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examples
Research vector :
Dynamic functioning and
quality assessment of
freshwater ecosystems
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information on scientific activities
Cemagref |
They are everywhere, or very nearly everywhere. Recreational ponds, wetlands, networks of ditches and streams with very low waters are quickly invaded by L. peploides. These exotic aquatic plants make very thick seagrass beds. The pollution they cause is substantial and managers have become increasingly alarmed over the past few years. However, they arrived in France more than one century ago. Coming from Latin America, they were introduced by accident in the South of France in the 1820s. They long remained in the southern part of the country, from Camargue to Aquitaine. For roughly 30 years, they have been migrating towards the north. Today, L. peploides has already reached the Belgian border. They have been located on a few sites in Belgium and the Netherlands.
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Photo
: Polder (Casier sud)
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| Invasion alert |
Photo : Etang du Turc (in the south of the Landes
department)
less than 10 km from the ocean |
In 1988, the Local Authorities of the Landes Department and the Joint Syndicate Géolandes appealed to Cemagref. The quick colonization of the aquatic environments by L. peploides and other invading plants has become alarming. The distribution of these plants has been mapped and a management plan has been proposed. Although it quickly became apparent that completely eradicating L. peploides is impossible, their development should nevertheless be controlled. |
Other zones colonized by these plants have been mapped, and the biomass has been measured. This is the first time a quantitative survey has been carried out in France. In the nature reserve of the Orx Marshes (Landes), Alain Dutartre and his team have measured the biomass to be roughly 2 kg of dry matter per square meter, very high for an aquatic plant. Doubling the biomass takes 15–20 days in stagnant environments and about 70 days in streams. Colonization of the Orx Marshes was very rapid: the seabeds of L. peploides progressed from a few dozen square meters in 1993 to 130 ha in 1998.
In order to fight the invader, Cemagref scientists propose solutions suitable to individual sites: pulling up and/or possibly spraying with a herbicide. The pulling up operation can be manual at the very beginning of colonization, but it is hard work. Once the plant in the seabed has become very well established, mechanization is necessary. Although a chemical spray treatment can replace or complete pulling up operations, its use is only conceivable as a last resort where the water use and the sensitivity of the environment make it possible. There is no magic formula that can be applied generally. Each site should have its own management plan according to its individual characteristics. Thus it has been possible to use arrivals of salt water in some wetlands of Languedoc to repel the attack of Ludwigia peploides, which do not resist salt. Nonetheless, these operations must be repeated: it is difficult to prevent a plant from
growing!
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| A strategy for conquest |
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The colonization of the freshwater environments by L. peploides is undemanding. A mere piece of stem is enough to regenerate a plant. Once the plant is established, it quickly proliferates. Even though L. peploides prefers areas where water is stagnant or areas with a low current, as they are not very demanding, they adapt outstandingly well to a large number of environments. A very dense grass bed forms itself into an inextricable network of large, stiff and ramified stems. The stems sometimes root as much as 3 metres deep and grow to 30–80 cm above water
level. |
Photo : Boudigau stream (Landes), an outlet of the nature reserve of the Orx Marshes |
As a consequence, flows are limited and boat traffic becomes impossible. Even irrigation and drainage are
disrupted. The less mobile waters favour the deposit of suspended matters and the freshwater environments fill in more
quickly. Even the banks are colonized. L. peploides thus invades some wet
grasslands, restricting extensive animal production, for cattle do not eat
them.
With their outstanding adaptability, L. peploides has quickly won the battle with indigenous plants. The ecological impact is
immediate. L. peploides progressively replaces the local species and biodiversity is
reduced. Some species are disappearing, such as the Myriophyllum
species, which accommodated a microflora eaten by fish.
In addition, the dense grass beds strongly influence the physico-chemical quality of the waters, particularly the dissolved oxygen content and the
pH. When the oxygen content in the water drops below 2 mg/L, no fish survive.
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| Defeating the invasion through knowledge |
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The scientists from Cemagref have followed the tracks of L. peploides for more than 10
years*. They must relentlessly study the development of the plants in the various sites in France. It is important to understand their ecological and biological characteristics in order to control the development of L. peploides over time.
Thus, recent results have refuted data from the literature.
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The fruit of L. peploides, until now considered sterile and not dangerous, have germinated in the laboratory. It remains to be verified whether the seeds can also germinate in the wild. If the answer is yes, in places where L. peploides have been disposed of through the above-mentioned works, several years of monitoring will be necessary to ensure that there is no resumption of the vegetation from the seeds.
Management plans set up from the beginning were the first act of sustainable management for these environments. By associating skills and combining initiatives, controlling the advance of these invading aquatic plants will be possible. Therefore, in partnership with the General Council of the Landes department, Cemagref organized a seminar on L. peploides, the 23rd and 24th of January 2004. Approximately 100 managers, technicians and scientists from all over France gathered to share information, opinions and questions.
But will not all these efforts remain fruitless if L. peploides remain freely on sale to individuals? Indeed, the sale of this plant, appreciated for its ornamental qualities, is another way of spreading it in the wild.
*
Two species of exotic Ludwigias are in France. They are Ludwigia peploides and Ludwigia uruguayensis or grandiflora. They can be differentiated by various features of their leaves and the size of their flowers (the second species has flowers about 5 cm diameter), but their ecology is rather close and the pollution they cause is highly similar.
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© Cemagref, updated June 2004,
Contact : info@cemagref.fr |
