Utricularia resupinata B.D. Greene ex Bigelow
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Basis for Listing
When Utricularia resupinata (lavender bladderwort) was listed as special concern in 1996, it was known from just five lakes in Minnesota. The hope was that unexplored potential habitat would produce more records, once further surveys were conducted. After all, this is a lake plant occurring in the lake-rich region of northeastern Minnesota (Laurentian Mixed Forest Province). Due to that reasoning, a more protective classification of threatened or endangered was considered premature. By 2015, botanical surveys of more than 2,000 lakes had been completed by the Minnesota Biological Survey, and only 29 lakes were found to harbor populations of U. resupinata. All but five of the lakes were clustered in a small area of the Border Lakes region. Suitable habitat exists only in those lakes where there is shallow and quiet water, over a sandy and gently sloping bottom. Clearly U. resupinata is not only a very rare species in Minnesota, it is also restricted to a very specific and vulnerable habitat type that appears to occur in only a very small portion of the state; therefore conflicts between recreation and conservation are a real concern. For these reasons, the status of U. resupinata in Minnesota was changed to threatened in 2013.
The stems of U. resupinata are very delicate and creep on or just below the surface of the sandy or peaty substrate, which is typically under several inches of water. Roots are absent. The leaves are alternate and divided into 3 segments, about 3.2 cm (1.3 in.) long. The center segment is linear and erect; the lateral segments are slender, root-like, and bear the bladders. The flowers occur singly and are purple and 0.6-1.3 cm (0.3-0.5 in.) long. They are found atop an upright stalk that averages 8-30 cm (3-12 in.) tall, though it can be taller in deep water. A bract that is modified as a tube surrounds the stalk at the base of the inflorescence. The lower lip of the flower is 3-lobed, and the spur is more-or-less horizontal.
The primary habitat of U. resupinata is shallow water in protected bays of large oligotrophic lakes (that is lakes with low nutrient concentration and low plant growth). In particular, it occurs where the lake bottom is sandy, gradually sloping, and largely free of competition from other plant species. It also occurs where the sand is overlain by unconsolidated organic sediments. In some of the habitats, the plants may become stranded above the water line as the shoreline recedes over the course of the summer. At one site, U. resupinata was found growing in a substrate of peat, rather than sand. The maximum water depth inhabited by U. resupinata is unknown, though one study in New York recorded a maximum depth of 3 meters (10 ft.) (Sheldon and Boylen 1977).
Biology / Life History
Plants in the genus Utricularia are notable for having tiny bladders that capture small invertebrates. The bladders have been described as deflated pear-shaped pouches; they are not air-filled or used for floatation. The bladders open abruptly when trigger hairs are disturbed, sucking in water along with the hapless aquatic creature responsible for touching the trigger (Marmottant 2011;video). Digestive enzymes and bacteria in the bladder then digest the prey for the nutritional use of the plant, a process typically taking fifteen minutes to two hours, depending on the size of the catch. When digestion is complete, special cells transfer the nutrient-rich water from the bladder into the stem, and in the process they reset the trap for the next quarry. Although U. resupinata is an aquatic species, it is sometimes found stranded on a sandy beach that has recently been exposed by receding water. This does not seem to harm the plants. In fact, it seems to stimulate flowering and may increase seed germination.
Conservation / Management
The fate of U. resupinata is closely linked to the fate of the sandy beaches and shallow-water habitat where it occurs. Natural water-level fluctuations that are a part of annual and seasonal precipitation patterns are also believed to be important to population maintenance, though there is little experimental data to substantiate this. In all cases, shoreline development that changes the contours of the beach or interferes with natural dynamics of the water-shore interface must be considered an impairment of the habitat. Changes in lake water chemistry or modifications to hydrological regimes would also be concerns.
A study conducted in New York identified U. resupinata as one of the aquatic species most vulnerable to invasion by the non-native Myriophyllum spicatum (Eurasian watermilfoil) (Boylen et al. 1999). It is not known whether any of the lakes where U. resupinata occurs in Minnesota are suitable habitat for Myriophyllum spicatum, however, it is a possibility that must be considered.
Best Time to Search
The best time to search for U. resupinata is when it is in flower, from mid-July to mid-September.
Conservation Efforts in Minnesota
Most of the occurrences of U. resupinata in Minnesota are within the Boundary Waters Canoe Area Wilderness (BWCAW). As a federally managed wilderness area, the BWCAW receives a very high degree of protection from large-scale human disturbance. However, it must be mentioned that this is the most heavily used unit in the federal wilderness system, with over 200,000 visitors a year, and the shallow sandy bay habitats of U. resupinata are a great attraction to visitors. While casual visitation, including canoeing, swimming, and fishing would probably not harm the species, establishing a campsite in an occupied bay or routing a portage to or from a bay could threaten the long-term viability of a population. All potential conflicts between recreational use of the lakes and protection of sensitive rare plant habitat needs to be continually monitored.
Welby Smith, MN DNR, 2008 and 2018
Boylen, C. W., L. W. Eichler, and J. D. Madsen. 1999. Loss of native aquatic plant species in a community dominated by Eurasian watermilfoil. Hydrobiologia 415:207-211.
Coffin, B., and L. Pfannmuller, editors. 1988. Minnesota's endangered flora and fauna. University of Minnesota Press, Minneapolis. 473 pp.
Fassett, N. C. 1957. A manual of aquatic plants. University of Wisconsin Press, Madison, Wisconsin. 405 pp.
Lloyd, F. E. 1942. The carnivorous plants. The Chronica Botanica Company, Waltham, Massachusetts. 352 pp.
Marmottant, P. 2011. The ultra-fast trap of an aquatic carnivorous plant. YouTube [web application]. <https://www.youtube.com/watch?v=Zb_SLZFsMyQ&t=63s>. Accessed 08 February 2017.
Minnesota Department of Natural Resources. 2003. Field guide to the native plant communities of Minnesota: the Laurentian mixed forest province. Ecological Land Classification Program, Minnesota County Biological Survey, and Natural Heritage and Nongame Research Program. Minnesota Department of Natural Resources, St. Paul, Minnesota. 352 pp.
Minnesota Department of Natural Resources, Division of Ecological Resources. 2008. Rare species guide: an online encyclopedia of Minnesota's rare native plants and animals [Web Application]. Minnesota Department of Natural Resources, St. Paul, Minnesota. Accessed 1 July 2009.
Minnesota Department of Natural Resources, Division of Fish and Wildlife. 1995. Statement of need and reasonableness in the matter of proposed amendment of Minnesota Rules, Chapter 6134: endangered and threatened species. Minnesota Department of Natural Resources, St. Paul, Minnesota. 336 pp.
Ownbey, G. B., and W. R. Smith. 1988. New and noteworthy plant records for Minnesota. Rhodora 90:369-377.
Sheldon, R. B., and C. W. Boylen. 1977. Maximum depth inhabited by aquatic vascular plants. American Midland Naturalist 97(1):248-254.
Taylor, P. 1989. The genus Utricularia: a taxonomic monograph. Kew Bulletin Additional Series XIV. Royal Botanic Gardens, Kew, London. 724 pp.