Article

Successional patterns of terrestrial wildlife following deglaciation

Two moose on the coast at the ocean edge. — Two moose captured on a remote camera explore new habitat in Tarr Inlet near the Grand Pacific Glacier. Brown bears and wolves have been observed on the park shoreline for 75–100 years, but moose are a relative newcomer, arriving from the east in the late 1960s. Large mammal colonization is ongoing.

Glacier Bay fjord was completely covered in ice just 300 years ago during the Little Ice Age. All plants and animals that occupy the shoreline of Glacier Bay have colonized the land since it emerged from the ice. Ice retreat in Glacier Bay has offered scientists a first-hand look at the successional processes that occur after glaciers retreat, and is one of the main reasons Glacier Bay National Monument (which later became a national park) was founded in 1925! The land in the lower bay near the mouth is older, whereas land in the upper bay near the glaciers is younger, providing an excellent platform to compare differences in landforms and species presence as the land matures. To date, most successional studies have focused on geology, botany, or aquatic succession, but park biologists and collaborators just published the first paper on the impacts of glacial recession on the successional dynamics of land mammals.

Researches spent two summers at ten sites across the shoreline of Glacier Bay from the mouth of the bay to the glacier faces to document what species had colonized various locations and habitats. They used vegetation surveys and distances from the mouth of the bay and the glaciers to characterize sites, and remote cameras and small mammal traps to document the species present. They also compiled relevant historic mammal observations to look at how mammal communities have changed over time. Researchers found that mammalian communities increase in diversity over time similar to plant and stream communities. However, changes in wildlife communities do not directly track changes in habitat, particularly if communities consist of species with differing dispersal abilities. Community assembly was more strongly related to site age than habitat, whereas species diversity was similarly affected by both factors. Findings indicate that changes in wildlife communities are not based on habitat succession alone, and that landscape connectivity, dispersal abilities, and even competition between species, may be important drivers of wildlife community succession following disturbances such as glaciation.

Glacier Bay Fjord and glacier-covered mountains are barriers to movement for many wildlife species. Results from this study indicate that certain species may have colonized the shoreline of Glacier Bay only from one side of the bay. For example, coyotes were only detected on the east side. However, historical observations show that coyotes were present in the west arm in the late 1960s, indicating some level of turnover in larger mammals. Similarly, red foxes were common in Bartlett Cove in the 1960s, but were not observed anywhere along the shoreline areas in this study. Brown bears, moose, and wolves were distributed across almost all sites, indicating that these species are able to disperse greater distances despite the fragmented landscape, and/or that they colonized from refugial populations on both sides of the bay.

A researcher holds up a small mouse. — A researcher prepares to measure a Keen’s mouse caught and then released in a small mammal trap near Marjorie Glacier. Keen’s mice, tundra voles, and montane shrews appear to be the first rodents to colonize new land after the ice retreats.
NPS/Tania Lewis

Interestingly, some of the smallest mammal species were also widely distributed. Montane shrews were detected at nearly all sites. Keen's mice and tundra voles were found only on newer land in both the upper east and west arms in our study, whereas common shrews were only found in older sites. These patterns suggest a relationship between species occupancy and habitat that leads to turnover for poorly dispersing small mammals as the habitat matures. However, the current distribution of small mammals calls to question the assumption that these animals are, indeed, poor dispersers, for although they may lack the ability to cross marine fjords on their own, they can disperse across fjords via logs or even vessels. This fact was brought to the researchers' attention when unpacking their kayaks near Reid Glacier they discovered a stowaway red-backed vole that had hitched a ride from the Beardslee Islands! The vole was liberated from the kayak hold and promptly ran off into the grass to colonize new land.