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.
Distribution
Map
Red dots indicate specimen records or confirmed breeding sites.
British Columbia
The isolated records in British Columbia may represent relict locations
in areas of what was once more widespread suitable micro-climate
and habitat.
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 occurs in Nevada, Utah, Montana, Illinois, Wisconsin
and even as far south as Bolivia.
Habitat
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.
Why is it endangered?
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.
Biology
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.
Sources for more
information
Related On-line Sites to Visit
Publications
Ferns of BC, TMC Taylor, RBCM, 1979, p. 140.
The SOCAP Workshop Summary, The Nature Trust, 1989.
"Mosquito Fern: Two New Records in British Columbia,"
Cordillera, Trevor Goward, #2, Dec. 1994, p. 23-25
COSEWIC Status Report on the Mosquito Fern, Daniel Brunton, 1984
Museum Specimens
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