Giant hogweed, American mink - two invaders in Czech nature

Giant hogweed, photo: Marie-Clarie, Creative Commons 3.0

So-called "invasive" species include plants and animals that have been brought from other parts of the world and harm native species, sometimes to the point of extinction. In today's Czech Science we look at two such aliens in the Czech Republic, the giant hogweed and the American mink, and at what exactly science can do to help tackle the problem.

The first record of the giant hogweed in this country comes from 1862 when it was first planted for ornamental purposes. Fifteen years later the plant was found growing in the wild. But a serious invasion of the giant hogweed, which can grow up to 4.5 metres in height, did not start until a few decades later.

"It took some time which is usually the case with invasive species or species that are not native to the region. They need time to adapt to local conditions and to get used to the habitats and to the native plants, so we estimate that it took about 80 years before the real invasion started. It was actually sitting there for 80 or 100 years doing nothing and the real invasion started in the 1940s and 1950s, which was also associated with a habitat change because these plants need disturbed habitats through which they can spread."

Petr Pysek, from the Institute of Botany of the Czech Academy of Sciences. Giant hogweed, like many other invasive plants, influences the invaded territories in many ways.

"Just imagine this plant grows to cover huge areas and there are almost no other species growing in these areas, so it occupies the space and changes the functioning of ecosystems, such as the nutrient regime or water regime. Simply there is no space for other, native, plants to create their communities. So the impact is quite obvious.

Giant hogweed,  photo: Marie-Clarie,  Creative Commons 3.0
"It also has impacts that are directly related to human health. It produces phototoxic saps that can create blisters, and human skin is sensitive to this. If it grows in riparian areas along river banks, it can increase erosion when it dies off in the autumn and so the soil gets washed into the rivers. So there are quite a wide range of impacts."

The Institute of Botany has taken part in an EU-funded project, called Giant Alien, which included seven other countries, in order to find a solution to the growing problem.

"It focused on achieving a broad understanding of how this plant is doing in both its native and invaded range in Europe. The plant is native to the Caucasus, so we also made some expeditions to the Caucasus to see how the plant behaves in its native area and we focused on the basic ecology of the plant: how many seeds it produces and what is the fate of the seeds once they are shed off the mother plant, what is the germination and what is the population dynamics, how many plants survive, what is the flowering pattern and so on."

The European project finished this year but scientists are still studying ways to curb the spreading of the giant hogweed.

Giant hogweed,  photo: GerardM,  Creative Commons 3.0
"Eradication is difficult. Imagine that an average plant produces 20,000-30,000 seeds, 90 percent of which germinate. If you start using these numbers and putting them into models, it gets wild. You find out that the landscape in the region of the Slavkovsky Forest where we had our study plots is actually completely full of seeds. But it's really difficult and the problem is that plant has a very good regeneration ability, and given this fecundity, this production of seeds and the fact that the plant is capable of self-pollination, you can have one single plant which gets dispersed to distant regions and it can start a new invasion.

"Also, the plant creates what we call seed banks. It means that some little portion of seeds persists in the soil for at least three years. So if you cut the plant and remove the individuals and don't come the following year, the population regenerates from the seed bank. And this can happen for several years. And a few regenerating plants can start a new invasion. Also, the plant is what we call a monocarpic plant. It flowers once in a lifetime and then dies. But if you keep it under grazing pressure and don't let if flower, it can postpone flowering for ten, twelve years. It has a very plastic strategy how to survive in invaded sites."

As Petr Pysek says complete eradication would be best achieved using biological control - which means making use of the plant's natural enemies. The European project studied these enemies in the original habitat of the giant hogweed in the Caucasus and looked into whether they could be used in Central Europe to control the expansion. However, any such interference in the environment could be extremely risky.

American mink,  photo: Marc Evans,  Creative Commons 2.0
A typical animal invader is the water-loving American mink. Originally, it was imported to Europe for its lustrous fur. The first fur farm was founded around 1925. Animals often escaped from the farms, and for example in the former Soviet Union thousands of mink were released to enrich the choice of wild fur animals. The first data about wild American mink in this country comes from the 1960s.

"The mink is very adaptable, it needs to be close to water but it can also adapt to urban environments. It is also very adaptable when it comes to food. It feeds on fish, amphibians, birds, mammals and reptiles. On top of that mink can travel as far as twenty kilometres in a single night. Compared to its domestic relatives that usually have around 4-5 young per litter, mink can have up to eight. All that plus the fact that it has no natural predator means it spreads and multiplies so quickly."

Vaclav Hlavac from the State Agency for Nature Conservation and Landscape Protection leads a project which monitors mink in Southeast Bohemia.

"Mink do not cause great economic damage but they have a bad effect on the indigenous species. They compete with other members of the weasel family, like the European polecat, and have pushed out the native European mink. In some areas of the Czech Republic it took the American mink 2-3 years to decimate populations of the critically endangered stone crayfish and dice snake. They are also a threat to water birds. The invasion of the mink in the Czech Republic is quite recent so we don't yet know its full implications. But it is sure mink are a great risk for our wildlife."

In 1995 mink were believed to inhabit 5 percent of the Czech Republic; in 2000 the figure was 30 percent. By now they have probably spread around the whole country.

"Mink are difficult to watch. Their track is very similar to the polecat or weasel. So you need to rely on what you see or catch. The population is expanding not only in terms of area but also number. Telemetric studies have shown that mink don't stay in one territory, especially in the mating season. Their number is estimated at tens of thousand, maybe it has exceeded 100,000. Just for comparison, the otter population is estimated at 2,000 animals."

In order to monitor the mink's behaviour closely, Vaclav Hlavac and his team started using radio transmitters to keep an eye on the invader.

"This method enables us to monitor all the animal's activities. Because mink often crawl through hideouts in the roots underwater, the transmitter needs to be implanted. It's so small it doesn't bother the animal but it enables us to see where the animal is around the clock. If we know where the animal has been moving, we can pick up its droppings and analyse what it has been eating. This method is very efficient but also time consuming."

American mink,  photo: Brendan Lally,  Creative Commons 2.0
The findings are supposed to help conservationists curb the mink population. As in the case of other invasive and introduced species, eradication is proving difficult.

"Mink are difficult to trap because they are so alert. Besides, we have to be careful not to confuse mink with polecats and otters. What we need is to amend the legislation, to include mink on the list of game. Being such a recent appearance, it does not legally exist."

At the moment, Vaclav Hlavac says, it is vital to keep the mink population under control in areas of biological value where it threatens to wipe out rare native species. The only places which achieved complete eradication were a few islands in the North Sea. As conservationist Vaclav Hlavac says, in continental Europe it is most probably impossible.