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Species-specific effects of dung beetle abundance on dung removal and leaf litter decomposition
Acta Oecologica 69 (2015) 31e34
Contents lists available at ScienceDirect
Acta Oecologica journal homepage: www.elsevier.com/locate/actoec
Short communication
Species-specific effects of dung beetle abundance on dung removal and leaf litter decomposition Thomas Tixier a, *, Juliette M.G. Bloor b, Jean-Pierre Lumaret a CEFE UMR 5175, CNRS, Universit e de Montpellier, Universit e Paul-Val ery Montpellier 3, Laboratoire Zoog eographie, Route de Mende, 34199 Montpellier cedex 5, France b INRA, UR874 Grassland Ecosystem Research Unit, 5 Chemin de Beaulieu, F-63039 Clermont-Ferrand, France a
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2014 Received in revised form 17 July 2015 Accepted 19 August 2015 Available online 31 August 2015
In grazed ecosystems, coprophagous beetles are known to play an important role in nutrient cycling, but interactions between species identity and dung beetle abundance on soil processes remain unclear. We conducted an outdoor mesocosm experiment to investigate the effects of three dung beetle species at four levels of abundance on dung incorporation into the soil. In addition we assessed indirect effects of dung beetle activity on leaf litter decomposition by microorganisms in soil. Both dung removal and leaf litter mass loss were positively correlated with initial dung beetle biomass and beetle abundance across species. However, the magnitude of beetle-induced increases in litter mass loss was very small compared to the magnitude of beetle effects on dung removal. Beetle effects on dung removal and litter decomposition also showed significant abundance species interactions, with strongest responses observed for Colobopterus erraticus Linnaeus. Our findings highlight the importance of interactive effects between species identity and abundance on dung removal and provide the first demonstration of indirect dung beetle effects on leaf litter decomposition in soil. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: Cattle dung Abundance effects Litter decomposition Scarabaeidae Soil carbon
1. Introduction Dung beetles play an important role for soil nutrient cycling and soil structure where dung is present as a result of tunneling, dung shredding and dung burial (Yokoyama et al., 1991; Bang et al., 2005; Brown et al., 2010). Dung beetle activity is of particular interest in grazed ecosystems, where dung beetles reduce the areas covered by animal waste and contribute to the sustainability of livestock production by increasing pasture productivity and suppressing livestock parasites (Nichols et al., 2008). However, mounting evidence suggests that the diversity and abundance of dung beetles may face threats due to changes in land-use and climatic conditions (Nichols et al., 2008; Nervo et al., 2014). Understanding the linkages between dung beetle community attributes and ecosystem processes is therefore critical for the prediction of ecosystem function in a changing environment (Nichols et al., 2008; Beynon et al., 2012; Braga et al., 2013). Dung beetles are a diverse group of insects (Scarabaeidae,
* Corresponding author. E-mail addresses: [email protected] (T. Tixier), juliette.bloor@clermont. inra.fr (J.M.G. Bloor), [email protected] (J.-P. Lumaret). http://dx.doi.org/10.1016/j.actao.2015.08.003 1146-609X/© 2015 Elsevier Masson SAS. All rights reserved.
Aphodiidae, Geotrupidae) which feed on animal excreta (Hanski and Cambefort, 1991). Dung beetles can be classed into three main functional groups: endocoprids (dwellers) whose larvae feed in dung pats or at the dungesoil interface, paracoprids (tunnellers) who bury brood balls in vertical tunnels under the dung pat, and telecoprids (rollers) who transfer dung away from the pat before burying brood balls under the soil surface (Hanski and Cambefort, 1991)Shredding, feeding-on and relocation of dung by dung beetles during tunneling has a direct effect on dung disappearance from the soil surface (Holter et al., 2002; Yamada et al., 2007; O'Hea et al., 2010). Moreover, dung beetle activity may have indirect effects on the decomposition of soil organic matter (OM) and plant litter fragments via beetle-induced changes in soil conditions such as aeration and soil water holding capacity (Bang et al., 2005; Brown et al., 2010), and/or stimulation of microbial biomass due to local inputs of labile carbon from dung (Bardgett et al., 1998; Hatch et al., 2000). Previous work has shown that dung beetle effects may vary depending on species composition, functional group richness and species identity (Bang et al., 2005; Slade et al., 2007; O'Hea et al., 2010; Beynon et al., 2012). Experiments with species mixtures have also shown faster dung removal in the presence of higher dung beetle abundance (Yamada et al., 2007). In theory, the magnitude of dung beetle effects may vary across species due to
32
T. Tixier et al. / Acta Oecologica 69 (2015) 31e34
differences in beetle burrowing activity or species body mass (Holter et al., 2002; Larsen et al., 2005; Nervo et al., 2014). Indeed, recent work suggests that effects of dung beetles on ecosystem function may reflect a balance between beetle abundance, biomass and presence of species with particular attributes (small-ve largebody size) (Braga et al., 2013). However, little is known about the influence of beetle species identity on the relationship between beetle abundance and dung removal, and indirect effects of dung beetles on leaf litter decomposition have never been assessed. Here we report a mesocosm study on the interactive effects of dung beetle species identity and beetle abundance on dung removal and decomposition of leaf litter in soil beneath dung pats. The study was conducted using three co-occurring dung beetle species which bury dung beneath the dung pat: Colobopterus erraticus (Linnaeus, 1758), Caccobius schreberi (Linnaeus, 1758) and Onthophagus vacca (Linnaeus, 1767). C. erraticus and C. schreberi are small-sized beetles (mean dry body mass of eight and seven mg respectively), whereas O. vacca is a large sized-species (mean body mass 41 mg) (Lumaret and Kirk, 1987). These three species are commonly found in the French Mediterranean region during the spring period (Lumaret and Kirk, 1987). We hypothesized that: (i) dung removal by dung beetles increases with increasing beetle biomass and abundance; (ii) dung removal by dung beetles varies depending on beetle species; (iii) high dung beetle biomass and abundance accelerates leaf litter decomposition in soil. 2. Materials and methods In April 2012, 51 experimental units were established in the CNRS-CEFE experimental field platform in Montpellier, France (43 380 16N, 3 51041, elevation 58 m). Each experimental unit comprised of a plastic pot (15 cm diameter, 25 cm deep) filled with a mix of fersiallitic brown Mediterranean soil and sand (pHH2O ¼ 6.8; 0.3% N for the soil/sand mix); the soil/sand mix was used for logistical reasons to be able to clearly distinguish dung, beetles and brood balls from the soil medium at the end of the study. Pots were placed in holes in the ground such that the surface of the pot was flush with the soil surface, and enclosed aboveground using 0.2 mm mesh cages (25 25 15 cm) nailed to the ground immediately adjacent to the buried pot. These cages were used to avoid i) dispersal of experimental beetles away from the pots and ii) colonization of the pots by macro- and mesoinvertebrates from the surrounding area. Litter decomposition was assessed following Bloor and Bardgett (2012) using 5 5 cm litterbags (25 mm mesh; ANKOM Technology, Macedon, NY, USA) filled with 0.5 g of standardised leaf litter (50:50 mix of Festuca rubra and Lolium perenne senescent leaf material, 1.6% N, 28C:N). Litter from each plant species was dried (60 C for 48 h), finely cut (1 mm), and combined to produce the standardised litter mix with equal dry mass of each plant species. One litter bag was inserted vertically into the soil of each pot to a depth of 5 cm, and 500 g of fresh, homogenised cow dung was then placed on the soil surface of each pot (dung composition 85.7% H2O, 2.7% N, 15.6C:N, collected from a nearby dairy farm). Dung was uncontaminated by antibiotics or anthelmintics since no veterinary medical products were administered to animals for at least 100 days before dung was collected. Adult dung beetles for use in the study were collected locally from grazed meadows using dung-baited pitfall traps, identified to species, sexed and stored in cool conditions for three weeks prior to the experiment. Four beetle density treatments were established per species. Beetles of C. erraticus Linnaeus, C. schreberi Linnaeus and O. vacca Linnaeus were added to mesocosms as 50:50 male:female pairs at densities of either two, four, eight or 12 individuals, consistent with variation in dung beetle abundance
observed in dung pats under natural conditions (Hanski and Cambefort, 1991). These four density treatments were replicated four times per species (total of 16 experimental units per species). Three remaining experimental units were kept as beetle-free controls. All experimental units were then left under natural rainfall conditions for five months. In September 2012, beetles were removed by hand from the experimental units. All remaining dung on the soil surface was collected, dried (48 h, 60 C) and weighed to determine residual dung dry mass. Organic matter content of this remaining dung was determined from loss of weight of dried dung samples after being ashed at 500 C for 12 h. Dung removal was calculated as the proportion of OM lost from the start to the end of the experiment following Holter (1979). Litter bags were retrieved from the soil, cleaned and dried (48 h, 60 C) before being weighed to determine litter mass loss during the five-month incubation period. Values for both dung removal and litter mass loss in the presence of beetles were corrected by values obtained in beetle-free controls (i.e. treatment values e mean control value) to separate out dung beetle effects. Effects of species identity and beetle abundance were assessed using two-way analysis of covariance (ANCOVA) with beetle species as a fixed factor and initial beetle abundance as a quantitative, independent variable, following Zar (1998). Effects of beetle biomass on dung removal and litter decomposition were examined using simple linear regression, and estimations of beetle biomass were based on mean beetle dry mass and initial beetle numbers. All statistical analyses were carried out using Statgraphics Plus 4.1 (Statistical Graphics Corp., Rockville, Maryland, USA). 3. Results Mean dung removal due to dung beetle presence ranged from 3.2 to 23.0% across species and abundance treatments over the fivemonth incubation period. Dung removal showed a positive relationship with dung beetle biomass estimated at the start of the study (R2 ¼ 0.56, P < 0.01; Fig. 1). In general, rates of dung removal increased with increasing beetle abundance across species treatments (Table 1, Fig. 2). However, dung removal also showed significant species abundance interactions (Table 1); dung removal increased with more individuals of C. erraticus, but showed no response to increasing abundance of C. schreberi or O. vacca (R2 ¼ 19.8, P < 0.05; Fig. 2). Leaf litter bags showed significant mass loss during the study (mean mass loss of 40.1% across treatments and controls), but very little of the litter mass loss in the beetle treatments could be directly attributed to beetle presence due to high litter mass loss in the control pots (Fig. 1). Nevertheless, leaf litter decomposition showed a positive relationship with initial beetle biomass (R2 ¼ 0.32, P < 0.05; Fig. 1) and increased with increasing beetle abundance across species treatments (Table 1, Fig. 2). As with dung removal, leaf litter decomposition showed significant species abundance interactions (Table 1). Litter mass loss increased in the presence of more C. erraticus individuals (R2 ¼ 0.88, P < 0.05; Fig. 2) but was unaffected by increasing abundance of C. schreberi. Litter decomposition showed a marginally significant positive relationship with increasing O. vacca abundance (P ¼ 0.08; Fig. 2). 4. Discussion The role of community attributes for ecosystem function has faced considerable interest in recent years due to growing concerns over altered species abundance and diversity patterns under global change (see Larsen et al., 2005; Slade et al., 2007; Nichols et al.,
T. Tixier et al. / Acta Oecologica 69 (2015) 31e34
6
A Litter mass loss (%)
Dung removal (%)
25 20 15 10 5 0
33
B
4 2 0 -2 -4 -6
0
100
200
300
400
500
Beetle biomass (mg)
0
100
200
300
400
500
Beetle biomass (mg)
Fig. 1. Relationship between dung beetle biomass estimated at the start of the experiment and (A) dung removal, or (B) mass loss of leaf litter across beetle species and abundance treatments. Values for dung removal and litter mass loss are the difference between beetle treatments and beetle-free controls. Means (±1 standard error) are shown after five months of incubation; n ¼ 4. The dashed lines represent significant linear regressions between biomass and function (P < 0.05).
Table 1 Effects of species identity and dung beetle abundance on dung removal and leaf litter decomposition. F values derived from ANCOVA are shown: significant effects (P < 0.05) are given in bold type. Effect
Df
Variable Dung removal
Litter mass loss
Species Abundance Species Abundance
2,42 1,42 2,42
0.87 4.33 3.25
2.65 10.14 3.51
2008; O'Hea et al., 2010 for examples with dung beetles as focal organisms). Based on results from previous work on dung decomposition using mixtures of beetle species at high densities (Yamada et al., 2007), we hypothesized that both beetle abundance and beetle biomass would have a positive effect on dung removal across species treatments. Our results were consistent with these predictions and indicate that effects of dung beetles can occur even at relatively low beetle numbers. Increasing evidence suggests significant species-level variation in dung beetle effects on dung removal and nutrient recycling (Bang et al., 2005; Beynon et al., 2012), but interactions between species identity and abundance have faced little attention to date. In the present study, we focused on ‘tunneler’ species which bury dung beneath the dung pat (Hanski and Cambefort, 1991) and we show significant interspecific variation in beetle abundance effects
on dung beetle removal within this functional guild. Our findings indicate that impacts of declining beetle abundance on soil function and sustainable soil management in grazed systems may vary depending on the species involved. These results have implications not only for agro-ecology, but also for the evaluation of potential side-effects of veterinary medical products (often found in dung) on dung organisms. Surprisingly, the pattern of interspecific variation in abundance effects recorded here for dung beetle removal showed no clear link with beetle size. Positive effects of beetle abundance were observed for C. erraticus, a small-sized beetle, but no significant effects were observed for C. schreberi, a similar-sized species. Such species-level variation in abundance effects could reflect differences in reproductive activity or survival among species during the study period (Finn and Gittings, 2003). Strong effects of C. erraticus might also be explained in part by the relatively high dung shredding activities and mobile larvae of this species compared to C. schreberi; unlike C. schreberi whose larvae remain confined in the dung reserve of the pedotrophic nest, late instars of C. erraticus larvae may move between the soil and dung to get food supplies (Rojewski, 1983; Vitner, 2000). Our results support the hypothesis that increased dung beetle abundance and biomass promote leaf litter decomposition by microorganisms in soil. Response patterns of litter mass loss mirrored those of dung removal, consistent with increased microbial activity when increased carbon inputs from dung are present (Bardgett et al., 1998). However, although significant positive relationships
Fig. 2. Relationship between initial dung beetle abundance and (A) dung removal, or (B) mass loss of leaf litter in three beetle species treatments. Values for dung removal and litter mass loss are the difference between beetle treatments and beetle-free controls. Means (±1 standard error) are shown after five months of incubation; n ¼ 4. Dashed lines represent significant linear regressions between biomass and function for C. erraticus (P < 0.05).
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between beetle abundance and litter mass loss were observed, the magnitude of beetle effects on leaf litter decomposition was very small. An experimental design with more replicates and additional ‘high beetle abundance’ treatments may be required to detect the signature of indirect beetle effects on litter decomposition more clearly. Given the fine mesh-size of the litter bags, it is unlikely that mesofauna such as springtails and mites would have played an important role in leaf litter decomposition, but we cannot rule out the possibility that such organisms were involved in dung decomposition. To date, few studies have addressed interactions between dung beetles and mesofauna on dung removal and subsequent dung decomposition (but see Bertrand and Lumaret, 1984). In conclusion, positive relationships between overall dung beetle abundance and dung removal mask interactive effects between dung beetle abundance and species identity. These results suggest that the impacts of within-guild, intraspecific variation in dung beetles on dung removal and ecosystem services may be more complex than previously suspected. We also show that dung beetle abundance has a limited but positive effect on litter decomposition, and demonstrate similar patterns of response to variation in beetle abundance and species for both litter decomposition and dung removal. Future studies should determine whether the patterns observed here can be generalised to different times in the growing season and to different soil types with undisturbed soil structure under field conditions. Acknowledgements The authors thank Alexandre Salcedo for help setting up the experiment. This study was supported by a project grant from the German Federal Agency (UBA Project-No. FKZ 371063412) awarded to JPL. We thank the three anonymous reviewers whose comments contributed to improve the final version of the manuscript. References Bang, H.S., Lee, J.-H., Kwon, O.S., Na, Y.E., Jang, Y.S., Kim, W.H., 2005. Effects of paracoprid dung beetles (Coleoptera: Scarabaeidae) on the growth of pasture herbage and on the underlying soil. Appl. Soil Ecol. 29, 165e171. Bardgett, R.D., Keiller, S., Cook, R., Gilburn, A.S., 1998. Dynamic interaction between soil animals and microorganisms in upland grassland soils amended with sheep dung: a microcosm experiment. Soil Biol. Biochem. 30, 531e539. action des populations de microarthropodes Bertrand, M., Lumaret, J.P., 1984. Re a
ces de mouton par les insectes Scarabaeidae en milieux a l'enfouissement des fe fortes contraintes climatiques. Pedobiologia 27, 51e66. Beynon, S.A., Mann, D.J., Slade, E.M., Lewis, O.T., 2012. Species-rich dung beetle communities buffer ecosystem services in perturbed agro-ecosystems. J. Appl. Ecol. 49, 1365e1372. Bloor, J.M.G., Bardgett, R.D., 2012. Stability of above-ground and below-ground processes to extreme drought in model grassland ecosystems: interactions with plant species diversity and soil nitrogen availability. Perspect. Plant Ecol. 14, 193e204. Braga, R.F., Koranski, V., Andresen, E., Louzada, J., 2013. Dung beetle community and functions along a habitat-disturbance gradient in the Amazon: a rapid assessment of ecological functions associated to biodiversity. PLOS One 8, e57786. Brown, J., Scholtz, C.H., Janeau, J.-L., Grellier, S., Podwojewski, P., 2010. Dung beetles (Coleoptera: Scarabaeidae) can improve soil hydrological properties. Appl. Soil Ecol. 46, 9e16. Finn, J.A., Gittings, T., 2003. A review of competition in north temperate dung beetle communities. Ecol. Entomol. 28, 1e13. Hanski, I., Cambefort, Y., 1991. Dung Beetle Ecology. Princeton University Press, New Jersey. Hatch, D.J., Lovell, R.D., Antil, R.S., Jarvis, S.C., Owen, P.M., 2000. Nitrogen mineralization and microbial activity in permanent pastures amended with nitrogen fertilizer or dung. Biol. Fert. Soils 30, 288e293. Holter, P., 1979. Effect of dung beetles (Aphodius spp) and earthworms on the disappearance of cattle dung. Oikos 32, 393e402. Holter, P., Scholtz, C.H., Wardhaugh, K.G., 2002. Dung feeding in adult scarabaeines (tunnelers and endocoprids): even large dung beetles eat small particles. Ecol. Entomol. 27, 169e176. Larsen, T., Williams, N., Kremen, C., 2005. Extinction order and altered community structure rapidly disrupt ecosystem functioning. Ecol. Lett. 8, 538e547. Lumaret, J.-P., Kirk, A., 1987. Ecology of dung beetles in the French Mediterranean region (Col. Scarabaeinae). Acta Zool. Mex. 24, 1e55. Nervo, B., Tocco, C., Caprio, E., Palestrini, C., Rolando, A., 2014. The effects of body mass on dung removal efficiency in dung beetles. PLoS ONE 9, e107699. Nichols, E., Spector, S., Louzada, J., Larsen, T., Amequita, S., Favila, M., Network, S.R., 2008. Ecological functions and ecosystem services provided by Scarabaeinae dung beetles. Biol. Conserv. 141, 1461e1474. O'Hea, N.M., Kirwan, L., Finn, J.A., 2010. Experimental mixtures of dung fauna affect dung decomposition through complex effects of species interactions. Oikos 119, 1081e1088. Rojewski, C., 1983. Observation on the nesting behaviour of Aphodius erraticus (L.) (Coleoptera, Scarabaeidae). Bull. Entomol. Pol. 53, 271e279. Slade, E.M., Mann, D.J., Villanueva, J.F., Lewis, O.T., 2007. Experimental evidence for the effects of dung beetle functional group richness and composition on ecosystem function in a tropical forest. J. Anim. Ecol. 76, 1094e1104. Vitner, J., 2000. Variability of behavior in the larvae of Aphodius (Colobopterus) erraticus (Coleoptera: Scarabaeidae). Acta Soc. Zool. Bohem. 64, 115e118. Yamada, D., Imura, O., Shi, K., Shibuya, T., 2007. Effect of tunneler dung beetles on cattle dung decomposition, soil nutrients and herbage growth. Grassl. Sci. 53, 121e129. Yokoyama, K., Kai, H., Koga, T., Aibe, T., 1991. Nitrogen mineralization and microbial populations in cow dung, dung balls and underlying soil affected by paracoprid dung beetles. Soil Biol. Biochem. 23, 649e653. Zar, J.H., 1998. Biostatistical Analysis. International Edition (NJ). Upper Saddle River, p. 663.
Contents lists available at ScienceDirect
Acta Oecologica journal homepage: www.elsevier.com/locate/actoec
Short communication
Species-specific effects of dung beetle abundance on dung removal and leaf litter decomposition Thomas Tixier a, *, Juliette M.G. Bloor b, Jean-Pierre Lumaret a CEFE UMR 5175, CNRS, Universit e de Montpellier, Universit e Paul-Val ery Montpellier 3, Laboratoire Zoog eographie, Route de Mende, 34199 Montpellier cedex 5, France b INRA, UR874 Grassland Ecosystem Research Unit, 5 Chemin de Beaulieu, F-63039 Clermont-Ferrand, France a
a r t i c l e i n f o
a b s t r a c t
Article history: Received 20 November 2014 Received in revised form 17 July 2015 Accepted 19 August 2015 Available online 31 August 2015
In grazed ecosystems, coprophagous beetles are known to play an important role in nutrient cycling, but interactions between species identity and dung beetle abundance on soil processes remain unclear. We conducted an outdoor mesocosm experiment to investigate the effects of three dung beetle species at four levels of abundance on dung incorporation into the soil. In addition we assessed indirect effects of dung beetle activity on leaf litter decomposition by microorganisms in soil. Both dung removal and leaf litter mass loss were positively correlated with initial dung beetle biomass and beetle abundance across species. However, the magnitude of beetle-induced increases in litter mass loss was very small compared to the magnitude of beetle effects on dung removal. Beetle effects on dung removal and litter decomposition also showed significant abundance species interactions, with strongest responses observed for Colobopterus erraticus Linnaeus. Our findings highlight the importance of interactive effects between species identity and abundance on dung removal and provide the first demonstration of indirect dung beetle effects on leaf litter decomposition in soil. © 2015 Elsevier Masson SAS. All rights reserved.
Keywords: Cattle dung Abundance effects Litter decomposition Scarabaeidae Soil carbon
1. Introduction Dung beetles play an important role for soil nutrient cycling and soil structure where dung is present as a result of tunneling, dung shredding and dung burial (Yokoyama et al., 1991; Bang et al., 2005; Brown et al., 2010). Dung beetle activity is of particular interest in grazed ecosystems, where dung beetles reduce the areas covered by animal waste and contribute to the sustainability of livestock production by increasing pasture productivity and suppressing livestock parasites (Nichols et al., 2008). However, mounting evidence suggests that the diversity and abundance of dung beetles may face threats due to changes in land-use and climatic conditions (Nichols et al., 2008; Nervo et al., 2014). Understanding the linkages between dung beetle community attributes and ecosystem processes is therefore critical for the prediction of ecosystem function in a changing environment (Nichols et al., 2008; Beynon et al., 2012; Braga et al., 2013). Dung beetles are a diverse group of insects (Scarabaeidae,
* Corresponding author. E-mail addresses: [email protected] (T. Tixier), juliette.bloor@clermont. inra.fr (J.M.G. Bloor), [email protected] (J.-P. Lumaret). http://dx.doi.org/10.1016/j.actao.2015.08.003 1146-609X/© 2015 Elsevier Masson SAS. All rights reserved.
Aphodiidae, Geotrupidae) which feed on animal excreta (Hanski and Cambefort, 1991). Dung beetles can be classed into three main functional groups: endocoprids (dwellers) whose larvae feed in dung pats or at the dungesoil interface, paracoprids (tunnellers) who bury brood balls in vertical tunnels under the dung pat, and telecoprids (rollers) who transfer dung away from the pat before burying brood balls under the soil surface (Hanski and Cambefort, 1991)Shredding, feeding-on and relocation of dung by dung beetles during tunneling has a direct effect on dung disappearance from the soil surface (Holter et al., 2002; Yamada et al., 2007; O'Hea et al., 2010). Moreover, dung beetle activity may have indirect effects on the decomposition of soil organic matter (OM) and plant litter fragments via beetle-induced changes in soil conditions such as aeration and soil water holding capacity (Bang et al., 2005; Brown et al., 2010), and/or stimulation of microbial biomass due to local inputs of labile carbon from dung (Bardgett et al., 1998; Hatch et al., 2000). Previous work has shown that dung beetle effects may vary depending on species composition, functional group richness and species identity (Bang et al., 2005; Slade et al., 2007; O'Hea et al., 2010; Beynon et al., 2012). Experiments with species mixtures have also shown faster dung removal in the presence of higher dung beetle abundance (Yamada et al., 2007). In theory, the magnitude of dung beetle effects may vary across species due to
32
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differences in beetle burrowing activity or species body mass (Holter et al., 2002; Larsen et al., 2005; Nervo et al., 2014). Indeed, recent work suggests that effects of dung beetles on ecosystem function may reflect a balance between beetle abundance, biomass and presence of species with particular attributes (small-ve largebody size) (Braga et al., 2013). However, little is known about the influence of beetle species identity on the relationship between beetle abundance and dung removal, and indirect effects of dung beetles on leaf litter decomposition have never been assessed. Here we report a mesocosm study on the interactive effects of dung beetle species identity and beetle abundance on dung removal and decomposition of leaf litter in soil beneath dung pats. The study was conducted using three co-occurring dung beetle species which bury dung beneath the dung pat: Colobopterus erraticus (Linnaeus, 1758), Caccobius schreberi (Linnaeus, 1758) and Onthophagus vacca (Linnaeus, 1767). C. erraticus and C. schreberi are small-sized beetles (mean dry body mass of eight and seven mg respectively), whereas O. vacca is a large sized-species (mean body mass 41 mg) (Lumaret and Kirk, 1987). These three species are commonly found in the French Mediterranean region during the spring period (Lumaret and Kirk, 1987). We hypothesized that: (i) dung removal by dung beetles increases with increasing beetle biomass and abundance; (ii) dung removal by dung beetles varies depending on beetle species; (iii) high dung beetle biomass and abundance accelerates leaf litter decomposition in soil. 2. Materials and methods In April 2012, 51 experimental units were established in the CNRS-CEFE experimental field platform in Montpellier, France (43 380 16N, 3 51041, elevation 58 m). Each experimental unit comprised of a plastic pot (15 cm diameter, 25 cm deep) filled with a mix of fersiallitic brown Mediterranean soil and sand (pHH2O ¼ 6.8; 0.3% N for the soil/sand mix); the soil/sand mix was used for logistical reasons to be able to clearly distinguish dung, beetles and brood balls from the soil medium at the end of the study. Pots were placed in holes in the ground such that the surface of the pot was flush with the soil surface, and enclosed aboveground using 0.2 mm mesh cages (25 25 15 cm) nailed to the ground immediately adjacent to the buried pot. These cages were used to avoid i) dispersal of experimental beetles away from the pots and ii) colonization of the pots by macro- and mesoinvertebrates from the surrounding area. Litter decomposition was assessed following Bloor and Bardgett (2012) using 5 5 cm litterbags (25 mm mesh; ANKOM Technology, Macedon, NY, USA) filled with 0.5 g of standardised leaf litter (50:50 mix of Festuca rubra and Lolium perenne senescent leaf material, 1.6% N, 28C:N). Litter from each plant species was dried (60 C for 48 h), finely cut (1 mm), and combined to produce the standardised litter mix with equal dry mass of each plant species. One litter bag was inserted vertically into the soil of each pot to a depth of 5 cm, and 500 g of fresh, homogenised cow dung was then placed on the soil surface of each pot (dung composition 85.7% H2O, 2.7% N, 15.6C:N, collected from a nearby dairy farm). Dung was uncontaminated by antibiotics or anthelmintics since no veterinary medical products were administered to animals for at least 100 days before dung was collected. Adult dung beetles for use in the study were collected locally from grazed meadows using dung-baited pitfall traps, identified to species, sexed and stored in cool conditions for three weeks prior to the experiment. Four beetle density treatments were established per species. Beetles of C. erraticus Linnaeus, C. schreberi Linnaeus and O. vacca Linnaeus were added to mesocosms as 50:50 male:female pairs at densities of either two, four, eight or 12 individuals, consistent with variation in dung beetle abundance
observed in dung pats under natural conditions (Hanski and Cambefort, 1991). These four density treatments were replicated four times per species (total of 16 experimental units per species). Three remaining experimental units were kept as beetle-free controls. All experimental units were then left under natural rainfall conditions for five months. In September 2012, beetles were removed by hand from the experimental units. All remaining dung on the soil surface was collected, dried (48 h, 60 C) and weighed to determine residual dung dry mass. Organic matter content of this remaining dung was determined from loss of weight of dried dung samples after being ashed at 500 C for 12 h. Dung removal was calculated as the proportion of OM lost from the start to the end of the experiment following Holter (1979). Litter bags were retrieved from the soil, cleaned and dried (48 h, 60 C) before being weighed to determine litter mass loss during the five-month incubation period. Values for both dung removal and litter mass loss in the presence of beetles were corrected by values obtained in beetle-free controls (i.e. treatment values e mean control value) to separate out dung beetle effects. Effects of species identity and beetle abundance were assessed using two-way analysis of covariance (ANCOVA) with beetle species as a fixed factor and initial beetle abundance as a quantitative, independent variable, following Zar (1998). Effects of beetle biomass on dung removal and litter decomposition were examined using simple linear regression, and estimations of beetle biomass were based on mean beetle dry mass and initial beetle numbers. All statistical analyses were carried out using Statgraphics Plus 4.1 (Statistical Graphics Corp., Rockville, Maryland, USA). 3. Results Mean dung removal due to dung beetle presence ranged from 3.2 to 23.0% across species and abundance treatments over the fivemonth incubation period. Dung removal showed a positive relationship with dung beetle biomass estimated at the start of the study (R2 ¼ 0.56, P < 0.01; Fig. 1). In general, rates of dung removal increased with increasing beetle abundance across species treatments (Table 1, Fig. 2). However, dung removal also showed significant species abundance interactions (Table 1); dung removal increased with more individuals of C. erraticus, but showed no response to increasing abundance of C. schreberi or O. vacca (R2 ¼ 19.8, P < 0.05; Fig. 2). Leaf litter bags showed significant mass loss during the study (mean mass loss of 40.1% across treatments and controls), but very little of the litter mass loss in the beetle treatments could be directly attributed to beetle presence due to high litter mass loss in the control pots (Fig. 1). Nevertheless, leaf litter decomposition showed a positive relationship with initial beetle biomass (R2 ¼ 0.32, P < 0.05; Fig. 1) and increased with increasing beetle abundance across species treatments (Table 1, Fig. 2). As with dung removal, leaf litter decomposition showed significant species abundance interactions (Table 1). Litter mass loss increased in the presence of more C. erraticus individuals (R2 ¼ 0.88, P < 0.05; Fig. 2) but was unaffected by increasing abundance of C. schreberi. Litter decomposition showed a marginally significant positive relationship with increasing O. vacca abundance (P ¼ 0.08; Fig. 2). 4. Discussion The role of community attributes for ecosystem function has faced considerable interest in recent years due to growing concerns over altered species abundance and diversity patterns under global change (see Larsen et al., 2005; Slade et al., 2007; Nichols et al.,
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6
A Litter mass loss (%)
Dung removal (%)
25 20 15 10 5 0
33
B
4 2 0 -2 -4 -6
0
100
200
300
400
500
Beetle biomass (mg)
0
100
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300
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Beetle biomass (mg)
Fig. 1. Relationship between dung beetle biomass estimated at the start of the experiment and (A) dung removal, or (B) mass loss of leaf litter across beetle species and abundance treatments. Values for dung removal and litter mass loss are the difference between beetle treatments and beetle-free controls. Means (±1 standard error) are shown after five months of incubation; n ¼ 4. The dashed lines represent significant linear regressions between biomass and function (P < 0.05).
Table 1 Effects of species identity and dung beetle abundance on dung removal and leaf litter decomposition. F values derived from ANCOVA are shown: significant effects (P < 0.05) are given in bold type. Effect
Df
Variable Dung removal
Litter mass loss
Species Abundance Species Abundance
2,42 1,42 2,42
0.87 4.33 3.25
2.65 10.14 3.51
2008; O'Hea et al., 2010 for examples with dung beetles as focal organisms). Based on results from previous work on dung decomposition using mixtures of beetle species at high densities (Yamada et al., 2007), we hypothesized that both beetle abundance and beetle biomass would have a positive effect on dung removal across species treatments. Our results were consistent with these predictions and indicate that effects of dung beetles can occur even at relatively low beetle numbers. Increasing evidence suggests significant species-level variation in dung beetle effects on dung removal and nutrient recycling (Bang et al., 2005; Beynon et al., 2012), but interactions between species identity and abundance have faced little attention to date. In the present study, we focused on ‘tunneler’ species which bury dung beneath the dung pat (Hanski and Cambefort, 1991) and we show significant interspecific variation in beetle abundance effects
on dung beetle removal within this functional guild. Our findings indicate that impacts of declining beetle abundance on soil function and sustainable soil management in grazed systems may vary depending on the species involved. These results have implications not only for agro-ecology, but also for the evaluation of potential side-effects of veterinary medical products (often found in dung) on dung organisms. Surprisingly, the pattern of interspecific variation in abundance effects recorded here for dung beetle removal showed no clear link with beetle size. Positive effects of beetle abundance were observed for C. erraticus, a small-sized beetle, but no significant effects were observed for C. schreberi, a similar-sized species. Such species-level variation in abundance effects could reflect differences in reproductive activity or survival among species during the study period (Finn and Gittings, 2003). Strong effects of C. erraticus might also be explained in part by the relatively high dung shredding activities and mobile larvae of this species compared to C. schreberi; unlike C. schreberi whose larvae remain confined in the dung reserve of the pedotrophic nest, late instars of C. erraticus larvae may move between the soil and dung to get food supplies (Rojewski, 1983; Vitner, 2000). Our results support the hypothesis that increased dung beetle abundance and biomass promote leaf litter decomposition by microorganisms in soil. Response patterns of litter mass loss mirrored those of dung removal, consistent with increased microbial activity when increased carbon inputs from dung are present (Bardgett et al., 1998). However, although significant positive relationships
Fig. 2. Relationship between initial dung beetle abundance and (A) dung removal, or (B) mass loss of leaf litter in three beetle species treatments. Values for dung removal and litter mass loss are the difference between beetle treatments and beetle-free controls. Means (±1 standard error) are shown after five months of incubation; n ¼ 4. Dashed lines represent significant linear regressions between biomass and function for C. erraticus (P < 0.05).
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between beetle abundance and litter mass loss were observed, the magnitude of beetle effects on leaf litter decomposition was very small. An experimental design with more replicates and additional ‘high beetle abundance’ treatments may be required to detect the signature of indirect beetle effects on litter decomposition more clearly. Given the fine mesh-size of the litter bags, it is unlikely that mesofauna such as springtails and mites would have played an important role in leaf litter decomposition, but we cannot rule out the possibility that such organisms were involved in dung decomposition. To date, few studies have addressed interactions between dung beetles and mesofauna on dung removal and subsequent dung decomposition (but see Bertrand and Lumaret, 1984). In conclusion, positive relationships between overall dung beetle abundance and dung removal mask interactive effects between dung beetle abundance and species identity. These results suggest that the impacts of within-guild, intraspecific variation in dung beetles on dung removal and ecosystem services may be more complex than previously suspected. We also show that dung beetle abundance has a limited but positive effect on litter decomposition, and demonstrate similar patterns of response to variation in beetle abundance and species for both litter decomposition and dung removal. Future studies should determine whether the patterns observed here can be generalised to different times in the growing season and to different soil types with undisturbed soil structure under field conditions. Acknowledgements The authors thank Alexandre Salcedo for help setting up the experiment. This study was supported by a project grant from the German Federal Agency (UBA Project-No. FKZ 371063412) awarded to JPL. We thank the three anonymous reviewers whose comments contributed to improve the final version of the manuscript. References Bang, H.S., Lee, J.-H., Kwon, O.S., Na, Y.E., Jang, Y.S., Kim, W.H., 2005. Effects of paracoprid dung beetles (Coleoptera: Scarabaeidae) on the growth of pasture herbage and on the underlying soil. Appl. Soil Ecol. 29, 165e171. Bardgett, R.D., Keiller, S., Cook, R., Gilburn, A.S., 1998. Dynamic interaction between soil animals and microorganisms in upland grassland soils amended with sheep dung: a microcosm experiment. Soil Biol. Biochem. 30, 531e539. action des populations de microarthropodes Bertrand, M., Lumaret, J.P., 1984. Re a
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