Cirsium pitcheri—A Thistle Worth Saving
As the Wisconsin ice sheet retreated into Canada more than 11,000 years ago, seed from Cirsium canescens (Platte thistle) found its way to the newly formed Great Lakes. The plant became established along the shoreline and, over the millennia, evolved into a distinct species: Cirsium pitcheri (Pitcher’s thistle). That’s one theory, at least, based on the genetic similarities between the two species. Regardless of its origins, Pitcher’s thistle was first recorded around 1820 in what is now Pictured Rocks National Lakeshore and is endemic to the sandy shores and sand dunes of Lakes Michigan, Huron, and Superior. The species is a monocarpic perennial that, for the majority of its life cycle, grows as an inconspicuous rosette of leaves. After 5-12 years, it flowers, distributes its seeds, and dies.
Pitcher’s thistle has been in decline for several decades and is currently listed as federally threatened. Inbreeding, habitat destruction, natural succession, and pressure from invasive species have taken a combined toll on its populations. Recently, a number of new threats have been identified and are being studied by scientists working for the United States Geologic Survey (USGS). American goldfinches consume a large proportion of Pitcher’s thistle seeds—an average of 60% in most years—and the bird’s growing numbers have negatively impacted C. pitcheri. Also, several weevils have been found to predate on Pitcher’s thistle. Seed eating weevils, Larinus planus and Rhinocyllus conicus, are used for biological control of noxious thistles, but have crossed over to C. pitcheri, invading the seed heads and destroying the maturing seeds. The larvae of a third weevil, Cleonis piger (sluggish weevil) bores down into the roots to eat, killing the plant altogether before it can set seed.
Pitcher’s thistle is an extraordinary plant which plays an important role in its sensitive Great Lakes sand dune ecosystem. But, it’s long term viability is precarious. Our actions have helped put it on the road to extinction and it will require a concerted effort on our part to help it escape that fate. Let’s hope we’re up to the task.