Grazing animals mow meadows to useful effect. From the results of experiments on newly established grassland, one such grazer, the little-considered slug, evidently has a big and beneficial influence on plant diversity.
Large mammalian grazers are allies of the conservation biologist. Sharp incisors, grinding molars and selectiveness in their choice of vegetation have made sheep, cattle and horses invaluable for grassland managers concerned with enhancing diversity or encouraging particular plant species.
But what of invertebrate grazers? Grasslands support large populations of small herbivores, from aphids to grasshoppers: what role do these lesser grazers play in the development of the plant composition of grassland habitats? This question lies at the heart of an experimental study, conducted by Buschmann and colleagues, and published in Functional Ecology1, on the impact of slugs on the development of an area of sown grassland near Zurich in Switzerland.
Sir Arthur Tansley2 recognized the significance of grazing in determining the structure and composition of grasslands when he published his first survey of British vegetation types in 1911. His attention was mainly directed at sheep and rabbits, which then roamed in large numbers over the chalk hills of southern Britain. Such grazers remove a substantial proportion of grass production, but also, and perhaps even more importantly, graze selectively on preferred species. Invertebrate grazers might be expected to operate in a similar, if less apparent, fashion. Biomass removal, within certain limits, can have a positive effect on overall diversity by restricting the development of highly productive and aggressive plants, thus giving the smaller and slower-growing species an opportunity to germinate and become established3. The outcome of such selectivity depends on which members of the plant assemblage prove most attractive to the grazer.
Buschmann and colleagues concentrated their attentions on slugs, in particular Arion lusitanicus (Fig. 1), a large species (up to 10 cm in length) of western and southwestern Europe and a generalist herbivore. Even slugs have their dietary preferences, as any gardener can testify, and the seedling stages of plant development are often the most vulnerable to slug attack. So it is reasonable to suppose that slug impact on vegetation composition would be most profound in the early stages of plant colonization. In the more established stages, on the other hand, the effect of dietary selectivity may be less, but the possible effect of general biomass removal could still influence grassland structure. To test these hypotheses, the researchers set up an experimental grassland sown with rye grass (Lolium perenne) and white clover (Trifolium repens) on a former arable field that contained its own residual seed bank of weed and other plant species.
The surface soil was thoroughly mixed to avoid local patchiness in the seed bank, and a series of experimental 2×2-m plots was established, each surrounded by a slug-proof fence. Local slugs were placed in selected plots at a density of 22 individuals per plot during the first year, with an additional 10 slugs in subsequent years; this represents a high but realistic concentration of the molluscs. Wooden slug shacks provided shelter for these easily desiccated creatures in times of drought. The control plots were treated with molluscicide to prevent any inadvertent slug invasion. Analysis of the vegetation composition over the following three years provided the data needed to determine the effect of slug grazing.
In the first two years, the species richness and the diversity were lower in the slug-grazed plots than in controls. (Species richness is the number of species per plot; diversity also takes into account the proportions of different species, and is measured by the Shannon diversity index.) This result confirms the expectation that slug selection of seedlings would reduce the number of species from the local seed bank that become established. In the third year of the experiment, however, species richness in the grazed plots was 23% higher than in the controls.
The reason for this enhancement of richness and diversity in the more mature stages can be attributed to the consistent removal of biomass by the slugs. The yield from primary productivity was reduced by around 25% as a result of slug grazing (comparable to the removal of biomass by sheep in a grazed pasture4). Holding back the development of dominance by fast-growing species provided an opportunity for the germination and establishment of less-competitive species, including annual plants. In other words, slug grazing permits the establishment of plant species that might otherwise find it difficult to maintain populations in developing grassland. So, on this account at least, slugs are good for diversity.
Slugs will never act as sheep substitutes by creating a pastorally idyllic landscape and inspiring poets. But they could well be an answer to the conservationist's prayer — silently grazing beneath our feet, they provide an alternative way to mow a meadow.
Buschmann, H., Keller, M., Porret, N., Dietz, H. & Edwards, P. J. Funct. Ecol. 19, 291–298 (2005).
Tansley, A. G. (ed.) Types of British Vegetation (Cambridge Univ. Press, 1911).
Grime, J. P. Plant Strategies, Vegetation Processes, and Ecosystem Properties (Wiley, Chichester, 2001).
Perkins, D. F. in Production Ecology of British Moors and Montane Grasslands (eds Heal, O. W. & Perkins, D. F.) 375–395 (Springer, Heidelberg, 1978).