J F Ponge

S Gillet

F Dubs

E Fedoroff

L Haese

J P Sousa

P Lavelle

COLLEMBOLAN COMMUNITIES AS BIOINDICATORS OF LAND USE INTENSIFICATION

Keywords: Land use, biodiversity, Humus Index

Springtail communities (Hexapoda: Collembola) were sampled in the Morvan Nature Regional Park (Burgundy, France) in six land use units (LUUs) one square kilometer each, which had been selected in order to cover a range of increasing intensity of land use. Human influence increased from LUU 1 (old deciduous forest) to LUU 6 (agricultural land mainly devoted to cereal crops), passing by planted coniferous forests (LUU 2) and variegated landscapes made of cereal crops, pastures, hay meadows, conifer plantations and small relict deciduous groves in varying proportion (LUUs 3 to 5).

Sixteen core samples were taken inside each LUU, at intersections of a regular grid. Species composition, species richness and total abundance of collembolan communities varied according to land use and landscape properties. Land use types affected these communities through changes in the degree of opening of woody landscape (woodland opposed to grassland) and changes in humus forms (measured by the Humus Index). A decrease in species richness and total abundance was observed from old deciduous forests to cereal crops. Although the regional species richness was not affected by the intensification gradient (40 to 50 species were recorded in every LUU), a marked decrease in local biodiversity was observed when the variety of land use types increased. In variegated landscapes the observed collapse in local species richness was not due to a different distribution of land use types, since it affected mainly woodland areas. Results indicated the detrimental influence of the rapid afforestation of previous agricultural land, which did not afford time for the development of better adapted soil animal communities.

Introduction

Collembolan communities have been shown to vary in abundance and species composition according to changes in vegetation and soil conditions [START_REF] Hågvar | Collembola in Norwegian coniferous forest soil. I. Relations to plant communities and soil fertility[END_REF][START_REF] Ponge | Biocenoses of Collembola in atlantic temperate grass-woodland ecosystems[END_REF][START_REF] Chagnon | Community structures of Collembola in sugar maple forests: relations to humus type and seasonal trends[END_REF]. Soil acidity, mainly through associated changes in food and habitat, but also through chemical composition or osmolarity of the soil solution, may favour or disfavour some species [START_REF] Hågvar | Collembola in Norwegian coniferous forest soils. III. Relations to soil chemistry[END_REF][START_REF] Vilkamaa | Effect of fertilization and pH on communities of Collembola in pine forest soil[END_REF][START_REF] Salmon | Earthworm excreta attract soil srpingtails: laboratory experiments on Heteromurus nitidus (Collembola: Entomobryidae)[END_REF], and pH 5 has been noted as a landmark between two distinct types of communities [START_REF] Ponge | Biocenoses of Collembola in atlantic temperate grass-woodland ecosystems[END_REF]. The opposition between grassland and woodland can also be traced by the species composition of springtail populations [START_REF] Gisin | Ökologie und Lebengemeinschaften der Collembolen im schweizerischen Exkursionsgebiet Basels[END_REF][START_REF] Rusek | Collembola and Protura in a meadow-forest ecotone[END_REF][START_REF] Ponge | Biocenoses of Collembola in atlantic temperate grass-woodland ecosystems[END_REF]. As a whole, Collembola are highly tolerant of a wide range of environmental conditions, including agricultural and industrial pollution, but species differ strongly in their sensitivity to environmental stress [START_REF] Lebrun | Effets écologiques de la pollution atmosphérique sur les populations et communautés de microarthropodes corticoles (Acariens, Collemboles et Ptérygotes)[END_REF][START_REF] Prasse | Indications of structural changes in the communities of microarthropods of the soil in an agro-ecosystem after applying herbicides[END_REF][START_REF] Sterzyńska | Communities of Collembola in natural and transformed soils of the linden-oakhornbeam sites of the Mazovian Lowland[END_REF]. The parthenogenetic collembolan Folsomia candida is now currently used as a standard in the assessment of environmental risk [START_REF] Riepert | Development of laboratory methods for testing effects of chemicals and pesticides on Collembola and earthworms[END_REF][START_REF] Cortet | The use of invertebrate soil fauna in monitoring pollutant effects[END_REF][START_REF] Crouau | The use of Folsomia candida (Collembola, Isotomidae) for the bioassay of xenobiotic substances and soil pollutants[END_REF]. In the search for indicators of environmental change, more especially those affecting biodiversity, abundant, diverse animal communities can be used to trace changes taking place at the landscape level, as this has been demonstrated in other arthropod groups [START_REF] Duelli | Population movements of arthropods between natural and cultivated areas[END_REF][START_REF] Halme | Carabid beetles in fragments of coniferous forest[END_REF].

The present study was undertaken within the European Community project BioAssess. Here we present springtail results (Hexapoda: Collembola) from the French sites, which were located in the Morvan Regional Nature Park (Burgundy). This central region was selected for its high variety of land use types, ranging from large areas of old forests or cereal crops to variegated landscapes with intricate deciduous and coniferous woodlands, pastures, hay meadows and agricultural fields [START_REF] Plaisance | Évolution des paysages forestiers et arborés en Bourgogne et Franche-Comté de l'an mille à nos jours[END_REF]. We asked whether there was a response of collembolan communities to land use intensification and, if yes, whether this effect was just a replacement of species or affected biodiversity patterns.

Material and methods

Study sites

Sampling took place in the Morvan Regional Nature Park, which covers most of the northern part of the Morvan natural region (western Burgundy, Centre of France). The climate is continental, with an annual rainfall averaging 1000 mm and a mean temperature of 9°C. The parent rock is granite.

The soil trophic level is poor, but despite moderate to strong acidity, the dominant humus form is mull [START_REF] Perrier | La forêt au Mont Beuvray: phytoscociologie[END_REF].

In the Morvan region, land use is shared between sylviculture (45%) and agriculture (55%).

Forested areas are comprised of coniferous stands (silver fir, Douglas fir, Norway spruce) with an artificial intensive management system (45%), and deciduous stands (beech, oak) with a semi-natural or a traditional management system (55 %). Agricultural areas are made up of grassland (80%, among which 40% are permanent pastures and 40 % are temporary hay meadows) and crops (20%) with dominance of cereals (wheat, barley) and conifer trees (Norway spruce, Douglas fir). Agricultural management systems exhibit a wide range of disturbance intensity (use of mineral fertilizers and pesticides to organic manure only). Several socio-economical and political driving forces influenced dynamics and composition of the landscape during the last five decades [START_REF] Plaisance | Évolution des paysages forestiers et arborés en Bourgogne et Franche-Comté de l'an mille à nos jours[END_REF]. Many old deciduous forests have been transformed to coniferous plantations and more recently forested areas expanded by afforestation of previous agricultural land, using European subsidies.

Six land use units (LUUs), one square kilometer each, have been chosen on the basis of aerial photographs, taking into account the distribution of forested areas (coniferous, deciduous), meadows and agricultural crops. LLUs 1 to 6 depicted a gradient of increasing influence of human activities:

 LUU 1 is within an old (100-150 year) deciduous forest landscape managed by the French National Office of Forests (public sector). This forested area is made of acidophilic beechwoods (Fagus sylvatica L.), oakwoods [Quercus petraea (Mattus.) Liebl.] and mixed stands, with holly (Ilex aquifolium L.) in the understory. The management system is based on natural regeneration and selection by man. LUU 1 is made up of stands at different stages of forest development.

 LUU 2 is within a more recent (20-50 year) coniferous forest landscape managed by the French Forest National Office (public sector), mostly made of silver fir plantations (Abies alba Mill.). Previous land use was deciduous forest. Where soils were too wet for coniferous growth spontaneous vegetation was let to grow (willow, alder, birch). The management system of coniferous stands is intensive and based on artificial regeneration (clear-cut followed by plantation).

 LUU 3 is comprised of meadows within a forested landscape. Originally farmers cleared the native forest. Currently, by the way of national subsidies for afforestation of agricultural land, Douglas fir [Pseudotsuga menziesii (Mirb.) Franco] and Norway spruce [Picea abies (L.)

Karst.] were planted fifty years ago on previous agricultural land purchased by private insurance companies. Remains of the old deciduous forest (now managed as beech and oak coppice) are also present, as well as a few cereal crops.

 LUU 4 is a mixed land use mosaic characterised by the presence of wet meadows. The agricultural system is based on organically manured meadows and intensive cereal crops (recently converted to organic farming). Some plots were afforested with Douglas fir and Norway spruce about thirty years ago.

 LUU 5 is a meadow landscape. The dominant agricultural system is based on organic farming.

A few plantations of Douglas fir or Norway spruce (20-50 years old) are also present, as well as a few relict deciduous thickets pastured by livestock.

 LUU 6 is an agricultural landscape dominated by cereal crops. The agricultural system is intensive with a range of intensity levels depending on the farmer, but pesticides and mineral fertilizers are used currently. Some plots are prescribed fallow, some others have recently turned to short rotation conifer crops (Christmas trees). Recently abandoned land (scrub) is also present.

Sampling procedure

Using aerial photographs, a grid of 16 regularly spaced plots (200 m) was identified in each of the six LUUs, and their position in the field was found by their spatial coordinates, given by a calibrated GPS system. Each sampling plot was indicated by a central post. Litter and soil springtails were sampled by taking a core (5 cm diameter, 10 cm depth) at a three-meter distance from the central post, in a northerly direction. Soil and litter were immediately sealed in a polythene bag then transported within three days to the laboratory. Sampling took place in June 2001. Extraction was performed within ten days using the dry funnel method. Animals collected under the desiccating soil were preserved in 95% ethyl alcohol before being sorted under a dissecting microscope. Collembola were mounted in chloral-lactophenol (25 ml lactic acid, 50 g chloral hydrate, 25 ml phenol) and identified to species under a phase contrast microscope at x400 magnification. Identification was done using [START_REF] Gisin | Collembolenfauna Europas[END_REF], [START_REF] Zimdars | Synopses on palaearctic Collembola. I. Tullbergiinae[END_REF], [START_REF] Jordana | Fauna Iberica. VIII. Collembola Poduromorpha[END_REF], [START_REF] Fjellberg | The Collembola of Fennoscandia and Denmark. I. Poduromorpha[END_REF][START_REF] Bretfeld | Synopses on palaearctic Collembola. II. Symphypleona[END_REF].

Humus forms (Table 1) were identified in the vicinity of core samples, after visual inspection of trenched soil, using morphological criteria defined by [START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF]. Mor was separated from Dysmoder using [START_REF] Ponge | Comment distinguer dysmoder et mor? L'exemple de la forêt domaniale de Perche-Trappe (Orne)[END_REF]. The Humus Index was measured at each sampling plot after scaling humus forms according to principles presented by [START_REF] Ponge | Humus Index: an integrated tool for the assessment of forest floor and topsoil properties[END_REF].

Amphimoder was defined for the first time in order to classify humus forms presenting both features of mull (crumby A horizon) and mor (litter with an OM horizon, without any visible signs of animal activity). Agricultural Moder was also defined for the first time to classify an agricultural solum with a spongy structure made of small enchytraeid faeces [START_REF] Didden | Involvment of Enchytraeidae (Oligochaeta) in soil structure evolution in agricultural fields[END_REF][START_REF] Topoliantz | Earthworm and enchytraeid activity under different arable farming systems, as exemplified by biogenic structures[END_REF], and was given a Humus Index of 6 as for Eumoder. Other agricultural soils exhibited a crumby structure made of faeces of earthworms or large enchytraeids. The Humus Index of these soils was assigned to 1 as for Eumull. Hydromorphic variants of humus forms such as Hydromull, Hydromoder and Hydromor were given the same Humus Index as their aerial counterparts exhibiting similar development of OL, OF, OH, and OM horizons.

Woody plant species growing in the vicinity of sampling plots were identified using [START_REF] Rameau | Flore Forestière Française. I. Plaines et Collines[END_REF].

Statistical analyses

Densities of the different collembolan species were analysed by simple correspondence analysis (CA), a multivariate method using the chi-square distance [START_REF] Greenacre | Theory and applications of correspondence analysis[END_REF]. Active (main) variables were species, coded by the number of individuals. Contrary to canonical correspondence analysis [START_REF] Ter Braak | The analysis of vegetation-environment relationships by canonical correspondence analysis[END_REF] passive (additional) variables were projected as if they had been used in the analysis but they did not influence to any extent the formation of the factorial axes. In the present study, additional variables were land use units (each coded as 1 or 0), land use types (each coded as 1 or 0), species richness and total abundance of collembolan populations (counts), woody species (each coded as 1 or 0) and the Humus Index (scoring from 1 to 9).

In order to give the same weight to all parameters, all variables (discrete as well as continuous) were transformed into X = (x-m)/s + 20, where x is the original value, m is the mean of a given variable, and s is its standard deviation. The addition to each standardized variable of a constant factor of 20 allows all values to be positive, correspondence analysis dealing only with positive numbers (commonly counts). Following this transformation, factorial coordinates of variables can be interpreted directly in term of their contribution to the factorial axes. Variables were doubled in order to allow for the dual nature of most parameters (the absence of a given species is as important as its presence, low values are as important as high values for measurement data). To each variable X was thus associated a twin X' varying in an opposite sense (X' = 40 -X). Such a doubling proved useful when dealing with ecological gradients [START_REF] Ponge | Soil fauna and site assessment in beech stands of the Belgian Ardennes[END_REF][START_REF] Loranger | Does soil acidity explain altitudinal sequences in collembolan communities[END_REF]. The transformations used here give to correspondence analysis most properties of the well-known principal components analysis [START_REF] Hotelling | Analysis of a complex of statistical variables into principal components[END_REF], while keeping the advantage of the simultaneous projection of rows (variables) and columns (samples) onto the same factorial axes and the robustness due to the principle of distributional equivalence.

In each LUU the variety of land use types was expressed by the Shannon Index, i.e. the number of binary digits (bits) measuring the information given by a sample according to the formula ∑p i .log(p i ), where p i is the probability given to land use type i among the 16 samples taken in a LUU.

One-way analyses of variance (ANOVA) followed by SNK procedure for comparisons among means were performed on some parameters [START_REF] Glantz | Primer of biostatistics[END_REF]. Homogeneity of variances between the different LUUs and normal distribution of residuals were tested prior to analysis. The absence of spatial autocorrelation was checked by computing Spearman rank correlation coefficients between adjacent rows and columns of each 16-pt sampling grid. None of these coefficients gave any significant value at the 0.05 level, thus the distance between adjacent samples (200 m) was judged enough to avoid autocorrelation. Given the absence of autocorrelation, the 16 samples taken in each LUU were considered as replicates.

Results

Table 2 shows the distribution of land use types among the 16 samples taken in each LUU.

One of the 16 plots could not be sampled in LUU 4, due to waterlogging. The most widespread land use types were deciduous forests, coniferous forests, and pastures.

Table 3 shows total numbers for springtail species found in every LUU. The most abundant species were the isotomids Folsomia quadrioculata (1742 ind.), Isotomiella minor (1517 ind.) and Parisotoma notabilis (1017 ind.).

Analysis of collembolan communities

The matrix analysed crossed 95 columns (samples) and 89 x 2 rows (species, doubled as mentioned above), as main (active) variables. Additional variables (84) were added, in order to facilitate interpretation of factorial axes. Only the first axis of correspondence analysis (6.5% of the total variance) was interpretable in terms of ecological factors. The second axis (5.0% of the total variance) was roughly a quadratic function of the first axis, i.e. when samples and variables were projected in the plane of the first two axes their cloud formed a parabola, i.e. they exhibited a Guttman or horsehoe effect [START_REF] Greenacre | Theory and applications of correspondence analysis[END_REF]. In this case, only the first axis (corresponding to the first eigen value of the distance matrix) was used for projecting the cloud of data. For the sake of clarity only main variables (collembolan species) and additional variables, but not individual samples, will be further considered.

Collembolan species could be projected on factorial axes both as high (original data, with their mean and standard deviation forced to 20 and 1, respectively) and low values (complement to 40), but for the sake of clarity only high values will be shown and discussed (Fig. 1). Collembolan species were continuously scaled along Axis 1, indicating that the first factorial axis expressed changes in collembolan communities according to some gradient. Significance of this gradient was shown by the projection of additional variables. The six LUUs were scaled in the order 2,1,3,5,4,6, with a large space between 3 and 5. This corresponded to an opposition between woodland (LUUs 1 to 3, positive side of Axis 1) and grassland environments (LUUs 4 to 6, negative side of Axis 1), with a slight departure from the original scaling of increasing intensity of land use (1 and 2 were inverted, 4 and 5 were inverted, too). The projection of land use types on Axis 1 reinforced the view that woodland areas were opposed to agricultural areas along Axis 1. This interpretation was strengthened by the fact that species typical of grassland environments [START_REF] Ponge | Les biocénoses des Collemboles de la forêt de Sénart[END_REF][START_REF] Ponge | Biocenoses of Collembola in atlantic temperate grass-woodland ecosystems[END_REF], such as Isotoma viridis, Lepidocyrtus cyaneus, Deuterosminthurus sulphureus, Sminthurus nigromaculatus, Brachystomella parvula, Sminthurus viridis and Isotoma tigrina, were all on the negative side of Axis 1, whereas species typical of woodland environments [START_REF] Ponge | Les biocénoses des Collemboles de la forêt de Sénart[END_REF][START_REF] Ponge | Biocenoses of Collembola in atlantic temperate grass-woodland ecosystems[END_REF], such as Pseudisotoma sensibilis, Xenylla tullbergi, Entomobrya nivalis and Orchesella cincta were all on the positive side (Table 2). Hedgerows exhibited an intermediate position between grassland and woodland environments. Coniferous woodlands did not exhibit profound changes in collembolan communities when compared to deciduous woodlands, as well as clearcut areas, but forest influence was at a maximum in deciduous forests, followed by coniferous forests then by clearcut areas. On the negative side, pastures, hay meadows and agricultural fields did not exhibit differences in collembolan communities, forming a homogeneous group on the negative side of Axis 1. Changes in total abundance and species richness were also depicted by Axis 1, more species and more individuals per unit surface being present in forested than in agricultural areas. The total abundance of Collembola and the species richness of individual samples were linearly correlated with Axis 1 (P < 0.001 and P < 0.01, respectively).

Collembolan communities of pastures and hay meadows did not change according to LUUs, contrary to agricultural fields and woodlands (Fig. 2). In LUU 3 and LUU 4 collembolan communities from agricultural fields were not very different from their coniferous woodland counterparts, as exemplified by the projection of the corresponding passive variables at the same level of Axis 1. Far from the origin on the positive side of Axis 1 (thus most typical for forest environments) were deciduous woodlands from LUU 1 and LLU 3, and coniferous woodlands from LUU 2, other forested sites being less different from open environments.

Discrepancies between forested sites were reflected in the projection of woody plant species on Axis 1 (Fig. 3). Although Quercus petraea, Fagus sylvatica and Abies alba were far from the origin on the positive side of Axis 1, other timber trees such as Picea abies and Pseudotsuga menziesii were near the origin, not far from open environments. Thus collembolan communities from spruce and Douglas fir plantations in agricultural landscapes (LUUs 3 to 6) differed less from agricultural fields and pastures than they differed from old beech and oak forests or from silver fir plantations in forested landscapes (LUUs 1 and 2). Trees typical of early stages of forest succession (abandoned fields) or of woodland borders, such as Prunus spinosa L., Crataegus monogyna Lacq., Malus sylvestris Mill., Pyrus pyraster Burgsd., Cytisus scoparius (L.) Link, Salix spp., Acer pseudoplatanus L., Prunus avium L., and Sambucus racemosa L., were nearly at the same position as planted spruce and Douglas fir, indicating that collembolan communities of Douglas fir and Norway spruce plantations did not differ to any great extent from early stages of forest succession (old fallows).

The projection of humus forms along Axis 1 revealed that forest samples exhibited thick organic horizons (typically Dysmoder and Amphimull) as opposed to agricultural fields and meadows which were characterized by Eumull (Fig. 4). The Humus Index exhibited a highly significant linear correlation with Axis 1 (P < 0.001). Thus Axis 1 reflected also a decreasing trend of soil biological activity from open to closed environments. This interpretation was reinforced by the position of all species known to live only in raw humus (Mor, Dysmoder) and other acid humus forms, i.e.

Sminthurinus signatus, Mesaphorura yosii, Willemia anophthalma, Proisotoma minima, Xenylla tullbergi, Pseudosinella mauli and Micraphorura absoloni on the positive side of Axis 1, and the projection of all species known to live only in Eumull, i.e. Sminthurinus aureus, Pseudosinella alba, Parisotoma notabilis, Onychiurus jubilarius, Heteromurus nitidus and Stenaphorura denisi, on the negative side of Axis 1. The position of Agricultural Moder is worthy of note, since it was projected not far from the origin, thus far from samples typical of agricultural fields (Fig. 1). This indicated that its species composition differed somewhat from Eumull, showing similarities with forest humus forms with thick litter horizons, despite the total absence of litter. Examination of individual samples revealed that acidophilic species such as Sminthurinus signatus, Willemia anophthalma and Mesaphorura yosii were present in Agricultural Moder (4 samples, all but one in LUU 4), and not in agricultural soils with Eumull (9 samples, all but one in LUU 6). On the contrary, the acido-intolerant species Pseudosinella alba was present in agricultural soils with Eumull, not in Agricultural Moder. In both agricultural crop environments, the open-habitat species Isotoma viridis and Lepidocyrtus cyaneus were present.

Waterlogging (and the associated humus forms Hydromull, Hydromoder and Hydromor) did not influence species composition to a great extent. All corresponding samples were not far from the origin (Fig. 4), and no other factorial axis was found to isolate these samples. It should be noticed that hygrophilic species such as Isotomurus palustris, Lepidocyrtus lignorum and Sminthurides schoetti were all far from the origin on the negative side of Axis 1, indicating that these species were present in open environments, even when soils were not waterlogged.

Biodiversity and land use variety

The total species richness (cf. 40-50 species found in each LUU) showed little variation between LUUs (Table 3), i.e. each contained around half the total number of species found in the whole sample (89). In contrast, the individual species richness (the number of species found in a core sample 5 cm diameter and 10 cm depth) varied markedly among the six LUUs (Fig. 5). Analysis of variance (ANOVA) revealed a significant heterogeneity according to LUUs (F = 2.7, P < 0.05), most difference (significant at 0.05 level) being between LUU 1 and LUU 4. The curve formed by the six mean values was saddle-shaped, indicating a continuous decrease from LUU 1 to LUU 4 followed by a continuous increase up to LUU 6, although the latter did not reach the level of species richness exhibited by LUU 1.

The distribution of land use types (Table 2) can be used in each LUU to measure the variety of the landscape. The Shannon Index [START_REF] Shannon | A mathematical theory of communications[END_REF] allowed to compare the species richness of individual samples with a quantitative landscape factor (Fig. 5). The curve of land use variety mirrored that of local species richness, the latter increasing then decreasing in contrast to the former, which was better exemplified by a correlation plot (Fig. 6). The least variation in land use occurred in a square kilometer, the more species occurred together at the scale of the core sampler.

The hypothesis that negative effects of landscape variety on local species richness could be due to changes in the dominant land use types was tested by examining individual trends followed by the main land use types when crossing several LUUs. Given results from correspondence analysis, coniferous and deciduous forests were pooled into an unique woodland category. Accordingly, pastures, hay meadows and agricultural crops (cereals, rape) formed the grassland category. It appeared that in grassland the species richness of individual samples exhibited only a slight increase from LUU 3 to LUU 6 (no grassland occurred in LUU 1 and LUU 2), while strong variation according to LUUs was observed in the woodland category (Table 4). The decrease observed from LUU 1 to LUU 4 when taking only woodland into account (approximating 50%) was more pronounced than when all land use types were included in the calculation (Fig. 5). Thus the decrease in biodiversity observed from LUU 1 to LUU 4 concerned only woodland.

Examination of individual data did not reveal any meaningful trend of extinction of species.

Rather, a collapse in the total population was observed in woodland samples taken in LUU 4 (Table 4), which could explain the observed fall in local species richness. Such changes in total abundance of Collembola were never observed in grassland samples.

Discussion

In the Morvan Nature Regional Park, land use intensification caused changes in species composition, total abundance and species richness of collembolan communities. The first axis of correspondence analysis showed a global trend contrasting forest sites (closed, with accumulation of organic matter at the ground surface) with agricultural sites (open, with rapid incorporation of organic matter). The Humus Index [START_REF] Ponge | Humus Index: an integrated tool for the assessment of forest floor and topsoil properties[END_REF] showed an improvement of soil biological activity in grassland, compared to woodland soils. This may result from a combination of factors, all of them acting in the same direction: choice of the best soils for crop and cattle production [START_REF] Braojos | Études sur l'agriculture autour du Mont Beuvray[END_REF], more heat and water in the soil [START_REF] Jansson | Simulated soil temperature and moisture at a clearcutting in Central Sweden[END_REF], use of organic manure or fertilizers to improve primary production [START_REF] Koerner | Influence of past land use on the vegetation and soils of present day forest in the Vosges mountains, France[END_REF]. Changes in species composition followed changes in both micro-climate and edaphic parameters, as revealed by the replacement of woodland species such as Pseudisotoma sensibilis by grassland species such as Isotoma viridis [START_REF] Szeptycki | Fauna of the springtails (Collembola) of the Ojców National Park in Poland[END_REF][START_REF] Ponge | Biocenoses of Collembola in atlantic temperate grass-woodland ecosystems[END_REF], and the replacement of acidophilic species, such as Pseudosinella mauli and Sminthurinus signatus, by acido-intolerant species such as Pseudosinella alba and Sminthurinus aureus [START_REF] Ponge | Biocenoses of Collembola in atlantic temperate grass-woodland ecosystems[END_REF]. This could allow the species composition of collembolan communities to be used as an indicator of the intensification of land use, provided underlying ecological factors are clearly identified. This is an important point to be highlighted, given the fuzzy contour of the human factor. For instance, [START_REF] Hågvar | Microarthropoda and Enchytraeidae (Oligochaeta) in naturally lead-contaminated soil: a gradient study[END_REF], studying a transect through a naturally lead-contaminated site (an abandoned mine), observed that the isotomid Isotoma olivacea (syn. I. tigrina, present in our sample) was favoured by lead contamination, compared to all other species, because of its higher abundance at the most polluted site. Examination of their site description allowed us to reinterpret the abundance of this species at the most polluted site as resulting from the collapse of tree vegetation. In a similar study on a zinc-polluted abandoned field [START_REF] Gillet | Changes in species assemblages and diets of Collembola along a gradient of metal pollution[END_REF] observed that typical grassland species such as Lepidocyrtus cyaneus were abundant at the most polluted site, due to collapse of the poplar plantation. Other instances concern atmospheric pollution, most effects of which on the soil are due to acidification, as this has been repeatedly observed in northern and Central Europe [START_REF] Tamm | Chanhges in soil acidity in two forest areas with different acid deposition: 1920s to 1980s[END_REF][START_REF] Hambuckers | A six year nutrient budget in a coniferous watershed under atmospheric pollution[END_REF][START_REF] Falkengren-Grerup | Long-term changes in pH of forest soils in southern Sweden[END_REF]. In this latter case, the effects of human activities are exactly opposed to those recorded in the present study, where most acid soils are those least subject to human influence, i.e. soils from old deciduous forests.

The decrease in local species richness observed when the landscape becomes more diversified, in the absence of any decrease in regional species richness, seems at first sight more difficult to interpret. The independence between regional () and local () diversity of collembolan populations has been already observed in temperate grassland communities [START_REF] Winkler | Local and regional species richness in communities of surfacedwelling grassland Collembola: indication of species saturation[END_REF] but it conflicts with studies on macroarthropod [START_REF] Halme | Carabid beetles in fragments of coniferous forest[END_REF][START_REF] Duelli | In search of the best correlates for local organismal biodiversity in cultivated areas[END_REF][START_REF] David | Belowground biodiversity in a Mediterranean landscape: relationships between saprophagous macroarthropod communities and vegetation structure[END_REF] and plant communities [START_REF] Tilman | Diversity and production in European grasslands[END_REF]. Some explanation can be found in the past history of the sites and in the scale at which these tiny soil animals are living. We have shown that the shift from woodland to grassland (and associated changes in climate, soil and vegetation) was the main factor explaining changes in soil collembolan communities. Other studies indicate the rates at which collembolan communities may recover (or shift to another equilibrium stage) following changes in vegetation cover. Cyclic changes in the species composition of collembolan communities have been observed to occur at the scale of centuries in near-natural mountain spruce forests, following cyclic changes in soil acidity in the course of vegetation dynamics [START_REF] Loranger | Does soil acidity explain altitudinal sequences in collembolan communities[END_REF]. In such forest mosaics, the rate of change and the availability of refuges allow the progressive recovery of communities as far as environmental conditions (micro-climate, soil chemistry, litter quality) return to original conditions. On the contrary, it has been observed that sudden deforestation [START_REF] Takeda | Effects of shifting cultivation on the soil meso-fauna with special reference to collembolan populations in the North-East Thailand[END_REF][START_REF] Gers | Rupture de l'équilibre biocénotique des populations de Collemboles à la station de ski de Calmazelle-Formiguères (Pyrénées Orientales)[END_REF][START_REF] Mateos | Efecto de los incendios forestales sobre las poblaciones de colémbolos edáficos en bosque mediterráneo[END_REF] as well as afforestation [START_REF] Jordana | Effect of reafforestation by conifers in natural biotopes of middle and South Navarra (Northern Spain)[END_REF] causes a rapid collapse in total abundance and species richness of collembolan communities. [START_REF] Cassagnau | Des Hexapodes vieux de 400 millions d'années: les Collemboles. II. Biogéographie et écologie[END_REF] underlined that in both cases rarefaction of species typical of past land use was more rapid than immigration of species typical of the new environment thus created, which could explain the decrease in biodiversity observed in landscapes most subject to recent changes in land use, compared to more stable landscapes. Along our gradient of intensification of land use (LUU 1 to LUU 6) both sides did not exhibit any profound changes over the last decades. For instance coniferous plantations in LUU 2 (mostly silver fir) occurred in previous old deciduous forests, as ascertained by the continuous presence of relict beech and oak. Thus no sharp transition occurred in the course of time, despite clear-cut operations and shift from hardwood to softwood (remind that clear-cut areas did not exhibit change in species composition, too). Most severe changes in land use occurred in zones intermediate between wide forested areas (on the less fertile soils) and plain land devoted to cereal crops for a long time (on the more fertile soils). The grassland past of present Norway spruce or Douglas fir plantations (more especially in LUU 4) can be ascertained by the presence of certain pasture plants still growing in the understory. Over the last ten decades the Morvan region has been subject to severe changes in land use [START_REF] Braojos | Études sur l'agriculture autour du Mont Beuvray[END_REF], due to i) abandonment of fire wood silviculture at the turn of the nineteenth century, ii) progressive abandonment of agriculture after the second world war, iii) recent increase of Christmas-tree fields. Variegated landscapes (LUUs 3 to 5) reflects best such recent shift in land use, afforestation of previous grassland occurring at a rate probably too rapid for the development of adapted soil animal communities. We hypothesize that the imbalance between immigration rates of new species and changes in land use will not occur for colonizers that have better dispersal mechanisms such as most plants and winged insects. The stability of regional biodiversity observed along our gradient of land use intensification can be ascribed to a compensation between an increase in the number of species in more variegated landscapes (through the addition of grassland to woodland communities) and the above mentioned lost of species resulting from too rapid changes in land use.

It was surprising to see that cereal crops, which undergo heavy disturbance by deep ploughing, pesticides and the absence of plant cover over a large part of the year, did not display any significant reduction in total abundance and species richness when compared with hay meadows and permanent pastures. Studies on agricultural soils showed a vertical redistribution of collembolan populations following annual ploughing and burying of crop residues [START_REF] Van Amelsvoort | The impact of Collembola on humification and mineralization of soil organic matter[END_REF][START_REF] Petersen | Collembola populations in an organic crop rotation: population dynamics and metabolism after conversion from clover-grass ley to spring barley[END_REF], which probably helps to maintain abundance of food at a depth where these tiny animals are protected from winter frost and summer drought. Thus may compensate for the absence of litter and plant cover during cold and dry seasons. Moreover the cyclicity of changes taking place in agricultural soils may allow communities to adapt themselves to changing conditions, by synchronizing their population dynamics with the cycle of change, as this has been demonstrated in forest soils [START_REF] Usher | Seasonal and vertical distribution of a population of soil arthropods: Collembola[END_REF][START_REF] Takeda | Dynamics and maintenance of collembolan community structure in a forest soil system[END_REF][START_REF] Gauer | Collembola in Central Amazon inundation forests: strategies for surviving floods[END_REF]. In short, it can be said that cyclic disturbance is not disturbance at all [START_REF] Odum | The strategy of ecosystem development[END_REF][START_REF] Chernova | Collembolan community organization and its temporal predictability[END_REF]. On the contrary, non-cyclic changes may damage communities until new, better adapted communities, develop, mainly through immigration of new species. Time is important in this respect [START_REF] Burges | Time and size as factors in ecology[END_REF]. Recovery of collembolan populations may last decades or centuries, depending on dispersal capabilities of the species, proximity of possible sources for the immigration of better adapted species, and absence of a new shift in land use during the meantime [START_REF] Bengtsson | Modelling dispersal distances in a soil gradient: the influence of metal resistance, competition, and experience[END_REF][START_REF] Mebes | A method for estimating the significance of surface dispersal for population fluctuations of Collembola in arable land[END_REF][START_REF] Ojala | Dispersal of microarthropods in forest soil[END_REF].

Another example of the effect of land use intensification on the structure and diversity of soil animal communities can be found in a study of nematodes in tropical soils by [START_REF] Bloemers | The effects of forest disturbance on diversity of tropical soil nematodes[END_REF].

These authors did not detect any profound influence of slash-and-burn and heavy machinery deforestation on the trophic structure of nematode communities, as expressed by the Maturity Index [START_REF] Bongers | The maturity index: an ecological measure of environmental disturbance based on nematode species composition[END_REF], but they observed a 40% decrease in species richness. This short-term depressive effect can be considered as in line with our results on temperate Collembola. Here too, most disturbance resulted from the abrupt passage from forest to open environments, the reverse (plantation of trees on previous agricultural soils) being not considered.

The results of this study show that a critical assessment is required over the choice of Collembola as bioindicators of land use intensification. On one hand we have seen that collembolan communities differ clearly between low (forest) and high (crop) land use intensity and that they could be used to some extent as bioindicators of this factor, in spite of a correlation with other ecological factors (soil acidity) not directly related to human activities. On the other hand we have also shown that it is necessary to take into account landscape dynamics. This second observation points to limits for the use of Collembola, and more generally animal communities, for the bioindication of land use intensity or land use type, since they do not adapt rapidly to changes in land use. The aim of bioindication must be clearly expressed before this interesting tool could be used for monitoring or predicting biodiversity. 2

Legends of figures

Fig. 1 .Fig. 2 .Fig. 3 .Fig. 4 .Fig. 5 .

 12345 Fig. 1. Correspondence analysis. Projection of collembolan species (three-letter codes according to Table 3), population parameters (higher values in bold type, lower values in italic type), land use units (numbers in boxes) and land use types (underlined) on Axis 1. Position of the origin is indicated by an arrow. Positive and negative Axis 1 coordinates are on the lower and upper side, respectively

  Fig.1

Humus Index Humus form OL horizon OM horizon OF horizon OH horizon A horizon

	Eumull	absent	absent	absent	absent	crumby
	Mesomull	present	absent	absent	absent	crumby
	Oligomull	present	absent	present but less than 1 cm absent	crumby
	Dysmull	present	absent	1 cm or more	absent	crumby
	Amphimull	present	absent	present	present	crumby
	Hemimoder	present	absent	present	absent	compact
	Eumoder	present	absent	present	present but less than 1 cm compact
	Dysmoder	present	absent	present	1 cm or more	compact
	Amphimoder present	present	absent	absent	crumby
	Mor	present	present	absent	absent	compact A or E horizon

Table 1 .

 1 Humus Index and morphological criteria used for the separation of humus forms according to[START_REF] Brêthes | Classification of forest humus forms: a French proposal[END_REF] andPonge et al.

	(2000, 2002)
	1
	2

Table 2 .

 2 Distribution of land use types among the six land use units (LUUs). Sixteen samples were taken in each LUU according to a regular grid, except LUU 4 with fifteen samples only due to flooding

	1
	2

Table 3 .

 3 Collembolan species and main features of collembolan populations in the six sampled land use units

	1

Table 4 .

 4 Total abundance and number of species of Collembola in individual core samples (5 cm diameter, 10 cm depth) according to main land use types in the six land use units investigated (mean followed by standard error)

Acknowledgements

This study was part of the European Community program BioAssess EVK2-CT-1999-00041 (directed by Allan Watt, CEH, Aberdeen, UK), which is greatly acknowledged for financial support and fruitful exchange of ideas between partners. Many thanks are due to Dr John Measey for improvement of the English language.


Jannik Laval
email: jannik.laval@inria.fr

Simon Denier
email: simon.denier@inria.fr

Stéphane Ducasse
email: stephane.ducasse@inria.fr

Alexandre Bergel
email: alexandre.bergel@inria.fr

Identifying cycle causes with Enriched Dependency Structural Matrix Accepted to WCRE 2009

Keywords: software visualization, reengineering, dependency structural matrix, package, dependency

Dependency Structure Matrix (DSM) has been successfully applied to identify software dependencies among packages and subsystems. A number of algorithms were proposed to compute the matrix so that it highlights patterns and problematic dependencies between subsystems. However, existing DSM implementations often miss important information to fully support reengineering effort. For example, they do not clearly qualify and quantify problematic relationships, information which is crucial to support remediation tasks.

In this paper we present enriched DSM (eDSM) where cells are enriched with contextual information about (i) the type of dependencies (inheritance, class reference . . . ), (ii) the proportion of referencing entities, (iii) the proportion of referenced entities. We distinguish independent cycles and stress potentially simple fixes for cycles using coloring information. This work is language independent and has been implemented on top of the Moose reengineering environment. It has been applied to non-trivial case studies among which ArgoUML, and Morphic the UI framework available in two open-source Smalltalks, Squeak and Pharo. Solution to problems identified by eDSM have been performed and retrofitted in Pharo main distribution.

I. INTRODUCTION

Understanding the package organization of an application is a challenging and critical task since it reflects the application structure. Many approaches have flourished to provide information on packages and their relationships, by visualizing software artefacts [START_REF] Wu | A multi-perspective software visualization environment[END_REF], metrics, their structure and their evolution. Distribution Map [START_REF] Ducasse | Distribution map[END_REF] shows how properties are spread over an application. Lanza et al. [START_REF] Ducasse | The class blueprint: Visually supporting the understanding of classes[END_REF] propose to recover high-level views by visualizing relationships. Package Surface Blueprint [START_REF] Ducasse | Package surface blueprints: Visually supporting the understanding of package relationships[END_REF] reveals the package internal structure and relationships among other packages -surface represents relations between the analyzed package and its provider packages. Dong and Godfrey [START_REF] Dong | System-level usage dependency analysis of object-oriented systems[END_REF] propose high-level object dependency graphs to represent and understand the system package structure.

Dependency Structure Matrix (DSM) is a well-know technique to identify cycles [START_REF] Sangal | Using dependency models to manage complex software architecture[END_REF]. Originally it has been developed for process optimization to identify dependencies between tasks. This method has been applied with success to identify software component dependencies [START_REF] Steward | The design structure matrix: A method for managing the design of complex systems[END_REF], [START_REF] Sullivan | The structure and value of modularity in software design[END_REF]. MacCormack et. al. [START_REF] Maccormack | Exploring the structure of complex software designs: An empirical study of open source and proprietary code[END_REF] have applied DSM to analyze modularity of the architecture of Mozilla and Linux.

While DSM is a robust solution to reveal software structure, DSMs have weaknesses too. DSM current implementations allow one to perform high-level inventory of a situation, but they are limited for fine-grained understandingtools just offer drop-down lists to show classes and methods creating dependencies between packages.

For example, current DSM implementations do not provide detailed information about interpackage dependencies. Cycles, which constitute a special target for dependency resolution, are commonly identified using the adjacency matrix power method [START_REF] Yassine | Engineering design management: an information structure approach[END_REF]. Unfortunately, this algorithm inaccurately identifies cycles since independent cycles are merged.

Our contribution is two-fold: first, we identify weaknesses of current DSM; second, we address these weaknesses. We propose, eDSM, a DSM with enriched cells 1 . eDSM cells contain contextual information which shows (i) the nature of dependencies (inheritance, class reference, invocation, and class extension), (ii) the referencing entities, (iii) the referenced entities, (iv) the spread of the dependency. We distinguish independent cycles and differentiate cycles using colors. We applied eDSM on several large systems, Ar-goUML, Morphic an UI framework and Seaside a dynamic web framework. The results of the analysis of Morphic were proposed to Pharo implementors (including two of the contributors) and integrated in Pharo 2 , a new open-source version of Squeak 3 . Squeak is an open-source implementation of Smalltalk programming language and environment. Our approach is language independent since it is based on the language independent FAMIX metamodel [START_REF] Demeyer | FAMIX 2.1 -The FAMOOS Information Exchange Model[END_REF], [START_REF] Ducasse | Moose: a collaborative and extensible reengineering environment[END_REF].

The paper is organized as follows: Section II introduces DSM and its limitations in existing implementations. Sections III and IV present eDSM specifications and its usage, from overview of an application to detailed view of interpackage dependencies. Section V reports an experiment on Morphic and identifies some patterns and solutions found in the study. Section VI discusses about our solution and future work.

II. DSM LIMITATIONS

DSMs are effective for detecting cycles between software components. The use of DSMs gives pertinent results for the verification of the independence of software components [START_REF] Sangal | Using dependency models to manage complex software architecture[END_REF]. However, in their current form, DSMs must be coupled with other tools to offer fine-grained information and support corrective actions.

Figure 1(a) shows a sample dependency graph and its corresponding binary DSM. A binary DSM shows the existence/lack of a dependency (or reference) by a mark or "1/0". The rule for reading the matrix is: element in column header references element in row header if there is a mark. In our context, A, B, C, and D are packages. The element in column header is also called the source and the one in row header the target. In Figure 1 We applied DSM on a couple large case studies and we identified two limitations with current DSM implementations: inaccurate merging of independent cycles by the adjacency matrix power method (Section II-A), lack of fine grained-overview of the dependency cause (Section II-B).

A. Problem with the adjacency matrix power method

A simple way to identify cycles in DSM is to use the adjacency matrix power method. The principle of this approach is to raise the binary DSM to its n th power to find elements which link back to themselves in n edges, thus making a cycle [START_REF] Yassine | Engineering design management: an information structure approach[END_REF]. A non-zero mark in the diagonal of the power matrix points to elements involved in a cycle of length n.

Figure 1(a) shows two distinct direct cycles (a cycle between two entities): one between A and B and one between C and D. However, the adjacency matrix power does not separately identify these different cycles, resulting in a single and inaccurate merged cycle. Figure 1(b) shows the partitioned matrix with a unique wrong cycle-a single gray zone representing the cycle. On the contrary, Figure 1(c) shows a correctly partitioned matrix with two distinct cycles.

The adjacency matrix power method produces inaccurate results when used to identify independent cycles because it computes the number of edges to come back to an element without considering the cycling path [START_REF] Gebala | Methods for analyzing design procedures[END_REF]. Instead, path searching algorithms have also been used to detect cycles in DSM and should be systematically preferred when the problem of identifying independent cycles is important.

B. Lack of fine-grained information

A traditional DSM offers an easily readable general overview but does not provide details about the situation it describes. We identify two weaknesses: lack of information on dependency causes and lack of information on dependency distribution. Dependency causes.: Fixing a cycle often means changing some dependencies involved in the cycle. However, the cost of fixing a cycle may vary with the cause of dependency e.g., changing a reference to a class is often easier than changing an inheritance relationship. Dependencies are of different natures (class reference, method invocation, inheritance relationship, and class extension) and a binary matrix (Figure 2(a)) or a matrix providing the number of dependencies in each cell (Figure 2(b)) do not provide such information.

A B C D A X B X C X X D X A B C D (a) A simple DSM A B C D A X B X C X X D X (b) Adjacency ma- trix power method A B C D A X B X C X X D X (
Annotating a DSM with the types of dependencies can give more fine-grained information and it supports a better understanding of the situation. However, a challenge with this solution is that the matrix should remain readable and should not be overloaded [START_REF] Bergel | Enhanced dependency structure matrix for moose[END_REF].

Dependency distribution.: Knowing that a package has 78 dependencies to another one (package Morphic-Widgets on Morphic-Basic in Figure 6) is a valuable but insufficient information.

The ratio of concerned classes in a package is important since it allows one to quantify the effort to fix a cycle. The intuition is that it is easier to target few classes with some dependencies rather than a lot of classes with few dependencies. For example in Figure 6, 16 classes of package Morphic-Widgets reference 10 classes of package Morphic-Basic, while only two classes of Morphic-Basic reference one class of Morphic-Widgets. Consequently, it should be faster to target the dependencies from Morphic-Basic to Morphic-Widgets rather than the ones in the opposite direction.

III. ENRICHED DSM (EDSM)

To address the problems previously mentioned, we enhance DSM with functionalities that are not present in current DSM implementation such as Lattix [START_REF] Sangal | Using dependency models to manage complex software architecture[END_REF]. Our solution (i) isolates independent cycles using colors (Section III-A), (ii) enriches contextual cell information (Section III-B) and (iii) supports class extensions (Section III-B).

In particular, enriched cells act as small multiples [START_REF] Tufte | Visual Explanations[END_REF] where similar looking side by side little visualizations provide a differentiating effect (see Figure 5). An important design feature is the use of color to focus on packages where it is easier to resolve a cyclic dependency. Therefore we use brighter colors for places having fewer dependencies. The tool is implemented on top of the Moose open-source reengineering environment. Since it is based on the FAMIX meta-model [START_REF] Demeyer | FAMIX 2.1 -The FAMOOS Information Exchange Model[END_REF], our eDSM works for mainstream objectoriented programming languages [START_REF] Ducasse | Moose: a collaborative and extensible reengineering environment[END_REF].

A. Enhanced cycle detection

Our approach enhances the traditional matrix by providing a number of new features: cycle distinctions, direct and indirect cycle identification, and hints for fixing cycles.

Cycle distinctions.: eDSM distinguishes independent cycles using a path searching algorithm [START_REF] Gebala | Methods for analyzing design procedures[END_REF]. With this method, two independent cycles are detected separately and remain isolated from each other in the DSM (Figure 1(c)).

Direct and indirect cycles.: We use color in DSM cells to identify cycles. DSM cells involved in a cycle have a yellow or red color (Figure 3). The red color means that the two concerned packages reference each other and thus create a direct cycle. Two packages in a direct cycle have two red cells symmetric against the diagonal. The yellow color means that the dependencies from one package to the other participate in an indirect cycle (a cycle with more than two elements). The pale blue background color frame all cells involved in a indirect cycle (visible in Figure 5). Its area is a visual indication of the number of packages in the cycle. On the contrary, rows and columns with white or gray colors indicate packages not involved in any cycle. The diagonal of the matrix, where a package may reference itself, is colored in gray to highlight the symmetry axis but is not used in the current version.

Color hint for targeting cycle.: We define a special rule to highlight cells of primary focus when resolving cyclic dependencies. The intuition is that it will be easier to fix a cycle by focusing on the side with fewer dependencies. A cell with much fewer dependencies is displayed with a bright red color whereas its symmetric cell is displayed with a light red/pink color (Figure 3). The ratio we currently use is 1 to 3. This rule only applies to direct cycles as it is easier to compare two packages side by side than an arbitrary number of packages involved in an indirect cycle.

Figure 3 illustrates the rules for cycle colors in cells. It shows two direct cycles with the red color, one between A B. Enriched contextual cell information eDSM enriches cell contents to give a detailed overview of dependencies from a source package to a target package. Thus each cell is the intersection between a source and a target. The objective is to create small multiples [START_REF] Tufte | Visual Explanations[END_REF] as shown in Figure 5.

The principle of small multiples is that "once viewers decode and comprehend the design for one slice of data, they have familiar access to data in all the other slices" [START_REF] Tufte | Visual Explanations[END_REF]. In eDSM, each cell represents a small context, which enforces comparison with others. Each cell represents a small dashboard with indicators about the situation between the source and the target. This section focuses on the cell contents.

It should be noted that an earlier version of this work did not use small multiples and only showed a summary of the cell contents [START_REF] Bergel | Enhanced dependency structure matrix for moose[END_REF]. The results were limited: It was difficult to see where to begin for resolving issues. To circumvent these limits, we designed enriched cells, which provide visual patterns.

Overall structure of an enriched cell.: An enriched cell is composed of four parts (see Figure 4). The top row gives an overview of the strength and nature of the dependencies. The bottom row presents cycle information as explained in the previous subsection. The two large boxes in the middle detail dependencies going from the top box to the bottom box i.e., from the source package to the target package. Each box contains squares that represent involved classes: referencing classes in the source package and referenced classes in the target package. Edges between squares link each source class (in top box) to its target classes (in bottom box) (Figure 4).

Colored information.: Enriched cells make use of colors to convey more information about the context in which dependencies occur. Our goal is to use preattentive visualization 4 as much as possible to help spotting important information [START_REF] Treisman | Preattentive processing in vision[END_REF], [START_REF] Healey | Visualization of multivariate data using preattentive processing[END_REF], [START_REF] Healey | Harnessing preattentive processes for multivariate data visualization[END_REF], [START_REF] Ware | Information Visualization[END_REF]. An enriched cell is composed of parts and shapes with different color schemas.

Cycle color (bottom row).:

It is the first information to see in a cell. The bottom row represents cycle information using color as explained in Section III-A. The red/pink indicates a direct cycle between the two packages, yellow and cyan an indirect cycle, and gray an unidirectional access from the source package to the target package-which means that there is no cycle.

Dependency overview.: An enriched cell shows an overview of the strength, nature, and distribution of the dependencies from the source to the target.

• Dependency strength and nature (top row). The top row gives a summary of the number and nature of dependencies to get an idea of their strength. We show the total number of dependencies (Tot) in black, inheritance dependencies (Inh) in blue, references to classes (Ref) in red, invocations (Msg) in green, and class extensions (Ext) made by the source package to the target one in cyan 5 . A brighter color highlights the strongest dependency type. The colors are used to reinforce the comprehension of links between classes (see below). In Figure 6 there are 78 directed dependencies 4 Researchers in psychology and vision have discovered a number of visual properties that are preattentively processed [START_REF] Healey | Visualization of multivariate data using preattentive processing[END_REF]. They are detected immediately by the visual system: viewers do not have to focus their attention on a specific region in an image to determine whether elements with the given property are present or absent. An example of a preattentive task is detecting a filled circle in a group of empty circles. Commonly used preattentive features include hue, curvature, size, intensity, orientation, length, motion, and depth of field. However, combining them can destroy their preattentive ability (in a context of filled squares and empty circles, a filled circle is usually not detected preattentively). 5 A class extension is a method defined in a package, for which the class is defined in a different package [START_REF] Bergel | Analyzing module diversity[END_REF]. Class extensions exist in Smalltalk, CLOS, Ruby, Python, Objective-C and C#3. They offer a convenient way to incrementally modify existing classes when subclassing is inappropriate. They support the layering of applications by grouping with a package its extensions to other packages. AspectJ inter-type declarations offer a similar mechanism.

from Morphic-Widgets to Morphic-Basic. There are 12 inheritances, 24 references and 42 invocations (in bright green).

• Dependency distribution (left bars). For each package, we are interested in the ratio of classes involved in dependencies with the other package. We map the height of the left bar of each package box to the percentage of classes involved in the package. The bar color is also mapped to this percentage to reinforce its impact (from green for low values to red for 100% involvement). A package showing a red bar is fully involved with the other package.

Class color (middle boxes).: Each square represents a class and displays two types of information using its fill color as well as its border (Figure 4).

• Border color and thickness: Internal usage. A gray thin border means that the class has a unidirectional dependency with the other package i.e., it either uses or is used by classes in the other package. In Figure 4, class B (resp. Z) has a thin border because it refers to X but is not referred in the target package (resp. is referred by A but does not refer to source package).

A black thick border means that the class has a bidirectional dependency with the other package: it both uses and is used by classes in the other package of the cell (not necessarily the same classes). In Figure 4, class A has a thick border because it is referred by class X of the target package and because it refers to class Z.

A dotted border represents a class extension as explained below.

• Color fill: Total usage. A class may be in dependency with other packages than the two represented by the cell, such as classes B or C in Figure 4. The color fill uses the colors of the traffic lights (green, orange, red) to qualify the relationships the class has with packages other than the two concerned. A class which has no dependency with external packages other than the concerned packages is displayed as green. A class which has dependencies in only one direction (i.e., either incoming dependencies or outgoing dependencies) is displayed as orange. A class which has dependencies in both directions is displayed as red. A class which only has internal dependencies is never displayed in a DSM. Thus, moving a green class between the two concerned packages does not have any impact on their external dependencies, whereas moving a red or orange class can change their respective external dependencies. Edge color.: Edges are the smallest details displayed by the eDSM. They give information on the nature and spread of dependencies between the classes in the cell (Figure 4). There are four basic natures, each one mapped to a primary color (synchronized with colors of information in top row of the cell): reference in red, inheritance in blue, invocation in green and class extension in cyan. When dependencies between two classes are of different natures, colors are mixed as follows: red is used for a dependency with both references and invocations because a reference is often following by invocations (a new color would make it more difficult to understand the figure). Black is used for any dependency involving inheritance with references and/or invocations. Indeed, an inheritance dependency mixed with other dependencies can be quite complex and we choose not to focus on such a combination.

Representation of class extension.: A class extension represents a method which is in another package than its class. In a cell, a class extension is represented by a square with dotted border linked to the original class by a cyan link and the same color information than the original class (Figure 4). This convention exists because a class extension is not a class. In addition if class extensions are not handled adequately, they can be considered as wrong cyclic dependencies.

C. Interaction and detailed view

While the eDSM offers on overview at the package level as shown by Figure 5, extracting all the information from a cell is sometimes difficult. There is a clear tension between getting a small multiple effect and details readability. We offer zoom and fly-by-help to improve usability.

Zooming on two packages.: Each cell in a DSM represents a single direction of dependency. To get the full picture of interactions between two packages, we compare two cells, one for each direction. Despite DSM intrinsic symmetry, it is not always easy to focus on the two concerned cells. We provide a selective zoom with a detailed view on the two concerned cells, as shown in Figure 6. Thus, we focus on a direct cycle which seems interesting from the overview, and analyze the details with the zooming view. Fly-by help.: Complementary to the overview and the zooming facility, fly-by-help includes the full name of concerned packages, the name of classes and the name of each concerned method. Figure 7 shows the pop-up information of the cell linking Morphic-Worlds to Morphic-Widget: there is one reference to the class HaloMorph from the method PasteUpMorph.acceptDroppingMorph:event:.

D. Fixing a cycle with eDSM

We now detail a first example of cycle resolution through the analysis of zoomed cells.

In Figure 6, there is a direct cycle between Morphic-Widgets and Morphic-Basic (named Widgets and Basic below). We can see that Widgets have lots of dependencies to Basic (pink cell) while only two classes in Basic use one class of Widgets (red cell). Moreover, there are only red edges (class references) in the red cell, whereas in the pink cell they are of multiple colors.

At first glance, it is thus easier to investigate the dependencies of the red cell, from Basic to Widgets. Let us look at the red cell. There are two referencing classes and one referenced class. All three are colored in red, which means they use and are used in other packages. Thus, it would be difficult to move these classes without further investigation. Instead, we focus on the dependencies between classes in the red cell, which are only class references. The fly-by help displays for each class in the cell the concerned methods (methods in the source package making class references in the target package). There are three such methods: Poly-gonMorph.customizeArrows:, TextMorph.setCurveBaseline:, TextMorph.changeMargins:. This provides entry points in the source code to find precisely where the target class HandleMorph is referenced.

It appears that each of these methods contains the line HandleMorph new to create an instance of HandleMorph. A possible solution is to create class extensions for TextMorph and PolygonMorph in the package Widgets and to put the three referencing methods in it. Then the dependencies would be reversed effectively breaking the cycle.

IV. SMALL MULTIPLES AT WORK

eDSM supports the understanding of the general structure of complex programs using structural element position. Since it is based on the idea of small multiples [START_REF] Tufte | Visual Explanations[END_REF], the cell visual aspect generates visual patterns. While performing our Morphic experiment, we have detected some patterns stressing characteristic situations.

We applied eDSM to the Morphic framework of Squeak. Morphic is composed of 46 packages and 325 classes. It was never packaged in a modular way, hence showing a lot of cyclic dependencies. We use this case study to show eDSM in practice (Figure 5).

The first use of the eDSM is to get a system overview to scan packages not involved in cycles (not shown in Figure 5) and how they interact with other packages. Subsequently, we spot packages involved in direct and indirect cycles. shows a large indirect cycle delimited by the pale blue area. At first glance, the bright red cells are good starting points for investigation of simple cyclic dependencies to resolve. In Figure 5 we can spot: A Packages in indirect cycles (yellow bottom bar). In a first step, it is not really interesting to fix them because the cycle probably comes from a direct cycle between two other packages. B Packages communicating heavily. There, the two packages interacts a lot, so intuitively it seems to be difficult to fix, the symmetric cell is probably less complex. A clear example of such situation was shown in Figure 6 where the symmetric cell is much simpler. C Packages referencing a lot of external classes (a lot of red links and header with bright red number). This pattern shows direct references to classes between the two packages in cycle. The symmetrical may be easier to fix, we can assess it rapidly, since it can be marked in red. Figure 8 shows an example of such situation. D Packages where only two classes are referring to each other (Thick border). Such pattern represents a direct cycle between two classes. In Figure 7, only one class of Morphic-Worlds is in cycle with only one class of Morphic-Widgets. In addition, they both have a thick border so it is clearly a direct cycle between these two classes. This pattern allows us to focus our attention on just two classes of the two packages. E Packages having a large percentage of classes involved in the dependency (left bar in red). When this pattern shows a high ratio in the referencing package (top), changing it can be complex since many classes should be modified. In the case of a high ratio in referenced (bottom) package, a first interpretation is that this package is highly necessary to the good working of the referencing package. F Packages with direct cycles which seem easy to fix (red bottom bar -low ratio of references). This pattern shows cycle created by a single class in one package. In Figure 8, the class labelled Pa is the only one appearing in Morphic-Worlds and both uses and is used by classes V. EXPERIMENTAL VALIDATION We experimented and validated our approach with nontrivial case studies: ArgoUML6 , Seaside7 and Morphic8 . We applied DSM on ArgoUML, a Java project which had been imported in Famix3.0 with the tool iPlasma 9 .

Seaside is a framework for dynamic web application development which matured over 8 years. Between version 2.8 and 2.9 the developers goal was to offer a more modular structure. They asked us to apply eDSM on the different versions. We reported the large improvement between the two versions: Seaside 2.9 contains 10 cycles when 2.8 contains 23 direct cycles. The work to fix residual cycles is underway.

ArgoUML is an open source UML modeling tool. We applied eDSM on the version 0.28 and the results include 90 direct cycles between 71 packages among the 97 imported packages. A visualization sample is available on http://www.janniklaval.eu/assets/files/softAnalysis/argoUml. It shows that ArgoUML has one big indirect cycle (in blue) with 77 packages.

Morphic is a large Swing-like graphic framework comprising 46 packages and 325 classes. It supports complex interface building. It was not designed in a modular fashion and has a 12 years long history of evolution and extension, making it complex to analyze by exhibiting an abnormal number of dependencies between packages. It contains 45 direct cycles between 28 packages in the studied version. Based on the eDSM output, we proposed solutions to fix cycles. Pharo developers performed and integrated the simple fixes and acknowledged the more complex problems we identified.

In this section, we present a global analysis of Morphic UI. First, we explain the process to analyze the identified cycles and subsequently, we give their possible resolutions, our results as well as the validation (in terms of feedback) by the Pharo maintainers. One goal of the Morphic refactoring is to reorganize classes in simpler, conceptually cleaner packages to make its maintenance easier. Cycles are then primary targets to resolve to get layers of packages.

A. Experiment Protocol

We followed the following protocol: we analyzed each cycle found with eDSM and tried to quickly identify the opportunity to fix them. For each direct cycle, we looked for one simple solution or marked the cycle as too complex to be easily fixed. To assess the aid provided by eDSM we took as a criterion to find a solution in five minutes. Solutions include: moving a class between two packages, converting a method into a class extension, removing a useless class or method, merging packages. The solutions found were sent as fix propositions to Pharo developers for review.

We make a classification of direct cycles according to the perceived complexity. We consider that it is easier to break a cycle when there are 10 dependencies on one class instead of 10 classes with one dependency on each. Thus the classification is based on the number and type of dependencies. Note that this classification is presented here just as an indication of the situation we found. We may refine it and use it in future work for refactoring assistance.

• Monotype (Mn) cycle: a direct cycle where a cell has only a single edge between two classes, representing either reference, inheritance, or invocation. There are 21 of them in Morphic UI: all are class references.

• Simple (S) cycle: a direct cycle where a cell has only a single edge between two classes, representing multiple types of dependencies (reference and invocation or inheritance and others). There are 12 of them in Morphic UI.

• Direct cycle with two edges (2L). There are five of them in Morphic UI.

• Complex direct cycles-with more than two edges (CC). There are seven of them in Morphic UI.

B. Results

We applied the previously described process on the 45 direct cycles identified in Morphic UI using eDSM. We proposed 25 cycle resolutions presented in Table I. Among the 20 cycles left, five cycles are judged visually too complex (with many dependencies on each side) and immediately left out; 15 cycles require a deeper exploration of the internals of Morphic UI, since we were not able to find a solution in five minutes. We checked our 25 proposals with one Pharo maintainer who commented, implemented or rejected them.

Among the 25 propositions, 18 have been accepted and integrated in the current Pharo release. The others are good propositions but have been replaced by merging large packages or by registration mechanisms. The propositions that were not integrated were also acknowledged as problems. However, their resolution requires more development. Here is a list of problems: missing menu registration mechanism in main tool bar, badly design button hierarchy and callback system.

Pharo developers acknowledge that the 20 cycles left out are real architectural problems: Kernel UI elements scattered over several packages with no particular reasons and left over of the monolithic architecture of Squeak.

VI. DISCUSSION

Language Independent Approach.: eDSM has been implemented on top of the Moose reengineering environment [START_REF] Ducasse | Moose: a collaborative and extensible reengineering environment[END_REF] and it is based on the FAMIX language independent source code metamodel [START_REF] Demeyer | FAMIX 2.1 -The FAMOOS Information Exchange Model[END_REF]. We applied eDSM on a Java case study and we expect it to work on C#, and C++ as well. Therefore while implemented in Smalltalk, eDSM can be applied to mainstream object-oriented languages.

A. Limits

There are still some limitations which we would like to overcome, with the objective to make eDSM more effective for reengineers.

When validating our proposals, the maintainer sometimes asked what was the impact of a merge or move between packages. He also asked to see other external references to packages in the cell before taking a decision. Currently we cannot show such valuable information. We plan to use a specific visualization, such as Package Blueprint [START_REF] Ducasse | Package surface blueprints: Visually supporting the understanding of package relationships[END_REF], showing all dependencies from/to one single package in a pop-up view.

Another problem is screen space limitation. A DSM uses a lot of useless space when there are empty cells. An interactive filter on packages may be useful with respect to this.

Professional DSMs such as Lattix support layer specification and violation detection. This is orthogonal to our work but definitively relevant to add to our approach.

B. Impact and cost of small multiples

One critic about eDSM is that it loses the simplicity of the original DSM. Our experience on real and complex software showed that DSM is powerful but limited. We were constantly losing time browsing code to understand to what exact situation a number in a cell was referring to. eDSM gives such information in a glance.

A related critic about eDSM is that it looks too complex as it needs one page to describe cell design. Cells have been especially designed to work as small multiples and micro-macro reading [START_REF] Tufte | Visual Explanations[END_REF] i.e., that variations of the same structure reveal information. The presented eDSM is already the second large iteration of this work. We developed a first version of eDSM with only header informations (type and number of dependencies) and simple cell colors [START_REF] Bergel | Enhanced dependency structure matrix for moose[END_REF]. A flyby-help text then gave the same information as the current cell design. However, it was cluttered: text was clearly not as efficient as little pictures. Then, it did not support small multiple effects or patterns. Comparing situations was tedious and understanding the problems too.

So, eDSM provides contextual information: in a global view, eDSM could be read similarly as the original DSM by looking the header for number of links and the bottom to see cycle context, it shows the global structure of the application. However, eDSM provides more information about the context of a dependency by displaying in a cell the complexity of the relation.

C. Comparison with other approaches

Package blueprint.: It takes the focus of a package and shows how such package uses and is used by the other package [START_REF] Ducasse | Package surface blueprints: Visually supporting the understanding of package relationships[END_REF]. It provides a fine-grained view, however package blueprint lacks (1) the identification of cycles at system level and (2) the detailed focus on classes actually doing the cycles.

Distribution Map.: It is a visualization that can be applied to packages [START_REF] Ducasse | Object-oriented legacy system trace-based logic testing[END_REF]. However, even if it removes the clutter due to edge representation Distribution Map is a generic visualization that focuses on how properties spread in a population of entities.

Oriented-graph.: Often graph-oriented visualizations are used to show dependencies among software entities. Several tools such as dotty/GraphViz, Walrus or Guess can be used. Using graph is intuitive and has a fast learning curve. One problem with oriented graph visualization is finding a good layout scaling well on large sets of nodes and edges: such a layout needs to preserve the readability of nodes, the ease of navigation following edges, and to minimize edge crossing. Even then, cycle identification is not trivial.

With DSM the visualization structure is preserved whatever the data size is, which enables the user to dive fast into the representation using the normal process. Cycles remain clearly identified by colored cells, there are no edges between packages, so this reduces clutter in the visualization. Moreover, eDSM enables fine-grained information about dependencies between packages. Classes in source package as well as in target package are shown in the cells of the DSM.

Dependence Clusters.: Brinkley and Harman proposed two visualizations for assessing program dependencies, both from a qualitative and quantitative point of view [?]. They identify global variables and formal parameters in software source-code. Subsequently, they visualize the effect dependencies. Additionally, the MSG visualization [?] helps finding dependence clusters and locating avoidable dependencies. Some aspects of their work is similar to our own. Granularity and the methodology employed differ: they operate on source-code and use slicing method, while we focused on coarse grained entities and use model analysis.

VII. CONCLUSION

This paper enhances Dependency Structure Matrix (DSM) using small multiples. First, colors are used to distinguish direct and indirect cycles. Second, cell contents are enriched with the nature and strength of the dependencies as well as with the classes involved. Such enhancements are based on small multiples [START_REF] Tufte | Visual Explanations[END_REF] and preattentive visualization principles [START_REF] Treisman | Preattentive processing in vision[END_REF], [START_REF] Healey | Visualization of multivariate data using preattentive processing[END_REF], [START_REF] Healey | Harnessing preattentive processes for multivariate data visualization[END_REF], [START_REF] Ware | Information Visualization[END_REF]. Thanks to these improvements, package organization and cycles are made explicit. We applied the eDSM on a complex system and systematically checked and tried to fix the cycles. Out of 45 direct cycles, we could propose 25 solutions to break the cycles. 18 got accepted and implemented by the maintainers of the Pharo open-source Smalltalk.

We believe this paper provides an appealing approach for identifying cycles. The experiment we conducted gave us the feeling that indirect cycles were more difficult to analyze than direct ones. This makes our future work focuses on getting better visualizations for indirect cycles. Currently, eDSM provides relevant indications for reengineers, but it appears that visualizing impact of changes in the matrix would greatly enhances reengineering tasks.

  (a), A references B and C, B references A, C references D and D references C.

Figure 1 .Figure 2 .

 12 Figure 1. Limitation of the adjacency matrix power method. Cycles are shown in gray.

Figure 3 .

 3 Figure 3. Cell color definition.

Figure 4 .

 4 Figure 4. Enriched cell structural information.

  Classes in Morphic-Widgets use classes in Morphic-Basic Classes in Morphic-Basic use classes in Morphic-Widgets

Figure 6 .

 6 Figure 6. Zoom on two packages in cycle.

Figure 5 DFigure 5 .

 55 Figure 5. An overview of a Morphic subset: Enriched cells in DSM provide a small multiple effect.

Figure 7 .

 7 Figure 7. A twin-class cycle, with fine-grained information.

Figure 8 .

 8 Figure 8. A one-hotspot cycle.

Figure 9 .

 9 Figure 9. A funnelled cycle.

a DSM cell represents the intersection of two packages

http://www.pharo-project.org

http://www.squeak.org

http://argouml.tigris.org/

http://www.seaside.st/

http://wiki.squeak.org/squeak/30

http://loose.upt.ro/iplasma/

Acknowledgements: We gratefully acknowledge the sponsoring of ESUG (the European Smalltalk User Group) http://www.esug.org/.