DOI: 10.5061/dryad.ngf1vhj2j
title: Mechanical traits as drivers of trophic interaction between macrodetritivores and leaf litter

description: In ecosystems, the rates of resource consumption by animals drive the
 flows of matter and energy. Consumption rates are known to vary according
 to consumer energy requirements, resource nutrient content and mechanical
 properties. The aim of our study is to determine how mechanical
 constraints, compared to energetic and nutritional constraints, explain
 the variation in leaf litter consumption rates by macrodetritivores. In
 particular, we focus on the impact of litter toughness. To this end, we
 propose a non-linear model describing leaf litter consumption rates of
 detritivore as a function of litter toughness. We also investigate a
 possible match between bite force and litter toughness, since
 consumer-resource co-occurrence is thought to be driven by the match
 between invertebrate mandibular traits and resource toughness. Our study
 was designed as follows: leaf litter from oak and hornbeam was exposed to
 field physical and microbial decomposition in aquatic and terrestrial
 ecosystems for selected time periods before it was offered to eight
 macrodetritivore taxa (three forest stream taxa and five forest soil taxa)
 in no-choice laboratory feeding experiments. Our findings show that,
 compared to energetic and nutritional constraints, mechanical traits have
 a greater impact on litter consumption rate by detritivores. After
 subtracting the contribution of the detritivore body mass, we report that
 litter consumption rates depend primarily on litter toughness. A sigmoid
 function is best suited to characterize the relationship between
 mass-independent consumption rate and litter toughness. We note that the
 parameters of our sigmoid model are taxon-specific, suggesting
 biomechanical thresholds and biological differences among taxa.
 Interestingly, we found no correlation with detritivore bite force,
 suggesting that food processing by detritivores does not only depend on
 mandibles strength.
description: <strong>Sites and experimental
 design</strong> Our sampling sites were located
 south of Toulouse (France), in the Pyrenean Piedmont, an area
 characterized by a calcareous bedrock and continental climate. Leaf litter
 and detritivores were collected in oak-hornbeam forests. For each
 ecosystem type (low order streams and forest floors), we selected two
 sites, one dominated by oak (Quercus petraea) and the other by hornbeam
 (Carpinus betulus). Hornbeam sites were both located in a hornbeam coppice
 stand. The terrestrial oak site was a mature even-aged oak forest. As
 there was no stream crossing it, we selected a nearby stream running along
 a forest edge dominated by oak trees. Leaf litter was collected at the
 time of abscission and stored air-dried at room temperature. Leaf litter
 was also collected later, after various physical and microbial
 decomposition times, to produce litter with varying toughness values. In
 our feeding experiments, detritivore individuals collected at a site were
 only offered the locally dominant leaf litter (either oak or hornbeam).
 Furthermore, we collected detritivores in the field at the same time as
 partially decomposed leaf litter. <strong>Leaf
 litter</strong> Freshly fallen leaves of oak were
 expected to be tougher than those of hornbeam and the toughness of both
 leaf species was expected to gradually decrease with length of exposure in
 the field. Five grams of air-dried leaves were enclosed in 0.5-mm nylon
 mesh bags (20 × 15 cm), thus preventing macrodetritivores to enter the
 bags. In autumn 2020, oak and hornbeam litter bags were incubated at oak
 and hornbeam sites, respectively. They were arranged in three blocks in
 each site. In streams, bags were secured to iron sticks anchored in the
 sediment, whereas on forest floor, bags were laid flat onto the topsoil
 and secured with bamboo sticks driven into the soil. Assuming that leaf
 litter disappears faster in streams than in soils, we collected litterbags
 after 54 and 99 days of exposure in the streams, and after 82, 111, 152
 (hornbeam only), 195 (oak only), 236 and 349 (oak only) days of exposure
 in terrestrial sites. At least one litterbag was retrieved from each block
 at each time point to cover a wide range of litter toughness. However, as
 hornbeam leaves decomposed more rapidly than oak leaves, only oak leaves
 bags were recovered from the terrestrial sites on the last sampling
 date. Partially decomposed leaves recovered from litter bags were rinsed
 with tap water to remove exogeneous particles. Discs were punched through
 the leaves with a 10-mm cork borer, avoiding central veins, to provide
 homogeneous food for detritivores. At each site, about 200 discs per
 sampling date were needed to assess litter traits (described thereafter)
 and perform feeding experiments. We also made around 300 discs from
 undecomposed oak and hornbeam leaves (t = 0). As the leaves were stored
 air-dried, they were soaked in water for an hour before being cut into
 discs. Leaf discs were freeze-dried, partitioned into sets of five (except
 for the first oak test for which we used sets of four discs, as
 consumption was very low), and weighed to the nearest 0.1 mg.
 We assessed litter toughness by measuring the force required for
 a 2.2-mm diameter steel punch to penetrate a leaf disc laid flat on a
 perpendicular plane. We used a custom-made penetrometer, such as described
 in Graça et al. (2005), fitted to a digital force tester (CS225 Series,
 Chatillon®). This device provides high accuracy (± 0.1 N) when measuring
 penetration force. Litter toughness is equal to the penetration pressure
 (kPa), i.e., the penetration force divided by the cross-sectional area of
 the punch. Litter softness (kPa−1) is the inverse of penetration pressure.
 We calculated the average of 15 replicate measurements for each batch of
 leaf litter (from a same site at a given sampling date).<br>Litter N
 content was considered an indicator of its nutritional value, assuming
 that (1) detritivores would experience strong N limitation and (2)
 contents of other limiting nutrients (e.g. Phosphorus) would positively
 covary with N content. The latter assumption was found to hold true when
 comparing undecomposed oak and hornbeam leaves based on analyses of litter
 N, P, K, Ca and Mg. Three replicates of 3–6 mg per leaf
 discs batch were grinded into powder by the means of a bead mill
 (FastPrep-24™ Classic Instrument). N content was quantified using a Total
 nitrogen and Organic Carbon analyzer (TOC L, Shimadzu) and values were
 reported on a dry weigh basis (mg N mg−1 litter), then expressed as a
 percentage.
 <strong>Macrodetritivores</strong>
 We collected macrodetritivores from natural accumulations of
 organic debris in the sites where litterbags were placed. We attempted to
 capture enough individuals of each taxon to ensure adequate levels of
 replication of experimental conditions (i.e., pairs of detritivore taxon
 and litter batch, at each time point). However, we did not achieve a
 fully-balanced design because the structure of detritivore assemblages
 differed between oak and hornbeam sites and population abundances
 exhibited large seasonal variations. We first conducted a feeding
 experiment with pre-weighed discs of undecomposed leaf litter (i.e.,
 air-dried leaves used to fill litter bags) and detritivores collected in
 the field at litterbag installation. Feeding experiments were repeated
 whenever litterbags and detritivores were retrieved from the
 sites. Individuals were assigned to coarse taxonomic
 categories in the field and were then identified at the lowest practicable
 taxonomic level, mostly genus, based on observations made on dead
 individuals under a dissecting microscope. We found case-bearing
 Trichoptera larvae and amphipods (Crustacea) at both stream sites, and
 Plecoptera larvae only at the hornbeam site. Amphipods and Plecoptera
 individuals were both assigned to a single genus, Gammarus and Capnia,
 respectively, and Trichoptera individuals to the Limnephilinae genus
 complex. Detritivores from the terrestrial sites were either Isopods
 (Philoscia and Porcellio) or Diplopods (Leptoiulus, Cylindroiulus, and
 Glomeris). Other genera (e.g., Polydesmus) were too scarce. Detritivores
 sorted by taxon and site of collection were held in plastic containers
 stored in the dark at 10 °C for 1–4 days. Containers with aquatic taxa
 were filled with permanently-aerated stream water.
 Pre-weighed leaf discs were rewetted for one hour before they
 were offered to one individual of either detritivore taxon, except Capnia.
 For this small-sized detritivore, we pooled together up to five
 individuals of the same size to ensure accurate quantification of per
 capita consumption of leaf litter. We used 10-cm large plastic containers
 filled with 50 g of clean sand as experimental units. The sand was sprayed
 with 5 mL of water to maintain wet conditions required for terrestrial
 detritivores, whereas 200 mL of stream water was added to containers with
 aquatic taxa. Detritivores were starved for at least 18 h prior to the
 introduction of pre-weighed leaf discs. Experimental feeding time was 4
 days and 2 days for terrestrial and aquatic detritivores, respectively, to
 account for their respective feeding rates. Feeding experiments were
 stopped earlier if disc area had been downsized by 75% (visual
 estimation). For each leaf litter species and ecosystem type, we set up
 five control experimental units with no detritivore, in order to correct
 for leaf mass loss caused by physical and microbial decomposition. Leaf
 mass loss in control containers did not exceed 10% of initial
 mass. At the end of feeding trials, remaining leaf
 discs and fragments larger than 1 mm were freeze-dried and weighed to the
 nearest 0.1 mg. Detritivores were blotted dry with paper towels and
 weighed to the nearest 0.1 mg. For each taxon, wet body mass was converted
 into dry mass using a linear relationship established with freeze-dried
 individuals (n ≥ 12, p &lt; 0.001, R2 ≥ 0.68). Five detritivore
 individuals per site, chosen to be representative of the diversity of
 conditions (temporal and geographical), were preserved in 70% ethanol,
 dissected and photographed under a stereomicroscope equipped with a
 digital camera (Olympus SZX10) to measure (0.01 mm accuracy) head width
 (H<sub>W</sub>), mandible length (M<sub>L</sub>),
 and width (M<sub>W</sub>). Mandible length was measured
 between the incisor tip and the axis of rotation, identified by the
 condyles. Mandible width was measured between the adductor muscle
 insertion and the axis of rotation. Head width was measured just behind
 the eyes. Each taxon’s bite force (F index) was calculated as follows
 (Wheater and Evans 1989): F = H<sub>W</sub>
 M<sub>w</sub> / M<sub>L</sub>
 Note that, as head width H<sub>W</sub> is a proxy for
 the force of the muscles involved in mandible motion, the unit of the F
 index is not relevant. A 2-D plot (F index versus dry body mass) shows
 morphological differences between dissected individuals.
 <strong>Data analyses</strong>
 Because it is more convenient to describe resource-mediated
 constraints on consumer performance by increasing functions (e.g., Monod
 equation, predator–prey function responses), we used the inverse of
 toughness (i.e., softness) as a predictor of litter consumption rates in
 statistical models. The conversion from toughness to softness made the
 data distribution less skewed, which helped to estimate parameters from
 linear and non-linear models. Our dataset consisted of
 291 values of litter consumption rate estimated on 63 pairs of detritivore
 taxon-litter batch. We calculated the mass-specific rate of litter
 consumption (?? in mg mg−1 day−1) as follows: ?? = (??−(???+?)) / ?Δ?,
 where ?? and ?? are the final and initial mass (mg) of the leaf discs,
 respectively; Δ? is the test duration; and B is the final dry body mass of
 the detritivore (mg). The equation term aMi + b was used to correct the
 initial litter mass for physical and microbial decomposition assessed in
 control containers, without detritivore. The
 coefficients <em>a</em> and <em>b</em> are the
 slope and intercept of the linear regression of final vs initial litter
 mass in controls (n = 5) for each condition (oak and hornbeam leaf discs
 incubated in terrestrial and aquatic habitats for each time point). As the
 lowest limit of quantification for litter consumption rate was estimated
 at 0.001 mg mg−1 day−1 (ca. 0.1 mg of litter weighing accuracy, divided by
 100 mg (order of magnitude) of detritivore body mass for larger
 individuals), lower values have been rounded down to zero (n = 46). No
 litter consumption rate was calculated whenever a detritivore died during
 a feeding experiment (5% occurrence; n = 20). We have developed a linear
 model to assess the relative impacts of litter softness, litter N content
 and detritivore body mass on mass-specific litter consumption rate (??
 &gt; 0.001 mg mg−1 day−1, n = 245). Litter N content reflects
 nutritional constraints, whereas detritivore body mass reflects
 energy-mass allometry. The dependent variable and all the predictors were
 log-transformed to linearize the expected power-law relationships and to
 achieve assumptions of homoscedasticity and normal distribution of
 residuals. Further analyses were carried out on
 mass-independent rate of litter consumption (?? in mg mg−1−c day−1),
 calculated as follows: ?? = ??∙?<sup>−?</sup> , where ?? is
 the mass-specific rate of litter consumption, B is the final dry body mass
 of detritivores (mg), and ? is the estimate for the effect of body mass in
 the linear model. We fitted a random-effects model to assess the
 contribution of experimental factors (ecosystem type, detritivore taxon,
 leaf species, and sampling date) to ?? variability. A square root
 transformation was applied to the data to ensure residual normality and
 homoscedasticity. We assumed that litter consumption rate (??) evolves as
 a function of litter softness (S) according to a S-shaped curve, with the
 transition phase informing on mechanical traits of both the consumer and
 the resource. A three-parameter sigmoid function was fitted to data for
 each detritivore taxon, using a non-linear regression: ?? = ? /
 (1+???((?−?) / ?))?, ? and ? are the maximum consumption rate, the
 softness value at the inflexion point, and a scale parameter,
 respectively. To assess the relevance of sigmoid models, we compared the
 fit of the sigmoid model with that of a linear model using root mean
 square error (RMSE) and Akaike Information Criterion (AIC). Parameter
 estimates were extracted of each model in order to assess cross-taxon
 variability. Graça MA, Bärlocher F, Gessner MO (2005)
 Methods to study litter decomposition: a practical guide. Springer Science
 & Business Media, Berlin Wheater CP, Evans MEG
 (1989) The mandibular forces and pressures of some predacious Coleoptera.J
 Insect Physiol
 35:815–820.https://doi.org/10.1016/0022-1910(89)90096-6
description: # Mechanical traits as drivers of trophic interaction between
 macrodetritivores and leaf litter
 [https://doi.org/10.5061/dryad.ngf1vhj2j](https://doi.org/10.5061/dryad.ngf1vhj2j) This table is the result of consumption tests aiming to quantify the consuption rate of leaf litter by different detritivore taxa. Leaf litter (Hornbeam or Oak) was left to decompose in a forest stream or on a forested soil and collected (after 0 to 349 days). Detritivores comprise 3 aquatic and 5 terrestrial taxa. Consumption rates were measured at the laboratory in microcosms. We performed controls with leaf litter discs without detritivores to correct consumption rates. We measured the mean litter toughness, softness, CN ratio, and N percent content for each litter batch. We measured a mean index for detritivore mandible strength for each detritivore taxon. We measured or estimated (from individual fresh body mass) each individual dry body mass. ## Description of each data column * Ecosystem: A=aquatical, T=Terrestrial * LeafLitter: C=Hornbeam (Caprinus), Q=Oak (Quercus) * Temporality: t0, t1, t2, t3, t4 (categorical) * Days: Number of days since t0 (days, numerical) * DetritivoreSp: Detritivore taxon (Capnia, Limnephilinae, Gammarus, Philoscia, Porcellio, Leptoiulus, Glomeris, Cylindroiulus) * Litter.CN: Litter Carbon / Nitrogen ratio (Mean value per litter batch) * Litter.pN: Litter nitrogen percent content (Mass %, Mean value per litter batch) * Litter.Toughness: Resistance to penetration, in kilopascals (kPa), computed as follows: Litter.Toughness = (S + m) / pi * r²*1000, with S = Strength in Newton (N) needed to punch a steel rod through the limb, m = rod weight (0.14N), r = radius of the rod (0.0011 m). (Mean value per litter batch) * Litter.Softness: Inverse of litter toughness (/kPa), computed as follows: Litter.Softness = 1 / Litter.Toughness (Mean value per litetr batch) * Detritivore.F: F index of detritivore mandible strength, computed as follows: Detritivore.F = HW * MW / ML, with HW = Detritivore Head width (behind the eyes), MW = mandible width, and ML = mandible length (MW / ML = lever arm of the mandible, see supplementary material). (Mean value for each detritivore taxon) * ConsumptionRate: Individual consumption rate of litter dry mass (mg/day), after correction for the mass lost (in percent of initial dry litter mass) by control leaf litter discs without detritivores. * Detrit.DryBodyMass: Individual dry body mass (mg), either measured or estimated with a taxon-specific linear regression