Of Slugs and Worms
Worms live underground and slugs above ground. Yet they may affect one another in ways not obvious. The lowly earthworm, well known for conditioning and improving soil, is great at protecting leaves from being chomped by slugs, suggests research in BioMed Central’s open access journal BMC Ecology. Although they lurk in the soil, they seem to protect the plants above ground. Increasing plant diversity also decreases the amount of damage slugs do to individual plants.
Slug is a common name for an apparently shell-less terrestrial gastropod mollusk. The great majority of slug species are harmless to humans and to their interests, but a small number of species are serious pests of agriculture and horticulture. They can destroy foliage faster than plants can grow, thus killing even fairly large plants.
The major benefits of earthworm activities to soil fertility are biological, physical and chemical.
In many soils, earthworms play a major role in the conversion of large pieces of organic matter into rich humus, thus improving soil fertility. When the worm excretes what it has eaten this in the form of casts, deposited on the surface or deeper in the soil, minerals and plant nutrients are changed to an accessible form for plants and thus provide excellent fertilizer.
The earthworm's burrowing creates a multitude of channels through the soil and is of great value to maintaining the soil structure, which allows the processes of aeration and drainage to occur.
Spanish slugs (Arion vulgaris) are among the top 100 worst alien species in Europe and are considered a pest almost everywhere. Arion vulgaris inhabits cultivated habitats of any kind, as well as natural habitats such as river and lake margins, margins of forests, forests in valleys or moderately dry meadows. In Switzerland it has been found up to 1700 m altitude.
A team of scientists from the University of Natural Resources and Life Sciences Vienna investigated what effect the presence of earthworms and plant diversity would have on the amount of damage these slugs caused.
Using large incubators to simulate grassland environments the researchers could regulate the diversity of plant species and time the introduction of earthworms and slugs. They found that the presence of worms increased nitrogen content of plants and reduced the number of leaves damaged due to slugs by 60%. Yet when they compared leaf area damaged the researchers found slugs also ate 40% less at high plant diversity than at low.
Explaining their results Dr Johann Zaller, who led the study, said, "Our results suggest that two processes might be going on. Firstly, earthworms improved the plant’s ability to protect itself against slugs perhaps through the build-up of nitrogen-containing toxic compounds. Secondly, even though these slugs are generalists they prefer widely available food and in high diverse ecosystems slugs eat less in total because they have to switch their diets more often since plants of the same species are less available. Therefore gardeners are to help protect earthworms by increasing plant diversity in the garden in order to keep slug damage low. In order to elucidate the mechanisms behind these complex interactions, all parts of an ecosystem need to be investigated."
For further information see Slugs and Worms.
Worm image via Wikipedia.