European Red List of Vascular Plants - European Commission

meres, have level and flow regimes tied to the behaviour
of the water table. Abstraction of water from the ground,
both for potable supply and agricultural use, is known to
affect both the levels and the periodicity of fluctuation of
water tables which in turn affect the timing and quantity
of supply to natural springs and seepages upon which
these wetland and river plants depend. The abstraction
of water from rivers and lakes leads as well to ecological
change with various consequences including the decrease
of surface area of flooded wetlands and the duration of
flooding.
Rivers and other wetlands have been modified since
humans first started to grow crops and keep livestock,
from minor diversions to form stock ponds up to hard
defences, channelization and damming of major rivers.
In many countries, even the smallest high altitude flushes
and headwaters have been modified. In the uplands of
the UK, for example, sheep farmers routinely dam and
divert the small streams which will eventually become
our main rivers; in many areas unregulated exploitation
of river gravels destroys the structure and vegetation of
river floodplains. It appears likely that in much of Europe
the vegetation of rivers has been in severe decline for
hundreds of years and it is difficult to establish which
species natural river systems would have supported.
Most wetland types are naturally highly dynamic,
resulting from natural processes at the ecosystem level,
e.g. seasonal and non-seasonal fluctuations of water
levels, succession to other habitats, the lateral movement
of rivers and the actions of large herbivores. Many aquatic
and wetland plant populations appear to function as
dynamic metapopulations; these populations are linked
by exchange of genetic material (e.g. pollen, propagules
or even plant fragments) thus increasing their resilience
to natural changes in the availability of suitable habitats.
Modification of wetland systems and complexes disrupts
connections between populations within metapopulations
by increasing the distance between patches further
enhancing the probability of extinction. Fragmentation
of wetland habitats also leads to the decrease in the total
surface area and thus in the total size of populations, as
well as the size of the remaining habitat patches which
increases their vulnerability.
Aquatic plants are often sensitive to changes in their
freshwater environments such as increases in nutrients,
changes in salinity, pH, temperature, etc. and they may be
important as indicators of ecosystem health. It is therefore
not surprising that pollution is a big threat and the main
cause is the use of fertilisers and herbicides or pesticides
in agricultural landscapes. Nutrient levels in wetlands are
increasing, through run-off from agriculture, sediment
leaching in from various practices which break up the
soil surface, from fish-farming and from atmospheric
deposition. Whilst the evidence for direct impacts of
increased nutrient loads on aquatic and wetland plants
is scant, they are extremely vulnerable to the knockon
effects of increased nutrient loads, developing
from an increase in productivity and the replacement
of oligotrophic species (which are often rare, such as
Eryngium viviparum and Thorella verticillato-inundata)
by meso- and eutrophic-species including aggressive
colonial perennial grasses and exotic invasive species;
if enrichment continues, most higher plants disappear,
displaced by algal mats, phytoplankton and eventually
anoxic crises. Eutrophication of large water bodies tends
to slow down or decrease in developed countries; however
eutrophication of headwaters is often increasing as is use
of xenobiotic pollutants such as herbicides. Pollution
from domestic or industrial sources and garbage disposal
is affecting the ecosystem in similar ways.
Recreational use of water bodies is another threat
factor; plants can suffer from excessive trampling due
to recreational activities or from work activities such as
removal of vegetation while “cleaning up” water bodies.
Similarly, water sports have, for example, been described
as a threat to Isoetes boryana which grows in shallow water
on the margins of large lakes
The invasion of exotic species such as Crassula helmsii,
Ludwigia species and Sagittaria subulata leads to increased
competition for space with native aquatic plants and
affects most threatened aquatic plant species. Climate
change and particularly an increase in droughts pose a
problem for aquatic plants especially in the Mediterranean
countries. The direct effect is that less suitable habitat will
be available but this be aggravated by higher demand on
the existing water resources in times of drought. Several
consecutive dry years may also adversely affect the
reproduction capacity of some species.
5.5 Population trends
In addition to the complexities of definition of aquatic
plants and taxonomic issues, there are fundamental
problems with attempts to quantify population trends
for aquatic plants. Many, if not most aquatic plants
reproduce mainly by vegetative means, either through
fragmentation followed by rooting of fragments, such
as water-starworts (Callitriche), or by turions, such as
pondweeds (Potamogeton); they may also reproduce by self-
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