FSD 2010 - Earthworm

5th International Symposium-Workshop on Frugivores and Seed Dispersal (1985-2010)

Earthworms are the Seed Dispersers of the Month

Earthworm is the common name for the largest members of Oligochaeta (which is either a class or subclass depending on the author) in the phylum Annelida. (Wikipedia). Earthworms travel underground. Thus earthworm activity aerates and mixes the soil, and is constructive to mineralization and nutrient uptake by vegetation. Their role as seed dispersers has been overlooked, and recent studies bring some light on this other ecological role in the darkness of soil depth, especially for the composition of seed bank. Today, Anecic earthworms start to be recognised as important dispersers and predators of plant seeds. Eisenhauer et al (2008) write : "Endogeic earthworms ingested and digested all of the studied plant seeds; however, both ingestion and digestion were earthworm and plant species specific. Moreover, passage through the gut of endogeic earthworms and their excreta modified plant seed germination". As a consequence, these earthworms may impact the composition of the soil seed bank and plant community assembly via direct and indirect effects on plant seeds. (Photo © Nico Eisenhauer ; Lumbricus terrestris and a plant seedling displaying the strong interactions between anecic earthworms and plant seeds and seedlings)

In another study, Regnier et al. (2008) conducted experiments to determine how seed foraging by the European exotic burrowing earthworm Lumbricus terrestris L., affected Ambrosia trifida (a highly damaging annual weed in the U.S. Corn Belt) seed burial and seedling recruitment, and to determine seed selectivity by L. terrestris. They found that Earthworms collected and buried over 90% of A. trifida seeds placed on the soil surface at a rate eightfold faster than abiotic seed burial. Earthworm may store seeds underground in caches as high as 127 seeds per burrow. Secondary dispersal of the native plant’s seeds in earthworm burrows may increase seed survival and contribute to the success of this weed. This result and other showed that this novel interaction of L. terrestris and A. trifida increases seed bank formation of A. trifida. (Photo © Kent Harrison : Lumbricus terrestris grips the seed of A.trifida and drags it toward its burrow.)

In the tropics, while studying the short-term influence of dung deposition and the further redistribution of dung by dung beetles under a resting place of the red howler monkey (Alouatta seniculus), Pouvelle et al. (2008) found that the composition of the soil matrix and the topsoil became more homogeneous by losing its litter, its content in earthworm faeces increasing in the course of time. Present knowledge of the effect of Earthworm on seed bank in tropical rainforests remains unknown (but see Decaëns et al., 2003; Jiménez & Decaëns 2004 for tropical savanna), and might be as vital for some minute seeds as observed in the temperates for some small-seeded earthworm-dispersed plants.

References

Decaëns T et al. 2003 Seed dispersion by surface casting activities of earthworms in Colombian grasslands. Acta Oecologica 24:175-185.

Eisenhauer, N et al. 2008. Animal ecosystem engineers modulate the diversity-invasibility relationship. PLoS ONE 3(10) e3489, 1-8.

Eisenhauer, N et al. 2009. Direct and indirect effects of endogeic earthworms on plant seeds. PEDOBIOLOGIA 52:151-162.

Eisenhauer, N. et al. 2009. Earthworm and belowground competition effects on plant productivity in a plant diversity gradient. Soil Biology and Biochimestry 161:291-301. (Full article)

Jiménez JJ, Decaëns T 2004. The impact of soil organisms on soil functioning undre neotropical pastures: a case study of a tropical anecic eartworm species. Agriculture, Ecosystems & Environment 103:329-342.

Milcu A, Schumacher J, Scheu S. 2006. Earthworms (Lumbricus terrestris) affect plant seedling recruitment and microhabitat heterogeneity. Functional Ecology 20:261-268.

Pouvelle, S et al. 2008. Topsoil as Affected by Dung Deposition Under Resting Places of Red Howler Monkey (Alouatta seniculus). Pedosphere 18:691-698.

Regnier, E et al. 2008. Impact of an exotic earthworm on seed dispersal of an indigenous US weed. J Applied Ecology 45:1621-1629. (Photo © Kent Harrison : Lumbricus terrestris grips the seed of A. trifida and drags it toward its burrow.)

Willems, JH, Huijsmans KGA. 1994. Vertical seed dispersal by earthworms - A quantative approcach. Ecography 17:124-130.

Zaller JG, Saxler N 2007.. Selective vertical seed transport by earthworms: Implications for the diversity of grassland ecosystems. European Journal of Soil Biology 43, Supplement 1, S86-S91.

Acknowledgements

Thanks to Emilie Regnier, Nico Eisenbauer and François Feer for pictures and additional references.