Time for Nature

Thistles And Flowing Sand

July Edition (July 3, 2002)

Summer Visitors

© Parks Canada

A bumblebee dances over the fine-haired Pitcher's Thistle flowering on a sand dune in Pukaskwa National Park of Canada. The bee hovers, as though suspended by an invisible thread, and gathers precious nectar from the pink blooms.

July, the Pitcher's Thistle's peak flowering period, brings another type of visitor to the shores where the thick-skinned, knee-high plant grows. These visitors come carrying clipboards, pens, tape measures and other instruments to gather nectar of a different sort – precious data on this endangered plant.

©   Parks Canada

A Species At Risk

Why is the Pitcher's Thistle so rare? It's a complicated picture.

• In Canada, the Pitcher's Thistle range is limited. It grows on shoreline sand dunes in Pukaskwa National Park, and at a few sites along Lake Huron.
• This shoreline habitat is popular with other species too. People use the beaches and develop the waterfront, interfering with the natural flow of sand. Other plants invade the shoreline, competing with the thistle for crucial open space.
• The plant's life history is delicate. It needs essentially stable sand dunes, where the sand is periodically disturbed. If the sand shifts too much, however, it buries the seeds and prevents them from germinating.
• The Pitcher's Thistle produces seeds only once during its lifetime -- between the ages of 3 and 10 -- then dies. That doesn't leave much margin for error.
• White-tailed Deer like to graze on the thistle's leaves and flowers. Overgrazing results in delayed reproduction, decreased seed production, and lower winter survival rates.
• The isolated populations face genetic constraints.

Trouble In The Park

© Parks Canada

At Pukaskwa, Pitcher Thistle numbers plunged from a high of 760 plants in 1981 to 153 in 1991. The decrease was partly the result of a beaver dam bursting in 1986 and re-routing a creek which washed out 63 per cent of the thistle population in Oiseau Bay.

Yet the thistle survived.

In fact, the population may have benefitted in the long run, because the high water levels of the washout discouraged other plant species from moving in. Still, the significant reduction in numbers caused concern in the park.

©   Parks Canada

Park Action

Park staff responded by developing an eco-management plan to save the plant from possible extinction in Pukaskwa.

Part of the plan involved starting a new colony at Oiseau Bay, where staff sowed seeds during the summer of 1992. The timing was good, because cool, moist weather conditions that season contributed to high germination rates. Park staff helped the seedlings grow by erecting protective barriers to prevent grazing and trampling.

Good News, And ...

The current population of Pitcher's Thistle at Oiseau Bay has rebounded and stabilized at an average of 422 plants. So the hard work yielded positive results.

©   Parks Canada

Yet the thistles are isolated from other populations. They need continued natural disturbances to survive. And other plant species threaten to crowd the dunes. That makes the plant's long-term status in Pukaskwa National Park difficult to predict.

Park staff continue to monitor the situation closely.

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