Illustration by Nancy Klaud
A Case Study
This muti-stemmed violet persists in a number of healthy populations, but its range in Illinois is essentially restricted to the northeast, and there is still much to be learned about its biology and ecology.
Life History: Perennial
Illinois Status: Rare (not listed)
Populations monitored: 21
Subpopulations monitored: 86
Counties monitored: 4
American dog violet (Viola conspersa, synonym: Viola labradorida) is native to the central and the northeastern parts of the United States. This stemmy violet reaches 4 to 8 inches tall and features alternate, heart-shaped leaves subtended by distinctively toothed stipules. Showy, insect-pollinated flowers develop from leaf axils early in the spring, while small, green unopened and self-pollinating flowers are produced throughout the season until the first frost1,2.
Habitat and Range
American dog violet occurs from Maine westward to North Dakota, and as far south as Alabama and the panhandle of Florida. It is most abundant in the northeastern part of this range; in Illinois it is found primarily in the northeast2, though a single downstate occurrence has been noted3. Its habitat includes moist to dry woodlands, perched morainal swamps and sandy floodplains4.
American dog violet blooms from April through May, producing two kinds of flowers. Open flowers, known as chasmogamous flowers, attract insects that transfer pollen between individuals. Closed, self-pollinating cleistogamous flowers are produced until first frost. American dog violet faces a trade-off in production of these flowers—open-pollinated flowers are more likely to produce genetically diverse seeds, but they depend on insect visitors to show up at the right time. Meanwhile, closed flowers are sure to produce seeds regardless of whether pollinators visit, but the lack of outcrossing may produce less healthy offspring. When ripe, seed capsules split into three parts and fling seed from the mother plant2. American dog violet can also hybridize with other aerial-stemmed violets, such as Viola striata4,5.
Habitat loss is a primary threat to many rare species, and American dog violet is no exception. However, where populations remain, they may experience additional threats from hydrological changes, invasion by exotic species, and lack of management.
Invasive species, like common buckthorn and other woody exotic species, pose a significant threat to American dog violet because they can dramatically decrease the amount of light that reaches the forest floor. A healthy dog violet plant can have up to 50 leaves; however, under buckthorn cover the average plant often produces closer to five to six leaves6. In some cases, POC has seen a significant positive response of American dog violet to invasive species removal7, but further analysis of POC data is needed to evaluate the generality of this trend.
Although considered globally secure, American dog violet is thought to be imperiled or vulnerable in a number of U.S. states8. It was first listed as threatened in Illinois in 1980, as its habitat was considered restricted and few populations were thought to exist9. Since that time, 15 additional populations in three additional counties have been recorded. As a result, the species was removed from Illinois’s list in 2014 because it was considered to be experiencing recovery or to be more abundant than previously thought10.
Monitoring and Research
Despite its rarity, surprisingly little research (outside of that conducted by POC) has focused on American dog violet. Since 2001, POC has monitored American dog violet at 20 sites across four Illinois counties. More than 90 subpopulations have been monitored by POC staff and volunteers, and many of these continue to be monitored on a yearly basis. In-depth demographic data, in which individual plants were tagged and followed across years, was collected through 2012.
Preliminary findings from POC’s data collection have shown that land management activities can affect American dog violet’s population dynamics. Shrub invasion can negatively impact both prairies and woodlands, and removal of shrubs is a common management tool. In addition to the potential for American dog violet populations to expand when invasive shrubs are removed7, the ratio of open to closed flowers can change depending upon forest health. More open flowers are produced under the open canopy of a well-managed forest, while more closed flowers are produced when forests are invaded with exotic species such as common buckthorn (Rhamnus cathartica). This demographic change is likely to have consequences for plant fitness. POC and partner researchers are continuing to investigate these dynamics to better understand how to support healthy populations of this species.
Removal of this species from Illinois’s list of endangered and threatened species referenced data collected by POC monitors that showed larger populations in more locations than were previously known. Although there are robust populations in our region, continued management that removes invasive woody shrubs seems to be an important factor for maintaining population health. Further, American dog violet’s restricted range in Illinois may indicate that it is at risk of range reduction due to climate change. Continued, regular monitoring by POC can help to detect population-level and region-wide changes to these currently positive trends.