Colorado Butterfly Plant and the US Air Force

Last week’s plant quiz featured the Colorado butterfly plant, growing in Wyoming.

Every summer for 28 years, field botanists have walked through riparian habitat on FE Warren Air Force Base counting Colorado butterfly plants.  It’s not always easy.  August days are hot, and there are thick stands of coyote willows and Canada thistle to fight through.  Butterfly plants are about as tall as the grass, so they’re hard to spot when it’s thick.  They’re a shifty bunch too, changing location from year to year.  Particularly difficult is the monotony of it all.  By afternoon, it can be tough to stay focused.

Butterfly plants trying to hide in the grass, but the white spots give them away. 

Colorado butterfly plant was first collected in 1894, near Fort Collins, Colorado, hence the “Colorado” part of its name.  “Butterfly” comes from the shape of the flowers.  They are bi-laterally symmetrical -- the two pairs of petals are mirror images -- and the petal pairs are reminiscent of butterfly wings.

Top to bottom:  buds, flowers, young fruit.  B. Heidel, WY Natural Diversity Database (WYNDD).

Taxonomic aside:  The butterfly plant was first called Gaura neomexicana spp. coloradensis, a name that lasted until 2007 when the genus was combined into Oenothera.  In 2013, the Colorado butterfly plant was recognized as a full species, and is now Oenothera coloradensis (source).  This change is so recent that you won’t find it in the usual online plant resources (e.g. ITIS and USDA PLANTS).

Colorado butterfly plant, from the Wyoming State Species Abstract.

You may have guessed why these butterfly plants are counted each year -- they are rare and threatened.  The species grows only in north-central Colorado, western Nebraska, and southeast Wyoming (where most of the populations occur, contrary to the plant’s name).  In 2000, it was listed as a threatened species under the Endangered Species Act (ESA) by the US Fish and Wildlife Service.  A listed-threatened species is  “likely to become in danger of extinction within the foreseeable future throughout all or a significant portion of its range. Federal law prohibits the removal or destruction of threatened plants on any federal land or as a result of federal actions.”

For plant species, ESA protection is applicable only on federal land.  Contrary to widespread fear, it does not cover populations on private land unless the owner happens to be receiving federal funds of some kind.  The only place where Colorado butterfly plant grows on federal land is FE Warren Air Force Base near Cheyenne, Wyoming.  Thus the Base has been become the focus for protection and research.

Warren Air Force Base is home to the 90th Missile Wing and the Colorado butterfly plant.

Colorado butterfly plant prefers moist sites in drainage bottoms.  Though the Base is small (630 acres, 255 ha), it has three drainages with suitable habitat, and supports one of the three largest populations known -- about 6000 to 8000 plants in flower in a given year (but see more below).

Butterfly plant with coyote willow, along small creek.

Even before the butterfly plant was listed under the ESA, the Air Force took steps toward protection.  Monitoring of population trends began in 1984.  As is commonly done, the population was sampled, rather counted in its entirety.  Permanent plots were established, each consisting of a bar driven into the ground at a chosen corner of the plot.  A metal tag stamped with the plot identification number was attached.  In subsequent years, field staff were to return to each location, set up a plot of the same size from the chosen corner, and count plants.  But then the very next year there were heavy rains, flooding and scouring of the drainage bottoms.  And there was a change of staff.

In 1986, I was hired as botanist for the state biological inventory, and one of my first projects was monitoring Colorado butterfly plants on Warren Air Force Base.  In spite of the flooding and scouring, I relocated many of the plots and duly counted butterfly plants.  Results showed a big decline.  Some plots had no plants whatsoever, in contrast with initial counts.  Yet looking around I saw lots of butterfly plants outside the plots.  Were we really capturing population trends?

Turns out not.  Colorado butterfly plant is a short-lived perennial that thrives with periodic disturbance.  It does best in fairly open habitat, without a lot of competition, and occupied habitat changes as the site changes (we say the plants move around but we really do know better!).  Sampling has to be intensive to deal with such "behavior".  So we switched to a straight census of all butterfly plants.  Given the limited area involved, this has proven to be quite doable.

Buds and flowers on left (flowers 1-1.5 cm across), fruit on right.

Colorado butterfly plant fruit.  Each capsule contains four seeds.  B. Heidel, WYNDD.

Many monitoring programs are started, but only a small percentage continue long-term.  Twenty-eight years of census data are exceptional.  So what’s up with butterfly plants on the Base?

Population size varies widely from year to year, and changes in distribution are common.  When the annual census program began in 1986, we recorded plant numbers for sections of drainages, and could therefore see major shifts in distribution.  Since 2002, the crews have used GPS to record and map patches of plants, recording changes more precisely.

Much of the open habitat created by the 1985 flood is now gone, taken over by coyote willow and other woody vegetation.  But the overall population hasn’t declined significantly, yet.  While Crow Creek numbers are way down, those on Diamond Creek and a tributary are up.  Even so, we know the butterfly plant is dependent on periodic disturbance for long-term survival, so we’re hoping for another big flood! (shhh ... don’t tell)

Monitoring will reveal the unexpected.  We can learn a lot about the biology and ecology of a target species just by being out there with it.  In 2007, field staff found almost no butterfly plants until they looked close and saw the dead and dying.  Remaining leaves were covered with shot holes indicative of flea beetles; stems and branches showed damage from their larvae.  However, it took only a few years for the population to recover.  In 2008, plant numbers were lower still, even though there was no herbivory, but by 2009, the population was clearly on the rebound (Heidel et al. 2011).

Excited field botanist strides hopefully toward red patch in left center of photo.

This summer I had the pleasure of revisiting Colorado butterfly plants on Warren Air Force Base with Bonnie Heidel, botanist for the Wyoming Natural Diversity Database.  During the drive to Cheyenne, she told me about the flea beetle scare, and worried out loud about what we might (not) find.  But as soon as we got out of the car I saw them in the distance -- not individual plants but the distinctive reddish patches (stems) with white spots seeming to float amid the grass.

Up close, we found the plants were both abundant and healthy.  And since that visit, the annual census results are in -- Colorado butterfly plant is doing quite well in 2014 :-)

We found butterfly plant haven at our first stop!

Sources

Heidel, B,  Fertig, W, Blomquist, F and Abbott, T.  2008.  Wyoming’s Threatened and Endangered Species: Colorado Butterfly Plant. Wyoming Bureau of Land Management, in collaboration with Wyoming Natural Diversity Database.  http://www.blm.gov/pgdata/etc/medialib/blm/wy/programs/pcp/species/docs.Par.58921.File.dat/ColoButterfly.pdf

Heidel, B, Tronstad, L and Handley, J. 2011. Flea beetle (Altica spp.) herbivory on a threatened plant, FE Warren Air Force Base, Wyoming. Natural Areas Journal 31:283-287.  http://www.uwyo.edu/wyndd/_files/docs/Reports/WYNDDReports/A11HEI03WYUS.pdf

Heidel, B and Handley, J.  2014.  26-year population trends of Colorado butterfly plant (Oenothera coloradensis; Onagraceae), a short-lived riparian species on F.E. Warren Air Force Base.  Prepared for F.E. Warren Air Force Base by the Wyoming Natural Diversity Database (University of Wyoming), Laramie, WY.  http://www.uwyo.edu/wyndd/_files/docs/reports/wynddreports/u14hei08wyus.pdf

Wyoming Natural Diversity Database.  2012 (last update).  State species abstract, Gaura neomexicana ssp. coloradensis.  http://www.uwyo.edu/wyndd/_files/docs/reports/speciesabstracts/gaura_neomexicana_coloradensis.pdf