Water howellia (Howellia aquatilis) Monitoring in Northern Idaho, 2005 to 2017

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Abstract:

Water howellia (Howellia aquatilis) is an annual aquatic macrophyte that is listed as
threatened by the US Fish and Wildlife Service (USFWS). Globally, it has a disjunct
distribution across the Pacific Northwest with known populations in Montana, Idaho,
Washington, and California. It occurs in small ponds that generally dry completely
during the summer. Only six extant occurrences are known in Idaho. (There is an
1892 herbarium specimen from near Spirit Lake, Kootenai County, but water howellia is
not presently known from that area.) All six extant occurrences are on floodplains in the
Palouse and Spokane river drainages, in small ponds formed by fluvial processes. The
earliest-known of the six was first documented in 1967, and consists of three ponds
along the Palouse River, just outside the town of Harvard. In 2005 we established
permanent transects to measure the frequency of water howellia in pond 2 and in 2006,
ponds 1 and 3. Transects were sited in a regular pattern in order to distribute them
evenly across the ponds. There is a total of 10 transects from 4 to 28 m in length.
Monitoring was done annually until 2010, then biennially. Monitoring was conducted in
late June or early July, when the plant was flowering. In 2012, a second monitoring site
was established at a separate occurrence on the Palouse River floodplain, near
Princeton, with seven ponds monitored. At this site, a single transect was randomly
located in each pond. Transects ranged from 4 to 30 m in length. At both sites, the
frequency of water howellia was measured along the permanent transects using 50 x 50
cm quadrats. McNemar’s test was used to detect significant changes in frequency
between years. In addition to frequency of water howellia, the frequency of reed
canarygrass (Phalaris arundinacea) was also measured, and water depth was
measured every 5 m. At the Harvard site, 2017 was the tenth year of data collection at
one pond, and the ninth at two others. Water howellia has not been observed at pond 1
since 2001, although the pond has been searched whenever the other two ponds are
monitored. At pond 2, water howellia frequency increased between the first two years
and remained high until 2009, when it began decreasing. In 2016, we recorded the
lowest frequencies since the beginning of monitoring, although not significantly different
than the first year. At pond 3, frequencies were relatively stable through 2014, then
decreased significantly in 2016 and 2017. At the Princeton site, ponds were sampled in
2012, 2014, 2016, and 2017. Transects at three of the seven ponds experienced
precipitous declines over the monitoring period, with two decreasing from more than
70% to zero. Both were completely dry at the time of monitoring in at least one
monitoring year. Reed canarygrass is the most serious threat to water howellia habitat
in Idaho. Over both sites the frequency of reed canarygrass ranged from 10 to 100%
with a mean of 66%. Protection and restoration efforts will need to focus on removing it
or curtailing its invasion of the ponds. We recommend continued monitoring of water
howellia populations, invasive associates, and water levels; and the identification of
sites suitable for introduction of new populations.

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Shelf Note: ELECTRONIC FILE - BOTANY: PLANT SPECIES: Howellia aquatilis

Lichthardt, J., and K. M. Pekas. 2019. Water howellia (Howellia aquatilis) monitoring in
northern Idaho, 2005-2017. Idaho Natural Heritage Program, Idaho Department of Fish
and Game, Boise, ID. 31 pp. plus appendices