- Email: [email protected]
Seed density and previous egestion affect earthworm seed ingestion: Preliminary observations using granivory of Alliaria petiolata by Lumbricus terrestris
Applied Soil Ecology 139 (2019) 29–31
Contents lists available at ScienceDirect
Applied Soil Ecology journal homepage: www.elsevier.com/locate/apsoil
Short communication
Seed density and previous egestion affect earthworm seed ingestion: Preliminary observations using granivory of Alliaria petiolata by Lumbricus terrestris
T
⁎
Michael J. McTavish , Stephen D. Murphy School of Environment, Resources and Sustainability, University of Waterloo, 200 University Avenue West, Waterloo N2L 3G1, ON, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Seed predation Density-dependent foraging Seed palatability Feeding experiment
With a growing body of research dedicated to understanding earthworm seed preferences, we conducted a preliminary investigation of two previously unconsidered factors: seed density and previous earthworm egestion. We used no-choice feeding experiments with the common European earthworm Lumbricus terrestris L. and seed of the biennial forb Alliaria petiolata (M. Bieb) Cavara & Grande. Earthworm seed ingestion increased exponentially when available seed density was doubled (+305%) and was reduced when encountering seed that had previously been egested by another earthworm (−30%). Impacts of seed density and previous earthworm egestion have broader ecological implications for earthworm granivory behaviour and earthworm-plant interactions in natural and managed ecosystems. We recommend both factors as promising candidates for future experimentation.
1. Introduction Earthworms are key soil biota in many terrestrial ecosystems around the world and their importance as granivores and seed dispersers is being increasingly recognized (Forey et al., 2011). The complex effects of earthworm-seed interactions on plant communities depend on the preferential ingestion of certain seeds over others and the various factors that influence seed ingestion (Shumway and Koide, 1994). Past research of earthworm seed ingestion has focused on the physical and chemical differences between seeds such as size, coat texture, and oil content (Clause et al., 2017; Forey et al., 2011). We have designed a preliminary study to investigate two factors that, to the best of our knowledge, have not been examined previously: seed density and previous earthworm egestion. Although density-dependent foraging is common to many taxa (Mitchell and Brown, 1990), it has not yet been documented for earthworm granivory. The frequently nocturnal and partially or fully belowground nature of earthworm feeding behaviour makes experimental study difficult and has limited our ability to describe how earthworms interact with resources such as seeds (Butt and Grigoropoulou, 2010). If earthworm granivory is strongly density-dependent, the relative ecological importance of earthworm-seed impacts from case to case will depend on the variability in seed density common to both natural seed rain and anthropogenic seeding (e.g., for ecological
⁎
restoration). Previous egestion of seeds by earthworms has similarly received no formal study that we are aware of. While egested seeds are generally considered to either germinate or decompose depending on their burial depth and physiological condition (Milcu et al., 2006; Regnier et al., 2008), re-ingestion may be an additional possibility with important ecological implications. Earthworm encounters with previously egested seed may be common due to high rates of egestion of ingested seeds (McRill and Sagar, 1973) and “stealing” of food resources from burrows of conspecifics (Butt et al., 2003). Previously egested seed may be more or less detectable or palatable for subsequent re-ingestion due to changes in the physical or chemical properties of seeds resulting from gut passage (Willems and Huijsmans, 1994). If previous egestion of seeds is followed by increased or decreased re-ingestion, the overall impacts of earthworms on the belowground fates of seeds may be more complex than formerly considered. The purpose of this study was to conduct preliminary tests of the impacts of two previously unstudied factors – seed density and previous earthworm egestion – on earthworm granivory using no-choice feeding experiments with Lumbricus terrestris L., a deep-burrowing anecic European earthworm frequently used in research (Addison, 2009; Eisenhauer et al., 2008), and seed of Alliaria petiolata (M. Bieb) Cavara & Grande, a biennial forb with seeds known to be ingested by L. terrestris (Cassin and Kotanen, 2016; Flinn, 2017).
Corresponding author. E-mail address: [email protected] (M.J. McTavish).
https://doi.org/10.1016/j.apsoil.2019.01.019 Received 14 August 2018; Received in revised form 14 January 2019; Accepted 17 January 2019 Available online 19 March 2019 0929-1393/ © 2019 Elsevier B.V. All rights reserved.
Applied Soil Ecology 139 (2019) 29–31
M.J. McTavish and S.D. Murphy
2. Materials and methods
Table 1 Results of no-choice feeding experiments assessing the effects of seed density (n = 20 replicate containers per density) and previous earthworm egestion (n = 17 replicate containers per seed history) on the ingestion of garlic mustard seeds by L. terrestris (number of seeds, % of seeds available). Seed ingestion is presented as mean ± standard deviation (SD).
We purchased mature L. terrestris from a commercial bait vendor and collected garlic mustard seeds from an urban woodlot (Waterloo, Ontario, Canada). Earthworms were subjected to a 24 h fast, weighed fresh, moved to an 18 h feeding period, then transferred to fresh containers for a 48 h egestion period (McRill and Sagar, 1973). We used Petri dishes (diameter = 15 cm) lined with filter paper (Grade 1, 11 μm), moistened with 10 mL of DI water, and placed randomly in a growth chamber (24 h dark, 80% relative humidity, 15 °C). In the seed density experiment, individual earthworms were offered a “low” density of 25 seeds∙container−1 or a doubled “high” density of 50 seeds∙container−1 (n = 20 replicate containers per density) chosen to reflect the range of densities used in other earthworm feeding experiments (Forey et al., 2011). In the previous egestion experiment, earthworms were offered 25 seeds∙container−1 that had no earthworm contact or that had previously been ingested and egested by a different L. terrestris during and collected following the earlier seed density experiment (n = 17 replicate containers per seed history). Seed ingestion was measured as the number of seeds removed during the feeding period and seed egestion as the percentage of ingested seeds found by hand searching casts collected during the egestion period. Given the estimated 8 h gut transit time of L. terrestris (Hartenstein and Amico, 1983), seeds not accounted for following the 48 h egestion period were likely digested following initial ingestion or else egested and re-ingested, the latter of which would make our reported seed egestion rates conservative estimates. Tests of association between earthworm fresh weight and seed ingestion or egestion were assessed for each treatment level using Pearson Correlation. The effects of seed density and previous earthworm egestion on seed ingestion and egestion were tested using One-Way ANOVA. The Anderson-Darling test and Levene's test were used to test for assumptions of normality and equal variance respectively to supplement visual assessment of residual plots. Means are reported as mean ± standard deviation (SD). All statistical tests were conducted at α = 0.05 in Minitab ® 18.1. Effect size for ANOVA models is reported as omega-squared (ω2) (Maxwell and Delaney, 2004).
Treatment
Seed density Previous egestion
Treatment level
“Low” “High” No contact Egested
Seeds available 25 50 25 25
Seed ingestion (no. of seeds) 8±5 34 ± 12 16 ± 5 11 ± 6
Seed ingestion (%) 33 67 62 43
± ± ± ±
22 24 21 23
concentrations of chemical cues used to detect and locate seeds (Clause et al., 2017; Willems and Huijsmans, 1994), or a behavioural response to increased food availability (Butt et al., 2003). Strongly density-dependent granivory may influence the role of earthworm-seed interactions in both naturally occurring plant communities and managed ecosystems. For example, earthworm granivory may have the greatest impacts on dominant plant taxa that produce higher quantities of seed, further contributing to their effects on overall plant community structure and function (Forey et al., 2011). Additionally, earthworm granivory may be a key consideration in the effective management of desired or undesired plant taxa (e.g., high fecundity weedy species) (Regnier et al., 2008) or the sudden introduction of large seed quantities when seeding for restoration. Furthermore, the “high” density trials produced the only significant correlation between earthworm weight and ingestion. While support for a general relationship between earthworm size and ingestion is surprisingly weak (Asshoff et al., 2010; Clause et al., 2017, 2011; Eisenhauer et al., 2010, 2009), our findings suggest that earthworm size may be a stronger predictor of ingestion when seed densities are higher and earthworm satiation becomes limiting. This may influence the relative contributions of different earthworm species to granivory when seed density is variable (Asshoff et al., 2010; Eisenhauer et al., 2009). We speculate that the reduced ingestion of previously egested seed may be due to physical or chemical changes seeds undergo passing through the earthworm gut that reduce earthworm detection or seed palatability (Willems and Huijsmans, 1994). Additionally, as earthworms use chemosensory cues from conspecifics for threat avoidance (Jiang et al., 1989), collective movement (Zirbes et al., 2010), and density-dependent habitat selection (McTavish et al., 2013), detection of other earthworms may similarly be altering foraging behaviour. These results suggest that the impacts of earthworm granivory do not necessarily stop after a single feeding encounter that results in either seed digestion or egestion. In this experiment, re-ingestion of previously egested seeds occurred at a lower rate of ingestion compared to fresh seeds, but still resulted in seed losses due to digestion and the creation of a new pool of twice egested seeds. Given that in our experiments previously egested seeds were collected only from other individuals after passing once through the gut, it would be interesting to experiment further with self-egested or multiply-egested seed to determine how seed survivourship and palatability may change after different histories of multiple ingestion and egestion. Further experimentation to determine how often earthworms encounter previously egested seed in natural systems will be necessary to assess overall earthworm impacts on the total pool of seed in a system. We emphasize that our experiments were a preliminary investigation that used a single pairing of earthworm and plant species, two factor levels for each of the predictor variables, and dish-based feeding trials that do not fully recreate natural feeding behaviours (Zaller and Saxler, 2007). While these results should not be over-generalized, our study provides a “proof of concept” demonstrating the effects of these two previously unconsidered factors on earthworm-seed interactions. Both factors introduce new ecological implications regarding
3. Results There was a statistically significant positive association between earthworm weight and seed ingestion in the “high” density treatments (Pearson Correlation, r(18) = 0.608, p = 0.004), with heavier earthworms ingesting more seeds. No significant correlations were found between earthworm weight and ingestion or egestion in any other treatments (data not shown). Seed ingestion was statistically significantly affected by both seed density (One-Way ANOVA, F1,38 = 73.45, p < 0.001, ω2 = 0.644) and previous egestion (One-Way ANOVA, F1,32 = 6.22, p = 0.018, ω2 = 0.133). For seed density, the number of seeds ingested was 305% greater in the doubled “high” density treatment compared to the “low” density treatment. For previous egestion, ingestion of seed that had been previously egested was 30% lower than that of seed that had no previous earthworm contact (Table 1). Egestion was not significantly affected by seed density (One-Way ANOVA, F1,38 = 1.69, p = 0.202) or previous egestion (One-Way ANOVA, F1,32 = 0.67, p = 0.419). Overall mean egestion across all trials was 68 ± 25% (n = 74). 4. Discussion While a positive association between seed ingestion and density was not particularly unexpected, the exponential increase in ingestion observed in this experiment (i.e., a four-fold increase for a two-fold increase in seed density) suggests L. terrestris granivory was strongly density-dependent. The increased ingestion may be due to a higher random encounter probability (Mitchell and Brown, 1990), higher 30
Applied Soil Ecology 139 (2019) 29–31
M.J. McTavish and S.D. Murphy
earthworm granivory behaviour and earthworm impacts on plant communities in natural and managed ecosystems. We propose that seed density and previous earthworm egestion are promising candidates for future experimentation using additional plant and earthworm species, a wider range of treatment levels, and a combination of laboratory and field trials.
Eisenhauer, N., Schuy, M., Butenschoen, O., Scheu, S., 2009. Direct and indirect effects of endogeic earthworms on plant seeds. Pedobiologia 52, 151–162. https://doi.org/10. 1016/j.pedobi.2008.07.002. Eisenhauer, N., Butenschoen, O., Radsick, S., Scheu, S., 2010. Earthworms as seedling predators: importance of seeds and seedlings for earthworm nutrition. Soil Biol. Biochem. 42, 1245–1252. https://doi.org/10.1016/j.soilbio.2010.04.012. Flinn, K.M., 2017. Invasive earthworms ingest and digest garlic mustard seeds at rates equal to native seeds. Northeast. Nat. 24, 413–420. https://doi.org/10.1656/045. 024.0403. Forey, E., Barot, S., Decaëns, T., Langlois, E., Laossi, K.-R., Margerie, P., Scheu, S., Eisenhauer, N., 2011. Importance of earthworm–seed interactions for the composition and structure of plant communities: a review. Acta Oecol. 37, 594–603. https:// doi.org/10.1016/j.actao.2011.03.001. Hartenstein, R., Amico, L., 1983. Production and carrying capacity for the earthworm Lumbricus terrestris in culture. Soil Biol. Biochem. 15, 51–54. https://doi.org/10. 1016/0038-0717(83)90118-9. Jiang, X.C., Wang, D., Halpern, M., 1989. Isolation and characterization of alarm pheromone from electric shock-induced earthworm secretion. Pharmacol. Biochem. Behav. 34, 213–221. https://doi.org/10.1016/0091-3057(89)90302-X. Maxwell, S.E., Delaney, H.D., 2004. Designing Experiments and Analyzing Data: A Model Comparison Perspective, 2nd ed. Lawrence Erlbaum Associates, Mahwah, New Jersey. McRill, M., Sagar, G.R., 1973. Earthworms and seeds. Nature 243, 482. https://doi.org/ 10.1038/243482a0. McTavish, M.J., Basiliko, N., Sackett, T.E., 2013. Environmental factors influencing immigration behaviour of the invasive earthworm Lumbricus terrestris. Can. J. Zool. 91, 859–865. https://doi.org/10.1139/cjz-2013-0153. Milcu, A., Schumacher, J., Scheu, S., 2006. Earthworms (Lumbricus terrestris) affect plant seedling recruitment and microhabitat heterogeneity. Funct. Ecol. 20, 261–268. https://doi.org/10.1111/j.1365-2435.2006.01098.x. Mitchell, W.A., Brown, J.S., 1990. Density-dependent harvest rates by optimal foragers. Oikos 57, 180–190. https://doi.org/10.2307/3565938. Regnier, E., Harrison, S.K., Liu, J., Schmoll, J.T., Edwards, C.A., Arancon, N., Holloman, C., 2008. Impact of an exotic earthworm on seed dispersal of an indigenous US weed. J. Appl. Ecol. 45, 1621–1629. https://doi.org/10.1111/j.1365-2664.2008.01489.x. Shumway, D.L., Koide, R.T., 1994. Seed preferences of Lumbricus terrestris L. Appl. Soil Ecol. 1, 11–15. https://doi.org/10.1016/0929-1393(94)90019-1. Willems, J.H., Huijsmans, K.G.A., 1994. Vertical seed dispersal by earthworms: a quantitative approach. Ecography 17, 124–130. https://doi.org/10.1111/j.1600-0587. 1994.tb00084.x. Zaller, J.G., Saxler, N., 2007. Selective vertical seed transport by earthworms: implications for the diversity of grassland ecosystems. Eur. J. Soil Biol. 43, S86–S91. https:// doi.org/10.1016/j.ejsobi.2007.08.010. Zirbes, L., Deneubourg, J.-L., Brostaux, Y., Haubruge, E., 2010. A new case of consensual decision: collective movement in earthworms. Ethology 116, 546–553. https://doi. org/10.1111/j.1439-0310.2010.01768.x.
Acknowledgments This work was supported by a Vanier Canada Graduate Scholarship (Vanier CGS) (McTavish) and via grants from the Canadian Foundation for Innovation, National Sciences and Engineering Research Council of Canada (NSERC), Ontario Ministry of Natural Resources and Forestry (OMNRF), Canadian Network for the Advancement of Research, Industry and Education (CANARIE), and the University of Waterloo via the Centre for Ecosystem Resilience and Adaptation (Murphy). References Addison, J.A., 2009. Distribution and impacts of invasive earthworms in Canadian forest ecosystems. Biol. Invasions 11, 59–79. https://doi.org/10.1007/s10530-008-9320-4. Asshoff, R., Scheu, S., Eisenhauer, N., 2010. Different earthworm ecological groups interactively impact seedling establishment. Eur. J. Soil Biol. 46, 330–334. https://doi. org/10.1016/j.ejsobi.2010.06.005. Butt, K.R., Grigoropoulou, N., 2010. Basic research tools for earthworm ecology [WWW document]. Appl. Environ. Soil Sci. https://doi.org/10.1155/2010/562816. Butt, K.R., Nuutinen, V., Sirén, T., 2003. Resource distribution and surface activity of adult Lumbricus terrestris L. in an experimental system. Pedobiologia 47, 548–553. https://doi.org/10.1078/0031-4056-00227. Cassin, C.M., Kotanen, P.M., 2016. Invasive earthworms as seed predators of temperate forest plants. Biol. Invasions 18, 1567–1580. https://doi.org/10.1007/s10530-0161101-x. Clause, J., Margerie, P., Langlois, E., Decaëns, T., Forey, E., 2011. Fat but slim: criteria of seed attractiveness for earthworms. Pedobiologia 54, S159–S165. https://doi.org/10. 1016/j.pedobi.2011.08.007. Clause, J., Forey, E., Eisenhauer, N., Seal, C.E., Soudey, A., Colville, L., Barot, S., 2017. Seed selection by earthworms: chemical seed properties matter more than morphological traits. Plant Soil 413, 97–110. https://doi.org/10.1007/s11104-016-3085-9. Eisenhauer, N., Marhan, S., Scheu, S., 2008. Assessment of anecic behavior in selected earthworm species: effects on wheat seed burial, seedling establishment, wheat growth and litter incorporation. Appl. Soil Ecol. 38, 79–82. https://doi.org/10.1016/ j.apsoil.2007.07.002.
31
Contents lists available at ScienceDirect
Applied Soil Ecology journal homepage: www.elsevier.com/locate/apsoil
Short communication
Seed density and previous egestion affect earthworm seed ingestion: Preliminary observations using granivory of Alliaria petiolata by Lumbricus terrestris
T
⁎
Michael J. McTavish , Stephen D. Murphy School of Environment, Resources and Sustainability, University of Waterloo, 200 University Avenue West, Waterloo N2L 3G1, ON, Canada
A R T I C LE I N FO
A B S T R A C T
Keywords: Seed predation Density-dependent foraging Seed palatability Feeding experiment
With a growing body of research dedicated to understanding earthworm seed preferences, we conducted a preliminary investigation of two previously unconsidered factors: seed density and previous earthworm egestion. We used no-choice feeding experiments with the common European earthworm Lumbricus terrestris L. and seed of the biennial forb Alliaria petiolata (M. Bieb) Cavara & Grande. Earthworm seed ingestion increased exponentially when available seed density was doubled (+305%) and was reduced when encountering seed that had previously been egested by another earthworm (−30%). Impacts of seed density and previous earthworm egestion have broader ecological implications for earthworm granivory behaviour and earthworm-plant interactions in natural and managed ecosystems. We recommend both factors as promising candidates for future experimentation.
1. Introduction Earthworms are key soil biota in many terrestrial ecosystems around the world and their importance as granivores and seed dispersers is being increasingly recognized (Forey et al., 2011). The complex effects of earthworm-seed interactions on plant communities depend on the preferential ingestion of certain seeds over others and the various factors that influence seed ingestion (Shumway and Koide, 1994). Past research of earthworm seed ingestion has focused on the physical and chemical differences between seeds such as size, coat texture, and oil content (Clause et al., 2017; Forey et al., 2011). We have designed a preliminary study to investigate two factors that, to the best of our knowledge, have not been examined previously: seed density and previous earthworm egestion. Although density-dependent foraging is common to many taxa (Mitchell and Brown, 1990), it has not yet been documented for earthworm granivory. The frequently nocturnal and partially or fully belowground nature of earthworm feeding behaviour makes experimental study difficult and has limited our ability to describe how earthworms interact with resources such as seeds (Butt and Grigoropoulou, 2010). If earthworm granivory is strongly density-dependent, the relative ecological importance of earthworm-seed impacts from case to case will depend on the variability in seed density common to both natural seed rain and anthropogenic seeding (e.g., for ecological
⁎
restoration). Previous egestion of seeds by earthworms has similarly received no formal study that we are aware of. While egested seeds are generally considered to either germinate or decompose depending on their burial depth and physiological condition (Milcu et al., 2006; Regnier et al., 2008), re-ingestion may be an additional possibility with important ecological implications. Earthworm encounters with previously egested seed may be common due to high rates of egestion of ingested seeds (McRill and Sagar, 1973) and “stealing” of food resources from burrows of conspecifics (Butt et al., 2003). Previously egested seed may be more or less detectable or palatable for subsequent re-ingestion due to changes in the physical or chemical properties of seeds resulting from gut passage (Willems and Huijsmans, 1994). If previous egestion of seeds is followed by increased or decreased re-ingestion, the overall impacts of earthworms on the belowground fates of seeds may be more complex than formerly considered. The purpose of this study was to conduct preliminary tests of the impacts of two previously unstudied factors – seed density and previous earthworm egestion – on earthworm granivory using no-choice feeding experiments with Lumbricus terrestris L., a deep-burrowing anecic European earthworm frequently used in research (Addison, 2009; Eisenhauer et al., 2008), and seed of Alliaria petiolata (M. Bieb) Cavara & Grande, a biennial forb with seeds known to be ingested by L. terrestris (Cassin and Kotanen, 2016; Flinn, 2017).
Corresponding author. E-mail address: [email protected] (M.J. McTavish).
https://doi.org/10.1016/j.apsoil.2019.01.019 Received 14 August 2018; Received in revised form 14 January 2019; Accepted 17 January 2019 Available online 19 March 2019 0929-1393/ © 2019 Elsevier B.V. All rights reserved.
Applied Soil Ecology 139 (2019) 29–31
M.J. McTavish and S.D. Murphy
2. Materials and methods
Table 1 Results of no-choice feeding experiments assessing the effects of seed density (n = 20 replicate containers per density) and previous earthworm egestion (n = 17 replicate containers per seed history) on the ingestion of garlic mustard seeds by L. terrestris (number of seeds, % of seeds available). Seed ingestion is presented as mean ± standard deviation (SD).
We purchased mature L. terrestris from a commercial bait vendor and collected garlic mustard seeds from an urban woodlot (Waterloo, Ontario, Canada). Earthworms were subjected to a 24 h fast, weighed fresh, moved to an 18 h feeding period, then transferred to fresh containers for a 48 h egestion period (McRill and Sagar, 1973). We used Petri dishes (diameter = 15 cm) lined with filter paper (Grade 1, 11 μm), moistened with 10 mL of DI water, and placed randomly in a growth chamber (24 h dark, 80% relative humidity, 15 °C). In the seed density experiment, individual earthworms were offered a “low” density of 25 seeds∙container−1 or a doubled “high” density of 50 seeds∙container−1 (n = 20 replicate containers per density) chosen to reflect the range of densities used in other earthworm feeding experiments (Forey et al., 2011). In the previous egestion experiment, earthworms were offered 25 seeds∙container−1 that had no earthworm contact or that had previously been ingested and egested by a different L. terrestris during and collected following the earlier seed density experiment (n = 17 replicate containers per seed history). Seed ingestion was measured as the number of seeds removed during the feeding period and seed egestion as the percentage of ingested seeds found by hand searching casts collected during the egestion period. Given the estimated 8 h gut transit time of L. terrestris (Hartenstein and Amico, 1983), seeds not accounted for following the 48 h egestion period were likely digested following initial ingestion or else egested and re-ingested, the latter of which would make our reported seed egestion rates conservative estimates. Tests of association between earthworm fresh weight and seed ingestion or egestion were assessed for each treatment level using Pearson Correlation. The effects of seed density and previous earthworm egestion on seed ingestion and egestion were tested using One-Way ANOVA. The Anderson-Darling test and Levene's test were used to test for assumptions of normality and equal variance respectively to supplement visual assessment of residual plots. Means are reported as mean ± standard deviation (SD). All statistical tests were conducted at α = 0.05 in Minitab ® 18.1. Effect size for ANOVA models is reported as omega-squared (ω2) (Maxwell and Delaney, 2004).
Treatment
Seed density Previous egestion
Treatment level
“Low” “High” No contact Egested
Seeds available 25 50 25 25
Seed ingestion (no. of seeds) 8±5 34 ± 12 16 ± 5 11 ± 6
Seed ingestion (%) 33 67 62 43
± ± ± ±
22 24 21 23
concentrations of chemical cues used to detect and locate seeds (Clause et al., 2017; Willems and Huijsmans, 1994), or a behavioural response to increased food availability (Butt et al., 2003). Strongly density-dependent granivory may influence the role of earthworm-seed interactions in both naturally occurring plant communities and managed ecosystems. For example, earthworm granivory may have the greatest impacts on dominant plant taxa that produce higher quantities of seed, further contributing to their effects on overall plant community structure and function (Forey et al., 2011). Additionally, earthworm granivory may be a key consideration in the effective management of desired or undesired plant taxa (e.g., high fecundity weedy species) (Regnier et al., 2008) or the sudden introduction of large seed quantities when seeding for restoration. Furthermore, the “high” density trials produced the only significant correlation between earthworm weight and ingestion. While support for a general relationship between earthworm size and ingestion is surprisingly weak (Asshoff et al., 2010; Clause et al., 2017, 2011; Eisenhauer et al., 2010, 2009), our findings suggest that earthworm size may be a stronger predictor of ingestion when seed densities are higher and earthworm satiation becomes limiting. This may influence the relative contributions of different earthworm species to granivory when seed density is variable (Asshoff et al., 2010; Eisenhauer et al., 2009). We speculate that the reduced ingestion of previously egested seed may be due to physical or chemical changes seeds undergo passing through the earthworm gut that reduce earthworm detection or seed palatability (Willems and Huijsmans, 1994). Additionally, as earthworms use chemosensory cues from conspecifics for threat avoidance (Jiang et al., 1989), collective movement (Zirbes et al., 2010), and density-dependent habitat selection (McTavish et al., 2013), detection of other earthworms may similarly be altering foraging behaviour. These results suggest that the impacts of earthworm granivory do not necessarily stop after a single feeding encounter that results in either seed digestion or egestion. In this experiment, re-ingestion of previously egested seeds occurred at a lower rate of ingestion compared to fresh seeds, but still resulted in seed losses due to digestion and the creation of a new pool of twice egested seeds. Given that in our experiments previously egested seeds were collected only from other individuals after passing once through the gut, it would be interesting to experiment further with self-egested or multiply-egested seed to determine how seed survivourship and palatability may change after different histories of multiple ingestion and egestion. Further experimentation to determine how often earthworms encounter previously egested seed in natural systems will be necessary to assess overall earthworm impacts on the total pool of seed in a system. We emphasize that our experiments were a preliminary investigation that used a single pairing of earthworm and plant species, two factor levels for each of the predictor variables, and dish-based feeding trials that do not fully recreate natural feeding behaviours (Zaller and Saxler, 2007). While these results should not be over-generalized, our study provides a “proof of concept” demonstrating the effects of these two previously unconsidered factors on earthworm-seed interactions. Both factors introduce new ecological implications regarding
3. Results There was a statistically significant positive association between earthworm weight and seed ingestion in the “high” density treatments (Pearson Correlation, r(18) = 0.608, p = 0.004), with heavier earthworms ingesting more seeds. No significant correlations were found between earthworm weight and ingestion or egestion in any other treatments (data not shown). Seed ingestion was statistically significantly affected by both seed density (One-Way ANOVA, F1,38 = 73.45, p < 0.001, ω2 = 0.644) and previous egestion (One-Way ANOVA, F1,32 = 6.22, p = 0.018, ω2 = 0.133). For seed density, the number of seeds ingested was 305% greater in the doubled “high” density treatment compared to the “low” density treatment. For previous egestion, ingestion of seed that had been previously egested was 30% lower than that of seed that had no previous earthworm contact (Table 1). Egestion was not significantly affected by seed density (One-Way ANOVA, F1,38 = 1.69, p = 0.202) or previous egestion (One-Way ANOVA, F1,32 = 0.67, p = 0.419). Overall mean egestion across all trials was 68 ± 25% (n = 74). 4. Discussion While a positive association between seed ingestion and density was not particularly unexpected, the exponential increase in ingestion observed in this experiment (i.e., a four-fold increase for a two-fold increase in seed density) suggests L. terrestris granivory was strongly density-dependent. The increased ingestion may be due to a higher random encounter probability (Mitchell and Brown, 1990), higher 30
Applied Soil Ecology 139 (2019) 29–31
M.J. McTavish and S.D. Murphy
earthworm granivory behaviour and earthworm impacts on plant communities in natural and managed ecosystems. We propose that seed density and previous earthworm egestion are promising candidates for future experimentation using additional plant and earthworm species, a wider range of treatment levels, and a combination of laboratory and field trials.
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Acknowledgments This work was supported by a Vanier Canada Graduate Scholarship (Vanier CGS) (McTavish) and via grants from the Canadian Foundation for Innovation, National Sciences and Engineering Research Council of Canada (NSERC), Ontario Ministry of Natural Resources and Forestry (OMNRF), Canadian Network for the Advancement of Research, Industry and Education (CANARIE), and the University of Waterloo via the Centre for Ecosystem Resilience and Adaptation (Murphy). References Addison, J.A., 2009. Distribution and impacts of invasive earthworms in Canadian forest ecosystems. Biol. Invasions 11, 59–79. https://doi.org/10.1007/s10530-008-9320-4. Asshoff, R., Scheu, S., Eisenhauer, N., 2010. Different earthworm ecological groups interactively impact seedling establishment. Eur. J. Soil Biol. 46, 330–334. https://doi. org/10.1016/j.ejsobi.2010.06.005. Butt, K.R., Grigoropoulou, N., 2010. Basic research tools for earthworm ecology [WWW document]. Appl. Environ. Soil Sci. https://doi.org/10.1155/2010/562816. Butt, K.R., Nuutinen, V., Sirén, T., 2003. Resource distribution and surface activity of adult Lumbricus terrestris L. in an experimental system. Pedobiologia 47, 548–553. https://doi.org/10.1078/0031-4056-00227. Cassin, C.M., Kotanen, P.M., 2016. Invasive earthworms as seed predators of temperate forest plants. Biol. Invasions 18, 1567–1580. https://doi.org/10.1007/s10530-0161101-x. Clause, J., Margerie, P., Langlois, E., Decaëns, T., Forey, E., 2011. Fat but slim: criteria of seed attractiveness for earthworms. Pedobiologia 54, S159–S165. https://doi.org/10. 1016/j.pedobi.2011.08.007. Clause, J., Forey, E., Eisenhauer, N., Seal, C.E., Soudey, A., Colville, L., Barot, S., 2017. Seed selection by earthworms: chemical seed properties matter more than morphological traits. Plant Soil 413, 97–110. https://doi.org/10.1007/s11104-016-3085-9. Eisenhauer, N., Marhan, S., Scheu, S., 2008. Assessment of anecic behavior in selected earthworm species: effects on wheat seed burial, seedling establishment, wheat growth and litter incorporation. Appl. Soil Ecol. 38, 79–82. https://doi.org/10.1016/ j.apsoil.2007.07.002.
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