Limosella aquatica L.
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Basis for Listing
The distribution of Limosella aquatica has been called interruptedly circumboreal (Gleason and Cronquist 1991), meaning that it occurs in a discontinuous band across the northern parts of North America, Europe, and Asia. In some areas it is reported to be common, in other areas it is rare, and in most areas it is absent; certainly, in our part of the world it is quite rare. It is also restricted to a very specialized and very limited habitat type, specifically ephemeral pools that develop in shallow depressions in bedrock outcrops and in small depressions in native prairies. These habitats are actually quite fragile, especially where they develop on true soil and can easily be drained and plowed. But even the habitats on rock outcrops can be damaged by intensive cattle grazing or destroyed by rock quarrying. There is even a growing trend to demolish outcrops to expand crop fields. Protection of these habitats takes on a special urgency when it is realized that a number of other rare species also occur in these habitats and face the same threats (Moore and Tryon 1946). Limosella aquatica was listed as a special concern species in Minnesota in 1984.
Limosella aquatica is a small "mud-plant" that grows only a few inches high. It does not have an upright stem, but it does produce runners (stolons) that grow across the surface of the ground and root at the nodes. At each of these rooted nodes it will produce a cluster of basal leaves that radiate out in all directions. Each leaf has a long, slender petiole 1-10+ cm (0.4-3.9+ in.) in length, and an elliptic blade 1-3 cm (0.4-1.2 in.) long and 2.5-12 mm (0.10-0.47 in.) wide. The shape of the whole leaf has been compared to that of a flattened spoon. There are also a number of 1-flowered peduncles that arise from the same point as the leaves. The flowers are tiny (2.5-3.5 mm (0.10-0.14 in.) long) and have 5 white petals that are fused together at the base. Each fertilized flower will develop into a spherical capsule 3-5 mm (0.12-0.20 in.) across that contains numerous small seeds. Fully developed specimens are rather easy to identify by the shape of the leaves and the small white flower, although plants often fail to fully develop.
Limosella aquatica is a wetland or perhaps a semi-aquatic species that occurs at the margins of shallow rainwater pools in lowland prairies and on rock outcrops. It usually occurs in a distinct zone that follows a particular moisture gradient around the margin of the pool. However, these small basins may fill up with water following a rain storm and submerge the plants under several inches of water. Then after a few weeks without rain, the basin will have dried out leaving L. aquatica stranded on exposed mud. All of these conditions are normal and, in fact, may actually be required for L. aquatica to complete its life-cycle.
Biology / Life History
Limosella aquatica is usually listed as an annual, and although the plant does produce runners that generate new individuals with separate root systems some distance from the parent plant, all the plants and their root systems apparently die at the end of each year. If this is actually the case, then reproduction occurs entirely by the production and dispersal of seeds. The seeds are small and probably disperse throughout the habitat on water currents, which can be significant after rain storms or spring snow melt. The vector for long-range dispersal is probably shore birds that forage in the pools and inadvertently transport the seeds in mud that adheres to their feet.
Conservation / Management
In general, management needs for a healthy population of L. aquatica are minimal. Most occurrences are in small aquatic habitats that are inclusions within larger prairie or grassland habitats, and therefore can be managed the same way a native prairie would be managed: livestock should be excluded, herbicide use curtailed, and prescribed dormant-season burns or mowing used to control woody plant invasion. However, monitoring the effects of management on populations of L. aquatica can be rather difficult. If seasonal weather conditions vary too much from established patterns, such as being too wet or too dry, it is quite plausible that no plants will appear during an entire growing season. And yet the population, existing as a seed bank in the soil, could still be healthy and reemerge the following year if weather conditions are favorable.
Conservation Efforts in Minnesota
In 2007, the Renville and Redwood County Soil and Water Conservation Districts in association with state and federal agencies coordinated an initiative to compensate interested landowners for permanently protecting the most ecologically intact rock outcrops and associated wetlands in those two counties. A total of 86 ha (212 ac.) were enrolled into perpetual conservation easements as a result of this initiative, and L. aquatica is one of the rock outcrop species most likely to benefit from the program. In 2009, the program was expanded to protect an additional 214 ha (530 ac.) within a five county area of the Upper Minnesota River valley. Conservation plans, which outline conservation practices such as removal of invasive species and livestock exclusion, are being developed for each of the easement sites. Habitat maintenance and improvement measures such as prescribed burning, fencing, rotational grazing, and/or seeding are also included in the plans.