Water Howellia

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Water Howellia

Howellia aquatilis

ListedJuly 14, 1994
FamilyCampanulaceae (Bellflower)
DescriptionBranched water plant with submerged or floating stems having narrow leaves and white flowers.
HabitatFirm, consolidated clay and organic sediments in shallow water or the edges of deep ponds.
ThreatsLoss of wetland habitat due to timber harvesting, livestock grazing, residential development, competition from alien plants.
RangeCalifornia, Idaho, Montana, Oregon, Washington


Howellia aquatilis (water howellia), a highly specialized wetlands plant, is a monotypic genus in the bellflower family (Campanulaceae). The plant was first described in 1879, from specimens collected in Multnomah County near Portland, Oregon. Water howellia is as an aquatic annual plant that grows 4-24 in (10-60 cm) in height, and has extensively branched, submerged or floating stems with narrow leaves 0.4-2 in (1-5 cm) in length. Two types of flowers are produced: small, inconspicuous flowers beneath the water's surface, and emergent white flowers 0.08-0.11 in (2-2.7 mm) in length. The plant predominantly self-pollinates with each fruit containing up to five large ( 0.08-1.6 in; 2-4 mm) brown seeds.


Water howellia grows in firm consolidated clay and organic sediments that occur in wetlands associated with ephemeral glacial pothole ponds and former river oxbows. These wetland habitats are filled by spring rains and snowmelt run-off. Depending on temperature and precipitation they may exhibit some drying during the growing season. This plant's micro habitats include shallow water and the edges of deep ponds partially surrounded by deciduous trees.

Howellia reproduces entirely from a seed. Germination only occurs when ponds dry out and the seeds are exposed to air. The size of an annual population varies in abundance depending on the extent of drying in the previous growing season.. Exceedingly wet or dry seasons can have a detrimental effect on plant numbers the following year. The length of time seeds remain viable is unknown. However, seeds that remain in the soil longer than 8 months have shown decreased rates of germination and vigor. Genetic variability in howellia populations is low throughout its range. This suggests that all populations of howellia most likely represent a single, narrowly adapted genotype. This low rate of genetic variability within populations may explain why the species is restricted to a highly specific habitat.


Water howellia has been extirpated from more than one-third of its former range including California, Oregon, and some sites in Washington (Mason and Thurston counties) and Idaho (Kootenai County). Populations are extant only in very small areas of Montana, Washington, and Idaho.

Only 79 small populations of this aquatic plant were known to exist when the proposed rule to list the species was published. Subsequent inventories conducted for howellia in Washington state located 28 new sites in Spokane County alone, thus expanding the number of known populations to 107. Nearly all of the remaining populations of howellia are clustered in two main population centers or metapopulations. They occur primarily in groups of closely adjacent ponds, although some ponds within the range of these metapopulations are unoccupied. One metapopulation near Spokane, Washington, consists of 46 individual populations in Spokane County, Washington, and one in Latah County, Idaho. A second metapopulation is found in the drainage of the Swan River in northwestern Montana (Lake and Missoula Counties) where 59 individual populations are found. This total represents a plant presence in only 13.5% of 437 potential habitats that have been surveyed since 1987. In addition to metapopulations, a third site near Vancouver in southwestern Washington (Clark County) contains two small populations in close proximity.

The large fluctuations in annual numbers, the low genetic variability, and habitat specificity indicates that isolated populations of howellia may be vulnerable to extirpation. However, the individual populations within the metapopulations appear interdependent and may act as founders. Most populations are extremely small. The fifty-nine populations found in Montana cover an area of only about 127 acres (51 hectares). Of this area, one population occurs in a 30-acre (12-hectare) pond, one in a 5-acre (2-hectare) pond, one in a 4-acre (1.6-hectare) pond, four in 3 acres (1.2 hectares) of ponds, 24 in ponds of 1-2 acres (0.4-0.8 hectares) in size, and the remaining 28 are in ponds of 1 acre (0.4 ha) or less. The U.S. Forest Service (Forest Service) estimates total area of occupied and suitable unoccupied habitat on Forest Service lands to be less than 200 acres (80 hectares).

Populations of howellia occur both on private and public lands. Of the 59 known populations in Montana, 21 (36%) are found on private lands, 34 (57%) occur on lands administered by the Forest Service, and four (7%) occur on a mixture of private and Forest Service lands. In Washington, 34 of the 47 populations (72%) are found on Service administered lands, 11 (24%) occur on private lands, one (2%) is on State land, and one (2%) is on Bureau of Land Management land. The one population in Idaho occurs solely on private property.


The major threats to Water howellia are loss of wetland habitat and habitat changes due to timber harvesting, livestock grazing, residential development, and competition by introduced plant species. The genetic similarity between different populations of howellia, its consequent confinement to extremely specific habitats, and its relative inability to adapt to abrupt environmental changes add to the vulnerability of the species.

Howellia populations can be threatened by the environmental effects associated with timber harvesting. The removal of trees from around ponds may increase water temperatures and evaporation, thus increasing wetland drying and influencing plant succession. Increased siltation occurs in wet-lands where logging or associated road building and maintenance is conducted, and this action changes bottom substrates and the vegetational composition of the sites. Water howellia occurs most frequently in ponds with firm and consolidated organic clay bottom sediments. It is also found in more open areas within these ponds. An increase in bottom sedimentation and subsequent competition from other vegetation could have an adverse effect on howellia populations. Of the 59 populations of howellia in the Swan Valley, Montana, 22 (37%) occur within areas where logging has taken place around the wetland margins. In Montana, 58% of the populations of howellia occur on Forest Service lands, and an additional 7% occur on lands partially owned by the Forest Service. The Plum Creek Timber Company owns 38% of the private lands in Montana where howellia occurs. Timber harvest has been increasing within the area of the Spokane metapopulation.

Livestock, by their grazing and trampling, can also adversely affect howellia populations due to the disturbance of shorelines and associated vegetation. Trampling of bottom sediments adversely affects the seed bank and the consolidated substrate which appears to be necessary for germination. Additionally, livestock waste increases nutrient loading in wetlands causing a change in the water quality that may alter pond vegetation composition. It is not known how much grazing impact can be tolerated by howellia, although the plant still exists in ponds that have been disturbed by grazing. The plant's ability to withstand grazing is evidently influenced by the timing, magnitude, and duration of grazing. The cumulative impacts of grazing and other human-induced disturbances threaten a number of populations. The California population may have been eliminated by cattle grazing and trampling, and two wetlands on private lands in Montana with populations of howellia have been heavily impacted by domestic livestock, especially horses. In Washington, 23% of the populations occur on private lands, many of which are subject to grazing. Grazing also took place on some of the lands administered by FWS until 1993. In Spokane County, Washington, several of the ponds containing howellia have been significantly altered by past and current grazing practices.

Sites where howellia was historically found in Oregon have been converted to urban areas, and an increase in residential development is occurring in the Spokane metapopulation area. Additionally, the construction of dams along the Columbia and Willamette Rivers has led to a loss of suitable wet-land habitats. Many wetlands within the historic range of water howellia have been drained, filled, or excavated for other uses.

Howellia has narrow ecological requirements, and even subtle habitat disturbances that alter the surface or subsurface hydrology of the pond could devastate a population. Activities that affect the ecology of a wetland bottom habitat also may affect wetland succession and the survival of howellia populations.

Howellia does not compete well with other aquatic vegetation and its wetland habitats are being threatened by Phalaris arundinacea (reed canary grass), a highly competitive, robust grass that invades wetlands. Howellia has been observed growing amongst reed canary grass stands, but only where these stands are sparse, or in openings entirely free of the grass. Reed canary grass has the potential to extirpate howellia populations due to its ability to rapidly form dense monocultures which cause the decline of nearly all other plants in a wetland. This exotic grass accelerates the rate of wetland succession causing significant changes in substrate and water table levels. Both native and exotic varieties of this grass occur in North America, and it is not known whether the variety occuring in wetlands within the range of howellia is native or exotic. However, due to the pernicious characteristic of the invasions and the lack of historical records of its presence in this region, some ecologists in the Pacific northwest believe this invasive variety of P. arundinacea is an exotic form that was introduced by humans. Reed canary grass is considered a major threat to howellia in the state of Washington since it occurs in 83% of the ponds where howellia is present. This exotic grass also threatens the howellia population in Idaho due to its presence in nearby ponds. Reed canary grass has also been found in several Montana ponds occupied by howellia.

Lythrum salicaria (purple loosestrife), another aggressive exotic plant, also poses a threat to howellia because it can out-compete and eliminate other aquatic plants. Purple loosestrife is present in Lake County, Montana, and also in the immediate vicinity of the Spokane howellia metapopulation.

Conservation and Recovery

Some protection already exists for this species since it is contained on the Forest Service's list of sensitive species for the Pacific Northwest region. However, populations that occur on private lands are not legally protected. Federal, state and local agencies have been notified of the localities of howellia, and landowners have been notified of the location and importance of protecting habitat of this species. FWS believes that federal involvement can be effective without the designation of critical habitat.


U.S. Fish and Wildlife Service
Regional Office, Division of Endangered Species
Eastside Federal Complex
911 N.E. 11th Ave. Portland, Oregon 97232
(503) 231-6121


U. S. Fish and Wildlife Service. July 14, 1994. "De-termination of The Plant Water Howellia (Howellia aquatilis ), Determined to Be a Threatened Species." Federal Register 59(134): 35860-35864.