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Keywords:

  • Decomposition;
  • functional diversity;
  • species interaction;
  • species richness;
  • synergistic effects

Summary

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References
  • 1
    In laboratory microcosms, we investigated the influence of diversity of both leaf litter and detritivores on decomposition processes. Either woodlice or earthworms, or a combination of woodlice and earthworms, fed on leaf litter of either oak or alder, or oak and alder for 8 weeks. Mass loss of leaf litter, soil microbial respiration and soil nutrient concentrations were determined every 2 weeks.
  • 2
    For four out of seven decomposition parameters, the joint effects of woodlice and earthworms were stronger than the sum of single-species effects when they had fed on alder litter. When feeding on oak litter, however, woodlice and earthworms together revealed lower decomposition rates than predicted from their single effects. Joint effects of detritivores on decomposition of mixed litter were always lower than predicted from the sum of their effects.
  • 3
    In mixed-litter assays, we obtained intermediate values of decomposition parameters, indicating that doubling the species richness of leaf litter from one to two species did not promote decomposition processes. Effects of mixing litter were, thus, mostly additive; essentially only when earthworms fed on mixed litter we observed, mostly positive, non-additive effects of diverse litter.
  • 4
    Our findings provide evidence for a potential effect on ecosystem functioning through joint action of detritivores even at low species diversity, while litter diversity seems to be less significant. On high-quality litter, isopods and earthworms are not functionally redundant but act synergistically on litter decomposition. The effects of detritivore diversity on ecosystem processes, however, are context-specific and depend on the quality and diversity of the available food sources, and on species-specific characteristics of the detritivores.

Introduction

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

In recent years, the effects of biodiversity in mediating ecosystem processes has been the topic of a plethora of studies. Besides the possible promotion of ecosystem stability and resilience (e.g. Naeem & Li 1997; Griffiths et al. 2000; Ives, Klug & Gross 2000; Wardle, Bonner & Barker 2000), there has been discussion of the extent to which different levels of biodiversity increase particular functions of ecosystems (e.g. Jonsson & Malmqvist 2000; Cragg & Bardgett 2001; Duffy et al. 2001; Jonsson, Malmqvist & Hoffstein 2001), but decomposition has been considered only rarely. Decomposition in terrestrial ecosystem occurs through detritivore feeding and digestion, and microbial degradation of detritus (e.g. Wood 1974; Facelli & Picket 1991).

Although a diverse soil fauna and microbiota are involved in the degradation of leaf litter, little is known about the significance of their diversity for the corresponding processes of nutrient cycling (but see Cragg & Bardgett 2001). Only if functional redundancy is reduced by species-specificity will species diversity increase functional diversity. Thus, interspecific differences of detritivores in their contribution to decomposition processes (Zimmer & Topp 1999; Zimmer et al. 2002, 2004) can be expected to result in non-additive effects of detritivore mixtures compared with single-species communities, i.e. the sum of single-species effects does not equal mixed-species effects.

However, the relative significance of detritivores for decomposition changes with varying diversity of detritus (Sulkava & Huhta 1998), and both microbial litter colonisation (Sulkava et al. 2001) and decomposition rates (Kautz & Topp 1998; Kaneko & Salamanca 1999; Conn & Dighton 2000; Zimmer 2002) are affected by detritus diversity. As early as 1984, Seastedt had suggested that differences in decomposition rates of mixed and single-species litter are due to species-specific resource qualities of litter to decomposing organisms. Subsequently, several students of decomposition processes experimentally approached this assumption (reviewed in Wardle, Giller & Barker 1999), but while, e.g. Hector et al. (2000) recently confirmed positive non-additive effects, Smith & Bradford (2003) observed negative non-additive effects of litter types that differed in quality. Knops, Wedin & Tilman (2001) did not find effects of litter diversity on decomposition.

In temperate regions, woodlice (Isopoda: Oniscidea) and earthworms (Oligochaeta: Lumbricidae) are the most prominent terrestrial detritivores, ingesting and processing litter and affecting biomass and activity of litter-colonizing microbiota in grassland and forests (e.g. Ronde 1960; Hassall et al. 1987; Scheu 1993; Van Wemsen, Verhoef & Van Straalen 1993; Zimmer & Topp 1999). They usually coexist, with isopods mainly inhabiting the litter layer, while lumbricids are mostly soil-dwelling, and exhibit different feeding strategies: above-ground fragmentation (isopods) vs. burying leaf litter fragments (earthworms). Essentially, however, nothing is known about potential synergistic interactions in litter decomposition.

Here, we investigate the effects of functional diversity of isopods and earthworms in terms of their different feeding strategies and of leaf litter quality and diversity (oak and alder) on selected parameters of leaf litter decomposition. Based on previous studies (see above), we expected non-additive effects of both detritivore species and litter species. We hypothesised that synergistic interactions of detritivore species that belong to different functional groups will promote decomposition processes, but that the decomposability and diversity of leaf litter will influence the significance of detritivore diversity.

Materials and methods

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

animals, leaf litter and soil

Woodlice (Porcellio scaber Latreille 1804) and earthworms (Lumbricus rubellus Hoffmeister 1843) were collected in the field near Cologne, Germany, in March 1997. In the laboratory, they were maintained under experimental conditions (see below) for 1 week to acclimate and to prevent input of non-experimental material (cf. Daniel & Anderson 1992). Pre-adult woodlice were used in each microcosm to avoid an effect of gravidity or sexual activity (cf. Lawlor 1976; Shachak 1980). Their experimental density equalled 909 individuals per m2. Their individual fresh mass of 25 ± 5 mg (median ± median absolute deviation) equalled a dry mass of c. 11 mg (N = 8; 60 °C for 24 h). Adult earthworms of 600 ± 100 mg (median ± MAD; fresh mass, equalling 90 mg dry mass, N = 8) were used in a density of 182 individuals per m2. The experimental densities of detritivores were similar to those frequently found in the field (cf. Judas et al. 1989; Topp et al. 1992; Irmler 1995; Auerswald et al. 1996).

Based on its chemical and physical characteristics (Table 1), leaf litter of oak (Quercus robur) is assumed a low-quality food for detritivores, whereas alder (Alnus glutinosa) litter is of high quality (Satchell & Lowe 1967; Neuhauser & Hartenstein 1978; Zimmer & Topp 1997, 2000). Leaf litter was collected in mixed deciduous forests near Cologne in October 1996, immediately after fall. In the laboratory, leaf litter was air-dried and stored in mesh bags to minimise microbial degradation (cf. Zimmer & Topp 1997, 2000). The difference between litter input (dry mass) to the microcosms and litter remnants (dry mass) after termination of the experiments served as a measure of litter degradation. These data were corrected (random pairing) for microbial degradation during the 1-week pre-treatment (see below) as determined in pre-experiments. Due to litter fragmentation and degradation, we were unable to distinguish between output of alder and oak in mixed-litter treatments.

Table 1.  Characteristics of experimental leaf litter. Data are median and median absolute deviation (M ± MAD) of eight replicates each
 UnitsOakAlder
Magnesiummg g−1 1·1 ± 0·1 1·5 ± 0·1
Calciummg g−1  21 ± 1  18 ± 2
Phosphatemg g−1 0·3 ± 0·1 2·5 ± 0·1
Nitrogenmg g−113·0 ± 0·625·7 ± 0·6
Organic carbonmg g−1 386 ± 12 409 ± 13
C:N   30 ± 215·6 ± 0·5
Ligninsmg g−1 290 ± 20 170 ± 50
Cellulosemg g−1 570 ± 60 410 ± 40
Phenolicsmg g−1 4·6 ± 0·1 4·5 ± 0·1
Hydrolysable tanninsmg g−1 5·3 ± 0·1 6·3 ± 0·6
Condensed tanninsmg g−1 6·2 ± 0·1 6·6 ± 0·2
pH [3 M KCl]  4·9 ± 0·2 5·7 ± 0·2
Physical toughnessg mm−2  73 ± 7  14 ± 3

Characteristics of leaf litter were determined once (N = 8) prior to microcosm experiments (Table 1). Total calcium and magnesium concentrations were determined by wet incineration, acidic digestion and atomic absorbance spectrometry after Steubing & Fangmeier (1992). Phosphate was quantified with the vanadate method (Steubing & Fangmeier 1992). The C:N ratio was calculated after determination of total nitrogen (Kjeldahl method) and of organic carbon (as mass loss after ashing at 500 °C). Leaf litter pH was measured in 3 m KCl (Schlichting & Blume 1966). Physical toughness of leaf litter fragments was determined with a penetrometer after Williams (1954; Graça & Zimmer 2005). Different classes of aromatic substances were quantified – phenolics after Julkunen-Tütto (1985), hydrolysable tannins after Barbehenn & Martin (1992), and condensed tannins after Price, Van Scoyoc & Butler (1978) – and the determination of cellulose and lignin followed the procedures described by Zimmer (1999).

Loamy sand soil (Table 2) was taken from a mineral soil layer (30–40 cm) beneath a mixed oak forest near Cologne, Germany. In the laboratory, the soil was air-dried to reduce microbial activity (cf. Whitford 1989), and subsequently sieved (2 mm mesh size) and thoroughly mixed. Prior to microcosm experiments, the soil was characterised as follows (N = 8). Particle size distribution and water-holding capacity (WHC) were determined according to Hartge & Horn (1989). Calcium, magnesium, phosphate and nitrogen concentrations were determined as above. Total C was determined after ashing at 1250 °C, and inorganic C was quantified as CO2 released from acidic digestion with H3PO4 and boiling; the difference between total and inorganic C served as a measure of organic C. After the experiment, the soil microbial respiration was measured (Skambracks & Zimmer 1998; modified for soil samples). Afterwards, soil samples were air-dried and used for chemical analyses (see above) again.

Table 2.  Characteristics of experimental soil, a loamy sand. Data are median and median absolute deviation (M ± MAD) of eight replicates each, and single measurements of particle size distribution, respectively
  Loamy sand
Particle size distribution:
 Sand (63–2000 µm)% 75
 Silt (2–63 µm)% 19
 Clay (< 2 µm)%  6
Water-holding capacity% 19
Magnesiummg kg−1 36 ± 1
Calciummg kg−1160 ± 4
Phosphatemg kg−1  9 ± 1
Nitrogenmg kg−1 70 ± 10
Organic carbonmg kg−1991 ± 86
C:N  14 ± 9

experimental design

Microcosms were made of PVC tubes with a height of 50 mm and a diameter of 84 mm, resulting in a soil surface area of 5542 mm2. To reduce water loss through evaporation, we used translucent Petri dishes as lids. To avoid stagnant moisture, the lower end of the microcosm tubes was sealed with gauze (mesh size 150 µm) instead of a plastic cover.

To obtain data for a time series of litter decomposition and nutrient release (8 weeks), and because microcosm analyses were destructive, we set up a total of 336 microcosms. From every treatment, subsets of 7 replicate microcosms were harvested every 2 weeks. Microcosms were stored at 15 ± 1 °C and a photoperiod of 12 h light:12 h dark in water-saturated atmosphere. During a 1-week pre-experimental treatment, 100 g (dry mass) of soil (see above; Table 2) with a moisture content of 50% WHC and leaf litter was added to each of the microcosms (N = 7 for each treatment) and moistened. In single-litter treatments, 2·5 g (dry mass) of either oak or alder litter was added; in mixed-litter treatment, the ratio of oak and alder litter of 1·5 : 1·0 (g) was chosen to prevent the sole consumption of alder, the preferred food source, throughout the entire experiment. According to pre-experiments, we expected the detritivores to consume the amount of alder in the mixed-litter treatments within 3–4 weeks.

To start the experiment, five woodlice were added to each of 84 litter-containing microcosms (N = 7 for oak, alder, and oak and alder, each, for four dates during a time series). To another 84 of litter-containing microcosms, one earthworm was added. Further, five woodlice and one earthworm were added to each of 84 litter-containing microcosms, and another 84 microcosms served as animal-free controls.

Once a week, 1·5 mL of distilled water was added to each microcosm (c. 3 mm m−2 per month) to compensate for evaporative water loss (as determined gravimetrically in pre-experiments). Microcosms were checked for dead animals weekly to replace them with similar-sized individuals that had been fed the same food source (either oak or alder or oak and alder). Overall mortality of detritivores was below 2% so that effects of replacing animals can be neglected. Different treatments did not differ from each other in terms of detritivore mortality.

After 2, 4, 6 and 8 weeks, seven microcosms of each treatment were used for microbiological and chemical analyses of the soil as described above. We therefore obtained time series of leaf litter decomposition and nutrient release through detritivores and in animal-free control systems over 8 weeks.

statistics

Most of our data were not normally distributed. Thus, we chose the median and the median absolute deviation (M ± MAD) for the presentation of results. Multiple comparison of treatments was performed by using non-parametric Kruskal–Wallis H-tests, and Bonferroni-corrected Mann–Whitney U-tests served for subsequent pair-wise comparisons.

We analysed the effects of the diversity of both detritivores and leaf litter on litter mass loss, soil microbial respiration, and soil concentrations of calcium and magnesium over the time series of our experiment using anova with ‘detritivore species’, ‘litter species’ and ‘time’ as factors. Since soil concentrations of organic carbon, total nitrogen and phosphate were only determined after 8 weeks, we excluded the factor ‘time’ here. Data were appropriately transformed to achieve approximate homoscedasticity prior to anova.

Comparison of the effects of single detritivore and litter species with the joint effects mixed detritivores and litter, respectively, was performed by using Wilcoxon tests (α = 0·05). For this, we added or averaged (weighted by 1·5 : 1·0, according to the mixture ratio of litter), respectively, random-paired samples of corresponding treatments (e.g. changes in N content in samples with isopods feeding on alder + changes in N content in samples with earthworms feeding on alder) and compared these computed data with the corresponding data obtained from randomly chosen samples of treatments with both detritivores or litters, respectively, present (all data randomly corrected for animal-free controls). When the sum or average, respectively, of single-species effects on litter decomposition was less than the two-species effects, we concluded that there were synergistic (positive non-additive) effects. When the sum or average, respectively, of single-species effects was higher than the two-species effects, joint effects were accepted as less than additive. When the computed data and the original data did not differ, we assumed the joint effects to be additive.

Results

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

leaf litter mass loss

Leaf litter mass loss during the course of the experiment was exponential in every treatment (Fig. 1; 0·963 < R < 0·999; highest coefficients yielded by exponential regression). In animal-free treatments, alder litter was degraded more rapidly than oak litter [after 8 weeks: 530 ± 40 mg (21%) mass loss vs. 174 ± 4 mg (7%) mass loss; P < 0·001]. Mass loss caused by feeding and incorporation of leaf fragments into the soil by detritivores was about twice that caused by microbial degradation in animal-free controls (Fig. 1). The diversity of both detritivores and litter significantly affected litter mass loss throughout the experiment (Table 3). The effect of isopods and earthworms on oak mass loss was additive, but detritivores had a synergistic (positive non-additive) effect on alder mass loss (Fig. 1; Table 5). Mixed-litter mass loss was intermediate between alder and oak mass loss for every treatment (compare Fig. 1c with a and b). Thus, mass loss of mixed leaf litter caused by the joint action of both detritivores was less than additive (Table 5). Effects of mixed litter on litter mass loss were less than additive with earthworms, but additive in any other combination (Table 6).

image

Figure 1. Leaf litter mass loss during 8 weeks of decomposition caused by different detritivores feeding on (a) oak, (b) alder, or (c) oak and alder. Data are median and median absolute deviation (M ± MAD) of seven replicates, each. Different lower case letters indicate differences between treatments after 8 weeks (α = 0·05). Note different vertical scales.

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Table 3. anova tables for leaf litter mass loss and changes in soil characteristics during 8 weeks of decomposition of different litter types through different detritivores (a), and after 8 weeks as influenced by the diversity of detritivores and leaf litter (b). Due to our experimental design, 3- and 4-way interactions were not calculated
 d.f.Litter mass lossMicrobial respirationCalciumMagnesium
FPFPFPFP
(a) Model121434·3<0·0011037·1<0·0013978·4<0·0013826·5<0·001
Woodlice 1330·3<0·001155·9<0·001273·1<0·001172·5<0·001
Earthworms 1455·1<0·001329·7<0·001935·9<0·001410·5<0·001
Oak 191·1<0·00131·6<0·0010·10·73810·10·002
Alder 1623·6<0·001174·8<0·001248·5<0·001259·1<0·001
Woodlice × Earthworms 11·50·2192·90·0900·10·9621·80·188
Woodlice × Oak 17·80·00722·3<0·00119·5<0·00120·6<0·001
Earthworms × Oak 18·20·0059·70·0030·40·5247·10·010
Woodlice × Alder 113·5<0·00114·1<0·0019·80·00320·8<0·001
Earthworms × Alder 193·8<0·00168·3<0·00185·7<0·00147·9<0·001
Oak × Alder 13·90·0517·40·00818·9<0·00123·6<0·001
(b) Diversity of:
 detritivores 222·6<0·00129·2<0·00175·3<0·00134·8<0·001
 litter 13·30·0721·40·2326·80·0114·20·044
Detritivores × Litter 20·30·7180·40·6551·40·2450·90·420
Table 5.  Joint effects of isopods and earthworms (after 8 weeks of leaf litter decomposition) on litter mass loss and soil characteristics compared with the sum of the effects of isopods and the effects of earthworms; ▾, joint effects less than additive; =, effects additive; ▴ joint effects more than additive (Wilcoxon; α = 0·05)
 OakAlderMixed litter
Leaf litter mass loss=
Soil microbial respiration=
Soil calcium concentration
Soil magnesium concentration
Soil nitrogen concentration=
Soil organic carbon concentration=
Soil phosphate concentration==
Table 6.  Effects of mixed litter (after 8 weeks of leaf litter decomposition) on litter mass loss and soil characteristics compared with the average effects of single-litter treatments; ▾, effects less than additive; =, effects additive; ▴ effects more than additive (Wilcoxon; α = 0·05)
 ControlIsopodsEarthwormsMixed detritivores
Leaf litter mass loss===
Soil microbial respiration===
Soil calcium concentration===
Soil magnesium concentration==
Soil nitrogen concentration===
Soil organic carbon concentration===
Soil phosphate concentration====

soil microbial activity

Soil microbial activity rapidly increased during the first 2 weeks of the experiment in every detritivore treatment, but not in animal-free controls (Fig. 2). Detritivores increased respiration 3- to 13-fold in single-species and single-litter treatments. The diversity of both detritivores and litter significantly affected soil microbial respiration throughout the experiment (Table 3). Joint effects of detritivores on soil microbial respiration were additive when they fed on oak litter, but were synergistic for alder litter (Fig. 2; Table 5). Although detritivore-mediated soil respiration was significantly greater in mixed-litter treatments than in oak treatments (compare Fig. 2a and c), there were no differences between alder treatments and mixed-litter treatments (compare Fig. 2b and c). Thus, the joint effect of earthworms and isopods was significantly less than additive (Table 5). Effects of mixed litter on soil microbial respiration were more than additive with earthworms, but additive in any other combination (Table 6).

image

Figure 2. Soil microbial respiration during 8 weeks of decomposition caused by different detritivores feeding on (a) oak, (b) alder, or (c) oak and alder. Data are median and median absolute deviation (M ± MAD) of seven replicates, each. Different lower case letters indicate differences between treatments after 8 weeks (α = 0·05). Note different vertical scales.

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soil nutrients

Changes in the soil concentrations of calcium and magnesium were exponential (Fig. 3; 0·705 < R < 0·995; highest coefficients yielded by exponential regression). While neither calcium nor magnesium concentrations changed in animal-free alder and mixed-litter treatments, and even decreased in animal-free oak treatments, detritivores increased the concentrations of basic cations. Diversity of both detritivores and litter significantly affected the release of calcium and magnesium from leaf litter (Table 3). However, the effect of detritivore diversity changed over time (Ca2+ and Mg2+) and depended on litter diversity (Ca2+), as indicated by statistical interactions of these factors. Joint effects of both detritivores were synergistic for calcium and magnesium on alder litter, but less than additive on oak litter (Fig. 3; Table 5). Nutrient release from mixed litter was intermediate in isopod and two-species treatments, but even greater than from alder litter in earthworm treatments (compare Fig. 3c,f with a/b and d/e, respectively). Calcium and magnesium release through joint action of detritivores was less than additive for mixed litter (Table 5). Effects of mixed litter on soil calcium concentration were more than additive with earthworms, but additive in any other combination (Table 6). Effects of mixed litter on soil magnesium concentration were more than additive with either of the detritivores, but were additive in animal-free controls and when both detritivores were present (Table 6).

image

Figure 3. Soil calcium (a–c) and magnesium (d–f) concentrations during 8 weeks of decomposition due to different detritivores feeding on oak (a, d), alder (b, e), or oak and alder (c, f). Data are median and median absolute deviation (M ± MAD) of seven replicates, each. Time series are represented by graphs of exponential equations. Different lower case letters indicate differences between treatments after 8 weeks (α = 0·05). Note different vertical scales.

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The total nitrogen concentration of the soil was increased by detritivores after 8 weeks (Fig. 4a). The diversity of detritivores, but not of litter, significantly affected nitrogen release from litter (Table 4). The joint effect of isopods and earthworms on soil N was additive for alder litter, but less than additive for oak litter (Table 5). In mixed-litter treatments, joint effects of detritivores did not differ from those of earthworms (Fig. 4a) and were less than additive (Table 5). Effects of mixed litter on soil nitrogen concentration were more than additive with earthworms, but additive in any other combination (Table 6).

image

Figure 4. Soil nitrogen (a), carbon (b) and phosphate (c) concentrations after 8 weeks of decomposition caused by different detritivores feeding on different litter types. Data are median and median absolute deviation (M ± MAD) of seven replicates, each. Different lower case letters indicate differences between treatments (α = 0·05). Note different vertical scales.

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Table 4. anova tables for changes in soil characteristics after 8 weeks of decomposition of different litter types through different detritivores (a), and as influenced by the diversity of detritivores and leaf litter (b). Due to our experimental design, 3- and 4–way interactions were not calculated
 d.f.NitrogenOrganic carbonPhosphate
FPFPFP
(a) Model12473.4<0.001162.5<0.001923.7<0.001
Woodlice 1163.3<0.001157.4<0.0012.7<0.102
Earthworms 1484.6<0.001181.4<0.00192.1<0.001
Oak 163.5<0.0015.2<0.02574.5<0.001
Alder 1158.6<0.00154.9<0.0011.20.275
Woodlice × Earthworms 10.10.9042.60.1089.10.004
Woodlice × Oak 146.5<0.00116.8<0.0013.50.067
Earthworms × Oak 11.10.3164.70.03324.1<0.001
Woodlice × Alder 12.0<0.1610.4<0.5304.50.038
Earthworms × Alder 154.5<0.00113.4<0.0013.20.080
Oak × Alder 130.8<0.00114.9<0.0010.10.736
(b) Diversity of:
 detritivores 227.2<0.00135.0<0.00175.3<0.001
 litter 10.80.3602.90.0906.80.008
Detritivores × Litter 21.90.1381.90.1331.40.126

The organic carbon content of experimental soils was always higher when detritivores were present than in animal-free controls (Fig. 4b). Detritivore diversity, but not litter diversity, proved significant for organic carbon release from decomposing litter (Table 4). Joint effects of detritivores were additive in oak and alder treatments (Table 5). Mixed-litter treatments did not differ from alder treatments considering single-species treatments (Fig. 4b); joint effects of detritivores were less than additive in mixed-litter treatments (Table 5). Effects of mixed litter on soil organic carbon concentration were more than additive with earthworms, but additive in any other combination (Fig. 4b; Table 6).

After 8 weeks, only earthworms had affected the phosphate content of the soil in oak treatments (Fig. 4c). Detritivores had no effect whatsoever on the soil phosphate concentration under mixed litter. Overall, however, soil phosphate concentration after 8 weeks of decomposition was significantly affected by the diversity of both detritivores and litter (Table 4). Detritivore effects were additive in alder treatments, but less than additive in oak treatments (Fig. 4c; Table 5). Effects of mixed litter on soil phosphate concentration were additive in every detritivore-litter combination (Fig. 4c; Table 6).

Discussion

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

Decomposition process rates increased when two different species of detritivores, belonging to different functional groups with respect to their feeding strategies, were present compared with a single-species soil fauna. Similarly, process rates were affected by increasing the diversity of litter. Our experimental design of increasing diversity and biomass of detritivores simultaneously has consequences for the definition of additivity of effects. Since we added detritivores, additivity of effects would have been achieved if the sum of single-species effects had equalled the mixed-species effects. According to this definition, additive effects of detritivores indicate that these species are not functionally redundant, while positive non-additive effects even suggest synergistic interactions of detritivores. By contrast, we replaced part of the litter mass of one species by litter of another species to increase litter diversity (without increasing total litter mass). In this case, additivity would have been achieved if the average single-litter process rate had equalled the mixed-litter rate.

Detritivores with different feeding strategies had additive or positive non-additive effects on decomposition of a high-quality food source, i.e. their actions were not functionally redundant or even synergistic, but they had additive or negative non-additive effects on decomposition of a low-quality food source. On mixed litter, joint effects of detritivores were negative non-additive; however, adding detritivores still resulted in increased decomposition process rates. Negative non-additive effects of detritivores may be explained by functional redundancy of detritivores or by decomposition processes being a saturation function of detritivore biomass. With respect to positive non-additive effects, we cannot entirely rule out the possibility of nonlinear biomass effects resulting in an over-proportional increase of decomposition parameters beyond some biomass threshold, but recent results on decomposition in laboratory microcosms with isopods and snails (M. Mews & M. Zimmer, unpublished) make this assumption unlikely. Reasons for different effects of detritivore diversity on different litter qualities remain unclear, but it is interesting to speculate on reciprocal facilitation (through distinct feeding strategies, e.g. fragmentation of alder leaves by isopods –vs skeleting of oak leaves – that facilitates burying of leaf fragments) that may prevail in the case of ad libitum available high-quality food. However, this may be counterbalanced or even overruled in the case of low-quality food or limited high-quality food supply (e.g. preferentially feeding on faeces rather than oak litter). While Jonsson et al. (2001) also observed different effects of detritivore diversity on low- vs. high-quality detritus in streams, our hypothesis warrants further testing in laboratory experiments.

By contrast, increasing the diversity of litter had mostly additive effects, i.e. decomposition process rates were neither promoted nor slowed down by litter diversity. However, the (mostly positive) non-additive effects of mixed litter in earthworm treatments are striking. Positive non-additive effects of mixed litter have been suggested to be due to translocation of nutrients (mostly nitrogen) from low-quality to high-quality litter (Seastedt 1984; Chapman, Whittaker & Heal 1988) via fungal hyphae (McTiernan, Ineson & Coward 1997). Negative non-additive effects (here, litter mass loss), by contrast, may be due to reduced microbial abundance in mixed litter (Blair, Parmelee & Beare 1990) or to spatial heterogeneity of litter quality, resulting in patchy distribution of specialized decomposer communities (Smith & Bradford 2003: field litter bags). Although we cannot provide evidence for supporting any explanation for our observation of non-additive effects, our overall conclusion is that litter diversity had weaker effects on decomposition processes than detritivore diversity, and only with earthworms but not with isopods.

Numerous other studies have presented evidence for a diverse fauna and flora to increase ecosystem stability and/or function (e.g. Naeem et al. 1994; Chapin et al. 1997; Tilman 1999; Hector et al. 2000; Ives et al. 2000; Wardle et al. 1999, 2000; Duffy et al. 2001; and references therein). The role of detritivore diversity in decomposition processes, however, has only recently begun to be addressed (e.g. Griffiths et al. 2000; Jonsson & Malmqvist 2000; Cragg & Bardgett 2001; Jonsson et al. 2001; Zimmer et al. 2002, 2004). Both earthworms (Hendriksen 1990; Scheu 1993; Cortez & Bouché 2001) and isopods (Hassall et al. 1987; Szlávecz 1993; Van Wemsen et al. 1993) are known to contribute to leaf litter decomposition and have been suggested to species-specifically affect soil microbiota and decomposition processes (Bernier 1998; Zimmer & Topp 1999; Kautz & Topp 2000; Scheu et al. 2002; Zimmer et al. 2002). By experimentally reducing the diversity of stream detritivores, Jonsson et al. (2001) demonstrated the significance of species combination, while species richness per se had no effects. In experimentally diversity-reduced microcosms, Griffiths et al. (2000) recently failed to find any relationship between (microbial) diversity and soil function. In contrast to these findings, Cragg & Bardgett (2001) observed species-specific effects on decomposition, as has also been shown for earthworms (Bernier 1998) and isopods (Zimmer et al. 2002), suggesting an effect of faunal diversity when species interact. Diverging conclusions deduced from different studies on detritivore diversity may be due to (1) the particular species or feeding group(s) studied, (2) the leaf litter type(s) studied, (3) the decomposition parameter studied, or (4) the time scale of the study.

  • 1
    In some cases, the joint effects of different species within the same trophic group were additive or less than additive, and the authors conclude that the composition of a detritivore community rather than its species richness is of importance for ecosystem functioning (Cragg & Bardgett 2001; Jonsson et al. 2001). Bardgett & Chan (1999) stressed the significance of different feeding strategies when studying below-ground detritivore systems. The present study is unique in combining representatives of below- and above-ground detritivores that, thus, belong to different trophic or functional groups (see Introduction).
  • 2
    While, e.g. Wardle et al. (1997) and Hector et al. (2000) found positive non-additive effects of increasing litter diversity, idiosyncratic effects are also common (e.g. Kaneko & Salamanca 1999; Wardle et al. 1999), and Smith & Bradford (2003) produced negative non-additive effects by mixing litter of different qualities (N content). The leaf litter type obviously decides whether increasing litter diversity gives rise to positive or negative effects (for discussion, see Wardle et al. 1997).
    Here, we provide evidence for both the diversity and identity of detritivores and litter being significant for decomposition process rates. For example, isopods and earthworms differed in litter removal: while isopods removed more oak litter than earthworms, mass loss of alder caused by earthworms was greater than that caused by isopods. On the other hand, both isopods and earthworms removed only about 30% of oak litter compared with alder. Soil microbial respiration in alder treatments was about 8 times greater than in oak treatments; effects of earthworms and isopods on soil microbiota were similar in oak treatments, but the increase of soil microbial respiration was more pronounced in earthworm- than in isopod-treatments with alder or mixed litter. Further, earthworms had stronger effects on nutrient release from litter than isopods, and more nutrients were released from alder than from oak litter. Sulkava & Huhta (1998), using litter of birch (Betula pendula), alder (Alnus incana) and spruce (Picea abies), state that the significance of detritivores increases in mixed-litter systems. In the present study, by contrast, joint effects of detritivores were always less than additive in mixed-litter treatments with oak and alder litter. Overall, we, thus, agree with Cragg & Bardgett (2001) and Jonsson et al. (2001) in that both species richness and species composition will be significant for ecosystem function. However, litter diversity seems to be of relatively little significance (cf. Wardle et al. 1997).
  • 3
    Most studies performed thus far focus on mass loss of leaf litter or only very few parameters of decomposition. The present study is unique in that we monitored seven parameters describing decomposition of, and nutrient release from, leaf litter, and we present evidence that the effects of diversity and identity of both detritivores and litter depend on the decomposition process in question.
  • 4
    Although the promotion of decomposition processes due to increased detritivore diversity was already obvious in early stages of our experiments, the magnitude of this diversity-effect (less than additive, additive or more than additive) only became clear at later stages of leaf litter decomposition. Thus, the duration of decomposition studies has to be taken into account when comparing different study systems.

Conclusions

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References

Our findings provide evidence for functional diversity or even synergistic effects of different detritivore species that belong to different feeding guilds, even at low diversities. Thus, we expect diverse detritivore communities to promote ecosystem processes such as decomposition and nutrient cycling. The effects of diversity, however, depend on the quality of the available food sources, being positive only on high-quality litter. On the other hand, we expect litter diversity to contribute little to decomposition process rates.

Overall, the ecosystem significance of diversity of both detritivores and litter is context-specific, depending on species-specific characteristics of both food and consumers. Different detritivore combinations will have different effects that will further depend on the diversity composition of the available litter. Thus, no general predictions on the effect of species diversity on litter decomposition can be made without considering species identity.

References

  1. Top of page
  2. Summary
  3. Introduction
  4. Materials and methods
  5. Results
  6. Discussion
  7. Conclusions
  8. References
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