Isoetes melanopoda Gay & Durieu ex Durieu
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Basis for Listing
Isoetes melanopoda occurs in Minnesota at the northern edge of its range, which is discontinuous, disjunct, and largely restricted to the Midwest. In fact, the Minnesota populations of this fern ally appear to be disjunct 250 km (155 mi.) or more from the nearest population. The potential for its occurrence in Minnesota is further limited by its unusual habitat requirements. It appears to be restricted to an aquatic microhabitat associated with bedrock outcrops of Sioux quartzite. Such outcrops themselves are limited to four or five counties in southwestern Minnesota, and the specific microhabitat itself is even more limited. Because the habitat occurs among rock outcrops, it is largely immune to land conversion practices such as plowing and ditching. However, these sites are frequently grazed by cattle, which may cause permanent trampling damage to the habitat. For these reasons, I. melanopoda was listed as an endangered species in Minnesota in 1984.
There is debate over the identity of I. melanopoda. Recent research suggests that significant genetic differences separate plants found east of the Appalachians from those found west of the Appalachians. Isoetes is an ancient genus that is morphologically distinct from anything else that might be encountered in Minnesota. And yet, this species is very difficult to spot among the grasses and rushes with which it grows. Leaves of Isoetes are homologous to the fronds of a fern; they are fertile leaves that bear the sporangia (structures containing spores). Leaves have a pale line running down the middle of the inner surface. On mature plants, the base of the leaf is black, dilated, and bears a sporangium on the inner surface; immature plants have white bases. Leaves are attached to a short, solid, globular rootstock. It is usually necessary to conduct a microscopic examination of the megaspores to differentiate species of Isoetes, but I. melanopoda is the only member of the genus Isoetes found in southwestern Minnesota.
Isoetes melanopoda is restricted to the prairie region of the state, specifically where Sioux quartzite bedrock is exposed at the surface. Where these outcrops are exposed as flat terraces on gentle slopes, vernal pools may form between the outcrops or in depressions in the rock itself. These pools are sustained by seepage from between the strata or directly from rain. Seepage pools generally retain water longer than rain pools and may be wet through the warm months of summer. Isoetes melanopoda occurs on the margins of deep rainwater pools that typically persist for at least 1.5 months in the spring, as well as other pools that are maintained by groundwater seepage. The plants are usually rooted in shallow soil or organic debris, which may be only 2.5 cm (1 in.) deep. During the early stages of development, the plants may be submerged or emergent in 5-10 cm (2-4 in.) of water. But by the time they mature in midsummer, they are usually stranded in mud. This is a very discrete and uncommon habitat type in Minnesota. It is also the preferred habitat of other rare species such as Heteranthera limosa (mud plantain), Plantago elongata (slender plantain), and Callitriche heterophylla (larger water-starwort). Elsewhere in the United States, I. melanopoda is found on non-granitic rocks, upland depression swamp forests, intermittent low woodland streams, low wet edges of agricultural fields, water-filled depressions and wet edges of rock outcrops, and other seasonally wet areas. This variety in habitats and range may explain the suspicion that I. melanopoda actually represents at least two distinct species (Cobb 1984; Lellinger 1985).
Biology / Life History
Mature I. melanopoda plants have fertile leaves that contain male spores, or microspores, and female spores, or megaspores. These spores produce unisexual structures, the male sex cells, and a structure containing the eggs. Upon fertilization, the egg develops within the female structure into a young I. melanopoda plant. In turn this plant produces microspores and megaspores. There has been little research done on Isoetes in comparison to other members of the ferns and fern-allies. It is known that this genus experiences a striking amount of hybridization. As a result, the taxonomic validity of the species is in question. Isoetes melanopoda is known to hybridize with I. engelmannii (Appalachian quillwort), which does not occur in Minnesota. Plants appear to require cold winter weather to thrive (Cobb 1984; Lellinger 1985).
Conservation / Management
Habitat degradation may be the greatest threat to I. melanopoda in Minnesota. While its habitats are not prime targets for agriculture, they are frequently grazed by livestock and are increasingly being mined, with the rock being crushed for gravel. Evidence has shown that cattle will trample the fragile habitats, which damages or destroys the native vegetation and allows non-native species to invade. The broadcast application of herbicides applied for pasture "improvement" is a potential threat, although the specific susceptibility of Isoetes to chemical herbicides is not known.
Conservation Efforts in Minnesota
Most known populations of I. melanopoda in Minnesota are on state and federal lands, including a National Wildlife Refuge, a National Monument, State Parks, and Wildlife Management Areas. A few populations persist in privately owned pastures.
References and Additional Information
Cobb, B. 1984. A field guide to ferns and their related families, northeastern and central North America. Houghton Mifflin Company, Boston, Massachusetts. xv + 281 pp.
Lellinger, D. B. 1985. A field manual of the ferns and fern-allies of the U.S. & Canada. Smithsonian Institution Press, Washington, D.C. 389 pp.