Mexican mosquito-fern (Azolla mexicana): COSEWIC assessment and status report

Photo of the Mexican Mosquito-fern Azolla Mexicana.

Threatened 2008

Committee on the Status of Endangered Wildlife in Canada

Comité sur la situation des espèces en péril au Canada

Table of contents

List of Figures

List of Tables

COSEWIC status reports are working documents used in assigning the status of wildlife species suspected of being at risk. This report may be cited as follows:

COSEWIC. 2008. COSEWIC assessment and update status report on the Mexican Mosquito-fern Azolla mexicana in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vi + 35 pages (pp).

Previous reports

COSEWIC. 2000. COSEWIC assessment and update status report on the Mexican mosquito-fern Azolla mexicana in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. vi + 11 p.

Martin, M.E. 2000. Update COSEWIC status report on the Mexican mosquito-fern Azolla mexicana in Canada, in COSEWIC assessment and update status report on the Mexican mosquito-fern Azolla mexicana in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. 1-11 p.

Brunton, D.F. 1984. COSEWIC status report on the mosquito fern Azolla mexicana in Canada. Committee on the Status of Endangered Wildlife in Canada. Ottawa. 36 p.

Production note

COSEWIC would like to acknowledge Brian Klinkenberg for writing the status report on the Mexican Mosquito-fern Azolla mexicana in Canada, prepared under contract with Environment Canada, overseen and edited by Erich Haber, Co-chair, COSEWIC Vascular Plants Specialist Subcommittee.

For additional copies contact:

COSEWIC Secretariat
c/o Canadian Wildlife Service
Environment Canada
Ottawa, ON
K1A 0H3
Tel.: 819-953-3215
Fax: 819-994-3684


Également disponible en français sous le titre Ếvaluation et Rapport de situation du COSEPAC sur l’azolle du Mexique Azolla mexicana au Canada – Mise à jour.

Cover photo:
Mexican Mosquito-fern--Photo by Brian Klinkenberg.

©Her Majesty the Queen in Right of Canada, 2009.
Catalogue Number CW69-14/568-2009E-PDF
ISBN 978-1-100-12452-0

COSEWIC Assessment Summary

Assessment Summary – November 2008

Common name: Mexican Mosquito-fern
Scientific name: Azolla mexicana
Status: Threatened
Reason for designation: This tiny floating aquatic fern of south-central British Columbia is restricted to 8 small water bodies where its populations undergo periodic fluctuations in numbers of individuals. Two populations and their habitat have been lost in recent years due to construction activities with most of the extant populations occurring adjacent to major highways or a railway where they are at potential risk from maintenance activities, including the use of chemicals such as road salt.
Occurrence: British Columbia
Status history:-Designated Threatened in April 1984. Status re-examined and confirmed in April 1998, May 2000, and November 2008. Last assessment based on an update status report.

COSEWIC Executive Summary

Mexican Mosquito-fern
Azolla mexicana

Species information

Mexican Mosquito-fern Azolla mexicana is a tiny floating aquatic fern found in wetlands and small wet areas. Individual plants range in size from 1-2 cm in length. Leaves are tiny and overlap like shingles. Spore-producing structures, borne below the water surface, are present in the Canadian populations. Roots are simple and short. Plants usually have three “floats”. This species forms extensive green or red mats on the water surface.


The natural range of Mexican Mosquito-fern is North, Central and South America (from Peru, northern Bolivia and Brazil north). In Canada, it is restricted in occurrence to British Columbia, where it occurs at the northern tip of its range. The eight extant British Columbia populations occur in three regions: the Little Fort area, the Shuswap Lake area, and Vernon. The extent of occurrence is 5400 km² and the area of occupancy for the small fragmented water bodies is 11 km² based on a 1 x 1 km grid and 40 km² based on a 2 x 2 km grid.


Mexican Mosquito-fern is a wetland species of sheltered still waters and is found in ponds, ditches, oxbow ponds, and lakeshores. It grows best in 50% sunlight, and is tolerant of semi-shade conditions. In BC, it is found in sites with pH ranging from 6.5-8.1, although it is tolerant of pH ranging from 3.5 to 10.


This aquatic fern grows in a symbiotic relationship with a species of blue-green alga (a cyanobacterium) Anabaena azollae. Primary reproduction is through fragmentation of plants. Secondary reproduction is sexual through spore production. This is a heterosporous fern: two types of spores (of different sizes and appearance) are produced that fall to the bottom of water bodies and eventually germinate, producing new plants. Dispersal is by wind, waterfowl, and accidentally by humans.

Population sizes and trends

Ten populations of this species have been confirmed from British Columbia. Two of these are now extirpated as a result of road construction and urban development. In spite of this, there has been an increase in known populations for this species since the 1984 status report, which reported only four sites for the species. Populations fluctuate dramatically from year to year in areal extent. Because of this, population trends are unknown and further study over several years is needed.

Limiting factors and threats

Mexican Mosquito-fern is naturally limited by factors such as water chemistry and temperature, which restrict its range in British Columbia. Threats to the populations include road repairs and construction, housing / urban development, accidental spills on roads and railways (which can influence water chemistry and could cause complete population die-off), invasive species, and eutrophication of sites.

Special significance of the species

This species is rare in BC and Canada and is found in British Columbia at the northern limits of its range. As populations peripheral to the main species range, the Canadian populations could be important to the species long-term survival in the face of climate warming. The species is economically important and is cultivated in many countries around the world as a green fertilizer, due to the presence of the symbiotic nitrogen-fixing cyanobacterium, and for use as a nutritional supplement in livestock feed.

Existing protection or other status designations

One population of Mexican Mosquito-fern occurs within a provincial park. Other populations have no existing formal protection. COSEWIC assessed Mexican Mosquito-fern as Threatened in 2000.


The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) was created in 1977 as a result of a recommendation at the Federal-Provincial Wildlife Conference held in 1976. It arose from the need for a single, official, scientifically sound, national listing of wildlife species at risk. In 1978, COSEWIC designated its first species and produced its first list of Canadian species at risk. Species designated at meetings of the full committee are added to the list. On June 5, 2003, the Species at Risk Act (SARA) was proclaimed. SARA establishes COSEWIC as an advisory body ensuring that species will continue to be assessed under a rigorous and independent scientific process.


The Committee on the Status of Endangered Wildlife in Canada (COSEWIC) assesses the national status of wild species, subspecies, varieties, or other designatable units that are considered to be at risk in Canada. Designations are made on native species for the following taxonomic groups: mammals, birds, reptiles, amphibians, fishes, arthropods, molluscs, vascular plants, mosses, and lichens.

COSEWIC Membership

COSEWIC comprises members from each provincial and territorial government wildlife agency, four federal entities (Canadian Wildlife Service, Parks Canada Agency, Department of Fisheries and Oceans, and the Federal Biodiversity Information Partnership, chaired by the Canadian Museum of Nature), three non-government science members and the co-chairs of the species specialist subcommittees and the Aboriginal Traditional Knowledge subcommittee. The Committee meets to consider status reports on candidate species.

Definitions (2008)

Wildlife Species

A species, subspecies, variety, or geographically or genetically distinct population of animal, plant or other organism, other than a bacterium or virus, that is wild by nature and is either native to Canada or has extended its range into Canada without human intervention and has been present in Canada for at least 50 years.

Extinct (X)

A wildlife species that no longer exists.

Extirpated (XT)

A wildlife species no longer existing in the wild in Canada, but occurring elsewhere.

Endangered (E)

A wildlife species facing imminent extirpation or extinction.

Threatened (T)

A wildlife species likely to become endangered if limiting factors are not reversed.

Special Concern (SC) Footnotea

A wildlife species that may become a threatened or an endangered species because of a combination of biological characteristics and identified threats.

Not at Risk (NAR) Footnoteb

A wildlife species that has been evaluated and found to be not at risk of extinction given the current circumstances.

Data Deficient (DD) Footnote c

A category that applies when the available information is insufficient (a) to resolve a species’ eligibility for assessment or (b) to permit an assessment of the species’ risk of extinction.


The Canadian Wildlife Service, Environment Canada, provides full administrative and financial support to the COSEWIC Secretariat.


Species information

Name and classification

Scientific Name:
Azolla mexicana Schlecht. & Cham. ex K. Presl.
English Common Name:
Mexican Mosquito-fern
French Common Name:
Azolle du Mexique
Family Name:
Major Plant Group:

Taxonomic circumscription of the genus Azolla is considered difficult and controversial by many researchers (Zimmerman et al. 1989; Pereira et al. 2001; Evrard and Van Hove 2004; Ceska personal communication (pers. comm.) 2005; Reid et al. 2006 and others). The most commonly accepted classification follows the work of Svenson (1944), who recognized seven species globally, and four New World species: Azolla caroliniana, A. filiculoides, A. mexicana, and A. microphylla. Difficulty is encountered when trying to identify sterile specimens lacking spores.

Recently, several researchers have reassessed the species of Azolla with varying outcomes (e.g., Zimmerman et al. 1989; Pereira et al. 2001; Reid et al. 2006). None has resolved the taxonomic difficulties and a clear picture has yet to emerge. Reid et al. (2006) indicate that cytology of the group has not helped because all species of Azolla are 2n = 44. These authors also indicate that initial circumscriptions of the species, and initial illustrations, were not based upon type specimens, and this has led to some confusion. Added to this is the natural variation in the species, and the environmental plasticity that is noticeable among populations, as well as the already-noted difficulty of working with sterile specimens (Zimmerman et al. 1989; Pereira et al. 2001; personal observation). This has led to much discussion about lumping species and reclassification.

However, the most recent DNA sequence work by Reid et al. (2006) has provided some insights. These authors have determined that Azolla mexicana is a good and distinct species. They have also determined that its lineage is identical to A. microphylla, and they recommend that the two species should be listed as a single species (A. microphylla, synonym A. mexicana). They cite experiments by Stergianou and Fowler (1990) that indicate that crosses between these two “species” yield normal offspring “which indicates conspecific status” (Reid et al. 2006, p. 537). However, a change in nomenclature and classification of these species is not yet in place.

For the purposes of this status report, A. mexicana is here recognized as a distinct species based on the taxonomic treatment by Svenson (1944) and the studies of Reid et al. (2006). Any taxonomic and nomenclatural update that results from their work may involve a future name change for this species. Regardless, this change in nomenclature would not affect the status of this species in Canada since A. microphylla is not known from North America.

Morphological description

Mexican Mosquito-fern is a tiny annual, aquatic, heterosporousFootnote1 fern that often forms extensive green or red-coloured mats in wetlands and small wet areas. It grows in a symbiotic relationship with the blue-green alga Anabaena azollae (a cyanobacterium), which lives in cavities on the dorsal lobes of Azolla leaves.

Plants are intricately branched with tiny overlapping leaves (Figure 1) and simple short roots, and range in size from 1-2 cm (Svenson 1944; Douglas et al. 2000; Martin In press). Upper leaf lobes are usually 0.7-0.9 mm long. Douglas et al. (2004, p. 302) describe the plants as “pinnately branched from a central axis, dichotomously branched only at the periphery of the plant, fronds overlapping like shingles.” Plants produce both micro- and mega-sporocarps (spore-bearing structures). A key character for separating Azolla species are the megasporangia, which are distinctively shallowly pitted at the base in A. mexicana, and are borne on short stalks on the undersides of floating lobes. These features are readily seen under a dissecting microscope. Generally, A. mexicana plants are larger and more compact than A. caroliniana, sporocarps are usually readily visible, and the floats Footnote2 on the leaves (usually three) are usually enlarged (Svenson 1944, Lomer pers. comm. 2007). A. filiculoides, the third Azolla species found in BC, has distinctively different megasporangia and is a much larger species than A. mexicana.

Figure 1. Mexican Mosquito-fern, photo by Brian Klinkenberg

Photo of the Mexican Mosquito-fern.

Genetic description

There are no apparent ecological or behavioural barriers to gene flow in this species in British Columbia. No genetic information on this genus in BC is available. Although the distances separating the three groups of populations (~80 km, ~50 km and ~125 km--see Figure 2 and Table 1) probably preclude much genetic interaction among them, within each group of populations waterfowl movements may ensure a degree of interaction as occurs with other aquatic species (Figuerola 2002; Green et al. 2008).

Figure 2. Canadian and BC distribution of Mexican Mosquito-fern

Map showing the distribution of the Mexican Mosquito-fern in Canada and British Columbia.
Table 1. Mexican Mosquito-fern populations in British Columbia
Locale Population Ownership/Protection status Population descriptiontable noted Status
Little Fort / North Thompson River Area 1) Little Fort, Round Top Road
(Conservation Data Centre (CDC) Extent of Occurrence (EO) #25545)
Private, owner unknown Covers pond (10 x 5m) 2005 (July). In 1997, the population at this site was thriving (500,000 plants), but in 2004 plants were not present. Plants present in both 2005 and 2007 (500,000 plants). Extant 2007.
Little Fort / North Thompson River Area 2) Little Fort, north of (CDC EO #25458)
(4 km south of Population 1)
Dunn Peak Provincial Park (based on the Conservation Data Centre Mapping Service map) Covers two large oxbows on west side of highway (30 x 18m). In some years (e.g., 2004, 2005, 2007) there is a high density of plants: 10,000 plants per m² for a total of over 1 million plants (Douglas 2004). The population has declined to zero visible plants in some years. It is impossible to determine trends from the annual fluctuations. It has persisted at this site for almost 25 years. Extant 2007. Not seen in oxbow on east side of the highway since prior to 1997 (although previously observed there), lately only observed on the west side.
Little Fort / North Thompson River Area 3) Little Fort, south of (CDC EO #25454)
(8 km south of Population 2)
Private, owners unknown (possibly different owners for each site) 3 sites in close proximity:
1) shallow pond at south end of pasture; (3 x 20m). In 2004, this site consisted of 40,000-60,000 plants in a 0.5 m ring around the edge of 3 m x 20 metre pond. 2) two oxbow lakes on west side of highway south of pasture; 20 x 150m very densely covered [over a million plants] 3) oxbow on east side of highway; 20 x 60m sparsely covered. In 1998, the eastern site consisted of 400-500 plants (40% coverage) over a 300 square metre area. In 2004, there was less than 1% coverage over a 100 square metre area at this site.
Extant 2007.
Site 1) no plants observed in 2007, plants observed in 2005; 2) had extensive cover in 2005 and 2007 but not observed previously (specifically in 2004); 3) had no visible plants in 2005, scattered plants present in 2007.
Little Fort / North Thompson River Area 4) Darfield (CDC EO #25478)
(13 km south of Population 3)
Not Applicable (N.A.) Known from a pond on the west side of the highway Extirpated. Douglas (2004) reported the site filled in by highway construction.
Little Fort / North Thompson River Area 5) Darfield, south of New population.
(3.6 km south of Population 4)
Private, owners unknown Plants present in oxbow on east side of highway only. 15 x 80m east, then 15 x 100+m north/south; sparsely covered [several 1000s of plants]. Extant 2007.
New population (2007) on east side of highway, 3.6 km to the south of previously known Darfield population.
Shuswap Lake Area 6) Tappen/White Creek (CDC EO #3786)
(80 km from the Little Fort / North Thompson River populations)
North Bay Indian Reserve 5 First observed at this site in 1974 on still water and stranded on the muddy bank. In 1997, small numbers of plants were observed among streamside vegetation along the lowest stretch of the creek and in a larger, but still small, concentration in a backwater at the first right-angled bend. Extant 1997.
Not visited in 2005 or 2007; no plants observed in 2004 (Douglas 2004), but habitat still intact.
Shuswap Lake Area 7) Salmon Arm (CDC EO #3782)
(9 km from Population 6)
(Although 1.8 km separates these two subpopulations, given the likely level of interaction between them--waterfowl flying from subpopulation 1 to subpopulation 2 on a daily basis--they were considered as a single population. Also, over the years other intervening subpopulations have sporadically been observed along the foreshore.)
Subpopulation 1: Switsemalph Indian Reserve with CPR interests and private lands. Possibly also Crown Land (lakebed) (based on the Conservation Data Centre Mapping Service map) Plants throughout a 5 x 85m wet area along RR tracks. Population size at mouth of Salmon River unknown, but previously stated at 5 x 30m.
Mexican Mosquito-fern has been observed in the area around the Salmon River at the outlet to Salmon Arm, Shuswap Lake, since 1890. In 1982, it was observed in the shallow water of an oxbow lake, and in 1993, it was observed at the mouth of the river. In 1994, it was observed in several sites around the lakeshore. Since then it has been sporadically observed in several sites throughout the general area.
Not visited in 2005 or 2007. Subpopulation observed in 2004 (Douglas 2004).
Shuswap Lake Area 7) Salmon Arm (CDC EO #3782) Subpopulation 2: Salmon Arm foreshore Municipality of Salmon Arm Plants observed in a 3 x 2m area in 2005; no plants observed in 2007. Extant 2005.
Shuswap Lake Area 8) Sicamous (CDC EO #3780)
(26 km from Population 7)
Private, owner unknown None Extirpated. Douglas (2004) reported the site filled in.
Shuswap Lake Area 9) Eagle River population
(9 km from Population 8)
(Less than 1 km separates the two subpopulations.)
Subpopulation 1: Cambie (along Eagle River) (CDC EO #3784)
Private, owner unknown
Azolla had been observed here in a large slough since 1972. Martin (1998) visited the site frequently from 1987 to 1997 and always found the plants fully covering the pond. However, no plants have been seen for the past couple of years (Douglas, 2004). Extant 1997.
No plants observed since 1997 (2004, 2005, 2007) but habitat still intact.
Shuswap Lake Area 9) Eagle River population Subpopulation 2: Eagle River / Solsqua
Private, owner unknown
None. Historical. No plants observed since 1974. However, a possible collection by W. Schofield in 1997 from south side of highway could be from this site (but there is some ambiguity in the site description).
Vernon 10) Vernon Creek
(50 km south of the Shuswap Lake populations)
Subpopulation 1: Okanagan Avenue, Vernon (CDC EO #21875)
Private: recently sold to a land development company.
2 10 x 20m patches in pasture. Extant 2007.
Observed by M. Martin (2007)
Vernon (Less than 1 km separates the two subpopulations.) Subpopulation 2: Vernon Creek (CDC EO #25670)
Owner unknown: possibly Vernon Airport; also possibly City of Vernon (Marshall Fields recreation area)
Dense subpopulation covered the surface of an oxbow 25 x 2m in size in 1997. Also seen sparsely scattered along the creek sides north (upstream 400m) of the larger, oxbow subpopulation. Extant 2007 [a few plants were observed in Sept., although no plants were present in Aug. 2007].

Sources: Fieldwork by the authors in 2005 and 2007, BC Conservation Data Centre Occurrence Reports, Goward (1994), Douglas (2004), and Martin (pers. comm. 2007).

Designatable units

Since all populations occur within the same ecozone (Southern Mountain Ecological Area recognized by COSEWIC), a single designatable unit is recognized for this species.


Global range

Mexican Mosquito-fern is found in disjunct patches in North, South and Central America (Svenson 1944; Lumpkin and Plucknett 1980; Brunton 1984; Douglas 2004). In the United States, it is found in several western and mid-western states: Arizona, Arkansas, California, Colorado, Illinois, Iowa, Kansas, Minnesota, Missouri, Nebraska, Nevada, New Mexico, Oklahoma, Oregon, Texas, Utah, Washington and Wisconsin (Figure 3).

Figure 3. North American distribution of Mexican Mosquito-fern. Source: Brunton (1984), Environmental Lab (2002) and United States Department of Agriculture (USDA) (2007).

Map showing the North American distribution of the Mexican Mosquito-fern.

Canadian range

In Canada, Mexican Mosquito-fern is found only in south-central British Columbia (Figure 2) where it reaches the northern limits of its distribution (Brunton 1984). It was first collected in British Columbia by John Macoun, from Sicamous, in 1889, and was reported by him from Salmon Arm in 1890 (Brunton 1984). There are now ten (10) populations of this species reported in British Columbia, in three areas: Little Fort / North Thompson River, Shuswap Lake area and Vernon (Figure 3). Two of the ten populations have been confirmed as extirpated (Goward 1994; Douglas 2004; British Columbia Conservation Data Centre (BCCDC) 2005; Southern Interior Rare Plants Recovery Team 2007; Martin In press).

All Canadian populations of Mexican Mosquito-fern lie within the Southern Interior Forest Region, in the Okanagan, Shuswap, and Kamloops Forest Districts. They are found within the Montane Cordillera Ecozone (Ogilvie 1998; Environment Canada 2005).

BC populations may be relictual from a warmer post-glacial period (Brunton 1984; Martin In press). Martin (In press) speculates that it may be a species still expanding its range. It is reported from the fossil flora of the Okanagan Highlands of the province (Greenwood et al. 2005). Less than 1% of the global population is found in Canada.

Historical reports that have been redetermined

  • One collection of this species from Cobble Hill, Victoria, Vancouver Island, 2002, has been redetermined as Azolla caroliniana. [Collection #: Royal British Columbia Museum (RBCM) (V) 184823. Sterile; plants all less than 0.5 cm in size]

Populations confirmed as destroyed

  • Population 4 (Darfield, south of): reported destroyed by Douglas (2004), confirmed as destroyed during fieldwork in 2005 and 2007.
  • Population 8 (Sicamous): reported destroyed by Douglas (2004), confirmed as destroyed during fieldwork in 2005 and 2007.

Populations added during 2007 fieldwork

  • One population (Population #5 in Table 1–South of Darfield)

Extent of occurrence

The current extent of occurrence (EO) for Mexican Mosquito-fern is 5,400 km². The extent of occurrence was calculated using Hawth’s Analysis Tools ‘create minimum convex polygon’ tool for ArcGIS 9.x (Beyer 2004). The point layer file containing the populations for all of the known populations (extant and extirpated) was used as the input file. The area of the convex hull as determined by the program was given as ~5,400 km². This value is only slightly larger (by ~100 km²) than the EO that would be estimated from the populations presented in Martin (In press). The discovery of a new population in Vernon in 1997 (50 km south of the Shuswap Lake population; Martin In press) increased the EO by 60% from an estimated EO of 3,400 km² as of 1984 (Brunton 1984, Goward 1994) to 5,300 km². The extirpation of two populations did not affect the EO.

Area of occupancy

The Index of Area of Occupancy (IAO), determined using the International Union for Conservation of Nature and Natural Resources (IUCN) methodology and 1 x 1 km grid cells, is 11 km². Using 2 x 2 km cells, the IAO is 40 km². However, the actual area occupied would be much less than 1 km² (about 10 ha) if one only considers the areas of the wetlands in which Mexican Mosquito-fern is found, since most of the wetland areas (e.g., oxbows, sloughs, ponds, wet depressions in fields) are less than 1 ha in size.

The IAO was determined by converting to a raster file the layer containing all of the known populations (extant and extirpated) using the ArcGIS 9.2 Spatial Analyst ‘convert features to raster’ tool. The cell size was specified first as 1000 x 1000 m (so that each cell would represent an area of 1 km² on the ground) and then as 2000 x 2000 m (so that each cell represents an area of 4 km² on the ground). The cells associated with the extirpated population were not included in the calculation of the IAO values presented above.


Habitat requirements

Mexican Mosquito-fern is a species of relatively narrow growing requirements that is susceptible to changes in water levels and water chemistry which may restrict its occurrence in what otherwise appears to be suitable habitat (Lumpkin 1993; Douglas et al. 2004). As with other species of Azolla, it is a shade-tolerant species (Wagner 1997), found in quiet still waters of lakes, ponds, streams and other wetlands (Figure 4). In BC, it is specifically found along the shores of quiet bays and inlets of lakes, oxbows of creeks, ponds, in wet pastures proximal to creek systems, and in ditches (Brunton 1984; Douglas 2000; Martin In press, 2007). Although it is primarily a still-water species (Svenson 1944; Tryon and Tryon 1982; Brunton 1984; Douglas et al. 2000; Martin In press), Martin (In press) has observed it in faster-flowing waters. However, this may simply represent dispersal.

Figure 4. Mexican Mosquito-fern on the surface of an oxbow near Little Fort (population 2) (the red mat of Azolla has been coloured white in this photo in order to illustrate how visible it would be in the field). Photo by Brian Klinkenberg.

Photo of Mexican Mosquito-fern on the surface of an oxbow near Little Fort, British Columbia.

This species grows well in shallow water with a depth of only a few centimetres, especially where the roots can touch the substrate (Wagner 1997; Watanabe 1997). This corresponds well with summer drawdown in areas of deep water, a period when plants appear to flourish. This was noticeable during fieldwork for the species. Throughout its range, and in BC, it occurs with other aquatic species that include Lemna minor (Common Duckweed), Lemna trisulca (Ivy-leaved Duckweed), Riccia fluitans (Crystalwort), Phalaris arundinacea (Reed Canary Grass) (Keddy 1976; Brunton 1984; Klinkenberg field observations). Phalaris arundinacea is particularly prominent in the Salmon Arm sites.

Douglas (2004) and Brunton (1984) describe the species as preferring cool, slightly acidic, partially shaded, phosphorus-rich, but otherwise nutrient-poor still waters with low salinity. Wind and wave action eventually fragments and kills mosquito-ferns in general, although periodic annual flooding probably aids in dispersal (Martin In press; field observation). It is known that excessive turbidity can inhibit spore germination in Azolla species. In BC, the Shuswap populations are described as occurring in glacial outwash plains with locally distributed calcareous deposits (Brunton 1984). Water tests conducted by Brunton (1984) found that the pH at Mexican Mosquito-fern sites ranged from 6.5 at Sicamous to 8.1 at Salmon Arm. Individual plants of Mexican Mosquito-fern vary in size from 1 cm to 2 cm, and this may reflect pH variation at each site. Other variations in water chemistry, temperature, etc., may also influence germination and growth.

Populations typically carpet areas where they occur (Figure 5), although abundance and thickness can vary dramatically from year to year.

Figure 5. Mexican Mosquito-fern completely covers the pond (area enclosed by the white line); population 1 near Little Fort. Photo by Brian Klinkenberg.

Photo of Mexican Mosquito-fern population 1 covering a pond near Little Fort, British Columbia.

Habitat trends

In the Shuswap locale, where there is moderate urban expansion occurring, one site for this species has been lost to site alteration for development. At the Vernon locale there is the threat of development for one subpopulation, based on the recent sale of the property to a land development company.

Overall, potential habitat for this species remains stable as the large majority of it occurs in rural areas outside of any immediate development potential. However, as a result of road and highway construction, many of the oxbows and sloughs associated with the occurrence of Mexican Mosquito-fern in British Columbia have been alienated from the natural forces that influence them, such as the natural shifting of the creek/riverbed over time that sometimes allows reincorporation of old oxbows and the creation of new ones.

Habitat protection/ownership

Mexican Mosquito-fern populations occur on private lands, federal Indian reserve lands, municipal lands, Crown land (possibly), and within a provincial park. A brief summary of ownership and protection levels is provided in Table 1. The section on search effort discuses the difficulty of identifying ownership in the field.


There is a substantial amount of biological information on Azolla species as a result of their use in cultivation. This literature covers growing requirements, including influences of various chemicals (phosphorus, iron, calcium and more), temperature ranges, pH requirements and much more. While only limited biological information specific to A. mexicana is available, the general Azolla literature does provide a better understanding of this species. Key biological information on Azolla species and Azolla mexicana is summarized below.

Life cycle and reproduction

Azolla species, in general, are considered annuals that die back or decompose in the fall (Martin In press).Footnote3 Reproduction in Azolla species is most commonly by vegetative reproduction, where fragments break off and multiply (Moore 1969; Wagner 1997). Sexual reproduction by spores also occurs, and sporocarps occur in pairs in the leaf axils of older plants, usually with a pair of either microsporocarps or megasporocarps (sometimes one of each; Moore 1969). Sporocarp development in Azolla species is generally associated with mat development and summer months in temperate regions (Lumpkin and Plucknett 1980). Megasporocarps and microsporocarps dehisce at maturity and the spores sink to the bottom where they can lie dormant; germination and fertilization take place under water (Lumpkin and Plucknett 1980). Schaffner (1905) provides a clear discussion of the life cycle of heterosporous pteridophytes, and indicates that, since there are no hermaphrodites, there is no possibility of self-fertilization.

Primary dispersal of Azolla mexicana, like other species of Azolla, is by both water movement (currents, high winds) and by waterfowl (Moore 1969; Green et al. 2008). Fish may also play a role in dispersing this species. Human transport also, no doubt, occurs.


Azolla species are preyed upon by many insect types, particularly Lepidoptera, Diptera, Coleoptera, Aphids, and as well by snails (Gomez-Pignataro 1978; Calilung and Lit 1986; Wagner 1997; Watanabe 1997 and others). Lumpkin and Plucknett (1980) indicate that insect attacks are more prevalent in the summer, especially above temperatures of 28° C, and Azolla crops are known to be destroyed by insects. Gomez–Pignataro (1978) reports the use of Azolla mexicana as shelter by beetles, and predation on the beetles by a wasp (Polybia rejecta forma belizensis). He speculates that the wasp may control the beetles and their predation on A. mexicana.


Several key features of the physiology of Azolla species, and Azolla mexicana specifically, emerge from the literature. Plants do best in 50% sunshine (Lumpkin and Plunknett 1980). Azolla generally can survive in water with a pH ranging from 3.5 to 10. Levels below 3.5 and above 10.0 are considered lethal (Lumpkin and Plucknett 1980). Johnson (1986) reports that Azolla mexicana grows best in slightly acidic waters, and grows most abundantly where the pH ranges between 6.0 and 7.0. Moore (1969) notes that in Azolla species, generally, the response to pH is significantly influenced by other factors such as light intensity, temperature and iron levels. Azolla mexicana can tolerate low levels of salinity, but is killed by high levels (Johnson 1986; Moore 1969). Growth rates of Azolla decrease with increasing salinity, and growth stops at salinity levels of 1.3%–die-off occurs at higher levels (Lumpkin and Plucknett 1980). A. mexicana has been found to be more tolerant of NaCl salinity than salinity associated with other salts. The addition of fixed nitrogen has resulted in greater salt tolerance (Johnson 1986).

Although Azolla species generally have an optimum temperature range of 18–28° C, some species can tolerate a wider temperature range of –5–35° C (Wagner 1997). The optimal temperature for Azolla reproduction is 25° C (Watanabe 1997). Nayak and Singh (2004) report abundant spore production for Azolla species under prolonged mat-forming conditions (summer months). Azolla mexicana, as with all Azolla species in general, is sensitive to desiccation (Douglas 2004; personal observation), and is killed by high temperatures (Vitousek et al. 2002 and others). However, A. mexicana is tolerant of higher temperatures than other species (Lumpkin and Plucknett 1980).

Low winter temperatures result in die-off of Azolla species at northern stations (Tryon and Tryon 1982; Brunton 1984). Although Azolla species generally are reported to be resistant to cold, freezing of the water surface can result in die-off Footnote4 (Tsujimura, Ideda and Tukamoto 1957 cited in Moore 1969). Lumpkin (1993), in Flora North America, indicates that A. mexicana is less cold tolerant and has a narrower environmental range than A. caroliniana. Cold tolerance is generally greatest with a higher pH in the range of 8 to 10 (Lumpkin and Plucknett 1980).

Based on cultivation information, and on other sources, phosphorus and iron may be limiting factors for growth and population establishment (Brunton 1984). Lack of iron turns plants yellow, while lack of phosphorus curbs growth and turns plants red (Lumpkin and Plucknett 1980). Azolla fronds also turn red in strong sunlight, while fronds that are growing in shade retain their green colour. However, red colouration may also be a result of stress factors such as insect damage, high pH, low temperature, or calcium deficiencies that can limit the ability of plants to use strong sunlight (Lumpkin and Plucknett 1980; Watanabe 1982; Wagner 1997).


The possibility of rescue effect for this species in Canada exists. Azolla mexicana is passively dispersed by wind, water currents and waterfowl, and dispersal by humans no doubt occurs if plants or spores are transported on clothing or attached to boats (Moore 1960; Lumpkin and Plucknett 1980). Long-distance dispersal is likely through transport by birds, both externally (e.g., carried in mud attached to the feet) (Cruden 1966; Figuerola 2002; Figuerola and Green 2003) and internally (endozoochory) (Figuerola 2002; Figuerola and Green 2002; Green et al. 2008). There is “little doubt that birds acts as dispersal agents” for many aquatic species (Cruden 1966, p. 518); even Darwin in 1859 noted that birds disperse plant propagules (Cruden 1966). The distances involved range from 10s of km on a daily basis to over 200 km during migration (Cruden 1966; Figuerola 2002). Although there have been no studies specifically studying the transportation of Mexican Mosquito-fern by birds, Green et al. (2008, p. 386) did observe in Australia that “swan and especially coot samples contained large amounts of Azolla filiculoides” megasporocarps that were potentially viable.

It is likely that, in the event of extirpation of Canadian populations, rescue could be effected naturally, especially during waterfowl migration periods.

Interspecific interactions

Azolla mexicana, like other species of Azolla, lives in symbiosis with a cyanobacterium--Anabaena azollae (Lumpkin and Plucknett (1980). Azolla provides nutrients and a protective cavity for the alga, while the alga provides fixed atmospheric nitrogen to the Azolla. Other interspecific interactions are unknown.


Azolla mexicana is one of several species of Azolla that are grown horticulturally and agriculturally as a green fertilizer and for use as a nutritional supplement in livestock feed. Cultivated stock may be readily used for transplanting, and is used in various countries in the Mediterranean, including Turkey, and others. However, the success of transplants and their adaptability under BC growing conditions is unknown. The species is very limited in the ponds and streams in which it appears.

Population sizes and trends

Search effort

Directed fieldwork for Mexican Mosquito-fern--over an area that encompasses the known range for the species in British Columbia--was conducted in July and August in two separate years--2005 and 2007. This is the time period when the species can be most easily identified in the field. In 2007, field searches were conducted from the 15th to the 21st of August in an area from Osoyoos north to Clearwater, and from just west of Revelstoke to Cache Creek (1800 km driven). In late July, 2005, the same area was also searched specifically for the Mexican Mosquito-fern for recovery strategy work (over 1800 km driven). Although typically populations are highly visible in August, it is possible that some (sub) populations may only appear in September and October (Brunton 1984; Martin pers. comm. 2007; personal observations 2007).

In both 2005 and 2007, all documented populations of Mexican Mosquito-fern (extant, assumed extirpated and historical) were visited. Close attention was paid, in particular, to bodies of water near previously identified populations since it was felt that those areas would be the most likely to contain undocumented populations. However, surveys were also conducted between sites for the species.

In addition, in 2002, 2003, and 2004, fieldwork conducted for other species (e.g., Symphyotrichum frondosum) by the author was carried out in the same region during the months of July and August, with a total of over 3000 km driven. Fieldwork for these species covered most of the same territory as that associated with Mexican Mosquito-fern, and included searching lakes, ponds and rivers for shoreline species. Other botanists, such as Malcolm Martin, Trevor Goward, George Douglas, Jenifer Penny, and Jamie Fenneman have also travelled extensively throughout the region over many years (Penny pers. comm. 2007). Douglas and Fenneman, in particular, searched for Mexican Mosquito-fern in the same areas in 2004. If Mexican Mosquito-fern had been present in any of the areas searched, it likely would have been documented.

The majority of the publicly accessible roads in the region have been traversed at least once and the major roads traversed many times over those years. All suitable ponds, rivers and wetland areas observable from a paved road, as well as accessible water bodies identified from topographic maps, were scanned, both on foot and using binoculars. If there was any indication that Mexican Mosquito-fern might be present (i.e., patches of red or green plants on the surface of the water body (Figure 4), a closer inspection was made where access was possible.

It is important to note that, outside of the main and secondary roads, there are relatively few accessible roads in this area of the province.

At the Vernon sites, the landowner of one subpopulation was approached, but permission to enter was denied. This was because the site was purchased by a development consortium (Martin pers. comm. 2007). However, it is also important to note that most Mexican Mosquito-fern populations occur in rural areas where it is often difficult to identify the property owner while in the field. Ownership for some sites is obscure and would require detailed investigation. This is particularly true in areas of open rangeland (no fencing) that were searched in the North Thompson River area. Lack of access to private property was a limitation as the full extent of populations at some sites is not visible from the road, and cannot be determined without a site visit, which in turn determines how many property owners are involved.

In most cases, for known populations, it was impossible to identify a residence associated with the property on which the population was observed (fences may or may not be present, and the closest residence may be several kilometres away). The owner or resident was not present at the one population that occurred on private lands and for which a residence could be easily identified (the Little Fort Round Top Road population). Two of the populations occur on Indian Reserves, and permission was not obtained for a site visit. It was thus not possible to document the full extent of any of the populations--only a general description of the population as viewed from the highway could be taken. Fortunately, most populations were visible from the highway. However, given the nature of the habitat of the species, it is possible that populations occur in oxbows and sloughs not visible from any highway.

Abundance, fluctuations and trends

For this status report, all reported sites for Mexican Mosquito-fern from previous status reports were assessed using a minimum separation of 1 km between populations. This distinction of populations is generally followed by the Vascular Plants Subcommittee (COSEWIC) for defining separate populations. This has resulted in a grouping of some “populations” and recognition of subpopulations (Table 1). Following this, there are now 10 populations of this species reported in British Columbia that can be identified within three areas: Little Fort / North Thompson River, Shuswap Lake area and Vernon (Figure 2). Two of the ten populations have been confirmed as extirpated (Goward 1994; Douglas 2004; BCCDC 2005; Southern Interior Rare Plants Recovery Team 2007; Martin In press).

Better documentation of known sites, and fieldwork since the first COSEWIC status report in 1984 (Brunton 1984), the update status report by Martin (In press), the national recovery strategy by Klinkenberg (2005) and for this status report, makes it appear that the number of Mexican Mosquito-fern populations has greatly expanded. However, while two populations in the Little Fort area (80 km to the north of the Shuswap Lake population) were not officially reported until 1994 (Goward 1994), they were known (and had been documented by collections) in 1982, which meant that, by 1984, there were actually 6 populations.

Between 1984 and 1997, an additional two populations were reported in the Little Fort area, bringing the total number of populations to 8. In 1997, a new population was reported from Vernon by Martin (In press), increasing the total number of known populations to 9. Note that Martin (In press) identifies two new populations from Vernon, and therefore reports ten populations in total. Assessment using a geographic information system (GIS) showed, however, that the records from Vernon should be considered as a single population, and therefore Martin’s population totals should be restated as 9. Given that Martin has lived in the Vernon area for several decades, the population he found in Vernon may represent a new population. However, due to dormancy of spores it is uncertain if this actually represents an expansion of the EO or renewed growth from dormant spores of a previously existing population.

The discovery of a new population south of Darfield during 2007 fieldwork brings the total number of known population for this species to 10. Since 1997, however, two populations were extirpated, leaving only 8 extant populations for this species as of 2007.

Because this species seems tied to the warmer temperature ranges, monitoring and on going inventory work will be needed to assess whether expansion of the species range is occurring.

Population fluctuations

Of note with this species is the appearance and disappearance of populations from year to year. Fieldwork in 2004, 2005 and 2007 has shown that while in some years plants appear absent from a site, they can reappear in following years. Goward (1994) particularly documents this fluctuation in appearance of Mexican Mosquito-fern in the Little Fort area between 1982 and 1994. Losses may be attributable to water chemistry and temperature fluctuations, and precipitation and/or draw-down in creeks and ponds. Re-appearance may be attributable to a persistent spore bank that reactivates when microclimate and water chemistry conditions are favourable. From this, it is important to note that absence of plants in any given year does not mean the extirpation of the population.

This responsiveness to annual and seasonal fluctuations in growing conditions influences searches for new populations and confirmation of known populations. Although many sites have been searched for the species with no success, this does not mean that populations are not present. Repeated searches over a span of several years are needed to assess populations of this species. It is notable that populations reported historically continue to be reported in the same sites, so populations appear to be persistent, and do not appear to have expanded in each locale. Also of note is the wide fluctuation in areal extent of individual populations. Based on fieldwork, this can vary from a few plants in one year to a thick continuous cover of plants in other years.

It should be apparent that determining the absolute population size--counting individual plants--is not practical for this species, given its extensive mat-forming nature. As illustrated in Figure 5, the plant, if present, most often covers the surface of the water body with thousands if not tens of thousands of plants (Table 1).

Furthermore, in spite of an extensive surface coverage in some years, plants can ‘disappear’ in intervening years. For example, in 2004 George Douglas did not observe any plants growing in the (western) oxbow in Figure 4, yet in 2005, when the photo was taken in the late summer, tens of thousands of plants were present. No plants were observed in the adjacent (eastern) oxbow in 2005, although Douglas observed several hundred plants there in 2004. He noted that in previous years the population at this site was much larger than our estimates for 2007.

This illustrates the problem inherent in confirming populations of mat-forming or clonal plants in years when they are dormant. If the habitat remains undisturbed, many years of consistent observation are required before a population can be declared extirpated. A population of a species such as Mexican Mosquito-fern should only be declared extirpated if the habitat has been significantly altered, as is the case for the Sicamous population and for the extirpated Darfield population (in both cases the land was filled in). Determining the total number of individuals and year-to-year changes in total population size and density would only be meaningful if those numbers were derived from many years of careful, consistent observation. Site-specific yearly monitoring of actual areas occupied would be an easier and more practical manner of determining population fluctuations than estimating numbers of individuals for this tiny aquatic fern.

Rescue effect

Since A. mexicana is not tracked in Washington State, information on its overall abundance is not available. However, based on information obtained from the collection labels for A. mexicana specimens held in the University of Washington(UW) Herbarium at the Burke Museum of Natural History and Culture (2007), the species does not appear to be abundant in that state as only eight collections are recorded from Washington State. Two of those collections are from the 1800s, and three of the more recent collections list the species as not common. The three collections from the two northerly-most counties do, however, indicate that the species is abundant at those sites. Assuming that waterfowl could transport plants or spores, it is possible that plants from Washington State populations could rescue Canadian populations. Such an event, however, would likely occur infrequently with long intervals between events. This is surmised on the basis that in the Okanagan only 10 locations are known and many other seemingly suitable water bodies are unoccupied by the species. If movement by waterfowl is the primary means of natural dispersal, then presumably more suitable habitats should be occupied.

Limiting factors and threats

Limiting factors

Several naturally limiting factors affect Mexican Mosquito-fern in BC.

  1. Mexican Mosquito-fern is reportedly sensitive to cold temperatures, which may affect populations during periods of temperature extremes and may account for population fluctuations from year to year. Plants growing in sites with higher pH are reportedly less susceptible to cold temperatures. However, see the discussion in Footnote #3, page 16.
  2. Mexican Mosquito-fern is also sensitive to changes in pH, salinity and water temperatures, as well as to phosphorus and iron levels in the water. Slight changes to these elements resulting from natural changes to site conditions (e.g., periodic flooding) could also account for population fluctuations and place distributional limitations.
  3. Nitrogen fertilizers adversely affect Azolla growth, but reports of the effects are inconsistent (Lumpkin and Plucknett 1980).


Several threats to Mexican Mosquito-fern and its habitat have been identified by Brunton (1984), Martin (In press) and during work for this status report:

  1. Road construction: Several populations of this species occur adjacent to highways and roads, and other public use areas, and this proximity to these areas poses a threat. One population was lost as a result of infilling related to road construction/road repairs (south of Darfield) (Douglas 2004).
  2. Housing/urban development: This is a threat in the Sicamous and Vernon areas. One of the two subpopulations in Vernon has recently been purchased by a development company, and one subpopulation in the Sicamous area has been infilled in an area of housing development.
  3. Eutrophication: Eutrophication is apparent at population 7, subpopulation 2 (Salmon Arm foreshore subpopulation). This site is located at the base of the public pier, along the foreshore of Lake Shuswap, immediately to the east of the park area. Algae bloom was apparent at this site in 2005 and 2006, although Azolla plants were present; the actual effect of a bloom on Azolla is, however, unknown.
  4. Invasive species: Encroachment of Reed Canary Grass (Phalaris arundinacea) is also evident at population 7, subpopulation 2 (Salmon Arm foreshore), and was reported by Douglas in 2004 and confirmed in 2005 and 2007.
  5. Accidental spills and runoff:
    1. Roadways/highways: Most populations of Mexican Mosquito-fern are found along highways and in public access areas where impact from adjacent roadwork, chemical spills, or development could potentially affect the wetland habitat and could alter water chemistry, conceivably destroying entire populations. Additionally, sensitivity to high levels of salinity use on adjacent highways would make salting of roadways and associated runoff a threat. Salt and other toxic runoff from roadways could impact plants directly.
    2. Railway spills: While the chances of a railway spill adjacent to the two populations located along the railway at Salmon arm are low, an occurrence would nevertheless have significant impact on the adjacent wetlands, and Azolla mexicana.

Special significance of the species

Azolla mexicana is a species at risk in BC and Canada that reaches its northern limits of distribution in the Little Fort area of British Columbia. BC populations are disjunct by 300 km from the nearest US population in Washington State. As populations, peripheral to the main species range, our populations could be important to the species long-term survival in the face of climate warming.

In other parts of the world, A. mexicana is grown as a crop and is of economic importance. It is used as a green manure and as a weed control measure in rice and other crops. It is also used as food for livestock. In some regions it is considered an invasive species. It is, for example, listed as a noxious weed in some counties in Montana.

Existing protection or other status designations

In Canada, Mexican Mosquito-fern is protected on federal lands by the Species at Risk Act (SARA). Specific conservation status in Canada and the US follows:

  • Global Status: G5 (Global: Secure)
    • Global Status Last Reviewed: 27 January 1987
    • Global Status Last Changed: 27 January 1987
    • Rounded Global Status: G5-Secure
  • Nation: United States
    • National Status: NNR (National: Not Ranked)
  • Nation: Canada
    • National Status: N2 (National: Imperiled)
  • COSEWIC: T (Threatened 2000)

United States

  • Arizona (SNR)
  • Arkansas (SNR)
  • California (S3.2?) (Subnational: Vulnerable to Imperiled?)
  • Colorado (S4) (Subnational: Apparently Secure)
  • Illinois (S4)
  • Iowa (S3) (Subnational: Vulnerable)
  • Kansas (SNR)
  • Minnesota (SNR)
  • Missouri (SNR)
  • Nebraska (SNR)
  • Nevada (SNR)
  • New Mexico (SNR)
  • Oklahoma (SNR)
  • Oregon (SNR)
  • Texas (SNR)
  • Utah (S2?) (Subnational: Imperiled?)
  • Washington (SNR)
  • Wisconsin (SNR)


  • British Columbia (S2)
  • SARA: Schedule 1, threatened

Technical summary

Azolla mexicana

Mexican Mosquito-fern – Azolle du Mexique

Range of Occurrence in Canada:

British Columbia

Demographic Information

Generation time (average age of parents in the population):
<1 yrs
Observed percent reduction in total number of mature individuals over the last 10 years.:
Projected percent reduction in total number of mature individuals over the next 10 years.:
Observed percent reduction in total number of mature individuals over any 10 years period, over a time period including both the past and the future.:
Are the causes of the decline clearly reversible?
Are the causes of the decline understood?
Have the causes of the decline ceased? - Observed trend in number of populations-Likely stable but uncertain since losses have occurred and new populations found due to increased search effort and population fluctuations with some absent some years.:
Are there extreme fluctuations in number of mature individuals? - Numbers of plants can fluctuate greatly at sites but due to the presence of dormant spores “extreme fluctuation” as defined by IUCN is not met.:
Are there extreme fluctuations in number of populations?

Number of mature individuals in each population

Millions of tiny aquatic ferns reproducing vegetatively at 8 extant sites with fluctuations noted at some sites.

Grand Total:
Millions likely in any one year

Extent and Area Information

Estimated extent of occurrence (km²):
5400 km²
Observed trend in extent of occurrence-Increased numbers of localities are likely due mainly to increased search effort.:
Are there extreme fluctuations in extent of occurrence?
Estimated area of occupancy (km²)-11 based on 1x1 km grid; 40 based on a 2x2 km grid:
1x1 grid = 11km², 2x2 grid = 40km²
Observed trend in area of occupancy-Perhaps stable but uncertain since new sites found and some lost.:
re there extreme fluctuations in area of occupancy? -There can be apparent visual fluctuation from year to year in the actual area of habitat occupied.:
Is the extent of occurrence or area of occupancy severely fragmented?-Sites are 50-80 km apart although the potential exists for occasional movement of propagules by waterfowl. “Severely fragmented” as defined by IUCN cannot be readily determined.:
Number of current locations:
Trend in number of locations-The current number is seemingly an increase but this is likely due to the more extensive fieldwork since the original status report.:
Are there extreme fluctuations in number of locations?
Observed trend in area of habitat-The species has undergone some population losses and fluctuations in size of populations makes it difficult to determine a trend.:

Quantitative Analysis

Ex.: % probability of extinction in 50 years

Threats (actual or imminent, to populations or habitats)

Road construction and associated infilling has resulted in the loss of one population (Darfield). Site development (infilling) has resulted in the loss of another population (Sicamous). One subpopulation in Vernon occurs on a property that was recent purchased by a land development company, so its future is very insecure. Several populations / subpopulations have not been observed for several years (i.e., Eagle River, Vernon Creek, White Creek) although no apparent causal factor can be identified. This could be a result of lack of observation at the right time, natural population variation, or actual extirpation of the population. The majority of the populations, with the exception of the Vernon and White Creek populations and some of the Salmon Arm subpopulations, are located adjacent to major highways or a railway, so accidental spills and maintenance activities and associated chemicals (e.g., road salt in the winter) are ever-present threats.

Rescue Effect (immigration from an outside source)

Status of outside population(s)?
USA : Not assessed nationally
Is immigration known or possible?-Possible but no evidence available:
Would immigrants be adapted to survive in Canada?
Is there sufficient habitat for immigrants in Canada?
Is rescue from outside populations likely?-Movement of propagules is possible but unlikely over 300 km distance during migration north from Washington. If this were a likelihood, more populations should have become established in BC.:

Current Status

COSEWIC: Threatened 2008

Status and Reasons for Designation

Status: Threatened

Alpha-numeric code: B1ab(iii,iv,v)+2ab(iii,iv,v)

Reasons for designation: This tiny floating aquatic fern of south-central British Columbia is restricted to 8 small water bodies where its populations undergo periodic fluctuations in numbers of individuals. Two populations and their habitat have been lost in recent years due to construction activities with most of the extant populations occurring adjacent to major highways or a railway where they are at potential risk from maintenance activities, including the use of chemicals such as road salt.

Applicability of Criteria

  • Criterion A (Decline in Total Number of Mature Individuals): Not applicable. No overall decline percentages are available although two populations have become extirpated.
  • Criterion B (Small Distribution Range and Decline or Fluctuation): Meets Threatened B1ab(iii,iv,v)+2ab(iii,iv,v); the extent of occurrence and area of occupancy are below critical values for Threatened; 8 extant populations are known with recent losses of two populations resulting in a loss of habitat, number of locations and mature individuals.
  • Criterion C (Small and Declining Number of Mature Individuals): Not applicable. Population size exceeds maximum criterion levels.
  • Criterion D (Very Small Population or Restricted Distribution): Not applicable. Population size is too large for D1. Criterion D2, even when based on the smaller IAO being <20 km², may not apply due to the presence of 8 extant populations and the potential for the discovery of a few additional small populations in areas not directly accessible by roads.
  • Criterion E (Quantitative Analysis): None available

Acknowledgements and authorities consulted

Several knowledgeable individuals were consulted during the preparation of this status report. The author would like to extend thanks to Adolf Ceska, Sharon Hartwell, Frank Lomer, Malcolm Martin, and Jenifer Penny for providing key information about Azolla mexicana and other Azolla species in BC. A special thank you to Malcolm Martin for providing ownership information on Vernon populations, specimens, and field accompaniment. Particular thanks go to Rose Klinkenberg for assistance with background research and fieldwork. The assistance of Linda Jennings, University of British Columbia (UBC) Herbarium, in obtaining specimens loans is very much appreciated.

Authorities consulted

  • Alain Filion. COSEWIC Secretariat, Map Preparation and Calculations. E-mailed August 8, 2007.
  • Kevin Fort (cc Bob Elner), Canadian Wildlife Service, Pacific and Yukon Region. E-mailed August 8, 2007.
  • Dave Fraser, BC Endangered Species Specialist, Biodiversity Branch, BC Ministry of Environment. E-mailed August 9, 2007.
  • Gloria Goulet, COSEWIC Secretariat, Aboriginal Traditional Knowledge. E-mailed August 9, 2007.
  • Ted Lea, Recovery Team, BC Ministry of Environment. E-mailed August 9, 2007.
  • Jenifer Penny, Provincial Botanist, BC Conservation Data Centre. E-mailed August 9, 2007.

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Green, A.J., K.M. Jenkins, D. Bell, P.J. Morris and R.T. Kingsford. 2008. The potential role of waterbirds in dispersing invertebrates and plants in arid Australia. Freshwater Biology 53: 380-392.

Greenwood, D.R., S.B. Archibald, R.W. Mathewes, and P.T. Moss. 2005. Fossil biotas from the Okanagan Highlands, southern British Columbia and northeastern Washington State: climates and ecosystems across an Eocene landscape. Canadian Journal of Earth Sciences 42: 167-185.

Johnson, G.V. 1986. Evaluation of Salt Tolerance in Azolla. [Accessed September 2007].

Keddy, P.A. 1976. Lakes as islands: the distributional ecology of two aquatic plants: Lemna minor L. and Lemna trisulca L. Ecology: 57: 353-359.

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Lomer, F. 2007. Personal communication. E-mail correspondence.

Lumpkin, T.A. 1993. Azollaceae. In: Flora North America, Volume 2: Pteridopytes and Gymnosperms. Oxford University Press, Oxford.

Lumpkin, T.A. and D.L. Plucknett. 1980. Azolla: Botany, physiology, and use as a green manure. Economic Botany 34(2): 111-153.

Martin, M.E., pers. comm. 2007. E-mail correspondence and direct communication to B. and R. Klinkenberg. Various e-mails and in-field discussions.

Martin, M.E. In press. Update COSEWIC status report on the Mexican Mosquito-fern Azolla mexicana in Canada. Committee on the Status of Endangered Wildlife in Canada, Ottawa. 11 p. Note: This status report has not yet been officially released to the public by COSEWIC and is therefore considered to be still “In press”.

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Biographical summary of report writer

Brian Klinkenberg is an Associate Professor of Geography at the University of British Columbia, and holds an M.Sc. in biogeography and a Ph.D. in landscape modelling and GIS. His research focuses on spatial analysis, landscape analysis, and the biogeography of rare plants, and has included exploration of the coincidence of rare plants and landform, the biogeography of the vascular flora of the Erie Islands, ecosystems services, effects of landscape and habitat fragmentation on forests, methods of evaluating restoration success, and nature reserve design. He is project coordinator and editor for E-Flora BC and E-Fauna BC, the biogeographic atlases of British Columbia. Brian is presently Chair of the Research Committee of the South Coast Conservation Program, and sits on the recovery teams for Streambank Lupine and Phantom Orchid. He has authored several national recovery plans, including Bidens amplissima, Cephalanthera austiniae, Azolla mexicana, Symphyotrichum frondosum, Epilobium torreyi, and Lupinus rivularis. He has also authored or co-authored several COSEWIC status reports, including Astragalus spaldingii, Bidens amplissima, Celtis tenuifolia, Cephalanthera austiniae (2), Collinsia verna, Desmodium illinoense, Opuntia humifusa, Lupinus rivularis, Ranunculus californicus, and Symphyotrichum frondosum. Additional technical reports include inventory and monitoring standards for rare plant species, and predictive mapping of rare species.

Collections examined

The following collections were examined by the author:


  1. Azolla mexicana. V219686, J.A. Calder, D.B.O. Savile and J.M. Ferguson, September 14, 1954, ½ mile west of Solsqua along road between Sicamous and Revelstoke, duplicate Ministry of Agriculture (DAO). Vegetative. Unable to confirm identification.
  2. Azolla mexicana. V213376, W.B. Schofield (his collection #109591), 1997, floating mats on pond ca. 6 mi E of Sicamous, pond in woodland. Fertile. Identification (ID) confirmed by R. Klinkenberg.
  3. Azolla mexicana. V140625, A & B Schwartz, September 22, 1972. 5 miles east of Sicamous opposite RR crossing. Fertile. ID confirmed by R. Klinkenberg.


  1. Azolla mexicana. 184823, J. Deniseger. February 14, 2002. Det. Richard Hebda. Vancouver Island, Cobble Hill, just outside of Manley Creek Community Park, north of Arbutus, Victoria. Golf Course. Plants less than .5 cm, vegetative. This specimen was re-determined as Azolla caroliniana by R. Klinkenberg 2007.
  2. Azolla mexicana. 174999, M.E. Martin, November 4, 1997, Vernon Creek, .5 km along creek from mouth, oxbow at bend of creek. Vegetative. Verified by G. W. Douglas.
  3. Azolla mexicana. 174998, M.E. Martin, November 4, 1997. Det. M.E. Martin. Vernon, 6141 Okanagan Avenue 82L-3, two low patches in field surrounded by rough pasture. Vegetative. Verified by G. W. Douglas.
  4. Azolla mexicana. 196897, Bob Scheer, September 3, 1998. Thompson-Okanagan, Little Fort, 1.13 km south of Little Fort, junction of highways 5 and 24, in abandoned stream channel at roadside in pond channel. Fertile. ID confirmed by R. Klinkenberg.

Other (unprocessed specimens to be deposited at UBC)


  1. Azolla mexicana. Malcolm Martin. August 24, 2005. Vernon Creek, north side, in oxbow across from Marshall Fields parking area, in stagnant shallow water. A few scattered but immature sporocarps.
  2. Azolla mexicana. Brian Klinkenberg. July 2005. Salmon Arm, in shallow water of the bay immediately adjacent to public pier, east side of park where it meets the pier. Many sporocarps. ID confirmed by R. Klinkenberg 2005/2007.
  3. Azolla mexicana. Brian Klinkenberg. July 2005. Little Fort, just north of, in shallow water in oxbow. Fertile. ID confirmed by R. Klinkenberg 2005/2007.


  1. Azolla mexicana. Brian Klinkenberg. August 2007. Salmon Arm, along railway tracks east of pier. Fertile. ID confirmed by R. Klinkenberg 2007.
  2. Azolla mexicana. Brian Klinkenberg. August 2007. Darfield. Twenty meters south of Darfield sign, in shallow water of oxbow on east side of road. Fertile. ID confirmed by R. Klinkenberg 2007.
  3. Azolla mexicana. Brian Klinkenberg. August 2007. Little Fort, south of, in shallow water in old oxbow, east side of road. Population continues in patches on west side of road in oxbows and ponded areas. Fertile. ID confirmed by R. Klinkenberg 2007.
  4. Azolla mexicana. Malcolm Martin. October 2007. Vernon Creek, in small drainage ditch into Vernon Creek. Disturbed area. Immature plants with no sporocarps. (No plants were present when the area was checked in August 2007).

Specimen examined by Frank Lomer

  1. Azolla mexicana. George DouglasFootnote5. Date Unknown (probably 2004). Site unknown. Fertile. ID confirmed by Frank Lomer 2007.


Azolla mexicana. V219683, J.A. Calder, September 14, 1954, ½ mile west of Solsqua along road between Sicamous and Revelstoke, duplicate DAO. Re-determined to A. caroliniana.

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