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FAQ (frequently asked questions)

This section aims at providing answers to several questions frequently asked concerning the invasive plants issue. These answers are not exhaustive and could be more fully developed. A few topics are controversial and opinions may differ following public perception. Here we present points of view shared by scientists involved in invasion biology. This section was prepared with the kind collaboration of Arnaud Monty (Biodiversity & Landscape Unit, ULg GxABT), Etienne Branquart (Cellule Espèces Invasives, Département de l'Etude du Milieu Naturel et Agricole) and Sonia Vanderhoeven (Belgian Biodiversity Platform).

1) What is an invasive plant (or invasive alien plant)?

Scientists have defined invasive plants following precise criteria. An invasive plant is a vegetal species (1) introduced by man intentionally or accidentally outside its natural distribution area (i.e. a so-called "alien" species); (2) capable of maintaining viable populations in natural habitats; (3) with important dispersal potential in the environment and (4) likely to form dense populations outcompeting indigenous species (and therefore having a negative impacts on biodiversity). Only species meeting all these criteria can be called "invasive". According to this definition, only a small part of alien plants introduced into Belgium really become invasive. Currently, out of the thousands of non-native plants having been imported into Belgium, only some sixty species are considered as invasive.

2) What is invasion biology?

Invasion biology is the scientific discipline studying invasive alien species, all biological groups included (animal and vegetal), as well as the processes inducing those species to become invasive.  Invasion biology integrates many fields of knowledge (biology, ecology, evolution biology, genetics, sociology, economics, …) because of the varied aspects examined: species biology study, impact assessment, definition of control methods, risk detection and assessment, etc. It is a relatively recent science since the first book specifically dedicated to this matter was published in 1958: The Ecology of invasions by Animals and Plants by the British ecologist Charles Elton (1900 – 1991). But it was not until the 1990s that the discipline really took off. Several international science magazines are currently specialised in the publication of such research fields.  In Belgium, research programmes have also been launched in the 1990s.

3) According to which criteria are invasive species classified in black list or watch list? Who makes this assessment?

Species are classified within the Harmonia information system, developed by the Belgian Biodiversity Platform. Their assessment is made by some twenty experts from different Belgian research institutions and universities.  Each species is assessed according to its invasion level in natural habitats and its impact on indigenous species and ecosystem processes. You will find more information on species assessment in the explanatory note on the Harmonia information system or on the website

4) What is the difference between an invasive plant and an indigenous plant with an expanding character?

Just like invasive alien species, some native species may become dominant in the vegetation and form very dense populations likely to outcompete other species. If so, they are often considered as undesirable in the same way as invasive alien species. In our regions, this is the case for bracken fern (Pteridium aquilinum) or bramble (Rubus fruticosus) in forests, purple moor-grass (Molinia caerulea) on peaty soils, tor-grass (Brachypodium pinnatum) or blackthorn (Prunus spinosa) on chalky grasslands, buckthorn (Hippophae rhamnoides) in dunes or common nettle (Urtica dioica) along rivers. Whether alien or native, all those species pose problems in numerous habitats. Most of them are very opportunistic and capable of taking advantage from disturbance, trampling or nitrogen enrichment of the habitat. Some experts even consider those plants rather as a symptom than as a cause of habitat alteration. It is not because problems with native and alien invasive species are similar that the introduction of new alien invasive species causes small damage to the environment. Their expansion is an additional difficulty for land managers on the field. This phenomenon seriously complicates management of natural habitats and actively contributes to their alteration.

5) What makes a plant to become invasive outside its original distribution area?

Although mechanisms are not yet fully understood, several theories have been put forward. One of these theories assumes that some non-native plants become invasive because they have been introduced in our regions without their natural enemies (phytophageous organisms and pathogens). In the absence of those enemies, they grow faster and gain in size, especially because they can allocate more resources to growth than defense. This is how they become more competitive. That mechanism lies at the basis of two scientific hypotheses called ERH (Enemy Release Hypothesis) and EICA (Evolution of Increase Competitive Ability). Moreover, some invasive plants release toxic substances (allelopathic substances) in the soil which inhibit the growth of other plants (this is the case for Fallopia japonica, F. sachalinensis, F. x bohamica, Ailanthus altissima, Solidago canadensis and S. gigantea, Ludwigia grandiflora and L. peploides). Such advantages cause excessive development of those species, enabling them to become dominant in ecosystems.

6) Is the spread of invasive plants not rather a symptom than a cause of alteration of our habitats?

It is true that invasive plants frequently establish and expand in disturbed environments, i.e. lands exposed to disturbance like eutrophization (pollution by nitrates and phosphates), soil removing, plant cutting, fires, floods, erosion, etc. Such disturbances are often induced by human activities. Waste lands, rubble, ballast grounds, polluted soils and rivers offer ideal homeland for invasive plants, which are pioneer species taking advantage of such disturbance regimes. They are well adapted to that kind of habitats and settle faster than native species. This is why disturbed areas are more vulnerable to invasions. In some cases, indeed, man unintentionally favours plant invasion by degrading the habitat. However, invasive plants do not exclusively colonize disturbed areas. They also settle in high ecological value natural habitats where they can cause major damages.

7) Is control invasive species not some kind of ecological racism?

As stated in question 1, most alien species introduced in our countries do not cause any problem and have no invasive potential. The majority of plants grown in our fields and orchards originate from other parts of the world and no one would think of questioning the services they provide us every day. Far from tempting to eradicate all non-native species, control of invasive organisms only focuses on a few species like Japanese knotweed or water pennywort, which are capable of causing damages because of their ability to establish dominant populations in semi-natural habitats. This has absolutely nothing to do with xenophobia.

8) Is the growth of so-called invasive species not some kind of a natural process? Finally, is it not normal that those plants are developing in our regions?

It is important to make a distinction between natural evolution of the species (in the way evolution biology considers that concept) and disturbance or modifications caused by human activities. The appearing and development of the different species on Earth is a complex and relatively slow process, which is measured over paleontological times. This phenomenon relies in particular on natural selection and survival of the most adapted individuals, a theory developed by Charles Darwin around 1859. That theory is based on natural factors (outside human intervention) causing species to adapt to their environment and coexist within an ecosystem, reaching some kind of ecological balance between populations self-regulated by means of competition, predation, mutual assistance or parasitism.The problem of biological invasion is totally different. In that issue, man has brought species together which would not have come in contact naturally. He has introduced species outside their original area, i.e. in ecosystems where they have not evolve naturally, so to say, breaking by this the above mentioned fragile balance. All that happened in a short period of time, in stark contrast to the slow natural evolution of the species. Such rapid, brutal and repeated introductions do not leave time for native species to adapt in order to counter the invasion.

9) Can biological invasions occur without human intervention?

There are, indeed, cases of intercontinental migrations (dispersal on very large distances) of vegetal species outside any kind of human intervention. Such migrations are often facilitated by large scale climatic or geological phenomena such as hurricanes, glacier retreat, continental drift, etc. However, in comparison with natural migrations, cases of man-induced displacement of species are much more frequent, over much longer distances and with a greater variety of organisms involved. Therefore, they considerably speed up planetary mixing of living beings and largely contribute to homogenization of biodiversity on the Earth's surface.

10) What are the consequences of global warming on the distribution of invasive species?

Belgian climate is supposed to warm up by 1 to 3 degrees by 2050. That scenario, considered as highly probable by most experts, is likely to deeply modify the distribution of vegetal species in our country. Everywhere in Low and Middle Belgium, oak (Fagus sylvatica) and other native draught sensitive species will largely weaken. Some non-native species which are already present on our territory will probably spread on larger areas (as it has already been demonstrated for species like butterfly bush, cherry laurel, black locust, etc.). Other ones, still unable to establish nowadays (common ragweed, pampas grass) may then find proper conditions to develop in our country. The problem of biological invasions is therefore likely to intensify in the future.

11) Some exotic plants have been introduced in our country since more than one century without ever causing damage to the environment. Might they become invasive one day?

It is hard to answer that question. Some non-native ornamental plants start spreading in the environment more than one century after they have been introduced in our parks and gardens. This is the case for walnut and black locust, of which a growing number of young plants have been observed lately in our semi-natural habitats.  An adaptation period more or less long, known as the lag phase, always separates the initial introduction of a plant from the time when it becomes truly invasive. That period varies according to species and environmental conditions. It can be a few decades or more than one century (an average of 147 years has been calculated for ligneous species). This phenomenon is almost unpredictable and no one can predict now with certainty which non-native species will be invasive in the future.

12) Is there also a risk for cultivars of an invasive species to become themselves invasive?

First of all, what is a cultivar? A cultivar is a cultivated form or type of a plant that has been deliberately selected through breeding programs for one (or several) desirable characteristic(s) from an aesthetical (colour of the flower, form of the leaves, ...) or technical (growth speed, disease resistance...) point of view. Lots of cultivars are created in ornamental horticulture.It is true that traits selected for creating cultivars may confer or even enhance invasiveness. Studies have shown, indeed, that characteristics such as resistance to diseases or pathogens, rusticity (resistance to cold), high amount of flowers, easy multiplication, germination quickness or size gain are particularly sought after by horticulturists. Improving those characteristics also favours their invasive potential. Lots of purely aesthetical characteristics are also sought after, such as colour and form of the flowers, leaves or stems. These characteristics, for their part, do not alter the reproduction capacities of the plant and do not modify, a priori, their invasive potential. However, certain cultivars may become less invasive, such as dwarf cultivars, late-blowing cultivars, sterile cultivars or cultivars with a reduced fertility. The issue of the invasive potential of cultivars is complex and is the subject of a lot of research work.

13) Surprisingly, Jerusalem artichoke (Helianthus tuberosus) is included  in the black list. This is an old vegetable variety which was once popular and is now coming "back into fashion". Why is it considered as invasive?

Jerusalem artichoke originates from North America. It was once (and still is) used as a food plant. Its tubers are eatable. Its culture was abandoned by farmers after World War II. But the species went on growing in the wild and has spread until now, particularly because if its tubers which are extremely resistant to cold, enabling the plant to easily regenerate in spring. Important populations have been observed in Central Europe. Jerusalem artichoke is mentioned on the list of invasive species in France, Austria, Germany, Slovakia, Poland. It grows preferably along riverbanks. It can form thick populations like those of Japanese knotweed, preventing native flora from growing. In Belgium, an important population can be found on the banks of the non-navigable part of the Meuse river, near the Dutch border. Actually, there are several cultivars or varieties of Jerusalem artichoke: ornamental Jerusalem artichokes, Jerusalem artichokes used as vegetable and Jerusalem artichokes used as biofuel. Currently, there is uncertainty about the identification of the invasive variety of Jerusalem artichoke found in natural habitats. Scientists have not yet answered that question, which certainly deserves more detailed research.

14) There has been buddleia in my garden for years. Still, this plant does not extend. Moreover, it is a beautiful shrub attracting butterflies. How can that species be problematic?

Buddleia is presently classified in the watch list. Its environmental impact is moderate. It is invasive in dry habitats like wastelands, quarries, railway embankments, abandoned lands. Those are habitats easily colonized by buddleia. Most of all, it is an invasive plant in urban areas, resistant to soil and air pollution. It may not extend in gardens if the environment does not favour its expansion. But buddleia easily escapes from gardens. One single individual can produce up to 3 million seeds, disseminated by wind (95% of the seeds fall back more than 10 metres away from the mother plant). In the above mentioned habitats, it can form very thick and impenetrable shrub populations, leaving few spaces for other vegetal species to grow.

15) They are some watch list species of which I have never seen an invaded site in nature. How can they be stated as invasive while there are no spectacular invasions to notice?

Some species may be in the beginning of the invasion process in Belgium. They do not form (yet) important populations in the wild. But their invasive potential is described abroad, in environmental conditions which can be found in Belgium. Those species might currently be in the lag phase (see question 11). Maybe one day their populations will start expanding, or maybe they will not. No one can predict this with certainty. By definition, species included in the watch list should be watched over because in the future they may develop extensively in natural habitats and have a greater impact on biodiversity. Other species on that list are invasive in restricted areas, in very particular habitats like bogs or heathlands, dry grasslands, coastal dunes. These habitats are not widespread in Belgium and cases of invasion are mostly known by naturalists.

16) Why is bamboo not mentioned on the list of invasive species?

There are lots of ornamental bamboo species. Three main groups can be distinguished: the Phyllostachys group (the biggest one), the Fargesia group and the Sasa group. Most of them originate from Asia. Those are rhizomatous species which can expand considerably in gardens. Up to now, however, there is a low presence of bamboo species in the wild, although there is evidence indicating they start to establish in natural habitats. They are presently not mentioned on the list of invasive plants. But that situation might change in the future if the species start to grow extensively. In that case, bamboo shall be assessed scientifically to determine the environmental hazard it represents.

17) Some invasive aquatic plants are oxygenating plants used for garden ponds. They favour aquatic life and improve water quality. How can they be harmful?

Some aquatic plants considered as invasive are used as oxygenating plants in ponds. Such are for instance Elodea canadensis, E. nutalii or Egeria densa. That oxygenating function is fulfilled as long as the plant does not grow excessively. Its development depends on several parameters: light, physical-chemical water quality, etc. When an aquatic plant is proliferating, it progressively asphyxiates the habitat by creating anoxic conditions. Indeed invasive aquatic plants start covering the whole pond, reducing light penetration and slowing down water circulation. Organic matters accumulate and decay slowly under the action of bacteria which consume the oxygen present in the water, then asphyxiating the habitat and, in the worst cases, finally killing aquatic life.

18) Lots of species mentioned on the list are used in cities for the planting of parks and green areas. Is there really a risk of invasion in natural habitats by species planted in urban areas, where the chance of escaping seems very limited?

Here is the rule: the more an invasive species is planted in the landscape (in gardens, cities, along roads, etc.), the greater it is likely to escape and establish one day into a natural habitat with favourable conditions for its expansion. Multiple and repeated introductions of invasive plants (through voluntary plantations for example) are one of the concepts explaining the invasion success of invasive ornamental plants. Unlike generally assumed, even species which are planted in cities may escape in the wild. Plants are no static organisms. They grow and reproduce. Seeds are dispersed by wind, vehicles or streams. Birds eat the fruits and spread seeds over long distances. Urban habitats are no closed bubbles completely cut off from natural environment. Linear elements like streams, roads, railways are dispersal corridors for vegetal species. Green spaces, gardens, wood in cities are part of an ecological network connecting these elements to the natural environment located outside of cities.

19) What does Belgian legislation stipulate about trade of invasive plants? Should we not simply ban the sale of these species?

Up to now, there is no legislation in Belgium limiting the trade of invasive species. This is why instruments should rapidly be implemented to avoid introduction of species recognized as detrimental for biodiversity, economics or public health. Two kinds of instruments can be developed to achieve this: legal instruments (a law) or self-regulation instruments ("code of conduct"). The second tool is based on the awareness and responsibility of professionals (producers, sellers) and consumers. The advantage of a code of conduct is to be more flexible and enable the involvement of a wide audience, including general public. It aims at inducing a progressive change of attitude concerning the use of invasive plants.

20) What is a Code of Conduct on invasive plants?

A Code of Conduct is a document recommending the adoption of best practices to limit the introductions and spread of invasive species in parks, gardens or along railways, roadways and river banks, which are the starting points of numerous invasions in natural habitats. It is a tool based on voluntary self-regulation (contrary to legislation, based on obligation). Everyone is free to subscribe. It is a tool for both horticulture professionals or garden amateurs whom are willing to do something for the environment. They can engage to the Code, available on this website (section Code of Conduct).

end faq