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MOSQUITO FERN
Image Credits: sketch in The Ferns and Fern-allies in British Columbia, T.M.C. Taylor, 1979.
The plants are
unfern-like free-floating groups of tiny bronzy green fronds attached to branching root stocks.
At only 1 to 2 cm across, the
Mexican Mosquito Fern more closely resembles a liverwort
than
a fern. Even so, this species is often hard to overlook, owing
to its habit of forming
dense vegetative colonies. Not unlike
Duckweed, which this mosquito fern, colonies
may cover
large expanses in the sheltered bays of ponds and lakes.
By late summer, the tiny scale-like
leaves have turned a deep
brick red, and so render the colonies even more conspicuous. In
late
Fall it forms red 'carpets' on the water surface.
Like lichens, it too has entered into
a
permanent symbiotic association with another organism. In this
case, the other member of the
relationship is actually a cyanobacterium
called Anabaena azollae. The fact that the
cyanobacterium
provides a ready source of nitrogen for the mosquito fern has
allowed the fern
to colonize nitrogen-poor aquatic habitats
not usually suitable for pteridophytes.
Map
British Columbia
Until recently, this fern was known only from
only
four localities in British Columbia - all of them within about 35 km of one another
on the
south shore of Shuswap Lake, near Sicamous, Tappen, and
Cambie.
Two new localities were
found in south-central British Columbia
on the floodplain of the North Thompson River,
approximately 60
and 80 km northwest of the closest Shuswap Lake localities.
North America
The Mosquito Fern prefers
moderately dry climates, but where saline
soils are not widespread (so that salt does not rise
to intolerable
levels during the water-level reduction which seems necessary
for its
development). Small ponds surrounded by wet meadows, still-water,
fresh ox-bow lakes over
sandy floodplain deposits and edges of
slow streams are where the Mosquito Ferns are found.
These locations
are usually surrounded by young hardwood and/or mixed forests
but can be in
the open amongst grasses and shrubs. The plants
are both free-floating and on logs and rotting
vegetation.
Its niche is a precise one, but the plant has the capacity to
"wait
out" unacceptable conditions - not unlike lake-bottom 'seed
banks' of some
emergent plant species in eastern Canada.
The Mexican Mosquito Fern is considered threatened on a provincial
and national status.
This status reflects both the few
sites and their susceptibility to disturbance. The North
Thompson
localities are apparently no more secure than other Canadian localities.
Potential
threats include herbicides, accumulating salt from winter
road de-icing programs, and leakage
of diesel oil or gasoline
in the event of an accident on the adjacent highway. Though
little
can be done to reduce the last of these threats, the North Thompson
sites should be
protected against salt and herbicides.
The fern fluctuates widely in abundance from year
to year; when
sparsely developed it may be easily overlooked. Drought conditions
cause the
ponds in which the ferns live to dry up much earlier
than usual.
Past threats to the
habitat required by the Mosquito Fern are
from in-filling of suitable backwater sites at
Sicamous for the
development of housing and road construction. Ponds where the
fern had been
reported to be abundant at the beginning of the
century have been damaged and destroyed.
Today major travel
corridors adjacent to the three best known sites are the main threats.
If a railway
tanker car containing toxic chemicals was to spill into the embayment
at the
Salmon Arm site, the plants could be eradicated. Similarly,
a railway tanker car accident at
or near the approach to the Sicamous
Narrows Bridge could destroy the Sicamous station. The
Sicamous
and Cambie sites are also vulnerable to a traffic accident involving
a toxic
chemicals and/or gasoline or oil delivery truck on the
Trans Canada Highway. The roadside
stations are also exposed to
accumulating salt from winter roadside de-icing programs which,
in suitable concentration, can be fatal to the Mosquito Fern.
This species along with
others of the grasslands communities are
endangered for a number of reasons. In addition to
the major impact
of livestock grazing, range re-seeding and off-road recreation
have modified
much of the remaining "undeveloped" grassland
areas. In addition, cultivation,
agricultural and urban development,
prescribed burning, forest encroachment, road and trail
development,
alien plant and animal species introductions, and hydro-electric
power projects
have caused outright, irreversible losses of native
grassland species.
Because grasslands
have been so influenced by human activities,
a relatively large number of wildlife species
associated with
grasslands (including this plant species) are listed as threatened
or
endangered. Due to these combined influences and the relatively
limited distribution of
grasslands, "ancient" grasslands
represent a much more endangered space in British
Columbia than
do "ancient" or old-growth forests.
Azolla is known from only a few small sites where many
thousands of individual plants blanket the surface of quiet backwater
pools in late summer. Vegetative
reproduction is the primary vehicle
for plant growth; the production of fertile material is proportionately
rare. Populations completely disappear each fall/winter
and do not show any
evidence of survival until conditions are
suitable for growth, then, the population expands quickly.
Different from most other pteridophytes, the Mexican Mosquito
Fern produces two kinds of spores: microspores and megaspores.
These function as reproductive packages for the species. Having
overwintered at the bottom of the pond, they eventually float
to the surface again, where they initiate sexual reproduction
and so produce a new generation of mosquito ferns. Though microspores
(sometimes referred to as the "male" spores) are said
to occur commonly in this species, megaspores (the "female"
spores) are rather rare, and have apparently not been reported
from Canadian collections. In this connection, it is noteworthy
that the North Thompson collections bear both types of spores.
A dramatically reduced water-level is apparently essential for
the development of the Mosquito Fern. The water must be somewhat
acidic for optimum Azolla growth and nitrogen-fixation.
As well, the blue-green algae which the fern has a symbiotic relationship
with cannot fix nitrogen without the proper levels of manganese.
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