Small Park, Big Resources: The Dunes, Lagoons and Prairies of American Camp

San Juan Island National Historical Park, in Washington state, was set up mainly for its history: it commemorates the arbitration and peaceful settlement of a boundary dispute between the United States and Great Britain. It has two units, American Camp (1,223 acres) and English Camp (915 acres). American Camp, at the island's southeastern end, has been lived on for thousands of years — from the ancestors of the Coast Salish Tribes to 19th-century homesteaders.

It also protects some remarkable natural places. Its remnant native prairie, coastal sand dunes and coastal lagoons are small, but big enough to support the endangered island marble butterfly, the imperiled sand verbena moth, and plants such as common camas, chocolate lily, biscuit root and American dune grass. A study funded by the National Park Service's Natural Resource Condition Assessment Program examined how these habitats are faring, to guide how the park cares for its vegetation and helps endangered species recover.

Field mustard, one of three host plants of the endangered island marble butterfly, blooming in the prairie after a rainy spring (NPS).

The sand dunes

Open, shifting dunes were once common around Puget Sound; American Camp's are now among only a handful of coastal dune ecosystems left in the region. They hold several rare plants, including the largest known population of yellow sand verbena — the host plant of the sand verbena moth, which is critically imperiled and known from only three sites in the United States (all in Washington) and five in Canada.

The moth lives its whole life on and around that one plant. Adults mate on it and lay eggs in its flowers; caterpillars eat it; caterpillars and pupae shelter beneath it; and winged adults drink its nectar before laying the next generation of eggs.

The dunes matter to the island marble butterfly too, because one of its host plants grows most densely there. The butterfly's only remaining population in the world lives on the south end of San Juan Island. Each year park staff improve its habitat, collect eggs and caterpillars, and raise butterflies to release back into the park.

The threat: dunes need sand that moves freely. Invasive plants such as Canada thistle and Himalayan blackberry pin the sand in place, open the way for other problem species, and can turn a dune into shrubland or grassland. The park is testing the best ways to control them without harming the dunes' native life.

The lagoons

Coastal lagoons are rare around Puget Sound and are recognized as especially important ecologically. San Juan Island has only three naturally formed ones, all at American Camp, and Jakle's Lagoon is the largest and deepest. Many plants around them are regionally rare, including native coastal peppergrass, another key host plant of the island marble butterfly. Winter storm surges and invasive plants both threaten that habitat, so every year the park counts butterflies at the lagoons, tracks changes in the habitat and removes invasive plants.

The prairies

American Camp's prairies are among the last natural prairies in the Northern Straits and Puget Sound regions, and they are part of the park's history. For millennia, the Coast Salish people of the area tended them, encouraging the native plants they used for food and medicine. Within the last 200 years, farming, ranching and the absence of fire changed much of the prairie, but remnants still hold a high diversity of native plants and rare species such as the island marble butterfly.

Those plants still matter to the Coast Salish Tribes. In 2021 the park and the Samish Indian Nation studied the cultural importance and medicinal uses of plants at American Camp, and found the area full of culturally significant species.

The threats: above all, open grassland turning into shrubland — along with invading conifers and non-native grasses that form a dense mat native plants cannot grow through. Each year the park surveys for new invasive plants, removes problem species and cuts young conifers before they spread.

How visitors can help

• Stay on established trails and keep pets on a leash.
• Leave plants and insects alone — including insects resting on plants, driftwood or the ground. The moth and the butterfly are easy to step on at every stage of life.
• Check your shoes and tires for seeds before you arrive, especially if you have been somewhere invasive plants grow.

Sources

• National Park Service, San Juan Island National Historical Park: "Small Park, Big Resources: The Dunes, Lagoons, and Prairies of American Camp," and the resource brief of the same name (June 17, 2022), based on Martin SJ and Martin RA, Focused Condition Assessment for San Juan Island National Historical Park: Special ecological areas at American Camp (Center for Natural Lands Management, 2021, prepared for the National Park Service). The article dates the boundary settlement to the mid-1880s and the brief to the mid-1800s, so no date is given here.