Photo courtesy of TNC Ancient volcanic bedrock along the shoreline at the Mary Macdonald Preserve, at Horseshoe Harbor, near Copper Harbor, in Grant Township.

GRANT TOWNSHIP — Horseshoe Harbor is home to the Mary Macdonald Preserve, the largest mainland bedrock beach preserve in Michigan. The 1,200-acre preserve was established in 1982, and is owned by The Nature Conservancy (TNC), which says the Mary Macdonald Preserve is the most visited preserve in Michigan.

On Wednesday, TNC announced that it has acquired an additional 24 acres of land that will connect the east and west sides of the preserve.

Helen Taylor, state director for TNC in Michigan, said in a Wednesday release that the acquisition establishes five continuous miles of shoreline in area she says is rich in biodiversity and home to numerous threatened or endangered plants, which are now protected forever.

The land was purchased from a private owner. The sum was not disclosed.

“To date, TNC and its partners have protected 19 miles, or 64%, of the nearly 30 miles of protected Keweenaw County mainland shoreline,” Taylor said in the release.

The 24-acre parcel that runs north to south from the shoreline into the for est, creates five miles of contiguously connected and protected shoreline, making it the second largest protected tract of shoreline in Keweenaw County.

The preserve is ecologically significant. It is home to 11 threatened or rare species.

“This land is part of a unique landscape you can only find at the tip of the Keweenaw Peninsula,” Emily Clegg, director of land and water management at TNC in Michigan, is quoted as saying in the release. “We’re thrilled to protect this additional stretch of Lake Superior shoreline and connect and expand one of our prized preserves in the U.P.”

The preserve now covers 1,224 acres. It is unique in that it comprises a landscape of volcanic bedrock lakeshore, which is considered a threatened community. Because its surface is rough and jagged, much of the lakeshore is sparsely populated by mosses and lichens as well as scattered trees and shrubs.

Along the shoreline, TNC states, a rocky ridge creates a barrier for inland species and slower-growing plants. Just inland from the rock beaches, forest thrives in this cool, moist climate. Balsam fir, white cedar, white spruce and white birch provide habitat for the black bear, snowshoe hare, peregrine falcon, ruffed grouse, golden-crown kinglet, black-throated green warbler and yellow-rumped warbler.