# Persistent macrodetritivore functional diversity and identity effects on litter mass loss under drought in a Mediterranean forest understory model ecosystem

Mika Lemoine, Stephan Hättenschwiler, Alexandru Milcu, Sandra Barantal, Johanne Nahmani, Pierre Ganault

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## To cite this version:

Mika Lemoine, Stephan Hättenschwiler, Alexandru Milcu, Sandra Barantal, Johanne Nahmani, et al. "Persistent macrodetritivore functional diversity and identity effects on litter mass loss under drought in a Mediterranean forest understory model ecosystem." Soil Biology and Biochemistry, 2025, 209, pp.109918. 10.1016/j.soilbio.2025.109918. hal-05369602

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https://hal.science/hal-05369602v1

Submitted on 17 Nov 2025

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Persistent macrodetritivore functional diversity and identity effects on litter mass loss under drought in a Mediterranean forest understory model ecosystem

Mika Lemoine<sup>a, *, b</sup>, Stephan Hättenschwiler<sup>a</sup>, Alexandru Milcu<sup>a, b</sup>, Sandra Barantal<sup>c, b</sup>,
Johanne Nahmani<sup>a, 1</sup>, Pierre Ganault<sup>d, 1</sup>

ARTICLE INFO

Keywords:

Climate change

Biodiversity-ecosystem functioning

Ecotron

Soil biodiversity

A B S T R A C T

Soil macrofauna's role in plant litter decomposition is increasingly recognized, with litterbag experiments in natural and lab settings highlighting its contribution in various pedoclimatic context. Recent findings indicate that macrofauna can counteract drought-induced slowing of decomposition and nutrient cycling, but the underlying mechanisms remain unclear. Characteristics of macrofauna communities, such as functional diversity (FD), community-weighted mean (CWM) traits, and species identity, may drive these effects and modulate decomposition responses to drought. In a controlled experiment simulating a Mediterranean forest understory, we tested how different macrodetritivore assemblages with increasing FD, based on five decomposition-related traits, affect decomposition under drought. We hypothesized that both FD and CWM of consumption rate would increase litter mass loss, with stronger effects under drought. Irrespective of species composition, results showed that higher FD significantly promoted litter mass loss, while consumption rate CWM had a small additional effect, supporting complementarity among species over the mass-ratio hypothesis. Species identity explained more variance than FD and CWM combined, highlighting the influence of specific species in the assemblages. Although drought reduced overall litter mass loss, the positive relationship between FD and mass loss persisted, suggesting that diverse macrodetritivore communities continue to promote decomposition and nutrient cycling under drought. This suggests that more functionally diverse macrodetritivore communities continue to promote litter decomposition and, consequently, nutrient cycling, even during periods of drought. These findings emphasize the importance of macrodetritivore communities in litter decomposition across climatic conditions and highlight the need to consider species identity in future biodiversity-ecosystem function studies.

**1. Introduction**

The contribution of soil fauna to plant litter decomposition received
renewed interest over the last decade followed by a substantially
improved general understanding of its importance in litter decomposi-
tion (García-Palacios et al., 2013; Handa et al., 2014; Frouz et al., 2015;
Chassain et al., 2021; Hedènec et al., 2022; Sagi and Hawlena, 2024;
Zeng et al., 2024) and of the underlying direct and indirect fauna-driven
mechanisms modifying litter decomposition and organic matter trans-
formations (David, 2014; Joly et al., 2020; Coq et al., 2022). This also

helped to establish a clearer conceptual framework for the interactions
between soil fauna, microbial communities and plant litter underpin-
ning the formation and stabilization of soil organic matter (Nielsen et al.,
2011; Cotrufo et al., 2015; Lehmann and Kleber, 2015; Coq et al., 2018;
Frouz, 2018; Angst et al., 2024). As part of the soil fauna, macro-
detritivores often have low assimilation rates while ingesting large
amounts of leaf litter with consequently most of it returning to the soil as
faeces (Wolters, 2000; David, 2014). This conversion into faeces
strongly affects organic matter physico-chemical properties that can
increase the following microbial decomposition and carbon and nutrient

<footnote><sup>a</sup> CEFE, Univ. Montpellier, CNRS, EPHE, IRD, Univ. Paul-Valéry Montpellier, Montpellier, France</footnote>

<footnote><sup>b</sup> Montpellier European Ecotron, Univ Montpellier, CNRS, Campus Baillarguet, Montferrier-sur-Lez, France</footnote>

<footnote><sup>c</sup> CEFE, Univ Paul Valéry Montpellier 3, Univ Montpellier, CNRS, EPHE, IRD, Montpellier, France</footnote>

<footnote><sup>d</sup> Ecodiv USC INRAE 1499, University of Rouen Normandy, France</footnote>

<footnote>* Corresponding author.</footnote>

<footnote>E-mail address: mika.lemoine@cefe.cnrs.fr (M. Lemoine).</footnote>

<footnote n="1">¹ Co-senior author.</footnote>
leaching, hence potentially promoting the physical and biological pathway of SOM formation (Joly et al., 2020; Angst et al., 2024). In addition to the overall mostly positive effects of macrodetritivores on litter decomposition, several studies showed that different species of macrodetritivores vary strongly in their effects on decomposition due to species-specific feeding preferences and rates of consumption, assimilation and faeces production (Zimmer, 2002; Coulis et al., 2009; David, 2014; Joly et al., 2020; Ganault et al., 2022).

Species-specific effects on decomposition subsequently leads to the question how detritivore communities composed of several species may influence decomposition and carbon and nutrient cycling at the ecosystem scale. The role of biodiversity on ecosystem functioning (BEF) is widely studied with a particularly strong focus on plant communities (e.g. Loreau et al., 2001; Cardinale et al., 2011; Tilman et al., 2014), but other groups of organisms, including soil organisms, received much less attention (de Bello et al., 2010; van der Plas, 2019). The few existing studies confirmed the importance of functional diversity (FD) over species richness to explain litter mass loss, and also showed that process rates saturated at low species number (Liiri et al., 2002; Heemsbergen et al., 2004; Hedde et al., 2010; Nielsen et al., 2011; Lubbers et al., 2020). These studies pointed to the *complementarity hypothesis* stating that communities of species with more dissimilar traits will exhibit resource partitioning use leading to increased process rates (Loreau and Hector, 2001; Barry et al., 2019). In addition to complementarity mechanism, facilitation may also operate, that is if one species benefits from the activity, e.g. litter transformation, by another species (Gessner et al., 2010). Alternatively, the *mass-ratio hypothesis* states that the influence of a species on a given ecosystem process is directly proportional to its relative abundance in the community (Grime, 1998). Hence, an ecosystem process could be predicted by the community trait composition and more specifically by the Community Weighted Mean (CWM) trait value, a metric determined by averaging the trait values of species present weighted by their relative abundances (Garnier et al., 2004; Ricotta and Moretti, 2011).

FD and CWM represent different facets of a community trait composition and both can contribute significantly to ecosystem processes (de Bello et al., 2010; Mouillot et al., 2011). However, FD and CWM are mathematically interdependent and show a hump-shaped relationship, with functional diversity being lowest when CWM values are either low or high. Therefore, observational data alone cannot disentangle their respective impacts. To address this knowledge gap, Dias et al. (2013a) proposed an experimental framework to separate the effects of CWM and FD on ecosystem processes. This framework was applied in a microcosm experiment replicating a temperate forest detritivore system with communities of woodlice varying in species richness, FD and CWM, and showed stronger support for the mass-ratio hypothesis (Bílá et al., 2014). Our knowledge is however very limited in other ecosystem types with different animal and plant species pools. Moreover, the relative importance of these two community trait metrics may depend on the environmental context, with drier or wetter climates or different temperatures potentially modifying also the relationship between trait values and process rates.

Mediterranean ecosystems are characterized by dry summers, which are expected to become even drier, with a projected 20 % decrease in precipitation by the end of the century (Philandras et al., 2011). This intensification of summer drought may have significant consequences for macrodetritivore activity (David and Handa, 2010; Collison et al., 2013). A recent study in China showed that a 50 % reduction in rainfall decreased carbon and nitrogen loss during litter decomposition, but increasing decomposer community complexity, particularly macrofauna, mitigated these effects (Luan et al., 2024). Similarly, a meta-analysis found that macrofauna presence enhances litter decomposition, especially in dry ecosystems (Njoroge et al., 2022), highlighting their importance under climate change. However, in experimental set up, millipede species or millipede and woodlice communities exhibited reduced feeding rates in response to drier conditions

(Coulis et al., 2013, 2015). Due to variations in experimental conditions, ecosystem types, and species used, comparisons across the limited number of available studies are challenging. Moreover, at the community level, interspecific interactions occur, which may change the response of certain species and determine the general outcome of changes in precipitation regimes. The stress gradient hypothesis suggests monotonic increased facilitation under stress (Bertness and Callaway, 1994; Lortie and Callaway, 2006), but its relevance to detritivore communities remains unclear. Coulis et al. (2015) found no difference in the relationship between macrodetritivore FD and litter decomposition under drought, while (Joly et al., 2021) showed that synergistic interaction between a millipede and a woodlice species disappeared and even switched to antagonist interactions under reduced rainfall. This aligns with the stress gradient hypothesis, which posits a stress threshold beyond which competition dominates (Dangles et al., 2013). This lack of knowledge is particularly problematic in view of the strong potential of macrofauna communities to alleviate drought effects on decomposition across multiple ecosystems (Luan et al., 2024).

With the present study we attempt to contribute to filling this gap of knowledge with an experiment under close to natural conditions in the European Ecotron in Montpellier. We tested drought effects on litter decomposition and specifically asked what characteristics of macrodetritivore communities may explain their impact on decomposition and their potential drought responses. We simulated a Mediterranean forest understory ecosystem in the Ecotron, composed of four different tree species. We included functional diversity, the consumption rate CWM and detritivore species identity in our assessment of drought effects on decomposition. We hypothesized that (H1) the community-weighted mean of consumption loss enhances litter mass loss more effectively than the functional diversity of macrodetritivores (Bílá et al., 2014). Our second hypothesis is that (H2) litter mass loss is reduced in microcosms facing drier conditions due to reduced microbial decomposition and macrodetritivore feeding activity under stress (Thakur et al., 2018). Furthermore, according to the insurance hypothesis (Yachi and Loreau, 1999), higher species richness hampers negative effects of stressing conditions on ecosystem processes depending and how the ecosystem process is influenced by individual species responses to environmental stress, and the extent of temporal asynchrony in these species-specific responses, which can both be predicted by traits. Therefore, a community of higher functional diversity may maintain and promote higher rates of an ecosystem function. Hence, we expected that (H3) detritivore assemblages with high functional diversity would enhance litter mass loss under drought conditions compared to assemblages with low functional diversity, despite the prediction in H1 that CWM is a stronger predictor than functional diversity. To test these hypotheses, we set-up a four-week experiment with six different assemblages of macrodetritivores, each represented by two different species and measured the mass loss of a mixture of four tree litter species under a control and a reduced precipitation quantity and frequency regimes.

## 2. Material and methods

Our experiment was conducted in 144 microcosms, which were nested within a larger experiment involving 16 mesocosms at the Montpellier European Ecotron experimental platform (Roy et al., 2021) (Fig. 1). The mesocosm experiment (described below) provided both the litter used in our study and the habitat in which the microcosms were incubated. However, the microcosms’ watering conditions were completely decoupled from the mesocosm conditions due to the following reasons: 1) a cover prevented direct rainfall from the mesocosms from affecting the microcosms’ water status, 2) the microcosms had an independent watering treatment, and 3) at the end of the experiment, soil moisture values in the microcosms did not differ between mesocosms (p = 0.34). Therefore, for the purposes of this study, including statistical analyses, the microcosm treatments were considered independent of the mesocosm experiment.
<figure><img src="image_4.png" /><figcaption>Fig. 1. Summary of the experimental design of the microcosm manipulation placed in ECOTRON mesocosms.</figcaption></figure>

## 2.1. Mesocosms setup

The mesocosm experiment that started in fall 2018 as part of the SoilForEUROPE project (https://websie.cefe.cnrs.fr/soilforeurope/en/), continuing from 2022 onwards as part of the BELOW project (https://biodivoc.edu.umontpellier.fr/recherche/projets-consortium/projet-consortium-below/). The goal of the larger project is to test the interactive effects of drought and soil fauna functional diversity, on the functioning of a Mediterranean forest ecosystem. Sixteen fully controlled mesocosms with a surface area of 1 m² and a volume of 4 m³ aboveground volume and 1 m³ lysimeters to host the belowground compartment were used. Within each mesocosm, two saplings (with the exception of *Quercus ilex* for which six were planted) of four Mediterranean tree species were planted: *Acer monspessulanum* L. 1753 (Sapindaceae), *Arbutus unedo* L. 1753 (Ericaceae), *Quercus ilex* L. 1753 (Fagaceae), and *Quercus pubescens* Willd. 1796 (Fagaceae). Tree species were selected because they typically co-occur in Mediterranean forests of southern France and because they produce leaf litter of distinct physical and chemical properties. In the year of this experiment, eight of the sixteen mesocosms underwent a drought treatment consisting of three drought-rewatering cycles. Each drought period was extended by 30 %, and precipitation was reduced by 30 % per cycle compared to the controls. Drought cycles were determined by soil water potential reaching the wilting point of -1.5 MPa (measured at 30 cm depth with a full-range tensiometer FR T15D, UGT, Germany) before rewatering. The 30 % reduction in rewatering was based on ecosystem water loss, measured through lysimeter weight changes. Before and after the drought cycles, both Control and Drought mesocosms were rewatered identically and maintained under the same general climatic conditions throughout the experiment. These conditions were based on the average temperatures and precipitation recorded over 15 years (2003–2017) at the long-term field site in Puéchabon (https://puechabon.cefe.cnrs.fr/), which served as the reference site.

## 2.2. Animal and plant material

We used six macrodetritivore species: three woodlice species (Crustacea, Malacostraca), *Armadillidium vulgare* Latreille 1804, *Armadillo officinalis* Duméril 1816, and *Porcellio laevis* Latreille 1804, as well as three millipede species (Myriapoda, Diplopoda), *Glomeris marginata* Villers 1789, *Ommatoiulus sabulosus* Linnaeus 1758, and *Cylindroiulus caeruleocinctus* Wood 1864. These same species were also used in the larger and longer-term mesocosm experiment. Three weeks prior to the installation of our experiment, 150 individuals of each species were collected in natural ecosystems nearby Montpellier (Montarnaud Forest, 3°40'E, 43°44'N and CEFE experimental field, 3°86'E, 43°63'N) and in Marseille (Massif de l'Étoile, 5°25'E, 43°22'N). They were stored in a plastic container under constant temperature (20 °C) and day length (12h) for two weeks, and fed with a mixture of litter composed of litter from their original habitat and of litter used later in our experiment (i.e. leaf litter from the four tree species planted in the mesocosms).

In March 2023, after five years of running the Ecotron mesocosm experiment, we collected litter produced by the four tree species in each of the sixteen Ecotron mesocosms using a 25 cm quadra placed at their center. We then pooled all leaf litter within each drought treatment. Keeping leaf litter from the two drought treatments separate appeared important because of the potential effects of climate on litter properties and litter feeding fauna (David and Handa, 2010). The litter was then dried in an oven at 30 °C (LMS LTD 303NP, Sevenoaks, UK). From each of the two litter pools we separated the litter of the four species to determine their respective proportions (across the eight mesocosms per drought treatment), which were reproduced in our experiment (Table 1).

## 2.3. Leaf litter chemical properties

The litter used for the chemical analysis came from the same pool
<caption>**Table 1**</caption>

Properties of leaf litter among the four tree species (mean ± standard deviation). Microbial decomposition = litter mass lost solely due to microbial activity, litter mass loss = litter mass lost due to the combination of microbial activity and detritivores. Letters indicate significant differences between litter species and drought treatment after a post hoc Tukey test.

<table><thead><tr><th rowspan="3">Trait</th><th colspan="8">Tree species</th></tr><tr><th colspan="2">A. monspessulanum</th><th colspan="2">Q. ilex</th><th colspan="2">Q. pubescens</th><th colspan="2">A. unedo</th></tr><tr><th>n</th><th>Control</th><th>Drought</th><th>Control</th><th>Drought</th><th>Control</th><th>Drought</th><th>Control</th><th>Drought</th></tr></thead><tbody><tr><td>Average initial proportion (%)</td><td>70</td><td>60.4 ± 0.4 e</td><td>60.8 ± 0.3 f</td><td>5.0 ± 0.2 a</td><td>5.0 ± 0.2 a</td><td>6.3 ± 0.2 b</td><td>6.3 ± 0.2 b</td><td>28.3 ± 0.4 d</td><td>27.9 ± 0.3 c</td></tr><tr><td>Initial dry litter mass (g)</td><td>70</td><td>1.8 ± 0.0 e</td><td>1.8 ± 0.0 f</td><td>0.2 ± 0.0 a</td><td>0.2 ± 0.0 a</td><td>0.2 ± 0.0 b</td><td>0.2 ± 0.0 b</td><td>0.9 ± 0.0 d</td><td>0.8 ± 0.0 c</td></tr><tr><td>Litter mass loss (%)</td><td>46</td><td>21.8 ± 4.7 a</td><td>19.7 ± 4.5 a</td><td>17.7 ± 13.7 a</td><td>18.8 ± 11.3 a</td><td>9.1 ± 7.9 ab</td><td>11.2 ± 8.0 b</td><td>7.0 ± 5.3 ab</td><td>4.1 ± 3.8 a</td></tr><tr><td>Microbial decomposition (%)</td><td>24</td><td>6.2 ± 2.3 b</td><td>6.8 ± 2.4 b</td><td>1.5 ± 3.1 a</td><td>1.6 ± 3.5 a</td><td>2.0 ± 2.3 a</td><td>2.0 ± 1.6 a</td><td>1.2 ± 1.1 a</td><td>1.8 ± 4.8 a</td></tr><tr><td>C:N ratio</td><td>5</td><td>64.6 ± 1.6 b</td><td>77.8 ± 1.9 cd</td><td>48.0 ± 0.7 a</td><td>43.6 ± 1.5 a</td><td>83.9 ± 1.4 d</td><td>71.8 ± 1.7 bc</td><td>159.9 ± 4.3 f</td><td>146.2 ± 8.9 e</td></tr><tr><td>C:P ratio</td><td>5</td><td>348.3 ± 32.8 a</td><td>401.4 ± 41.4 a</td><td>686.2 ± 82.6 bc</td><td>766.9 ± 52.3 c</td><td>618.1 ± 71.5 b</td><td>895.9 ± 96.5 d</td><td>282.4 ± 20.9 a</td><td>286.6 ± 31.5 a</td></tr><tr><td>Tannin (%)</td><td>5</td><td>2.0 ± 0.3 a</td><td>2.6 ± 0.1 a</td><td>0.3 ± 0.0 a</td><td>0.4 ± 0.1 a</td><td>1.4 ± 0.2 a</td><td>1.3 ± 0.2 a</td><td>7.6 ± 3.6 b</td><td>12.6 ± 5.5 c</td></tr><tr><td>Hemicellulose</td><td>5</td><td>4.9 ± 0.5 cd</td><td>4.9 ± 0.6 d</td><td>0.9 ± 0.9 a</td><td>4.0 ± 0.3 c</td><td>4.3 ± 0.3 cd</td><td>7.0 ± 0.2 e</td><td>2.3 ± 0.5 b</td><td>4.6 ± 0.3 cd</td></tr><tr><td>Cellulose</td><td>5</td><td>19.9 ± 0.7 c</td><td>18.7 ± 0.6 c</td><td>23.2 ± 1.1 d</td><td>23.4 ± 0.8 d</td><td>23.5 ± 0.3 d</td><td>23.4 ± 0.4 d</td><td>15.3 ± 1.2 b</td><td>10.5 ± 1.9 a</td></tr><tr><td>Lignine</td><td>5</td><td>20.5 ± 0.6 c</td><td>13.7 ± 0.3 a</td><td>30.4 ± 1.4 f</td><td>29.9 ± 1.2 f</td><td>17.4 ± 0.6 b</td><td>18.4 ± 0.4 b</td><td>26.9 ± 0.6 e</td><td>24.3 ± 0.3 d</td></tr></tbody></table>

that was used for the microcosm experiment. For each of the separated eight types of litter (four tree species from two drought treatments), five subsamples were used for the analyses of chemical properties (Table 1). Subsamples were ground to 1 mm (Cyclotec Sample Mill, Tecator, Hoganas, Sweden) and then analyzed for carbon and nitrogen concentrations using an elemental analyzer (Flash EA1112 Series, Thermo-Finnigan, Milan, Italy). Phosphorus concentration was measured colorimetrically after a mineralization phase using an automatic analyzer (SmartChem 200, Alliance Instruments, Roma, Italy). Fiber content was measured through a fully automated boiling and filtration process using the Fibertherm FT12 (Königswinter, Germany). Finally, tannin concentration was measured with the protein-precipitable phenolics microplate assay, a microplate protocol adapted from Hagerman and Butler (1978) following Hagerman (2011).

## 2.4. Detritivore assemblages

To select the specific species assemblages used in our experiment, we followed the framework developed by Dias et al. (2013a) and used traits of the same six macrodetritivore species feeding on the same four leaf litter types measured by Ganault et al. (2022). We selected traits quantifying the ability of detritivores to process litter and to influence its decomposition through their faeces characteristics, while avoiding redundancy among them. Out of nine available traits (Ganault et al., 2022), we selected five based on a correlational approach and an ordination method (see Fig. S1). The five selected traits (Table S1) include mass-specific consumption rate and assimilation rate (directly related to the impact of detritivores on litter mass loss), faeces C:N ratio (linked to faeces mass loss, Coq et al., 2022), faeces water retention capacity (accounting for the importance of water availability for microbial activity) and fresh body mass (directly related to consumption rate with smaller individuals consuming more per unit of body mass than larger individuals and to their drought response with smaller individuals typically more sensitive to drought than larger individuals) (Astor et al., 2015; Ardestani et al., 2019).

Functional diversity was calculated according to the Rao Q quadratic entropy index representing the total dissimilarity in trait space among all possible species pairs weighted by the product of their relative abundances (Ricotta and Moretti, 2011). The community-weighted mean of a trait is the average of a trait value, weighted by the relative abundances or biomasses of each species of a community (Garnier et al., 2004; Ricotta and Moretti, 2011). Rao Q and CWM of the mass-specific consumption rate (CWM<sub>Cm</sub>) weighed by species initial biomass were calculated for all 15 possible combinations of two detritivore species using the dbFD function of the FD package (Laliberté et al., 2014). Rao Q

and CWM exhibited the expected hump-shaped relationship although not significant (Fig. S2, Table S2, Dias et al., 2013a). We used CWM<sub>Cm</sub> as it is well linked to litter mass loss and was recently recommended as a mandatory, major trait to include in experimental studies linking soil fauna and organic matter transformations (Bonfanti et al., 2024). Following Coulis et al. (2015), six combinations out of the 15 were chosen to maximize the gradient of both functional diversity and CWM<sub>Cm</sub> while also ensuring equal occurrence of each species to avoid species imbalance (Fig. S2, Table S2). Only combinations with one woodlouse and one millipede species were selected to limit any potential bias due to differences in phylogenetic proximity (Joly et al., 2020; Ganault et al., 2022). Applying these rules, the following six species assemblages resulted for the experiment, ordered by increasing functional diversity: A. officinalis/O. sabulosus (RaoQ = 1.21), A. vulgare/C. caeruleocinctus (RaoQ = 1.75), A. vulgare/O. sabulosus (RaoQ = 2.57), A. officinalis/G. marginata (RaoQ = 2.89), P. laevis/G. marginata (RaoQ = 3.23), and P. laevis/C. caeruleocinctus (RaoQ = 3.45).

## 2.5. Microcosms experimental design

Six microcosms were randomly placed around the center of each mesocosm, presenting each of the six detritivore species assemblages, along with three microcosms without fauna as controls to quantify litter mass loss resulting from microbial activity only (Fig. 1). This led to 9 microcosms replicated 8 times for both drought treatments, resulting in a total of 144 microcosms. The microcosms were made from polyvinyl chloride tubes (10 cm in diameter x 5 cm in height) closed at the top, bottom, and sides with 1 mm mesh to prevent fauna from escaping. At the base of each microcosm, a 3 cm layer (120 g) of the same soil used in the mesocosms was placed (Fig. 1). This soil had been previously sieved (1 cm mesh) and sterilized (3 cycles of freezing/oven at 60 °C following Huhta et al. (1989)). The soil exhibits an alkaline pH of 8.6, an organic carbon content of 11.9 g/kg, a C:N ratio of 11.7 and a 20.6 g/kg of organic matter. Its texture comprises 27.7 % clay, 31.4 % silt, and 40.9 % sand, classifying it as loam. On the soil surface, 3g of leaf litter mixture was added, representing a non-limiting amount of food for the animals during our experiment (David, 2014). The amount of each litter type corresponded to the average proportion of each species found in the drought and control mesocosms separately (Table 1). All microcosms were prepared by filling the respective amount of soil that was then humidified two days before the experiment. To do so, each soil-filled microcosm was placed in a large plastic container, and water was added up to half the soil height for 5 min. In parallel we also humidified the leaf litter to approach water retention capacity before adding it to the microcosms. Then just before we placed the microcosms in the
mesocosms we again humidified the litter with 40 ml using a pipette, and added three previously weighed individuals of each of the two animal species according to the designated functional diversity. A total of 96 individuals per animal species were selected from the pool of 150 collected individuals aiming at homogenizing initial biomass within each species between drought treatments.

The microcosms were incubated within the mesocosms for four weeks, from April to May 2023. During this period, the control treatment microcosms were watered every three days using a 40 mL micro-pipette, while the drought treatment microcosms were watered every six days only. At each watering event, water quantity was the same (40 mL added) and was based on the average precipitation in the Mediterranean forest of Puechabon during the same season. Ultimately, throughout the experiment, the control treatment microcosms received 280 ml, compared to 160 ml for the drought treatment. At the end of the fourth week, the microcosms were collected. The litter was retrieved and placed in an oven at 30 °C for 72 h, after which the leaf litter was brushed to remove any adhering soil particles. The litter was then sorted and weighted by species. The soil was weighed for each microcosm, dried in an oven at 30 °C for 72 h, and weighed again to measure the soil water content (SWC) at the end of the experiment. The fauna was extracted and weighed to determine the final fresh mass of the living individuals per species. We observed a few rare dead animals at the end of the experiment, but the mortality rate was less than 5 % overall. Half of all dead animals occurred in *P. laevis*. Mortality was not influenced by the watering treatment (p = 0.81). All assemblages except the *A. vulgare/C. caeruleocinctus* combination lost mass during the experiment (Table 2). The calculation of the RaoQ index was recalculated by using the mean species biomass for each assemblage over the entire duration of the experiment, therefore accounting for both weight loss of animals and mortality (Table 2). Assemblages with 3 individuals of the same species dead (a total of four microcosms) were excluded from the analysis as the RaoQ index for the assemblage was equal to 0.

## 2.6. Statistical analyses

To test the relative importance of FD and CWM and how drought could modulate their effect on litter mass loss, we used a generalized linear mixed model with accounting for the mesocosms as a random factor, using the *lme* function from the *nlme* package (Pinheiro et al., 2013). To reach homoscedasticity and normality of the variance, different error structures were tested under restricted maximum likelihood (REML) and compared with AIC criteria, and litter mass loss (expressed as a percentage of initial weight) was log-transformed using the function *logit* from the *car* package (Fox and Weisberg, 2011). Weights are typically used to account for variables that influence data variability and thus affect homoscedasticity. After exploring different options, we retained weights for moisture level and CWM because they resulted in the lowest AIC. Biologically, this choice is justified by the fact

that variance increases with the mean value of litter mass loss. Since litter mass loss is strongly linked to moisture level and CWM, these factors can influence both its mean and variance. Then different fixed parameter structures were then tested under maximum likelihood (ML) and model selection was based on the AIC criteria. Using the function *StepAIC* from the MASS package (Venables and Ripley, 2002), we determined the ultimate set of terms in the model through a backward model selection process. We initially allowed interactions among FD, CWM and Moisture level (M), which was finally not retained in the minimal adequate model. The final selected model structure is:

Eq. (1): $lme(logit(litter mass loss) \sim FD + CWM + Moisture level, random = \sim 1 | mesocosm, weights = varComb(varIdent(form = \sim 1 Moisture level), varExp(form = \sim CWM)))$

A second model was constructed following the same procedure, adding species identity as an additional variable. Taking into account species identity in the model helps avoid overinterpretation of the effect of FD, where litter mass loss can also be partly driven by the trait of individual species in addition to macrodetritivore assemblage effect. To identify which species contribute to the general model, we initially ran a model including all and only detritivore species coded as a presence/absence variable. Three of them (*A. officinalis*, *P. laevis*, *C. caeruleocinctus*) significantly impacted litter mass loss and were then added to the general model (Eq. (1)). The most parsimonious model was selected using the function *StepAIC*, which only kept the species *A. officinalis*. The final selected model is:

Eq. (2): $lme(logit(litter mass loss) \sim A. officinalis + FD + CWM + Moisture level, random = \sim 1 | mesocosm, weights = varComb(varIdent(form = \sim 1 | Moisture level), varExp(form = \sim CWM)))$

Ordinary least squares regression (OLS) was employed to examine the correlation between the loss of litter mass and either functional diversity indices or consumption rate CWM indices of detritivore assemblages. Variance partitioning was performed using linear mixed-effects models. The sum of squares from ANOVA outputs was used to calculate the percentage of variance explained by each fixed effect. All analyses were conducted using R software version 4.3.2.

## 3. Results

### 3.1. Leaf litter chemical traits and decomposition

The litter used in our study comes from the on-going long-term mesocosm experiment that started in fall 2018 exposed the four tree species to reduced and more rare precipitation. We therefore explored the effect of this drought on the initial leaf litter traits in addition to interspecific differences. All chemical traits differed significantly between plant species (Table 1, Fig. S4, Fig. S5). The drought treatment and its interaction with plant species significantly influenced the C:N ratio, C:P ratio, and hemicellulose concentration, while the effect on tannin was marginally significant (Table S3, Fig. S4). Q. ilex had the

<caption>**Table 2**</caption>

Traits of detritivore assemblages (mean ± standard deviation, n = 16). RaoQ recalculated = RaoQ recalculated per microcosm, taking mortality into account. Letters indicate significant differences between assemblages after a post hoc Tukey test. A. off refers to *Armadillo officinalis*, A. vul to *Armadillidium vulgare*, O. sab to *Ommatoidius sabulosus*, G. mar to *Glomeris marginata*, P. lae to *Porcellio laevis* and C. cae to *Cylindroiulus caeruleocinctus*.

<table><thead><tr><th rowspan="2"></th><th colspan="6">Assemblage</th></tr><tr><th>A.off/O.sab</th><th>A.vul/C.cea</th><th>A.vul/O.sab</th><th>A.off/G.mar</th><th>G.mar/P.lae</th><th>C.cea/P.lae</th></tr></thead><tbody><tr><td>RaoQ</td><td>1.21</td><td>1.75</td><td>2.57</td><td>2.89</td><td>3.22</td><td>3.45</td></tr><tr><td>RaoQ recalculated</td><td>1.21</td><td>1.74</td><td>2.53</td><td>2.85</td><td>3.21</td><td>3.40</td></tr><tr><td>CWM</td><td>7.04</td><td>18.01</td><td>14.59</td><td>7.11</td><td>11.5</td><td>15.03</td></tr><tr><td>Initial fresh mass assemblage (g)</td><td>1.21 ± 0.14 d</td><td>0.54 ± 0.12 a</td><td>0.81 ± 0.26 b</td><td>1.23 ± 0.18 d</td><td>0.97 ± 0.14 c</td><td>0.65 ± 0.06 b</td></tr><tr><td>Final fresh mass assemblage (g)</td><td>1.20 ± 0.16 d</td><td>0.56 ± 0.15 a</td><td>0.76 ± 0.11 bc</td><td>1.19 ± 0.22 d</td><td>0.94 ± 0.22 cd</td><td>0.63 ± 0.20 ab</td></tr><tr><td>Litter mass loss (%)</td><td>12.25 ± 3.03 a</td><td>14.66 ± 1.74 ab</td><td>17.21 ± 3.54 bc</td><td>13.93 ± 2.59 a</td><td>18.8 ± 2.89 c</td><td>17.79 ± 3.24 bc</td></tr><tr><td>Mortality (n)</td><td>A.off: 0/48 O.sab: 0/48</td><td>A.vul: 1/48 C.cae: 1/48</td><td>A.vul: 2/48 O.sab: 0/48</td><td>A.off: 3/48 G.mar: 2/48</td><td>G.mar: 0/48 P.lae: 8/48</td><td>C.cae: 3/48 P.lae: 5/48</td></tr></tbody></table>
lowest C:N ratio and *A. unedo* the highest and drought decreased this ratio except for *A. monspessulanum*. C:P ratio was the highest for *Q. pubescens* and strongly increased under drought while changes for other litter species were not significant. Tannin's percentage was lower than 2 % and similar for all species except *A. unedo*, for which values increased from 7.6 ± 3.6 % to 12.6 ± 5.5 % when exposed to reduced precipitation. Cellulose and lignin contents responded similarly with a decrease for *A. monspessulanum*, an increase for *A. unedo*, no change for *Q. ilex*, and a significant increase of lignin only for *Q. pubescens*. Hemicellulose content increased in drought conditions except for *A. monspessulanum*.

In control microcosms without fauna, microbial decomposition significantly differed among plant species, ranging from 1 ± 1.2 % for *A. unedo* to 6.4 ± 2.2 % for *A. monspessulanum* (Table 1). The drought treatment, however, did not significantly impact microbial decomposition across litter species. In the presence of macrodetritivores, litter mass loss increased significantly ranging from 5.4 ± 4.7 % for *A. unedo* to 20 ± 4.3 % for *A. monspessulanum* (Table 2). Total litter mass loss was lower under reduced precipitation in presence of macrodetritivore by 1.8 %. Finally, species-specific mass loss was not affected by the drought treatment either in absence or presence of macrodetritivore.

## 3.2. Effects of macrodetritivore assemblage, diversity and identity

All drought treatments combined, litter mass loss varied significantly among different fauna assemblages ranging from 12.3 ± 3 % for the least functionally diverse assemblage with *A. officinalis/O. sabulosus* to 18.8 ± 2.9 % for the second most functionally diverse assemblage with *G. marginata/P. laevis* (Figs. 2 and 3, Table 2). The two detritivore assemblages with lower litter mass loss values both include *A. officinalis*, while higher mass loss was observed in the two assemblages containing *P. laevis*. Overall, litter mass loss increased with higher macrodetritivore FD, while CWM<sub>Cm</sub> also had a positive significant but smaller effect (R<sup>2</sup> = 0.46, p-value <0.001, and R<sup>2</sup> = 0.31, p-value = 0.003, respectively, Table 3, Figs. 2 and 3). In the second modelling step where species identity was included as an additional potential driving factor of litter mass loss, the presence of *A. officinalis* significantly reduced litter mass loss by 2.90 % (p-value <0.001) which corresponds to a 18 % decrease to mass loss in assemblage where it was absent. This effect was the most important factor compared to FD, CWM and drought (Table 3, see Fig. 4).

## 3.3. Effects of precipitation reduction

In the absence of detritivores, litter mass loss did not significantly

<figure><img src="image_5.png" /></figure>

**Fig. 3.** Litter mass loss according to the moisture level (A) in relation to functional dissimilarity of detritivore assemblages (B). Litterbags without fauna were not considered. Black line with gray confidence intervals represent the linear regression across all treatments. Single color lines without confidence intervals show the linear regression for each moisture treatment separately. R<sup>2</sup> values, computed using the Spearman coefficient, are displayed for each linear regression, with colors corresponding to the solid line. (For interpretation of the references to color in this figure legend, the reader is referred to the Web version of this article.)

differ between the drought and the control treatment (p = 0.3) and was on average 4.54 ± 2.01 %. However, in the presence of detritivores we observed a significantly smaller litter mass loss in the drought compared to the control treatment (Table 3, Fig. 2), with an average total mass loss of 14.75 ± 2.94 %, and of 16.55 ± 4.04 % in the drought and the control treatment, respectively. Macrodetritivore FD effect was significant for both the control treatment (R<sup>2</sup> = 0.41, p-value <0.01) and the drought treatment (R<sup>2</sup> = 0.6, p-value <0.001, Fig. 3), with a similar slope for the two treatment-specific regression lines (Fig. 3), in accordance with a non-significant interaction between FD and drought in the models. CWM<sub>Cm</sub> effect of litter mass loss was significant in the drought treatment (R<sup>2</sup> = 0.4, p-value = 0.005) and marginally significant in the control treatment (R<sup>2</sup> = 0.27, p-value = 0.07). The reduction in watering frequency and quantity significantly decreased SWC in the drought treatment (p = 0.01) at the end of the experiment, with an average SWC of 15.31 ± 3.09 % in the drought treatment and 16.68 ± 2.01 % in the

<figure><img src="image_6.png" /></figure>

**Fig. 2.** Effect of functional diversity (A) and CWM<sub>Cm</sub> (B) on litter mass loss according to the detritivore assemblage. A Tukey post hoc test was used to assess pairwise differences between detritivore assemblages. The letter indicates a significant difference in the mean value between assemblages. *A.off* refers to *Armadillo officinalis* (woodlice), *O.sab* to *Ommatobiulus sabulosus* (millipede), *A.vul* to *Armadillidium vulgare* (woodlice), *C.cae* to *Cylindroiulus caeruleocinctus* (millipede), *G.mar* to *Glomeris marginata* (millipede) and *P.lae* to *Porcellio laevis* (woodlice).
<figure><img src="image_7.png" /><figcaption>Fig. 4. Explained variance including and excluding species identity (presence or absence of *Armadillo officinalis*) from the models.</figcaption></figure>

control treatment. However, neither FD nor assemblage composition had an effect on SWC (p = 0.47 and p = 0.07, respectively).

### 3.4. Variance partitioning

With a variance partitioning approach, we observed that including *A. officinalis* increased the proportion of explained variance by the model from 39 % to 48 % and species identity accounted for 32 % of the variance. The drought treatment consistently explained 6 % of the variance, whether species identity was accounted for or not. On the other hand, variance explained by CWM<sub>Cm</sub> decreased from 8 % to 2 % and variance explained by FD decreased from 25 to 8 % when accounting for species identity.

## 4. Discussion

Our study investigated the joint effect of macrodetritivore functional diversity, species identity and community weighted mean consumption rate on leaf litter mass loss under two precipitation regimes in an ecotron experiment simulating a Mediterranean forest understory ecosystem. Our results support the importance of functional diversity of litter feeding macrofauna for decomposition, while providing little support for the hypothesis that community-weighted mean traits are stronger predictors in contradiction to our initial first hypothesis. Instead, our findings suggest that functional diversity plays a more central role than expected, especially when considering species identity effects that also significantly influenced litter mass loss. Increased drought through the reduction in the amount and frequency of precipitation reduced litter mass loss in line with our second hypothesis, but contrary to our expectations and our third hypothesis, and somewhat in contrast to the stress gradient hypothesis, the functional diversity effect was not higher

under drought conditions.

### 4.1. Persistent functional diversity effects

Litter mass loss significantly increased with functional diversity of detritivores assemblage explaining up to 25 % of the variance in litter mass loss. This positive relationship is in agreement with the literature, even if the other existing studied used different animal taxa, including earthworms (Heemsbergen et al., 2004; Hedde et al., 2010) or snails (Coulis et al., 2015) in addition to woodlice and millipedes, and used different metrics to quantify functional diversity (Heemsbergen et al., 2004; Hedde et al., 2010). These consistent findings across studies with distinct experimental setups and general context make a strong case for a robust positive relationship between macro-detritivore functional diversity and litter mass loss (Bardgett and van der Putten, 2014). Such a positive relationship can be explained by at least two mechanisms, facilitation and resource partitioning (Barry et al., 2019) among macrodetritivore species, which are increasing with increasing differences in functional traits. In our system, facilitation could arise if a given species alters the leaf litter in a way that makes it more easily consumable by another species in the assemblage (Gessner et al., 2010; Gobat et al., 2010). For example, the conversion of litter into faeces may facilitate microbial activity and microbial colonization which may favor macrodetritivore litter consumption (Schaefer et al., 2009; Joly et al., 2020). In our case, we observed that the mass loss of individual litter species mass loss was higher in presence of functionally more diverse detritivore assemblages (Fig. S3). This may indicate that a litter species' individual leaves were made more edible for a species after consumption by another one. Concomitantly, resource partitioning could occur if assemblages with higher functional diversity feed on litter from different plant species or different parts of the leaf litter due to different mouthpart traits (Ranta and Lundberg, 1980; Brousseau et al., 2018). In our experiment the proportion of litter mass loss increased with macrodetritivore FD for all litter species and especially for *Q. ilex*, but not for *A. unedo*. This shows that mass loss was more even across litter species, likely from a more even consumption of each litter type in more diverse macrodetritivore assemblages. To test this hypothesis, we calculated litter consumption evenness using Pielou's index. Consumption evenness showed a non-significant relationship with macrodetritivore FD (R<sup>2</sup> < 0.01, p = 0.36, Fig. S6), failing to support this mechanism experimentally. A better relationship may have emerged if the Rao Q FD index had been computed with traits more directly linked to resource partitioning such as mouthpart traits (e.g. opening width, incision strength). Further research is needed to identify key traits and test these relationships experimentally (Brousseau et al., 2018).

### 4.2. Weak community weighted mean effects

In our experiment, higher CWM<sub>Cm</sub> increased litter mass loss, but, contrary to our first hypothesis, explained a smaller proportion of variance compared to functional diversity with only up to 8 % of variance explained, compared to 25 % by functional diversity. Our study provides strong support for the diversity hypothesis (Loreau and Hector,

<caption>**Table 3**</caption>

Results of an ANCOVAs testing the effect of detritivore dissimilarity, moisture level, consumption rate CWM without and including the species identity (presence or absence of *Armadillo officinalis*) on litter mass loss (%). The interaction, annotated NA, was not retained in the minimal adequate model.

<table><thead><tr><th rowspan="3"></th><th colspan="4">Litter mass loss</th><th colspan="4">Including species identity</th></tr><tr><th colspan="4">Without species identity</th><th colspan="4">Including species identity</th></tr><tr><th>den DF</th><th>num DF</th><th>Sum Sq</th><th>F-value</th><th>den DF</th><th>num DF</th><th>Sum Sq</th><th>F-value</th></tr></thead><tbody><tr><td>Functional diversity (D)</td><td>73</td><td>1</td><td>1.82</td><td>35.64***</td><td>73</td><td>1</td><td>0.56</td><td>12.76***</td></tr><tr><td>Moisture level (M)</td><td>14</td><td>1</td><td>0.47</td><td>10.83**</td><td>14</td><td>1</td><td>0.43</td><td>9.65**</td></tr><tr><td>CWM<sub>Cm</sub></td><td>73</td><td>1</td><td>0.55</td><td>9.20**</td><td>73</td><td>1</td><td>0.18</td><td>3.97*</td></tr><tr><td>Armadillo officinalis</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>73</td><td>1</td><td>2.37</td><td>52.76***</td></tr><tr><td>D x M</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td><td>NA</td></tr></tbody></table>
2001) over the mass-ratio hypothesis (Grime, 1998). This finding contrasts with those of Bílá et al. (2014), who showed that CWM<sub>Cm</sub> was the primary factor driving variations in ash leaf litter mass loss, with trait dissimilarity contributing only minimally to these variations. This difference may arise first from the fact that they provided a single litter species to macrodetritivore, strongly limiting the resource diversity available, in terms of physico-chemical characteristics that influence macrodetritivore feeding behavior. This likely hampered resource use complementarity to occur and therefore any effect of functional diversity. Second, their study focused on trait variation within a single taxonomic group only (woodlice), whereas our study also included millipedes. Having two classes of arthropods likely resulted in a wider range of trait diversity values among our selected species. We cannot fully assert this idea given that different trait and functional diversity indexes were used. Finally, plant and animal species and experimental set-up reproduced a temperate forest with less harsh conditions for animal activity. It is possible that functional diversity effects occur only at a certain threshold of stress, as stated by the stress gradient hypothesis (Bertness and Callaway, 1994), which likely wasn't reached in the experiment by Bílá et al. (2014) but was in ours. Our study is only the second to precisely test the relative contribution of these two mechanisms and further work is needed to better understand the importance of FD and CWM across environmental contexts.

### 4.3. Species identity effects

Our statistical analysis, which included tests for the presence and absence of each species, highlighted a significant negative effect of *A. officinalis*. This negative sampling effect means that in assemblages containing this species, litter consumption was generally slowed. Accounting for the presence of *A. officinalis* in the model accounted for 32 % of the variation in litter mass loss. *A. officinalis* was actually the species with the lowest consumption rate in the study by Ganault et al. (2022) which was also confirmed in an earlier study (Coulis et al., 2015), and it was associated to the two assemblages with the lowest CWM of consumption rates in our study. The inclusion of the presence of *A. officinalis* in the model reduced the amount of variation explained by FD and CWM<sub>Cm</sub> in litter mass loss, highlighting that assemblage properties may be, at least in part, explained by species identity effects on litter consumption. Although not kept in the final minimal adequate model, the presence of *P. laevis* had a similar, but opposing effect on litter mass loss with its higher consumption rate. These observations collectively show that several properties of macrodetritivore communities affect litter mass loss with a particularly strong contribution of species identity in support of earlier work (Vos et al., 2011). This is in line with the "sampling effect" or "selection probability effect" (Wardle, 1999) that has been a widely debated topic in the context of BEF relationships. It refers to the hypothesis that ecosystems with greater species richness are more likely to contain species that perform a particular function exceptionally well or bad, thus potentially shaping the BEF relationship. While the sampling effect was initially considered a statistical artefact, it is now largely accepted as a biodiversity mechanism. The reasoning is that high biodiversity increases the likelihood of species with traits beneficial to ecosystem function being present, and this is a genuine ecological process (Flombaum et al., 2014). Existing studies on BEF relationships in detritus-based systems (Heemsbergen et al., 2004; Hedde et al., 2010; Bílá et al., 2014; Coulis et al., 2015; Lubbers et al., 2020; Joly et al., 2021) never included species identity effects in their experimental design or statistical analyses and therefore may have overestimated the effect of functional diversity.

### 4.4. Drought effects

Consistent with our second hypothesis, litter mass loss was significantly lower under drier conditions. However, this negative drought effect was entirely driven by the presence of macrodetritivores, as litter

mass loss in fauna-free controls was similar in both treatments. This contrasts with several studies showing that microbial activity is less resistant to drought than macrodetritivore activity (Schimel et al., 1999; Coulis et al., 2013, 2015; Thakur et al., 2018; Joly et al., 2019). The observed unchanged microbial activity may be explained first by the short duration of the experiment (four weeks) and the use of already pre-decomposed litter. Indeed, litter decomposition is typically a slow process occurring over months and even years depending on litter type and environmental conditions with decomposition rates decreasing over time (Berg and McLaugherty, 2008). Another potential explanation could be that we used litter produced from trees that had been exposed to a control or a drought treatment in the Ecotron mesocosm experiment for five consecutive years. This likely selected microbial communities that are already adapted to drier conditions, illustrating the legacy effect in microbial communities previously exposed to drought (Yuste et al., 2011; Hawkes and Keitt, 2015; Kaisermann et al., 2017).

The decrease in macrodetritivore feeding activity under drought treatment is opposed to several recent studies showing resistance of fauna activity to drought (Coulis et al., 2013; Thakur et al., 2018; Joly et al., 2019), but does align with others studies (Bertrand et al., 1986; David and Gillon, 2002; Collison et al., 2013). These conflicting results suggest that we do not yet have a complete understanding of how macrofauna activity will respond to drought, perhaps because of strong context dependency, different drought treatment, and calls for more studies on litter feeding fauna responses to drought, in particular under natural conditions that would allow excluding potential artifacts occurring in controlled environment studies. Although the Ecotron mesocosm experiment was a relatively realistic simulation of a natural ecosystem, the small microcosms we used to enclose the fauna did not provide the diversity and heterogeneity of microhabitats of a natural forest, which may have altered their behavior and limited sheltering possibilities (Dias et al., 2013b; Joly et al., 2019), leading to less time spent feeding and lower litter mass loss. Identifying macrodetritivore traits that drive specific differences in effect and in response to climate changes would greatly improve our understanding of how detritivores contribute to decomposition (Dias et al., 2013b; De Smedt et al., 2018).

Our results showed an almost identical relationship between decomposition and macrodetritivore functional diversity under control and drought conditions. This finding parallels our result that drought did not affect the relationship between litter mass loss and CWM<sub>Cm</sub>, which is in line with the sole other existing study on the relationship between CWM and litter mass loss under varying moisture level (da Silva et al., 2020). Our results are in accordance with the study by Coulis et al. (2015), who also reported unchanged relationships between soil processes, including decomposition, and functional diversity of detritivore assemblages. More generally, this does not seem to support the stress gradient hypothesis (Bertness and Callaway, 1994) stating that biodiversity is having a stronger effect on ecosystem processes with increasing environmental constraints. The positive effect of functional diversity, persisting in drier conditions, suggests a robust relationship between litter mass loss and macrodetritivore functional diversity, independent of contrasting environmental conditions. More diverse communities may therefore counteract negative drought effects and maintain ecosystem processes under stressing conditions (Luan et al., 2024). These effects, however, may not hold for other ecosystem types, environmental conditions, and detritivore species. For a temperate ecosystem for example, Joly et al. (2021) showed that synergistic interactions between detritivore species may switch to antagonistic with a reduction in the amount and frequency of water addition. It is possible that changes in precipitation may have ecosystem- and/or species-dependent effects on how detritivore species interact during litter decomposition requires much larger comparative studies across ecosystems with variable species composition. The meta-analysis by (Sagi and Hawlena, 2024), for instance, showed that macrofaunal effects on decomposition should increase with temperature and aridity, because larger animals are more tolerant to aridity than smaller organisms. This
may point towards Mediterranean ecosystems and other ecosystem types with naturally occurring drought periods having a particularly strong contribution of macrodetritivore communities to decomposition that are less susceptible to drought.

## 5. Conclusion

Our experiment showed that multiple characteristics of detritivore assemblages affect leaf litter decomposition in a Mediterranean forest understory ecosystem. Within the limits of the environmental context and macrodetritivore species considered in our study, species identity appeared to be the strongest determinant of litter mass loss, followed by functional diversity. The community weighted mean consumption rate contributed relatively little to the measured litter mass loss and accounted for about the same amount of variance as the drought treatment. This result stresses the importance of jointly evaluating different characteristics of litter feeding fauna communities for a better understanding of their effects on ecosystem processes such as decomposition and their responses to climate change. The fact that species-identity was particularly important questions on the ability of traits to explain community effect on ecosystem functioning as initially thought (Lavorel and Garnier, 2002). This is especially true for soil fauna since the major challenge for such an approach across is the identification of traits common to the extremely diverse and phylogenetically distant soil taxa spanning orders of magnitude in size, and the constitution of an comprehensive trait database (Brousseau et al., 2018; Wong et al., 2019; Hedde et al., 2022; Angst et al., 2024).

### CRediT authorship contribution statement

**Mika Lemoine:** Writing – original draft, Methodology, Investigation, Formal analysis, Data curation, Conceptualization. **Stephan Hättenschwiler:** Writing – review & editing, Funding acquisition, Conceptualization. **Alexandru Milcu:** Writing – review & editing, Methodology. **Sandra Barantal:** Writing – review & editing. **Johanne Nahmani:** Writing – review & editing, Methodology, Conceptualization. **Pierre Ganault:** Writing – review & editing, Validation, Software, Formal analysis.

### Declaration of competing interest

The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Stephan Hattenschwiler reports financial support was provided by French National Research Institute for Sustainable Development Occitanie Regional Delegation. If there are other authors, they declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

### Acknowledgements

This research was part of the BELOW project with the support of the Occitanie Regional Council's program "Key challenge BiodivOc". The setup and maintenance of the mesocosm experiment between 2018 and 2021 was funded through the 2015–2016 BiodivERsA COFUND call with the national funders Agence Nationale de la Recherche (ANR, France), Belgian Science Policy Office (BELSPO, Belgium), Deutsche Forschungsgemeinschaft (DFG, Germany), Research Foundation Flanders (FWO, Belgium), and The Swedish Research Council (FORMAS, Sweden) for the SoilForEUROPE project. Chemical analyses were performed at the Plateforme d'Analyses Chimiques en Ecologie, technical facilities of the LabEx Centre Mediterranéen de l'Environnement et de la Biodiversité. Special thanks to the coordinator of the Mesocosms platform M. Gritti for experimental support, to P. Schevin for setting up the microcosm, and to R. Leclerc for the chemical analyses. This study

benefited from the Centre Nationnal de la Recherche Scientifique human and technical resources allocated to the Ecotrons research infrastructures ('Investissement d'Avenir' ANR-11-INBS-0001) and AnaEE France.

## Appendix A. Supplementary data

Supplementary data to this article can be found online at https://doi.org/10.1016/j.soilbio.2025.109918.

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