Will the Rare Aquatic Carnivorous Plant Aldrovanda vesiculosa Survive in Europe?

Aquaphyte Newsletter (Univ. Florida, Gainesville, FL, USA) 17: 4-5 (1997)

Lubomír Adamec

 Institute of Botany of the Academy of Sciences of the Czech Republic, Section of Plant Ecology, Dukelská 135, CZ-37982 Třeboň, Czech Republic, adamec@butbn.cas.cz

The waterwheel plant, Aldrovanda vesiculosa L. (Droseraceae), is a very rare aquatic carnivorous plant that is rapidly vanishing from Europe. Yet it is spread over a vast territory throughout the Old World. It is a rootless plant, usually 6-18 cm long, free-floating just below the water surface. It has always excited curiosity among botanists due to its attractive snapping traps (3-6 mm long). Aldrovanda has the same life form end growth strategy as temperate aquatic Utricularia species: a fast apical growth (1-2 whorls a day) and simultaneous basal decay. In temperate countries, it propagates only by apical branching of shoots. Apical winter buds (turions) are formed in autumn.

Distribution

Aldrovanda is spread irregularly and sparsely from temperate to tropical zones from Europe to Australia. The origin of its recent population in Europe is still unclear. Here, on the basis of palaeontological records, it is usually considered a Tertiary relict. Since its turions probably are spread by migratory waterfowl to new sites, another theory explains the origin of its European temperate populations as postglacial naturalization of African plants that were transported by birds. In any case, the historical postglacial spread of Aldrovanda in Europe was highly irregular, variable in time and area, and probably dependent on migratory routes of waterfowl. Recent data on its distribution in Africa and Asia are not available (except for Japan) and only a few recent sites are known from Australia. In Europe, it occurred more frequently and was recorded at about 150 sites in the last two centuries. However, the view on the recent map of its sites is alarming. It has declined dramatically in the last 30 years, vanishing from Germany, France, Italy, and Slovakia. It is now probable that Europe (excluding the former USSR) has no more than 15-20 sites, with a few dozen sites in the Ukraine and Russia. There are ten sites still in Poland (from a previous record of 78), one in Hungary, and possibly, a very few in the Balcans. Two artificial sites are in Switzerland where it was successfully introduced in 1908. In all European countries, it has had a status of "critically endangered species" and has been under strict state protection for at least twenty years. Yet, this has not helped much!

Ecological Requirements and Reasons for Decline

Aldrovanda is highly sensitive to competition with filamentous algae and higher aquatic plants that form denser stands. Very fast apical growth and vegetative propagation are the only way to overcome the competition. The most important ecological requirements of Aldrovanda include: a) free-CO₂ concentration >0.1 mM as the plant is a strict CO₂ user; pH may be within 5.0-7.6; b) a medium humic acid concentration (2-30 mg.l^-1); c) high biomass of plant litter from reeds or sedges; d) water surface free of a dense biomass of submersed or floating macrophytes; e) transparent water free of suspended matter or phytoplanlaon; f) relative irradiance >20 % of full sun; g) relatively warm water in summer (optimum 25-28°C); h) shallow water (0.15-0.6 m, but summer minimum 5-10 cm); i) abundant zooplankton as prey; j) oligo-mesotrophic water.

Aldrovanda vesiculosa L. by I. Pencak (used with permission)

Aldrovanda grows in shallow standing dystrophic waters: lakes, bogs, fens, backwater pools, and peaty fishponds, usually on very limited plots in wind-exposed littorals. It never grows in open water, but only in shallow, loose stands of emergent vegetation (Phragmites, Typha, Carex) or in little bays among tussocks of denser vegetation. The strict habitat requirements of Aldrovanda characterize it as a stenotopic species. Its preferred sites are those subject to slow, permanent succession wherein the emergent vegetation becomes dense. Generally, small habitat changes may result in the decline of Aldrovanda. That is why its occurrence at a site usually is limited in time and it may be found at other proximate site(s) where favourable conditions have developed (a "fugitive strategy"). In the last decades, however, the number of potential suitable sites has decreased drastically in many European countries and its chance to "jump" to new sites has greatly diminished. Its rapid decline has been caused mainly by water eutrophication, drainage, and filling in of water bodies. Other reasons may be more general land-use changes, particularly intensive agriculture, water level fluctuations, and wetland afforestation. Besides the evident reasons, high amounts of NO₃^-, NH₄⁺, and SO₄^2- in acid precipitates might accelerate eutrophication and, hence, native succession of shallow wetlands. This is suspected because Aldrovanda has been vanishing even from those sites which are evidently quite unimpacted by man. Also, the velocity of its decline has been much higher within the last 30 years than before.

Active Protection - Selection of New Sites

Elaboration of reliable outdoor culture of Aldrovanda was a necessary prerequisite for both ecophysiological study and selection of its suitable substitute sites in the Czech Republic. The culture mimics habitat conditions at natural sites. In a 1-2.5 m² plastic container, about a 3 cm layer of litter of Carex gracilis (or similar species), placed over 5-8 cm of sand, is used as the bottom substrate. The container is loosely planted with sedges or common reed. Water depth is 20-30 cm. As Aldrovanda is susceptible to boron deficiency, boric acid must be added. Turions overwinter well in the refrigerator.

Great effort has been made to select new suitable sites in the Czech Republic. The plants placed in nylon enclosures in three shallow dystrophic wetlands in North and South Bohemia grew rapidly and reproduced 8-34 times over the 1994 season. Approximately 10-50% of the turions overwintered. When 30 Aldrovanda plants were introduced to the suitable sites in South Bohemia in 1995, the plants grew rapidly only in the Carex rostrata-dominated pool at Ptačí blato 1C. However, turions overwintered perfectly and in the 1996-1997 seasons, the plants propagated richly, forming an abundant population. The character of the suitable pools at Ptačí blato reminds us of some natural Polish sites. Water level at the sites in summer has been found to be the crucial factor for rapid growth and propagation of Aldrovanda. Both of the dystrophic sites are firmly connected with hypereutrophic fishponds with fluctuating water levels due to summer rains. The water level was very low in 1995, but high in 1996 and 1997.

Thus, a new prolific site arose in South Bohemia, where Aldrovanda had never grown. However, this success should be accepted cautiously as the suitable pools tend towards eutrophication and overgrowing and some management will be necessary in future years to keep up its rich population. Similar introduction of Aldrovanda succeeded in Switzerland as early as 1908 and a stable population has been growing there since. Nowadays, when its natural spread by waterfowl within vast areas of Europe is almost excluded and its natural populations are declining, introduction to new sites is probably the only effective way to keep and propagate the European population, in spite of some success with growing it in tissue culture.

To increase the feasibility of introductions of Aldrovanda to new sites, it is essential to grow it in outdoor culture, e.g. in botanical gardens or research institutes (so far Třeboň, Wroclaw, Berlin, Kiel, Arras, and Strasbourg) and select potential sites. It also might be a project for Nature Conservation Unions. These organizations in European countries are challenged to consider their participation in the conservation of Aldrovanda. I am able to provide them with sufficient plant material for cultivation as well as know-how. Aldrovanda is still waiting ...!

Note: The author asks colleagues working in Africa to please send living plants or seeds of Aldrovanda to the above address. The study of African plants could provide more clues to the origin of European populations. Any information on recent African spread of Aldrovanda is greatly welcome.

≈ means guesses

Summary of free introduction of A. vesiculosa (June 1995, 30 plants) to dystrophic pools in Třeboň region, South Bohemia, Czech Republic. Total number of apices (AP) or turions (TUR) are shown.

References:

Adamec L., 1995. Ecological requirements and recent European distribution of the aquatic carnivorous plant Aldrovanda vesiculosa L. - a review. Folia Geobot. Phytotax. 30: 53-61.

Adamec L., 1997. Photosynthetic characteristics of the aquatic carnivorous plant Aldrovanda vesiculosa. Aquat. Bot. 59: 297-306.

Kamiński R., 1987. Studies on the ecology of Aldrovanda vesiculosa L. I. Ecological differentiation of A. vesiculosa population under the influence of chemical factors in the habitat. Ekol. Pol. 35: 559-590.

Carniv. Plant Newslett. (1997) 26: (special September issue on Aldrovanda).

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