-
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 and 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 phytoplankton; f) relative irradiance >20 % of full sun; g) relatively warm water in summer
(optimum 25-28oC); h)
shallow water (0.15-0.6 m, but summer minimum 5-10 cm); i) abundant zooplankton as prey; j)
oligo-mesotrophic water.
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 SO42- 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 Pta ¡ 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 Pta ¡ 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 T ebo , 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.
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. (in
press).
Kaminski 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).