Sluggish Invasion

The Iberian slug or killer slug as it has become popularly known is one among a hundred various species that’s been blacklisted as serious threats to the environment. The aggressive nature of these invasive species threatens an already thinly stretched balance, in the worst case resulting in a loss of biodiversity.

Pests
Alien species come in all shapes and sizes. Some are deliberately introduced and economically important, others have little impact but quite a few have been a disaster.

”They can quickly become a threat, either by reproducing at a wild and uninhibited rate and seize habitats, food sources or other resources, or by threatening existing species in the region”, says Hans Peter Ravn, Senior Scientist and a specialist in the fields of zoology/entomology, and biological diversity in woodlands, parks and landscapes.

“Iberian Slugs are usually considered as good for the environment, as they help decompose dead organic material. But as killer slugs often appear in such great numbers, they can cause massive damages on gardens and crops – and is thus considered by most people to be pests.”

In the Beginning
The by now infamous killer slug started to spread around Europe about 30 years ago, traveling as eggs in the soil of potted plants. It has been given its nickname, perhaps due to its tendency to eat dead or weaker individuals of the species, although its destructive impact on gardens may seem just as appropriate a reason for the name.

The main reason behind the problematic invasions of gardens by the Spanish slug is that it is a hermaphrodite, meaning that any snail can carry fertilized eggs and start an infestation with offspring numbering up to 100 eggs. When two snails meet and sweet music ensues, both animals will lay fertilized eggs after a period of gestation, which then proceed to hatch after a development period. Snails typically reproduce in early spring and late autumn.

Click here to see the expansion pattern of the Killer Slug by year of discovery. Includes both reported and unreported sightings of the slug. Initial images courtesy of Google Earth.

In Denmark, like so many places, the slug has most likely been spread by tourists who, without considering possible consequences bring home plants potentially ridden with fertilized snail eggs.

”Tourists who are abroad has a tendency to bring home plant-life they believe to be enticing to have in their garden. These plants may have dozens of fertilized eggs hidden under leaves or in the soil”, says Hans Peter Ravn.

“This is a common scenario that has been seen many times in various cases.”

DAISIE
The slug is just one example of a much wider threat to Europe's biodiversity as alien or non-native species establish and spread across the continent as a result of human activities. Most arrive as stowaways and are transported unwittingly around the globe; others have deliberately been put out in nature, without realization of potential calamities.
The UN Convention on Biological Diversity identifies the threat of invasive alien species as one of the major threats to biodiversity worldwide

The Delivering Alien Invasive Species In Europe program (DAISIE), is a project funded by the sixth framework programme of the European Commission. It provides information on biological invasions in Europe, delivered via an international team of leading experts in the field of biological invasions, latest technological developments in database design and display, and an extensive network of European collaborators and stakeholders.

The general objectives of DAISIE are to create an inventory of invasive species such as the Iberian slug, that threaten European environments, and asses and summarize the ecological, economic impact as well as health risks these invasive species might bring.

DAISIE has compiled a list of these threats, including the 100 most threatening species to the environment. This list named “100 of the worst” also includes the Comb Jelly originally from American waters, which now plague the Azov, Black, Caspian, North, Baltic and Mediterranean (north-eastern part) seas, and the Kamchatka or Red King Crabs, originally native to the Pacific Ocean, and introduced by Josef Stalin to the Bering Sea area in the 1930s to provide for tasty meals to the Russian people.

A threat to us all
Killer slugs are important plant defoliators. Outcompetes native slug species due to its large size and its high population densities and hybridizes with the native slug. In recent years, it has been rumored that the Spanish slug has bred with the indigenous black slug to produce a more frost-resistant variety in the more northerly regions, though nothing conclusive have been seen on that front yet.

It is the most important slug pest in Europe causing severe damage to the plant life in private and public gardens and cultivated crops in agriculture, thus aiding in the loss of biodiversity, we are now feeling on a global scale.

By Thorsten Weitling