Fig. 25) but were never observed in springtails.

SPECIFICITY IN FOOD DIETS

Observation of a great number of soil animals living in the same environment supported the idea that very few species were specialized feeders and that the bulk of food resources were consumed without any discrimination. This held especially true for oligochaeta, i.e., enchytraeid and lumbricid species. Nevertheless this view must not be taken as the negation of choice by soil animals.

The epigeous worm Dendrobaena octaedra (Savigny, 1826) was probably the only lumbricid species present in the sample examined. These animals did not show any choice in their food diet, as could be judged from their faecal pellets, and gut contents reflect undiscriminating consumption of the material present in the microhabitat occupied by the worm.

The animal species that had been more extensively studied here is the enchytreid worm Cognettia sphagnetorum (128 individuals). These animals showed differences in the ingestion of moss leaves between the three layers investigated: green leaves were consumed in the L 1 layer, dead but uncolonized leaves were ignored in the L 2 layer, and leaves colonized by fungi were consumed in the F 1 layer [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume, premiers résultats. I. La couche L 1 d'un moder sous pin sylvestre[END_REF](Ponge , 1985a[START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume. III. La couche F 1 d'un moder sous Pinus sylvestris[END_REF]. In addition, filamentous cyanobacteria ( = blue-green "algae"), testate amoebae, pollen grains and resin were more commonly found in the gut of these animals. The absence of pine needles in the animals present in the L 1 layer was probably due to their early stage of fungal decomposition (especially the strong cuticle which impeded penetration). This was also the case for the sc1erotia of the fungus Cenococcum geophilum, which were certainly too hard structures. With these exceptions all materials available to these animals were actively ingested.

Sciarid larvae (22 individuals) actively consumed moss leaves in the L 1 layer, fungal hyphae in the L 2 layer and pine needles in the F 1 layer. We cannot prove that the same sciarid species was involved but this was probably true, since the individuals seemed to be morphologically identical and belonged to the same colonial group.

Collembola were represented by several groups of species, with distinct food diets. Isotomid species (Folsomia manolachei, 30 ind.; Parisotoma notabilis, 24 ind.; Isotomiella minor, 5 ind.) seemed to be charaeterized in our litter sample by their coprophagy. It is difficult to say that this diet was specialized, since the composition of the pellets so ingested was highly variable, but from a behavioural point of view, these animals might be classified as specialized feeders. Pseudosinella terricola (8 ind.) and Willemia anophthalma (8 ind.)

were strictly fungal feeders and the degree of specialization of the second species was higher: this animal ingested only the hyaline hyphae of the basidiomycete fungus Hyphodontia. The other species of Collembola were in too low numbers to ascertain their food diet.

Oribatid mites, whatever the taxonomie group they belonged to, were the most specialized animals.

Phthiracarids (Rhysotritia duplicata, 14 ind.; Phthiracarus sp., 10 ind.) ate only pine material (needles and bark), tunnelling within plant tissues. Nevertheless it must be noticed that adults seemed to have less specialized requirements, since some fungal material (hyphae of mycorrhizal fungi in the present case) was eaten to a little extent. Oppiids [Oppiella nova (Oudemans, 1902), 7 ind.; Oppia subpectinata (Oudemans, 1901), 9 ind.; undetermined nymphs, 3 ind.] fed only on fungal hyphae, the ratio of hyaline versus dematiaceous hyphae varying with the size of the animals (dematiaceous hyphae of the mycorrhizal Cenococcum geophilum needed animals with stronger buccal pieces to break them off). Camisiids sensu lato [Platynothrus peltifer (Koch, 1839), 25 ind.; Nothrus sylvestris (Koch, 1839), 5 ind.] seemed to be specialized in our sample on the mycorrhizal C. geophilum. It is worthy to note that, although the first species ate only mycelia in the L 1 layer [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume, premiers résultats. I. La couche L 1 d'un moder sous pin sylvestre[END_REF], the second one preferred the mycorrhizae formed by the same fungus in the F 1 layer, ingesting also a small quantity of some root tissues [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume. III. La couche F 1 d'un moder sous Pinus sylvestris[END_REF].

If we wanted to c1assify the different groups investigated according to their degree of specialization on food resources, we could obtain the following series (groups with too few data have been excluded):

Lumbricidae < Enchytraeidae < Sciaridae < Collembola < Oribatida DISCUSSION A great deal of work has already been done on the feeding habits of soil animals. Most conclusive studies concerned only one taxonomic group or even one single species. The works of Zachariae (1985) and [START_REF] Bal | Morphological investigation in two moder-humus profiles and the role of the soil fauna in their genesis[END_REF][START_REF] Bal | Zoological Ripening ofSoils[END_REF] were nevertheless closely related to the aim of the present study. Both of these scientists used thin slides of the upper horizons of forest soil to study trophic relationships that occurred during the decomposition of leaf litter. As was done in the present study, they attempted to reconstruct a dynamic process from instantaneous photography. Unfortunately the optic properties of the hard resins used to embed soil profiles were so questionable that observation could only be made at the lowest magnification of the light microscope. Other shortcomings were the absence of fauna in the studied profiles, due both to the process of desiccation and to the fact that on a given section the probability to find animals was very feable. Consequently animal feeding activities were traced only through their excrements, thus giving no results on digestive processes.

Enchytraeid worms

In the present study, enchytreid worms were the more thoroughly investigated group (because it was the more abundant at the time of sampling, 129 ind.), more precisely the acidophilic species Cognettia sphagnetorum. In the aforementioned work of [START_REF] Zachariae | Spuren tierischer Tätigkeit im Boden des Buchenwaldes[END_REF], Enchytraeidae were not given a decisive role in the transformation of beech litter, and their feeding habits were interpreted as mainly coprophagous. In the work of Bal they were quite absent [START_REF] Bal | Morphological investigation in two moder-humus profiles and the role of the soil fauna in their genesis[END_REF] or considered as negligible [START_REF] Bal | Zoological Ripening ofSoils[END_REF]. This was also the case in Jacot (1939)'s observations on spruce and fir litter. We cannot dispute on these points, but our own experience in temperate forests raises doubts to the contention that enchytraeid worms do not play a key role in the comminution of leaf litter. The most conclusive work on the feeding habits and digestion abilities of enchytraeids (also C. sphagnetorum) is the study made by [START_REF] Latter | Decomposition of a moorland litter, in relation to Marasmius androsaceus and soil fauna[END_REF], [START_REF] Standen | Distribution of a population of Cognettia sphagnetorum (Enchytraeidae) in relation to micro habitats in a blanket bog[END_REF] and [START_REF] Latter | Studies on the microfauna of blanket bog with particular reference to Enchytraeidae. II. Growth and surviva1 of Cognettia sphagnetorum on various substrates[END_REF]. Latter and co-workers proved, both by field and laboratory experiments and observation of intestinal guts, that this species thrived on leaf litter (Rubus, Eriophorum or Calluna), and that leaf tissues were consumed and finely comminuted. Disagreement with our own observations was only with the respective fate of fungal and plant cell walls. In the present study we did not register any significant change in the appearance of plant cell walls, although crushing was pronounced. On the contrary, hyaline fungal walls were always partially digested. Comparison with sciarid larvae, for instance, allowed us to say that pine cell walls did not seem strongly affected by their passage through enchytraeid intestines, apart from their comminution [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume. III. La couche F 1 d'un moder sous Pinus sylvestris[END_REF].

Observation by light microscopy of the disintegration of cellulosic walls is difficult, due to their high transparency, but use of phase contrast helped to detect changes in refringency that might be related to changes in the cristalline structure of cellulose. Observations by light microscopy on the collembolan Paratullbergia callipygos (Börner, 1902) ( = Tullbergia callipygos) suggested that this species was able to dissolve cellulose to some extent. This was confirmed by transmission electron microscopy [START_REF] Saur | Alimentary studies on the Collembolan Paratullbergia callipygos using transmission electron microscopy[END_REF]. Concerning the fungal cell walls and their fate, no distinction was made between hyaline and dematiaceous fungi in Latter's studies, and we showed that only hyaline fungal walls were transformed in the intestine of Cognettia sphagnetorum. Ultrastructural studies on the enchytraeid worm Fridericia striata (Levinsen, 1884) [START_REF] Toutain | Etude ultrastructurale des processus de biodégradation. II. Modèle enchytraeides-litière de feuillus[END_REF], fed on aspen leaves, concluded that plant cell walls were little affected in its gut (apart from some changes in the microfibrillar arrangement), contrary to fungal walls that were partly destroyed. Oxalate crystals that covered the hyphae of Hyphodontia did not seem to be dissolved in the gut of C. sphagnetorum. Thus these animals probably do not take a great part in the cycling of Ca through fungus-animal food chains, in contrast to what has been claimed by [START_REF] Cromack | Soil microorganism-arthropod interactions: fungi as major calcium and sodium sources[END_REF] for most soil animals.

Concerning the feeding behaviour of C. sphagnetorum, we observed the tunnelling activity of this species in pine needles, similar to the same behaviour inside the cylindrical leaves of the cotton grass [START_REF] Latter | Studies on the microfauna of blanket bog with particular reference to Enchytraeidae. II. Growth and surviva1 of Cognettia sphagnetorum on various substrates[END_REF]. Penetration between bark and wood (phloem part) of dead pine branches was similarly recorded by these authors on heather woody stems. It must be noticed that the deposition of faecal pellets at the inside of the tunnelled needles was rarely observed, contrary to phthiracarid mites. Exception is in the L 1 layer, where desiccation probably delayed animals escaping from the needles.

Intestinal microflora was commonly observed, with several morphological types often occurring together in the same intestine (but in distinctive metameres, Ponge, 1985a).

Oribatid mites

Oribatid mites were the second most abundant group (116 ind.). Rhysotritia duplicata (14 ind.) and Phthiracarus sp. (10 ind.) fed on dead plant tissues. They tunnelled pine needles, bark and faeces of epigeous earthworms and deposited their own faecal pellets inside the so-formed cavities. Tunnelling activity of phthiracarid mites inside litter debris has been recorded time and again [START_REF] Jacot | Reduction of spruce and fir litter by minute animals[END_REF][START_REF] Kubiëna | L'investigation microscopique de l'humus (Die mikroskopische Humusuntersuchung)[END_REF][START_REF] Kubiëna | The Soils of Europe[END_REF][START_REF] Handley | Mull and mor formation in relation to forest soils[END_REF][START_REF] Kubiëna | Animal activity in soil as a decisive factor in establishment of humus forms[END_REF][START_REF] Kendrick | Biological aspects of the decay of Pinus sylvestris leaf litter[END_REF][START_REF] Zachariae | Spuren tierischer Tätigkeit im Boden des Buchenwaldes[END_REF][START_REF] Bal | Morphological investigation in two moder-humus profiles and the role of the soil fauna in their genesis[END_REF][START_REF] Babel | Micromorphology of soil organic matter[END_REF][START_REF] Rusek | Die bodenbildende Funktion von Collembolen und Acarina[END_REF]Kubikova and Rusek, 1976;[START_REF] Toutain | Les humus forestiers. Structures et modes de fonctionnement[END_REF][START_REF] Bal | Zoological Ripening ofSoils[END_REF] and therefore nothing needs to be added about the importance of these animals in the mechanical reduction of plant litter. We observed that pine needles were not penetrated by these animals until they reached the F 1 layer, contrary to the enchytraeid worms, tunnelling by them having been observed as early as the L 1 layer. The question is whether phthiracarid mites were unable to feed on fresher needles or not, compared to enchyraeid worms. [START_REF] Jacot | Reduction of spruce and fir litter by minute animals[END_REF] indicated that coniferous needles needed to be softened by fungi before any penetration by mites occurred. In the field study made by [START_REF] Kendrick | Biological aspects of the decay of Pinus sylvestris leaf litter[END_REF] on Pinus sylvestris litter, needles were not attacked by oribatid mites (presumably phthiracarid species) until the "F 1 " layer, which corresponded in fact to our L 2 layer. The laboratory study by [START_REF] Hayes | Studies on the feeding preferences of some phthiracarid mites (Acari: Oribatidae)[END_REF] on the coniferous species (including Scots pine) and three phthiracarid species concluded that needles needed to achieve a particular stage of fungal decomposition in order to be actively consumed by these animals. Thus our results did not agree exactly with other studies, although intense fungal penetration of the needles was considered by these authors as a prerequisite for penetration by phthiracrid mites. This discrepancy might be partly explained by the fact that our observations took place on a sample collected in August. This was the time of intense activity of enchytraeid worms, given the high level of their population density, probably due to rainy and overcast condition of the weather in the summer of 1981. Summer was recorded as a period of intense vegetative multiplication of Cognettia sphagnetorum [START_REF] Springett | The distribution of life histories of some moorland Enchytraeidae (Oligochaeta)[END_REF]. Reproduction of phthiracarid species (Rhysotritia duplicata and Phthiracarus) was also effective at the time of sampling (presence of larval instars of the two species and eggs of the first), but not at the same rate as Enchytraeidae and they were probably more evenly distributed over the year: traces of phthiracarid activity (pellet deposition) were found in a lot of needles that were no longer inhabited by these animals. Thus we could say that enchytraeid activity was contemporary and phthiracarid activity rather a remnant of a past one at the time of sampling. Pine needles that were incorporated into the L layers might not have been on the ground long enough to be significantly colonized by phthiracarids. Other species may be classified as mycophagous or "microphytophagous" in the sense of Schuster (in [START_REF] Luxton | Studies on the oribatid mites of a Danish beech wood soil. I. Nutritional biology[END_REF]. The distinction between plant feeder species (phthiracarids) and fungal feeders (other species, except perhaps Suctobelba which seemed to ingest a fluid food) in our sample corresponded roughly to what was known from the literature on oribatid food diets. Nevertheless, some species considered as "panphytophagous" (plant-fungal feeders) in other sites, were here strictly or mainly fungal feeders.

Platynothrus peltifer (25 ind.) was found to eat exclusively the hyphae of the dematiaceous mycorrhizal Cenococcum geophilum in the L 1 layer [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume, premiers résultats. I. La couche L 1 d'un moder sous pin sylvestre[END_REF] and faecal material containing the same fungus in the L 2 layer (Ponge, 1985a). In the literature, this species was known to eat both plant and fungal material (Hartenstein, 1962a;[START_REF] Pande | Studies on the food and feeding habits of soil Oribatei in a black pine plantation[END_REF][START_REF] Anderson | Succession, diversity and trophic relationships of some soil animals in decomposing leaf litter[END_REF][START_REF] Behan-Pelletier | Feeding habits of sixteen species of Oribatei (Acari) from an acid peat bog, Glenamoy, Ireland[END_REF]. Nothrus sylvestris (5 ind.) was interpreted as a root-fungal feeder (black mycorrhizae of C. geophilum), but mainly browsing the fungal mantle [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume. III. La couche F 1 d'un moder sous Pinus sylvestris[END_REF]. This species was generally considered as a plant-fungal feeder [START_REF] Luxton | Studies on the oribatid mites of a Danish beech wood soil. I. Nutritional biology[END_REF][START_REF] Pande | Studies on the food and feeding habits of soil Oribatei in a black pine plantation[END_REF][START_REF] Hågvar | Succession, diversity and feeding habits of microarthropods in decomposing birch leaves[END_REF][START_REF] Behan-Pelletier | Feeding habits of sixteen species of Oribatei (Acari) from an acid peat bog, Glenamoy, Ireland[END_REF], with plant feeding dominating. Other species, like Oribatula tibialis (Nicolet, 1855) (5 ind.) and Nanhermannia nanus (Nicolet, 1855) (5 ind.) ate a mixture of the two main mycorhizal fungi present in the studied sample [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume. III. La couche F 1 d'un moder sous Pinus sylvestris[END_REF], but were considered as plant-fungal feeders by other authors [START_REF] Pande | Studies on the food and feeding habits of soil Oribatei in a black pine plantation[END_REF][START_REF] Hågvar | Succession, diversity and feeding habits of microarthropods in decomposing birch leaves[END_REF][START_REF] Behan-Pelletier | Feeding habits of sixteen species of Oribatei (Acari) from an acid peat bog, Glenamoy, Ireland[END_REF]. Perhaps pine needles were especially repellent for these species in our study site, nevertheless it must be remembered that [START_REF] Pande | Studies on the food and feeding habits of soil Oribatei in a black pine plantation[END_REF] and Hartenstein (1962a) worked also in pine stands. We prefer to suggest that the so-called plant-fungal feeder species favoured fungi which were far in excess of faunal requirements in the volume under study. Pine needles might perhaps be ingested in other times or in other places when preferred a food was not available, in contrast to phthiracarid mites which seemed to eat only plant material. Good agreement with other studies was found only with oppiid species (Oppia subpectinata, 9 ind.;

Oppiella nova, 7 ind.), which were considered as pure fungal feeders (Hartenstein, 1962a;[START_REF] Haq | Feeding habits of ten species of oribatid mites (Acari: Oribatei) from Malabar, South India[END_REF], as in our observations, but it must be remembered that these small species seemed to choose hyaline hyphae rather than dematiaceous ones, probably because of the toughness of the latter.

Food partitioning between co-existing species was studied by [START_REF] Mitchell | Fungal feeding of oribatid mites (Acari: Cryptostigmata) in an aspen woodland soil[END_REF]. They found that differences in habitats between species were reflected in their gut contents throughout the year, both qualitatively and quantitatively. Microcosm experiments of [START_REF] Anderson | Competition between two unrelated species of soil Cryptostigmata (Acari) in experimental microcosms[END_REF] on two related species demonstrated that at high densities habitat specialization was influenced by the presence of a competing species and that gut contents were largely determined by habitats. From our results, several facts may be ascertained. In this small soil volume, we were sure that differences between species in habitat and food were determined by environmental constraints and food preferences and not by differences in food availability. The exception was when food availability was influenced by micro-environment factors, for instance an animal physiologically unable to live in the L layers might not consume living mosses. In the L 1 layer, where plant material (mainly pine needles and moss leaves) and fungal material were co-existing, Platynothrus peltifer was found to eat only dematiaceous hyphae although this species was known to consume also plant material, including pine needles (see above). We may suppose either that pine needles and moss leaves were not in a good decay state or that these fungi (presumably Cenococcum geophilum) were better liked. In the L 2 layer, a few millimetres below, the same mite species was poorly present and apparently ate only the same fungus. In the F 1 layer this mite was absent, although this was the place where C. geophilum was well developed and decaying pine and moss material were present. Thus we may tentatively suppose that the mite had (at the time of sampling) moved from the F 1 layer and was thus forced to feed in an environment where fungi were undoubtedly preferred to fresh plant material. Allelopathy (from enchytraeid worms for instance, with a higher bio-volume) might account for this. In the F 1 layer, where most faunal species were present (except Platynothrus peltifer), food partitioning was evident between phthiracarid species, which fed only on plant material, and fungal-feeding species. Some oppiid species were seen only to ingest hyaline fungi. In this case, buccal anatomy and size of the animals were probably the main factors responsible for the observed segregation, but food niches for both plant-feeding and fungal-feeding species would be overlapping.

The fate of the materials ingested by oribatid mites was followed to some extent. Unfortunately, there is no conclusive study on this problem in the literature. Plant cell walls seemed to be highly transformed in phthiracarid species, especially during the transit time in the post-colon, where they lose their refrigency and sharp contour and became brownish. Ultrastructural studies would be needed to study these cellulosic and lignocellulosic walls at the fibrillar level. Fungal walls without melanin were destroyed, except in Nothrus sylvestris [START_REF] Ponge | Etude écologique d'un humus forestier par l'observation d'un petit volume. III. La couche F 1 d'un moder sous Pinus sylvestris[END_REF]. Oxalate crystals seemed to be dissolved together with the fungal walls, in contrast to enchytraeid worms. Disintegration of dematiaceous walls was observed in the gut of N. sylvestris and inside faecal pellets of the other fungal feeding species (Ponge, 1985a), but this was not observed in any other group.

Presence of an intestinal microflora was commonly observed in Platynothrus peltifer and Rhysotritia duplicata, where the same morphological strain seemed to be present in each of these two species, but appeared to be quite absent in Nothrus sylvestris and Phthiracarus sp. In the literature, associated bacteria were reported from numerous oribatid species (Hartenstein, 1962b;[START_REF] Dinsdale | The digestive activity ofa phthiracarid mite mesenteron[END_REF][START_REF] Stefaniak | Intestinal microflora in representatives of different feeding groups of soil moss mites (Acarida, Oribatida)[END_REF]) and the latter showed selection of associated bacteria and actinomycetes was strongly related to the feeding habits of the oribatid species.

Collembola

Feeding habits of springtails have been intensively studied, but several of our species are uncommon (acidophilic species, Ponge, 1980Ponge, , 1983) ) and have not been investigated by other scientists. The present study showed that three common isotomid species (Folsomia manolachei, Parisotoma notabilis and Isotomiella minor) seemed to be mainly coprophagous, contrasting with mycophagous species such as Willemia anophthalma and Pseudosinella terricola. Comparison with other studies is rather difficult for these species, since faecal material was mostly overlooked by authors and in the best cases only described as "unidentifiable" [START_REF] Poole | Studies on the food of Collembola in a Douglas fir plantation[END_REF][START_REF] Gilmore | Foods ingested by Tomocerus spp. (Collembola, Entomobryidae), in relation to habitat[END_REF], "humus" [START_REF] Knight | A preliminary study of the dietary requirements of Tomocerus (Collembola)[END_REF][START_REF] Gilmore | Foods ingested by Tomocerus spp. (Collembola, Entomobryidae), in relation to habitat[END_REF] or "amorphous" material [START_REF] Knight | A preliminary study of the dietary requirements of Tomocerus (Collembola)[END_REF][START_REF] Bödvarsson | Alimentary studies of seven common soil-inhabiting Collembola of southern Sweden[END_REF][START_REF] Marshall | Gut content analysis of the Collembolan Bourletiella hortensis (Fitch) from a forest nursery[END_REF][START_REF] Hågvar | Succession, diversity and feeding habits of microarthropods in decomposing birch leaves[END_REF].

Nevertheless several authors assumed that this material had undergone previous digestion [START_REF] Poole | Studies on the food of Collembola in a Douglas fir plantation[END_REF][START_REF] Knight | A preliminary study of the dietary requirements of Tomocerus (Collembola)[END_REF]. The most important fact in determining the faecal origin of ingested material was the mixture of distinct strongly decayed material (mainly plant and fungal), associated with bacteria dispersed in the food bolus. Intestinal microflora when present (in other animal groups) were always observed as monospecific clouds of bacteria which were never dispersed in the food bolus.

Food specialization has been studied by some authors. Most of these works indicated that coexisting species exhibited differences in resource utilization but with tremendous overlapping between their respective food spectra. MacMillan and Healey (1971) pointed out the fact that some closely related species (here belonging to the genus Onychiurus) did not exhibit significant differences in their feeding habits, contrary to what was expected. The work of [START_REF] Bengtsson | Fungal odour attracts soil Collembola[END_REF] demonstrated that these animals were influenced by smell in their choice of food, but their choice was different for animals reared on agar plates to those reared on soil contaminated with known fungal species. [START_REF] Hassall | Vertical migration of Onychiurus subtenuis in relation to rainfall and microbial activity[END_REF] observed that the vertical migration of one Onychiurus species into freshly remoistened litter was strongly influenced by microbial colonization of the decaying leaves. Feeding preferences were measured by [START_REF] Shaw | A consistent hierarchy in the fungal preferences of the Collembola Onychiurus armatus[END_REF] with a great care in the statistical treatment: there was undoubtedly a constant hierarchy between the different fungal strains to be tested, but some ranks were not consistent with mortality results, indicating that the animals did not always feed on the most suitable fungal species. [START_REF] Verhoef | Relative importance of fungi and algae in the diet and nitrogen nutrition of Orchesella cincta (L.) and Tomocerus minor (Lubbock) (Collembola)[END_REF] reared two sympatric species on algal, fungal and mixed diets and compared their growth and fecundity. They found that the most suitable diets were not necessarily those that were used by field animals. In laboratory experiments, [START_REF] Ponge | Etude des relations microflore-microfaune: expériences sur Pseudosinella alba (Packard), Collembole mycophage[END_REF] concluded that spores were always preferred to hyphae in cultural tests of Pseudosinella alba fed with known species of fungi, although spores were rarely found in the gut contents of field animals. [START_REF] Vegter | Food and habitat specialization in coexisting springtails (Collembola, Entomobryidae)[END_REF] compared field and cultural observations and concluded that food specialization between co-existing species was mainly a reflection of differences in their micro-habitats. Such a conclusion was also supported by the work of [START_REF] Saur | Alimentary studies on the Collembolan Paratullbergia callipygos using transmission electron microscopy[END_REF] where the proportion of different foods in the gut contents were shown to be dependent both on season and on vertical distribution of individuals of the same species. Our own observations showed in addition that food preferences existed between species or groups of species living in the same micro-habitat. The fungal content of Pseudosinella terricola and Willemia anophthalma guts compared to the algal content of Lepidocyrtus lanuginosus and to the faecal content in the three isotomid species was not a consequence of differences in their micro-habitats. Nevertheless it must not be argued that each Collembola species living in the same micro-habitat is specialized in its food habits. Observations on the three related isotomid species Folsomia manolachei, Parisotoma notabilis and Isotomiella minor showed that no food specialization existed between them in the studied sample. From the literature and the present study we must recognize that no general law currently explains or predicts soil food webs, apart the minimal evidence that food webs are the convergence point of resources and feeding preferences of animals that are present in a given place at a given time. Since choices change with the food resources available to the animals, no model can explain changes in the present state of our knowledge. Nevertheless it must be remembered that provided the scale of observation is sufficiently reduced, most significant trends are quite easy to perceive, although not applicable in samples some distance away.

Observations on the fate of ingested materials were made by a few authors with the help of transmission electron microscopy (Kilbertus andVannier, 1979, 1981;[START_REF] Saur | Alimentary studies on the Collembolan Paratullbergia callipygos using transmission electron microscopy[END_REF]. They did not correspond to our species, so unfortunately no comparison could be made with our results. Nevertheless it must be noticed that soil bacteria may constitute a food for Collembola [START_REF] Saur | Alimentary studies on the Collembolan Paratullbergia callipygos using transmission electron microscopy[END_REF], as is probably the case in coprophagous and geophagous species, and that intestinal microflora were never observed in empty guts [START_REF] Kilbertus | Relations microflore-microfaune dans la grotte de Sainte-Catherine (Pyrénées ariégeoises). II. Le régime alimentaire de Tomocerus minor (Lubbock) et Tomocerus problematicus Cassagnau (Insectes Collemboles)[END_REF][START_REF] Saur | Alimentary studies on the Collembolan Paratullbergia callipygos using transmission electron microscopy[END_REF], strengthening the idea that Collembola do not digest with the help of associated bacteria or actinomycetes.

Sciarid larvae

The most interesting feature was the succession of dominant materials which were observed in the gut contents of animals found from the surface to the deeper regions of the studied profile. Moss leaves (ingested in a green state) were replaced by fungal hyphae from the L 1 to the L 2 layer, then by plant tissues (coming from pine needles) in the F 1 layer. This trend paralleled similar features observed in enchytraeid worms (see above), except that pine needles were ingested by Enchytraeidae at an earlier stage of decomposition and that consequently there was not a so sharp boundary between the food diets observed in the three sub-layers. Other differences with enchytraeid worms were that fungal walls, even when hyaline, were never observed to be digested by sciarid larvae. On the contrary, sciarid larvae (at least the single unidentified species which was present in our sample) seemed to be able to digest cellulose, which was not the case for the enchytraeid Cognettia sphagnetorum. Studies on feeding habits of sciarid species are scarce. The observations of Healey and Russel-Smith (1971) and [START_REF] Bal | Morphological investigation in two moder-humus profiles and the role of the soil fauna in their genesis[END_REF] and the experimental work of [START_REF] Deleporte | Rôle du Diptère Sciaridae Bradysia conjinis (Winn., Frey) dans la dégradation d'une litière de feuillus[END_REF] established that some species fed actively on leaf litter. The "small diptera larvae" observed by [START_REF] Zachariae | Spuren tierischer Tätigkeit im Boden des Buchenwaldes[END_REF] to feed on litter between the L and the F layers belonged probably also to the same family, which we observed to be very common in acid soils, especially in acid mull or weak moder humus. Further discussion of the present results is impossible, since the species which was found had not been identified. Only other work is numerous papers on the damages sciarid larvae cause to mushrooms beds.

Earthworms

Food diet of the species Dendrobaena octaedra was traced only through its faeces. This animal did not seem to chose between the available food resources and, apart from comminution, no transformation of plant and fungal material occurred during the intestinal transit time. Ingestion of leaves by earthworm species has been recorded many times, but the impact of these animals on the litter they commonly eat in forests has been very poorly studied. [START_REF] Rafidison | Rôle de la faune dans l'humification: transformations des feuilles de hêtre par un ver anécique[END_REF] compared the ultrastructure of the plant material (beach leaves) before and after ingestion by the anecic worm Nicodrilus velox (Bouché, 1967) and found that the action of the worm on plant cell walls, apart from comminution, seemed to be negligible, except in the case of beech leaves which had been previously decayed by white-rot fungi, similar to our results on an epigeous species. The same author detected a strong modification of the tannin-protein complexes, which was not possible to assess with our techniques, and it must be noticed that he observed a contact between beech leaves and soil bacteria, which was only possible in the gut of such a soil-dwelling species.

Nematoda

The food diet and digestion of some free-living nematodes has been studied by mean of transmission electron microscopy [START_REF] Arpin | Ultrastructure du contenu digestif et de l'épithélium intestinal chez quelques nématodes prédateurs (Mononchida) et bactériophages[END_REF][START_REF] Saur | Ultrastructural analysis of the intestinal contents of Clarkus papillatus (Nemata: Mononchina): ecological interest of the survey[END_REF]. These authors accounted for the importance of bacteria in the food diet of predatory species belonging to the family Mononchidae and Saur and Arpin (1989) followed the digestive process by comparing different parts of the same intestine, as in our study.

Thus our results were in fairly good agreement with theirs.

Legends of figures

Fig. 1 .

 1 Fig. 1. Population density and bio-volume (upper estimate, see text) of the five main mesofaunal groups in the L 1 , L 2 and F 1 layers.

Fig. 2 .

 2 Fig. 2.Oribatid faecal pellets at the inside of a pine needle (after partial dissection). F 1 layer.

Fig. 3 .

 3 Fig. 3. Two enchytraeid worms (Cognettia sphagnetorum) tunnelling through the same pine needle, indicated by arrows (pellets have been deposited at the outside). F 1 layer.

Fig. 4 .

 4 Fig. 4. Protoxylem reticulate tracheid (arrow 1) and dissociated cell walls (arrow 2) from pine needles in the gut of an enchytraeid worm. Same species as above. L 2 layer. Phase contrast. Bar=50 µm.

Fig. 5 .

 5 Fig. 5. Accumulation of bordered pits (arrow 1) from pine transfusion tissue (stele parenchyma of needles) and emptied basidiomycete hyphae (arrow 2) in the gut of an enchytraeid worm. Same species as above F 1 layer. Phase contrast. Bar= 50 µm.

Fig. 6 .

 6 Fig.6. Faecal pellet, attributed to a woodlouse. Pine needles, cutted into small pieces and compacted. F 1 layer.

Fig. 7 .

 7 Fig. 7. Faecal material, attributed to a slug. Pine needles, ingested as entire or large pieces. F 1 layer.

Fig. 8 .

 8 Fig. 8. Pine needle mesophyll in the abovementioned faecal material. Cells have their wall intact, but their content has disappeared. F 1 layer. Phase contrast. Bar= 50 µm.

Fig. 9 .

 9 Fig. 9. Resin pieces in the gut of the enchytreid worm Cognettia sphagnetorum. L 2 layer. Phase contrast. Bar= 50 µm.

Fig. 10 .

 10 Fig. 10. Two pine pollen grains (exin) in the gut of the same species. L 1 layer. Phase contrast. Bar=50 µm.

Fig. 11 .

 11 Fig. 11. Fragment of a pine pollen grain (exin, arrow 1), together with fungal wall remnants (arrow 2) and bacteria in the gut of the same species. L 2 layer. Phase contrast. Bar = 50 µm.

Fig. 12 .

 12 Fig. 12. Pine pollen grain (exin) in the gut of a sciarid larva. L 1 layer. Phase contrast. Bar= 50 µm.

Fig. 13 .Fig. 14 .

 1314 Fig. 13. Hyaline hyphae partially digested in the gut of the enchytraeid worm Cognettia sphagnetorum. Compare with intact cell walls of the mycorrhizal fungus Cenococcum geophilum (arrow). L 2 1ayer. Phase

Fig. 15 .

 15 Fig. 15. Hyaline hyphae of the mycorrhizal fungus Hyphodontia sp., with intact cell walls (arrow), in the gut of a sciarid larva. L 2 layer. Phase contrast. Bar = 50 µm.

Fig. 16 .

 16 Fig. 16. Me1anized hyphae of the mycorrhiza1 fungus Cenococcum geophilum, with intact cell walls, in the gut of the enchytraeid worm Cognettia sphagnetorum. L 1 layer. Phase contrast. Bar=50 µm.

Fig. 17 .

 17 Fig. 17. Same material in the same animal species as above. Note small holes (arrows) in elsewhere intact cell walls. L 2 layer. Phase contrast. Bar= 50 µm.

Fig. 18 .

 18 Fig. 18. Melanized hyphae, with intact cell walls, of the mycorrhizal fungus Cenococcum geophilum, in an oribatid faecal pellet. L 2 1ayer. Phase contrast. Bar= 50 µm.

Fig. 19 .

 19 Fig. 19. Filamentous cyanobacteria du ring their digestion in the gut of the enchytraeid worm Cognettia sphagnetorum. Cells (arrows) have been separated and emptied. In other respects same as Fig. 18.

Fig. 20 .

 20 Fig. 20. Chlorella-like algae in the gut of the same species. Intact cells, with opaque cytoplasm. In other respects same as Fig. 18.

Fig. 21 .

 21 Fig. 21. Digestion of Chlorella-like algae in the gut of the same species. Covering of the cells with symbiotic bacteria (1) was followed by loss of cytoplasm opacity (2) then by collapsing (3). L 2 layer. Phase contrast. Bar = 50 µm.

Fig. 22 .

 22 Fig. 22. Soil algae, without cell contents, in the gut of the springtail Lepidocyrtus lanuginosus. F 1 layer. Phase contrast. Bar= 50 µm.

Fig. 23 .

 23 Fig. 23. Soil bacteria together with some minute particles in the food bolus of the springtail Megalothorax minimus. In other respects same as Fig. 22.

Fig. 24 .

 24 Fig. 24.Two oribatid faeces (arrows) in the gut of the enchytraeid worm Cognettia sphagnetorum. In other respects same as Fig. 22.

Fig. 25 .

 25 Fig. 25. Associated bacteria (right) near the food bolus (left) in the gut of the oribatid mite Platynothrus peltifer. L 2 layer. Phase contrast. Bar= 50 µm.

  Fig. 1

Fig. 3

 3 Fig. 3

  Fig. 4

Fig. 8

 8 Fig. 8

Fig. 9

 9 Fig. 9

Fig. 10

 10 Fig. 10

Fig. 11

 11 Fig. 11

  Fig. 12

Fig. 14

 14 Fig. 14

Fig. 15

 15 Fig. 15

  Fig. 16

Fig. 18

 18 Fig. 18

Fig. 19

 19 Fig. 19

Fig. 20

 20 Fig. 20

Fig. 21

 21 Fig. 21

Fig. 22

 22 Fig. 22

  Fig. 23

Fig. 25

 25 Fig. 25

TABLE 1

 1 Food resources and their ingestion by animal groups.

	Food	Enchytraeids Sciarids Oribatids	Springtails Earthworms Woodlice Slugs
				Phthir. Others
	Pine needles	*	*(D)	*(T)		*	*	*
	Pine resin	*
	Pine pollen	*(D)	*			*
	Pine wood	*
	Pine bark			*
	Bracken	*
	Moss living	*	*
	Moss dead	*
	Fungus hyaline	*(D)	*	*(D)	*
	Fungus dematiaceous *		*(T)
	Cyanobacteria	*
	Algae	*(D)			*
	Bacteria				*
	Faeces	*		*	*	*
	Testacea	*(D)
	* = Ingested; D = digested; T = transformed.

ACKNOWLEDGEMENTS

The author is gratefully indebted to Dr. P.M. Latter (Institute of Terrestrial Ecology, Merlewood, England) and Dr. J.M. Anderson (University of Exeter, England) for revising of the English language.


Jean-François David

Jean-François Ponge

Pierre Arpin

Guy Vannier

Reactions of the macrofauna of a forest mull to experimental perturbations of litter supply

The effects on forest soil macrofauna of two treatments, viz. litter interception and twofold litter supply, were studied for five yr in the field.

Results concern five saprophagous groups (Lumbricidae, Diplopoda , Isopoda , Coleoptera larvae, Diptera larvae) and five zoophagous groups (Geophilomorpha, Lithobiomorpha, Pseudoscorpionida, Coleoptera larvae , Diptera larvae) . The litter interception had a negative effect on the abundance of most taxa, but was rather slow; there was no significant decrease in abundance before at least 1 yr in Lumbricidae, 2 yr in lsopoda and zoophagous Diptera larvae, 2.5 yr in Diplopoda, saprophagous Diptera larvae, Geophilomorpha and Lithobiomorpha. The twofold litter supply had no significant effect on the abundance of most taxa; on the other hand, it initiated a process of dead leaf accumulation in relation to control, with an annual decomposition rate (k') higher on the control plot than on the plot with an increased litter supply. The results do not support the assumption that saprophagous soil macrofauna are food-limited in acid mull conditions.

 for meso-and microfauna. In 1984, the latter authors began another study on the effects of the amount of litter and herbs on the meso-and microfauna of a forest mull, in a site more acidic than that formerly studied, where the annual litter fall was thought to be of greater importance owing to the lower organic matter content of the soil. This has given an opportunity for measuring the variations in abundance of the soil macrofauna in the same site, after decrease or increase in litter supply, which is the subject of the present paper.

 are of the opinion that there is density-dependent regulation due to competition for the best quality food. In addition, Van der Drift (1963) and Anderson and Healey (1972) consider that the amount of available food may vary greatly from year to year, between limiting conditions and excess resource. Experiments modifying the litter supply in forest cannot settle those questions decisively, for too many factors escape control; nevertheless, they are interesting insofar as they test the conflicting hypotheses under natural conditions.

Study site and methods

Site description

The study site is located in parcel No. 921 of the Ingrannes Massif, in the State forest of Orleans, France.

Temperature fluctuates between 0.2°C (average minimum in January) and 23.9°C (average maximum in July), and annual rainfall, rather evenly distributed, amounts to 624 mm (regional averages from 1951 to 1980).

The forest stand consists mainly of oak (Quercus petraea), which gives 87% of the mass of dead leaves, with a little beech (Fagus sylvatica) and hornbeam (Carpinus betulus). The herb layer, not very developed, consists mainly of bramble (Rubus schleicheri).

The humus is an acid mull (pH = 5.2), with AoL and AoF layers 0.5 to 1 cm and 0.2 to 0.5 cm thick respectively. The percentage of organic matter in the A1 layer is low (3.3%), as is its C/N ratio (13.2).

The soil is a leached brown earth, with a temporary water table which is usually more than 1 m deep, but can rise to 14 cm below ground level during rainy winters. The silty texture of the upper soil layer reduces its drying capacity in summer; the mean soil moisture was 15% from July to September 1985, a figure to be compared to 22% for the field capacity (pF = 2.5) and 6% for the wilting point (pF = 4.2).

Perturbations of organic matter supply

The first part of the study site (12 x 6 m) was covered in February 1985 with 1 m 2 detachable baskets made of 1 cm 2 plastic mesh, set 50 cm above ground level. This area was divided into four 6 x 3 m plots, corresponding to four treatments pursued fortnightly until September 1990:

(a)

LoHo plot. Baskets turned upside down (normal litter supply) and herb layer undamaged.

(b) LoH-plot. Baskets turned upside down (normal litter supply) and herb layer cut away.

(c) L-Ho plot. Baskets emptied outside the plot (decrease in litter supply) and herb layer undamaged.

(d) L-Hplot. Baskets emptied outside the plot (decrease in litter supply) and herb layer cut away.

The second part of the study site (4 x 4 m), 6 m away from the first one and without baskets, was supplied from the autumn of 1985 with the litter of sixteen baskets from the Lplots , in addition to normal litter fall. This part was the L+ plot, and the herb layer was left undamaged.

Some remarks are necessary about the treatments:

(a) Taking into account the phenology of litter fall, litter interception (L-) actually began in the autumn of 1985.

(b)

The Ltreatment was a drastic decrease in litter supply, but the mesh let through leachates and small solid debris from canopy. In addition, there were other inputs of organic matter on the Lplots, such as dead leaves from the herb layer on the L-Ho plot, and also algae and moss which developed on the bare ground.

(c)

The cutting of the herb layer (H-) led to its decay in a few months. Therefore, its influence was progressively suppressed on the Hplots from the autumn of 1985, as the decay of roots proceeded.

(d)

The L+ treatment was nearly equivalent to a doubling of litter supply from canopy, except for the small debris.

Measurements of litter amounts

The annual litter fall and its spatial variation within the site were estimated by weighing air-dried dead leaves, dead wood and fruits, collected fortnightly in twelve baskets taken at random on the Lplots. These measurements were made for three years, from mid-September 1985 to mid-September 1988.

The parameter k' of [START_REF] Jenny | Comparative study of decomposition rates of organic matter in temperate and tropical regions[END_REF], which is equal to the average fraction of litter which disappears from the ground annually, was estimated on the Lo and L+ plots at the end of the experiment. For this purpose, seven 1/16 m 2 sampling units were taken in September 1990, cleaned of mineral particles and air-dried, in order to measure the mass of old litter (F) just before the fall; k' was calculated as k' = A / (A+F), where A is the mean annual litter fall.

Faunal sampling

Sampling of the whole soil macrofauna, deep burrowing earthworms included, requires destructive methods, which were used only for the last sample, in the spring of 1990. Earlier in the experiment, 10 cm deep probes were taken twice a year, in spring and autumn; from the autumn of 1984 to the spring of 1989 on the LoHo, LoH-, L-Ho and L-Hplots, and from the autumn of 1985 to the spring of 1989 on the L+ plot. The sampling units (s. u.) were as small as possible in order to limit disturbance of the study site. At first they were taken with a 10 cm cubic corer (1/100 m 2 ) in the autumn of 1984, and subsequently with a cylindrical corer 10 cm in diameter (1/127.3 m 2 ). On each sampling occasion, 5 s.u. per plot were taken at random without replacement, but not processed separately. Invertebrates were extracted using Tullgren funnels.

For the last sample, in the spring of 1990, each s. u. was taken as follows: the litter and the upper soil layer were taken in a 1/11 m 2 quadrat, and then transferred to Tullgren funnels. The same depth of soil was removed from a 1/4 m 2 quadrat, and 5 l of 0.3% formaldehyde solution was then applied to this area to expel the deep soil macrofauna. 5 l of 0.4% and 0.5% solution were added at 10 minute intervals, and the underlying soil was finally dug from a 1/11 m 2 quadrat to sort invertebrates by hand on a cloth. 7 s.u. per plot were taken at random, on the same days for all the plots, and processed separately. All the saprophagous invertebrates were weighed in the laboratory to estimate the fresh biomass of litter-feeding macrofauna.

Statistical analysis

The abundance data were log transformed, in order to reduce departures from the basic assumptions of the statistical tests used, viz. normality and homogeneity of variance. However, the tests were considered to be robust to departures from normality [START_REF] Sokal | Biometry[END_REF].

In order to compensate for lack of spatial replication of treatments, making it possible to take into account spatial heterogeneity in the data analysis, the homogeneity of abundances on the plots before treatment can be checked. A set of one-factor analyses of variance (ANOVA) in blocks was carried out on the data obtained before any effect of perturbations could be determined, i.e. from the autumn of 1984 to the autumn of 1985. The different sampling occasions were treated as blocks, so as to separate temporal variation from variation due to the plots. Owing to the fact that there were no data for the L+ plot in the autumn of 1984 and spring of 1985, the first data collected in the autumn of 1985 were compared to the mean of the other four plots by t-tests.

A set of two-factor ANOVA in blocks was carried out to compare the abundances on the LoHo, LoH-, L-Ho and L-Hplots, for different periods from the spring of 1986 to the spring of 1990. The two factors were the interception of litter and the cutting of the herb layer; the sampling occasions were treated as blocks. The abundances on the same plots in the last sample, in the spring of 1990, were also compared with the help of twofactor ANOVA.

A set of one-factor ANOVA in blocks was carried out to compare the abundances under three levels of litter supply (Lo, L-, L+), for different periods from the spring of 1986 to the spring of 1990. As the cutting of the herb layer, tested beforehand , had no significant effect on the abundance of taxa, all the values obtained for the Lo and Lplots were included in the analyses, whether there was a herb layer or not. The abundances under the three levels of litter supply in the last sample, in the spring of 1990, were also compared with the help of onefactor ANOVA. The differences between sample means were tested using the Student-Newman-Keuls procedure (SNK) at the 5% level of significance.

Results

General data

Soil macrofauna community and variation in time

Ten taxa were studied throughout the experiment. Other taxa were not taken into account because they were either far less numerous, or considered too small in size on average to belong to the macrofauna. The main taxa can be roughly divided into saprophages and zoophages, feeding primarily on plant debris or prey respectively. (e) Asilid, Rhagionid and Tipulid (Limoniinae only) larvae (Diptera).

The insect larvae were highly dominant in number (44% of the total number of individuals collected on the Lo plots), even omitting the exceptional abundance of Bibionid larvae in one s.u., the autumn of 1985. Other numerous groups were Lumbricidae (16%) and both orders of Chilopoda (11% each), before Diplopoda (7%), Pseudoscorpionida (6%) and Isopoda (5%). Due to the high average individual biomass within the taxa (cf.

Petersen and Luxton 1982 for dry masses), Lumbricidae were highly dominant, coming far before Diplopoda and Chilopoda. Actual measurements of fresh biomass on the Lo plots in the spring of 1990 gave 28.6 ± 2.9 (s.e.) g m -2 for Lumbricid earthworms, 3.8 ± 1 g m -2 for Diplopoda, and less than 0.3 g m -2 for other saprophagous groups.

The variation in abundance in time, irrespective of any treatment effect, was substantial in most taxa and in the soil macrofauna as a whole (Fig. 1). On the one hand, there was some seasonal periodicity, showing maxima in autumn and minima in spring; on the other hand, there was great variation according to year, which can be ascribed both to chance as regards the sampling conditions, and to real year-to-year fluctuations in community abundance. The last sample, taken in the spring of 1990 with the purpose of collecting the soil macrofauna more completely than in probes, actually gave low densities for most taxa. In this particular case, the decrease in abundance might also be due to the change of sampling methods.

Litter amounts

The results of litter fall measurements on the Lplots are given in Table 1. The annual mean was 648 g m -2 for the total litter and 423 g m -2 for dead leaves only. Year-to-year fluctuations were about 10%. The dispersion of the measures, as given by the standard error of the mean in Table 1, shows that litter fall was heterogeneous as regards dead wood and fruits, but very homogeneous as regards dead leaves. This allows dead leaf quantities to be extrapolated to the whole site. It will be admitted that (a) the annual leaf litter supply was 423 g m -2 on the Lo plots, equalling the quantity removed from the L-plots; (b) the annual leaf litter supply was 2 x 423 = 846 g m -2

on the L+ plot.

After five years of treatments, in September 1990, the mass of litter on the ground before the fall was 508 ± 44 g m -2 and 1505 ± 118 g m -2 on the Lo and L+ plots respectively, which leads to a k' value higher on the control plots for litter supply (0.56) than on the plot with an increased litter supply (0.46). For dead leaves only, the mass on the ground before the fall was 296 ± 38 g m -2 and 744 ± 38 g m -2 on the Lo and L+ plots respectively, which also leads to a k' value higher on the control plots (0.59 vs 0.53). On the L-plots, nearly the whole of the litter layer had disappeared, the ground growing bare from the spring of 1987.

Variations in soil macrofauna abundance after perturbations

Initial data

The results of ANOVA carried out to check the homogeneity of abundance of taxa before treatments are given in Table 2. In view of the small number of sampling occasions (three blacks between the autumn of 1984 and the autumn of 1985), the power of the tests is low and there is some risk of type II errors. However, in any taxon, abundance appears homogeneous on the four plots covered with baskets (LoHo, LoH-, L-Ho and L-H-); the probability that the four samples were drawn from the same population is generally high (P ≥ 0.21)except for zoophagous Coleoptera larvae, and saprophagous Diptera larvae owing to their outstandingly contagious distribution (P = 0.06).

Effects of herb layer removal

ANOVA of the data obtained between the spring of 1986 and the spring of 1990 on the LoHo, LoH-, L-Ho and L-Hplots, show that no significant effect of cutting the herb layer can be found in the studied taxa, for any peri ad analysed (Table 3). This contrasts strongly with the effect of the decrease in litter supply, when tested for the same periods.

The result is confirmed by ANOVA on the data from the last sample, in the spring of 1990, both expressed as number of individuals or biomass. On this sampling occasion, only one taxon -Lithobiomorphawas significantly less abundant where the herb layer had been removed, and on one Hplot only (interaction with the other treatment is significant).

The simplest interpretation of this result is that herb layer has little influence on the soil macrofauna of the studied site, and its presence or absence can be disregarded when studying the effects of litter supply.

Effects of changes of litter supply

The variations in the abundance of taxa through time for three levels of litter supply (Lo, L-, L+) are represented in Figs 2 and3. The time at which a treatment began to produce an effect can be determined graphically, and ANOVA of data obtained from this time onwards makes it possible to test the significance of the apparent heterogeneity (Table 4).

In many taxa, abundance was significantly decreased on the Lplots, while there was no significant difference between the Lo and L+ plots. This result concerns Lumbricidae, Diplopoda, Isopoda and Diptera larvae among saprophagous groups, and Geophilomorpha, Lithobiomorpha, Coleoptera larvae and Diptera larvae among zoophagous groups. In the case of Diplopoda, the significance level is barely reached, and the result is only based on the least significant difference (LSD), not on the SNK procedure. The effect seems to have begun: The perturbations acted differently on the other two taxa. The abundance of saprophagous Coleoptera larvae was significantly increased and that of Pseudoscorpionida was significantly decreased on the L+ plot, but there was no significant difference between the Lo and L-plots. The effect seems to have begun in the autumn of 1986 in saprophagous Coleoptera larvae and in the spring of 1987 in Pseudoscorpionida.

ANOVA of data from the last sample, in the spring of 1990, confirm the results of the treatment effects for most taxa (Lumbricidae, Diplopoda, Isopoda, saprophagous Diptera larvae, Geophilomorpha, Lithobiomorpha), regarding individual numbers as well as biomass. On this definite sampling occasion, other taxa showed a distribution somewhat different, though not inconsistent with their previous trends.

Discussion

Effects of decrease in litter supply

The comparison between the control plots, with a normal litter supply (Lo) , and the plots where litter was intercepted (L-) shows that the existence of many taxa in the soil macrofauna depends, directly or indirectly, on litter. Whereas their abundance on the different plots showed no significant heterogeneity before treatment, the perturbation resulted in a drop in individual numbers and biomass. That was the case not only for saprophagous groups (Lumbricidae, Diplopoda, Isopoda, Diptera larvae), but also for zoophagous groups as far as individual numbers are concerned (Geophilomorpha, Lithobiomorpha, Coleoptera and Diptera larvae). Such a result, which confirms those in the literature [START_REF] Nielsen | Earthworms and the development of coprogenous A1 horizons in forest soils of Wisconsin[END_REF][START_REF] Hövemeyer | Der Einflu von Streumenge und Streuqualität auf die Siedlungsdichte von Dipterenlarven: ein Freilandexperiment im Kalkbuchenwald[END_REF][START_REF] Garay | Study of a mixed forest litter of hornbeam (Carpinus betulus L.) and oak (Quercus sessiliflora Smith) III. Organization of the edaphic macroarthropod community as a function of litter quantity[END_REF][START_REF] Judas | The development of earthworm following manipulation of the canopy leaf litter in a beechwood on limestone[END_REF], Poser 1990), may seem trivial. However, an important point is that, in most cases, there was no close dependence on the most recent litter fall. In saprophagous groups in particular, decrease in abundance was usually evident only after two yr since the perturbation started: for example, after 2 yr in Isopoda; after 2.5 yr in Diplopoda and Diptera larvae; and the decrease was more pronounced after 2 yr in Lumbricidae (cf. Fig. 2).

The reaction time of the main components of the soil macrofauna coincided approximately with the beginning of litter disappearance on the L-plots. It was a gradual process, which cannot be dated precisely, but the soil began to become bare from the spring of 1987 onwards, i.e. 1.5 yr after the perturbation had started.

When the amount of litter on the ground became very small, the soil community was likely to be influenced, a priori, by three sorts of factors: There is no objective means of separating the influence of these different factors in our experiment, but it can be affirmed that, in most cases, they did not act after the very first interception of litter. In case of stress as soon as the first interception, the members of the soil macrofauna would have responded by a decrease in abundanceowing to the fact that they are very mobile, as shown by pitfall trapping.

This implies that saprophagous groups do not feed, or feed very little, on litter lying on the ground for less than 1 yr, but depend primarily on the most decomposed debris. This is in accordance with results of feeding behaviour experiments, which show that many species of the soil macrofauna (except in some Isopoda), prefer to eat dead leaves from lower litter layers [START_REF] Van Der Drift | Analysis of the animal community in a beech forest floor[END_REF][START_REF] Kheirallah | Behavioural preference of Iulus scandinavius (Myriapoda) to different species of leaf litter[END_REF][START_REF] Soma | Ecological studies of soil organisms with references to the decomposition of pine needles. II. Litter feeding and breakdown by the woodlouse Porcellio scaber[END_REF][START_REF] David | Influence de la durée du séjour dans la litière des feuilles mortes de chêne (Quercus petraea Liebl.) sur leur consommation par le Diplopode Cylindroiulus nitidus (Verhoeff, 1891)[END_REF]. It follows that the saprophagous macrofauna feed on the organic matter reduced by leaching, activity of mesofauna, microfauna and microflora, and thus much below 423 g m -2 , the mean annual leaf fall.

Effects of increase in litter supply

Dead leaves and associated microflora are actively eaten by many species of the forest soil macrofauna.

Therefore, an increase in litter supply constitutes undeniably an increase in food for saprophagous populationseven if the additional food is not directly available and does not equal the amount of dead leaves which are added (see above).

However, the comparison between the control plots (Lo) and the plot with a twofold litter supply (L+)

shows that this treatment had no effect on the abundance of the main taxa. Only the abundance of saprophagous Coleoptera larvae, a group of minor importance with respect to biomass, was significantly increased on the L+ plot; and, though this observation is in accordance with the natural distribution of Elaterid larvae in thick litter layers [START_REF] Bornebusch | The fauna of forest soil. -Forst[END_REF], one cannot be certain that it is due to the increase in litter supply. In this particular case, the lack of baskets on the plot makes the comparison difficult, as egg-laying conditions were not the same as on the Lo and Lplots for winged adult insects. In all the other saprophagous groups, abundance did not deviate significantly from the control value. Not only was there no significant difference between individual numbers and biomasses on the Lo and L+ plots in the last sample (4.5 yr after the treatment had started), but also the variations in abundance through time were remarkably similar on both kinds of plots (cf. Fig. 2).

As regards the question of the amount of resource at the disposal of the macrofauna in mull conditions, this outcome may be interpreted in several ways.

(a) According to the simplest interpretation, there is no limiting resource and the abundance of populations is controlled by density-independent factors. This assumption is reinforced by the fact that closely related species, with regards to both taxonomy and feeding habits, were often found in the same sampling unit;

for instance, Glomeris marginata and G. intermedia in Diplopoda, Lumbricus centralis and L. festivus in anecic earthworms.

However, given this interpretation, it must be explained why there is apparently no litter accumulation in the mull site. If the saprophagous soil macrofauna are not food-limited, how is it that excess food does not accumulate year by year? There are two possible answers to this argument, frequently discussed after [START_REF] Slobodkin | Regulation in terrestrial ecosystems and the implied balance of Nature[END_REF].

(1) The first one, which is not at variance with the last authors' opinion, is that food uneaten by macrofauna would be utilized by other communities in the site, such as soil mesofauna, or fungal and bacterial microflora, the real decomposers; these communities would be saturated and food-limited, as assumed by [START_REF] Vannier | Modes d'exploitation et de partage des ressources alimentaires dans le système saprophage par les microarthropodes du sol[END_REF] in mesofauna, but the soil macrofauna would not.

(2) The second one is that no soil community would be food-limited, and that, logically, organic matter would be in excess in forest soils; but while that could lead to some perceptible accumulation in mor sites where the excess litter is considerable (Van der Drift 1963), it would not be the same in mull sites; first because the excess, if any, is presumably slighter; secondly because accumulation, in ecosystems which are not closed, may be delayed or balanced by organic matter exports, notably by drainage.

Therefore, the assumption that the soil macrofauna, or even the whole decomposer community, is controlled by density-independent factors, cannot be ruled out. It would even be unquestionable if the variations in abundance through time on the Lo and L+ plots really represented year-to-year fluctuations. The extent of the variation in abundance of Lumbricidae, for example, would imply that they were below the carrying capacity of the environment for most of the time. Unfortunately, it is not evident that the variations observed in probes were real year-to-year fluctuations.

(b) According to a second interpretationcompatible with the results if the variations in abundance through time could be ascribed to sampling conditionsspace would be the major limiting factor, with the soil macrofauna being already saturated with individuals. This assumption would explain why the abundance of the main taxa did not increase significantly, in spite of an additional food supply. However, this is not very realistic, owing to the fact that higher biomasses and individual numbers have been measured in other forest mull communities (Bornebusch 1930, Petersen and[START_REF] Petersen | A comparative analysis of soil fauna populations and their role in decomposition processes[END_REF].

(c) Finally, the assumption that food would be the limiting factor cannot be completely discarded, but with the condition that other changes brought about by the increase in litter thickness had negative effects, balancing the positive effect of the increase in food supply. Though this more complicated process is also more unlikely, there is at least one reason for not precluding it. When the increase in litter supply was more extreme than in our experiment, a negative effect was effectively observed, in Lumbricidae in particular [START_REF] Judas | The development of earthworm following manipulation of the canopy leaf litter in a beechwood on limestone[END_REF], which gives weight to the idea that opposing influences may govern the resulting abundance of macrofauna. This would also lead to regard the conflicting conclusions in the literature as equally plausible, the equilibrium abundance after perturbation changing according to conditions and populations involved. For instance, [START_REF] Nielsen | Earthworms and the development of coprogenous A1 horizons in forest soils of Wisconsin[END_REF] found that a twofold litter supply resulted in twice as much Lumbricid activity, preventing any accumulation; the same perturbation had no effect on the abundance of Lumbricidae in our experiment, whereas [START_REF] Judas | The development of earthworm following manipulation of the canopy leaf litter in a beechwood on limestone[END_REF] found that abundance decreased after two successive fivefold increases in litter supply. Under the last conditions, [START_REF] Hövemeyer | Der Einflu von Streumenge und Streuqualität auf die Siedlungsdichte von Dipterenlarven: ein Freilandexperiment im Kalkbuchenwald[END_REF] found that the abundance of saprophagous Diptera larvae increased, whereas a twofold litter supply had no effect on them in our experiment.

A negative effect is very unlikely to have originated in an increase in predation on the L+ plot.

Predators capable of feeding, at least in part, on saprophagous macrofaunasuch as Chilopoda [START_REF] Poser | Chilopoden als Prädatoren in einem Laubwald[END_REF]) -

were not significantly more numerous than on the control plots. Further studies are in progress in order to detect possible noxious factors associated with the thicker litter layer on the L+ plot, notably from the chemical point of view. But it could be very difficult to prove that food is a limiting factor in forest mull communities, if other factors actually tend to inhibit the effect of an increment in food supplies.

Soil saprophagous macrofauna and processes of litter decomposition

The five-yr treatment on the L+ plot clearly initiated a process of dead leaf accumulation in relation to the control plots. The difference between the k' values for dead leaves of the L+ and Lo plots, 0.53 and 0.59 respectively, reflects that process, which is even more pronounced if the whole litter is taken into account, dead wood included.

However, in a forest mull with soil macrofauna in abundance, k' should not be regarded as the parameter of a simple exponential model of decomposition, in which litter would lose a fairly constant percentage of mass per year. The measured k' value results from several decomposition processes (Witkamp and Olson 1963):

(a) Microprocesses (consumption by microfauna and mesofauna, microbial decomposition, leaching), the effects of which may approximate, more or less, to an exponential model, as in fine mesh bag experiments [START_REF] Anderson | The breakdown and decomposition of sweet chestnut (Castanea sativa Mill.) and beech (Fagus sylvatica L.) leaf litter in two deciduous woodland soils[END_REF][START_REF] Lousier | Litter decomposition in a cool temperate deciduous forest. -Can[END_REF][START_REF] Berg | Changes in organic-chemical components and ingrowth of fungal mycelium in decomposing birch leaf litter as compared to pine needles[END_REF].

(b) A removal of dead leaves by mull macrofauna, at a given time during decomposition, for example 1 or 2 years after litter fall; the term removal underlines the fact that soil macrofauna not only break down leaf litter in faeces (Macroarthropoda, epigeic and anecic Lumbricidae, Gastropoda), but also mix faeces in the A1 layer (endogeic and anecic Lumbricidae); anecic Lumbricidae also bury complete leaves in the soil, without eating (e.g. [START_REF] Satchell | Selection of leaf litter by Lumbricus terrestris[END_REF]Lowe 1967, Szlavecz 1985).

It is quite possible that the microprocesses of litter decomposition were enhanced on the L+ plot, due to microclimatic changes connected with litter thickness [START_REF] Dwyer | Influence of topographic heterogeneity on deciduous litter decomposition[END_REF]. In spite of this, dead leaves accumulated on this plot in relation to the control. This is to be compared with the fact that the abundance of saprophagous macrofauna was not higher on the L+ than on the Lo plots during the experiment, in both individual numbers and biomass, from which it can be inferred that the same amount of leaf litter was removed annually by the soil macrofauna on both kinds of plots. The dead leaf accumulation on the L+ plot in relation to the control may therefore have been due to the discrepancy between litter supply and consumption by saprophagous macrofauna, however high the decomposition due to microprocesses may have been.

Conclusions

The saprophagous macrofauna of acid mull in a temperate forest feeds primarily on litter lying on the ground for more than one year. Taking into account decomposition and leaching which occur until the time when litter is consumed, the mass of available food is appreciably lower than the mass of litter fall.

In spite of this, there is no evidence of food being a limiting factor in the main taxa of saprophagous macrofauna. The community, when followed through 4.5 yr, does not respond to an increase in litter supply by an increase in abundance, either in individual numbers or in biomass. The implicit acceptance of some exploitation competition in soil macrofauna communities (either intraspecific or interspecific if several species feed on the same resource) is, at least, hazardous.

A twofold litter supply results in an accumulation process, in relation to control conditions, which is likely to be caused by the inability of the soil macrofauna to eat or remove the additional food supply.

Legends of figures

  and Staphylinid larvae (Coleoptera).

  the spring of 1986 in zoophagous Coleoptera larvae; (b) in the autumn of 1986 in Lumbricidae, though becoming much more evident in the autumn of 1987 (in both cases P < 0.001); (c) in the autumn of 1987 in Isopoda and zoophagous Diptera larvae; (d) in the spring of 1988 in Diplopoda, saprophagous Diptera larvae, Lithobiomorpha, and in the spring or autumn of 1988 in Geophilomorpha (in both cases P < 0.001).

  Microclimatic factors. The buffer action of litter towards temperature and moisture fluctuations no longer existed on the L-plots, and some light directly reached the soil. (b) Trophic factors. Litter-feeding populations were deprived of food on the Lplots; the case of zoophagous groups is less clear, because they feed mainly on mesofauna, for which data are not yet available.(c) Structural factors. The complexity of litter, which affects the abundance of some predators[START_REF] Bultman | Effect of structure and nutritional quality of litter on abundances of litterdwelling Arthropods[END_REF], was lowered on the Lplots.

Fig. 1 .

 1 Fig. 1. Variations in abundance of the whole soil macrofauna through time. Data from the Lo plots. (A: Autumn. S: Spring. B.I.: Bibionid larvae separated from other saprophages).

Fig. 2 .

 2 Fig. 2. Variations in abundance of the saprophagous taxa through time, under three levels of litter supply: Lo (white squares), L-(black squares), L+ (triangles). (Vertical line: beginning of treatments. Arrow: beginning of effect to test. A: Autumn. S: Spring).

Fig. 3 .

 3 Fig. 3. Variations in abundance of the zoophagous taxa through time, under three levels of titter supply: Lo (white squares), L-(black squares), L+ (triangles). (Vertical tine: beginning of treatments. Arrow: beginning of effect to test. A: Autumn. S: Spring).

Fig. 1

 1 Fig. 1

Fig. 3

 3 Fig. 3

Table 2

 2 also gives the results of t-tests comparing the first sample from the L+ plot with those from the other four plots for the autumn of 1985. They show that, in any taxon, the probability that the L+ estimate was from the same normal distribution as the other sample means is high (P ≥ 0.19).

Table 1 .

 1 Average masses (g air-dried) of annual litter fall on the L-plots (± standard error).

	Year			dead leaves		dead wood and fruits	Total
		Oak Beech Hornbeam Total
	Sept. 85-Sept. 86 339	31	29	399±4	225±33	625±34
	Sept. 86-Sept. 87 386	30	19	435±5	202±23	637±23
	Sept. 87-Sept. 88 376	39	19	434±6	248±32	683±35
	Mean	367	33	23	423	225	648

Table 2 .

 2 Statistical analysis of abundances before treatments. P is the probability that the sample means are from the same population.

		1-factor anova in blocks on four plots with	t-test comparing L+ plot to other four
		baskets (autumn 84-autumn 85)	plots (autumn 85)
		P-value	P-value
	Lumbricidae	0.21	0.63
	Diplopoda	0.61	0.37
	Isopoda	0.22	0.46
	sapro. Coleoptera l.	0.34	0.61
	sapro. Diptera l.	0.06	0.86
	Geophilomorpha	0.57	0.19
	Lithobiomorpha	0.33	0.81
	Pseudoscorpionida	0.65	0.19
	zoo. Coleoptera l.	0.06	0.68
	zoo. Diptera l.	0.23	0.36

Acknowledgements -We are very grateful to H. Read for having checked the English text. We are indebted to F. Arpin, R. Botalla, M. Dolin and A. Munsch (C.N.R.S., URA 689) for their technical cooperation.

Table 3. Statistical analysis of abundances under two treatments: litter interception (Lo vs L-) and herb-layer removal (Ho vs H-). P is the probability that the sample means are from the same population. The analysed period is the one that yields the lower P-value for the H factor. (A: autumn; S: spring; *: significant; **: highly significant; ***: very highly significant).

2-factor anova in blocks (spring 1986-spring 1990)