DOI: 10.5061/dryad.6m905qg6z
title: Litter consumption by macrodetritivores depends more on mechanical than on nutritional constraints

description: Ecosystem functions greatly depend on trophic interactions between
 consumers and their resources. Resource consumption depends on ingestion,
 digestion, and allocation processes. Mechanical constraints are expected
 to influence ingestion, while metabolic and nutritional constraints are
 expected to influence allocation. Leaf litter are resources presenting a
 high mechanical and nutritional heterogeneity that depends on plant
 identity and on physical and microbial processing over the course of
 decomposition. Litter consumption by detritivores is known to depend on
 metabolic and nutritional constraints but the importance of mechanical
 constraints is yet unknown. After accounting for metabolic constraints on
 consumption rate, we tested the relative importance of mechanical and
 nutritional constraints in explaining litter consumption rates by
 detritivores. For this, we exposed 16 leaf treatments (8 leaf species
 either just leached or leached and microbially conditioned) to 4 aquatic
 and 5 terrestrial detritivore taxa in laboratory no-choice consumption
 experiments. We investigated two mechanical constraints: grabbing and
 fragmenting the resource, by measuring suitable couples of mechanical
 traits for both litter and detritivores. We also investigated four
 nutritional constraints related to N, P, K, and Ca contents in both
 detritivores and litter. For each constraint, we also tested if trait
 matching significantly contribute to explain consumption. Our analysis
 revealed that both mechanical and nutritional constraints are influencing
 mass-independent consumption rate but that mechanical constraints
 predominate over nutritional constraints. Litter fragmentation, studied
 through litter toughness and detritivore biting force, was especially
 important to explain consumption rate. Nutritional constraints were
 dominated by P constraints. Trait-matching had very weak importance and
 was significant only for P constraints. Our findings highlight the
 importance of mechanical constraints for litter consumption by
 detritivores.
description: This dataset is based on consumption tests: In small plastic
 boxes we quantified the consumption rate of a given litter by a given
 detritivore individual. Detritivore species and leaf litter batches were
 characterized by the measurements of traits.
 <strong>Experimental design</strong>
 We selected 8 leaf litter species with a large range of
 mechanical and nutritional trait values to cover a wide spectrum of
 potentially different constraints. Each leaf litter species was then
 either just leached, or leached and microbially conditioned before being
 offered to detritivores. This ensured enlarging the range of initial
 mechanical and nutritional litter traits values while limiting the content
 of chemical deterrents. We performed leaf litter consumption tests by
 offering one of 16 different leaf litter types to one of 9
 macrodetritivore taxa (four aquatic and five terrestrial species).
 Detritivore taxa were chosen to be representative of coarse lifeforms
 commonly encountered in aquatic and terrestrial ecosystems. As we aimed to
 assess fundamental rules of pairwise interactions between detritivores and
 litter, we tested the consumption of a single litter by a single
 detritivore individual at the time. We also offered litter species that
 detritivores could not encounter in their natural habitat. We thus sampled
 litter in a geographical site (Canal du Midi: Toulouse, 31000 and
 Ramonville-Sainte-Agne, 31520, France) distant from the geographical sites
 where we sampled detritivores (Montagne Noire, France). We performed a
 total of 576 consumption tests, corresponding to 144 detritivore-litter
 pairs: 9 detritivores taxa * 16 litter treatments (8 leaf litter species *
 2 litter conditioning treatments (leaching or leaching plus microbial
 conditioning)) * 4 replicates. Replicates spread out from week to week and
 were associated with corresponding control tests without detritivores
 (Supplementary Material, Table S3). We performed a total of 128 control
 tests without detritivores: 16 litter treatments (8 litter species * 2
 litter conditioning treatments (leaching or microbial conditioning)) * 2
 ecosystem types (aquatic or terrestrial) * 4 replicates.
 We performed consumption tests in microcosms made of clean
 plastic containers with 50 g of clean sand in the dark at 10°C. For
 aquatic detritivores, we added 200 mL of water collected from their
 stream. For terrestrial ones, we sprayed 5 mL of tap water on the sand. We
 starved detritivores for 3 days prior to the consumption test. In each
 microcosm we placed one individual (assigned randomly) and five discs of
 one litter treatment (litter species * conditioning treatment) that were
 previously freeze-dried, weighted, and rehydrated in tap water for 1 h.
 Tests were stopped when consumption visually reached 75% of initial discs
 surface or after three days. At the end of the consumption tests,
 remaining discs fragments larger than 1 mm were collected, freeze-dried
 and weighted. When one individual died during the first 24 h of the test,
 we immediately replaced it with a new individual. When the individual died
 later during the test, we repeated the consumption test the week after,
 with corresponding control treatments. At the end of consumption tests,
 detritivores were starved for 24 h, and were weighted (aquatic animals
 were gently blotted with paper towel). We converted fresh body mass into
 dry mass using a linear relationship established for each taxon (See
 Macrodetritivores’ section). For each detritivore taxon, we conserved half
 of the individuals in 70% ethanol for dissection and we froze the other
 half for chemical analyses. All weight measurements were determined at the
 nearest 0.1 mg. <strong>Leaf
 Litter</strong> We collected dead leaves at
 abscission from October to November along the Canal du Midi (Toulouse,
 31000, and Ramonville-Sainte-Agne, 31520, France) from a limited number of
 individuals (£ 5) for each species. We used 8 tree species belonging to 8
 different families, namely <em>Ailanthus altissima</em>
 (Simaroubacea), <em>Robinia pseudoacacia</em> (Fabacea),
 <em>Juglans regia</em> (Juglandacea), <em>Carpinus
 betulus </em>(Betulacea), <em>Acer platanoides</em>
 (Aceracea), <em>Prunus avium</em> (Rosacea), <em>Quercus
 petrea </em>(Fagacea), <em>Platanus ×hispanica</em>
 (Platanacea). Leaves were air-dried and stored in the dark until being
 leached with tap water during 24 h. We then cut 1-cm diameter leaf discs
 with a cork-borer, avoiding main veins. Some leached discs were
 microbially conditioned. They were incubated in a mix of decomposing dead
 leaves until they were visually microbially colonized (soft and
 discolored). For aquatic conditioning specifically, we collected 50 L of
 stream water and dead leaves from the same stream where we collected
 aquatic individuals. We added Fertiligène Naturen® fertilizer (NPK: 3 – 2
 – 5) at 0.5 mL.L<sup>-1</sup>. We left it three days in a
 greenhouse with constant oxygenation for microorganisms’ development. We
 then filled tanks with filtered (63 mm) water and placed one fine-mesh bag
 of monospecific litter discs of each litter species per tank with constant
 oxygenation. For terrestrial conditioning, we collected dead leaves from
 Montagne Noire forest soil (beech, chestnut, hazelnut) and grinded it with
 a garden shredder. We left it three days in a green house after
 humidification with the same fertilizer at 0.5
 mL.L<sup>-1</sup>. We then placed monospecific leaf discs in a
 fine-mesh bag of each litter species between two layers of fragmented
 litter in each tank. Tanks were regularly humidified with the same
 fertilizer. To test for mechanical constraints of
 grabbing and fragmenting litter, we measured thickness and toughness on 8
 discs from controls, respectively. We measured limb thickness to the
 nearest 0.001 mm with a Helios-Preisser® digital micrometer, avoiding main
 veins. We measured litter toughness as the penetration pressure (kPa)
 needed for a 2.2-mm diameter steel rod to penetrate through a leaf disc.
 We used a custom-made penetrometer, such as described in Graça et al.
 (2005), fitted to a digital force tester (CS225 Series, Chatillon®)
 measuring force to the nearest 0.1 N. To test for nutritional constraints,
 we quantified nitrogen (N) content using a Total nitrogen and Organic
 Carbon analyzer (TOC L, Shimadzu) on three replicates of 20 mg. We
 quantified phosphorus (P), potassium (K) and calcium (Ca) content with an
 Induced Coupled Plasma – Mass Spectrometer (ICP-MS) on three replicates of
 5 mg.
 <strong>Macrodetritivores</strong>
 We collected macrodetritivores in la Montagne Noire, a
 metamorphic forested massif east of Toulouse (France) (Supplementary
 Material, Table S1 and Table S2). We hand-captured aquatic detritivores
 from February to March in a stream mainly boarded by beech
 (<em>Fagus sp.)</em>, hazelnut (<em>Corylus
 sp.</em>), and chestnut (<em>Castanea sp.</em>) trees.
 We hand-captured terrestrial detritivores in April in a site dominated by
 ash (<em>Fraxinus sp.</em>). We identified all taxa to the
 lowest taxonomic level, mostly species. Only Plecoptera and Tipula larvae
 were identified to the genus, <em>Nemoura</em> and
 <em>Tipula</em>, respectively (Tachet et al., 2010).
 Case-bearing Trichoptera larvae were identified as <em>Potamophylax
 cingulatus</em> (Stephens, 1837) (Waringer and Graf,
 2011)<em>,</em> and Amphipods as <em>Gammarus
 fossarum</em> (Koch, 1835) (Tachet et al., 2010). Terrestrial
 detritivores were either Isopods (<em>Philoscia affinis</em>
 (Verhoeff, 1908)<em> </em>and <em>Porcellio monticola
 </em>(Lereboullet, 1853)) (Oliver and Meechan, 1993; Vandel, 1962)
 or Diplopods (<em>Cylindroiulus londinensis</em> (Leach,
 1815), <em>Polydesmus inconstans</em> (Latzel, 1884), and
 <em>Glomeris marginata</em> (Villiers, 1789)) (Blower, 1985;
 David, 1995). At the laboratory we sorted them by taxon, and left them 1 –
 3 days in the dark at 10°C for acclimatation before being
 starved. We performed dissections on at least five
 individuals per taxon, from several random experimental conditions, under
 an Olympus SZX10 stereomicroscope equipped with a digital camera. To
 assess the ability of detritivores to overcome litter thickness
 constraints, we measured mandible gape (distance between the axis of
 rotation of both<em> </em>mandibles; Supplementary Material,
 Figure S1) to the nearest 0.01 mm. To asses biting force as the ability to
 overcome litter toughness, we measured head width
 (H<sub>W</sub>, behind the eyes), mandible length
 (M<sub>L</sub>, distance between the incisive tip and the axis
 of rotation which was identified with condyles), and mandible width
 (M<sub>W</sub>, distance between the adductor muscle insertion
 and the axis of rotation) to the nearest 0.01 mm (Clissold, 2007;
 Supplementary Material, Figure S1). We used these metrics to calculate an
 index of biting force at the taxa level (<em>F</em>,
 (Brousseau et al., 2018; Wheater and Evans, 1989)): F =
 H<sub>W</sub> * M<sub>W</sub> /
 M<sub>L</sub> To assess metabolic
 constraints, we converted detritivore fresh body mass into dry mass using
 a linear relationship established for each taxon with a subset of
 individuals both fresh- and freeze-dry-weighted (n ≥ 30, p &lt; 0.001,
 R<sup>2</sup> ≥ 0.57). Other individuals were only
 fresh-weighted before being conserved in 70%-ethanol for dissection. Their
 dry mass was then inferred with the estimated linear relationship. To
 assess nutritional constraints, four pools of at least four individuals
 for each detritivore taxon (resulting in a total of four analytical
 replicates per taxon) were grinded into powder and analyzed following the
 same procedure as for litter samples. For <em>Nemoura</em>
 alone, only two pools were analyzed due to the very small body weight of
 this taxon.
description: # Litter consumption by macrodetritivores depends more on mechanical than
 on nutritional constraints 
 This dataset is based on consumption tests: in small plastic boxes, we
 quantified the consumption rate of a given litter by a given detritivore
 individual. Detritivore species (n = 9) and leaf litter batches (from 8
 species, with 2 different treatments: either just leached, or microbially
 conditionned, resulting in n = 16 batches) were characterized by the
 measurements of traits. ## Description of the data and file structure Each
 line corresponds to a unique detritivore individual - litter batch couple.
 Here is a description of all columns, with units if relevant: *
 DetritivoreSp = Detritivore taxon * Id = Unique Id for each line. First
 letter = Litter species (A=Ailanthus; C=Carpinus; N=Juglans; M=Prunus;
 R=Robinia; E=Maple; Q=Quercus; P=Platanus). Second letter = Conditioning
 treatment (A=Aquatic, T=Terrestrial, nothing=no conditioning) * LitterId =
 Litter Id First letter = Species (A=Ailanthus; C=Carpinus; N=Juglans;
 M=Prunus; R=Robinia; E=Maple; Q=Quercus; P=Platanus). Second letter:
 "a" = leached litter for an aquatic detrtitivore, "t"
 = leached litter, for a terrestrial detritivore ; "A" =
 microbial conditioning in aquatic conditions ; "T" = microbial
 conditioning in terrestrial conditions *  LeafSpecies: A=Ailanthus;
 C=Carpinus; N=Juglans; M=Prunus; R=Robinia; E=Maple; Q=Quercus; P=Platanus
 * Conditioning = Litter conditionning treartment. A=Aquatic ;
 T=Terrestrial ; leaching=Leaching only, no condityioning * Litter = Litter
 (Leaf + treatment) identity. First letter = leaf species (A=Ailanthus;
 C=Carpinus; N=Juglans; M=Prunus; R=Robinia; E=Maple; Q=Quercus;
 P=Platanus). Second letter = conditioning treatment (A=Aquatic ;
 T=Terrestrial; nothing = leached only, no conditioning) * Replicate =
 Replicate number for each detritivore species - litter batch couple * Mi =
 Initial dry mass of leaf litter discs (gramms) * Mf = Final dry mass of
 leaf litter discs (gramms) * FreshBodyMass = Detritivore individual fresh
 body mass (milligrams) * DryBodyMass = Detritivore individual dry body
 mass. Either measured or estimated with a species-specific linear
 regressions (milligrams)  * Duration = Individual consumption test
 duration (days) * Mloss = Leaf litter discs dry mass loss, after
 correction by controls without detritivores. Equation: Mloss = Mf - M1*(1
 - %dry mass loss in controls) (milligrams) * ConsumptionRate = Individual
 consumption rate (ConsumptionRate = Mloss / Duration) (milligrams / day) *
 ConsumptionRateByBodyMass = Individual consumption rate divided by each
 individual detritivore dry body mass (ConsumptionRateByBodyMass =
 ConsumptionRate / DryBodyMass) (milligrams / milligram / day) *
 LitterToughness = Strength needed to puch through a leaf litter disc with
 a steel rod. Mean value for each litter batch (see LitterId column)
 (Newtons) * LitterToughnesskPa = Pressure needed to punch through a leaf
 litter disc with a steel rod (diameter = 1.1mm and mass = 0.14N).
 LitterToughnesskPa = (LitterToughness + 0.14)/((pi*(0.0011)^2)*1000). Mean
 value for each litter batch (see LitterId column). (kiloPascals) *
 LitterSoftness = Inverse of LitterToughnesskPa. Mean value for each litter
 batch (see LitterId column). (/ kiloPascal) * LitterThickness = Leaf
 litter discs thickness, avoiding main leaves. Mean value for each litter
 batch (see LitterId column). (millimiters) * DetritSpHeadWidth =
 Detritivore Head width (behind the eyes). Mean value for each detritivore
 species. (millimeters) * DetritSpMdGape = Detritivore Mandible Gape (see
 supplementary material). Mean value for each detritivore species.
 (millimeters) * DetritSpLarg.LongMd = Ratio between mandible width and
 mandible length (lever arm of the mandible, see supplementary material).
 Mean value for each detritivore species. (no unit) * DetritSpC =
 Detritivore body Carbon concentration. Mean value for each detritivore
 species. (parts per billion) * DetritSpN  = Detritivore body Nitrogen
 concentration. Mean value for each detritivore species. (parts per
 billion)          * DetritSpP = Detritivore body Phosphorus concentration.
 Mean value for each detritivore species. (parts per billion) * DetritSpK =
 Detritivore body Potassium concentration. Mean value for each detritivore
 species. (parts per billion) * DetritSpCa = Detritivore body Calcium
 concentration. Mean value for each detritivore species. (parts per
 billion) *  LitterC = Litter Carbon concentration. Mean value for each
 litter batch (see "Litter" column). (parts per billion) *
 LitterN = Litter Nitrogen concentration. Mean value for each litter batch
 (see "Litter" column). (parts per billion) * LitterP = Litter
 Phosphorus concentration. Mean value for each litter batch (see
 "Litter" column). (parts per billion) * LitterK = Litter
 Potassium concentration. Mean value for each litter batch (see
 "Litter" column). (parts per billion) * LitterCa = Litter
 Calcium concentration. Mean value for each litter batch (see
 "Litter" column). (parts per billion)