How Wildlife Colonizes Land Left by Glaciers in Glacier Bay

Measuring a Keen's mouse, caught and released near Marjorie Glacier. Keen's mice, tundra voles and montane shrews seem to be the first rodents to colonize land after the ice retreats. NPS / Tania Lewis.

Just 300 years ago, during the Little Ice Age, Glacier Bay was entirely under ice. Every plant and animal on its shores has arrived since. That made it a natural laboratory — and one reason Glacier Bay National Monument was created in 1925. Land near the bay's mouth is older; land near the glaciers, younger. Most studies of such succession have looked at geology, plants or streams; this is the first on land mammals.

The study

Over two summers (2017 and 2018), researchers surveyed ten sites from the bay's mouth to the glacier faces, using vegetation surveys, remote cameras and small-mammal traps, and gathered historical mammal records.

What they found

• Diversity grows with time, as with plants and stream life — but mammal communities don't simply track habitat.
• Age over habitat: which species were present related more to how long ago the land was freed of ice than to its habitat; overall diversity depended on both.
• Other forces: connectivity, how well species disperse, and even competition shape who arrives after a disturbance like deglaciation.
• Body size matters: smaller, less mobile mammals turned over more between sites than larger ones.

Barriers and travelers

The fjord and ice-covered mountains block many animals.

• Coyotes turned up only on the east side, though they were seen in the west arm in the late 1960s. Red foxes, common at Bartlett Cove in the 1960s, weren't found at all.
• Brown bears, moose and wolves were at almost every site — able to travel far, or colonizing from both sides. Bears and wolves have been seen on the shore for 75–100 years; moose are newcomers, arriving from the east in the late 1960s.
• Small mammals: montane shrews were almost everywhere; Keen's mice and tundra voles only on young land in the upper arms; common shrews only on older sites — turnover as habitat matures. But small mammals may travel better than assumed, rafting on logs or riding boats: unpacking kayaks near Reid Glacier, the researchers found a red-backed vole stowaway from the Beardslee Islands, which ran off to colonize new land.

Two moose, caught on a remote camera, exploring new habitat at Tarr Inlet near Grand Pacific Glacier. NPS.

Why it matters

Wildlife may not follow vegetation after a disturbance if there are barriers or animals disperse poorly — so landscape connectivity and species traits belong in conservation planning, and can improve predictions of how communities rebuild.

Sources

Based on "Successional Patterns of Terrestrial Wildlife Following Deglaciation," Glacier Bay National Park and Preserve, National Park Service, summarizing Sytsma, Lewis, Bakker and Prugh (2023), Journal of Animal Ecology; a work of the United States government in the public domain.