In nature, a lot can change on a largely uninhabited Great Lakes island over the course of a century.
And a lot can stay the same.
That includes the disappearance and appearance of wildlife species.
That’s what scientists discovered when they inventoried mammals and amphibians on Charity Island, a 252-acre speck near the mouth of Saginaw Bay in Lake Huron. It’s one of about 35,000 islands in the Great Lakes, most of them even smaller than Charity Island, according to a recent study.
There’s been little recorded human engagement with the island. A lighthouse operated there from 1857 to 1939, and its decommissioning marked the end of the only year-round human occupation.
It also served as a fishing outpost in the 1920s-1930s.
Charity Island is within the Shiawassee National Wildlife Refuge, and the U.S. Fish & Wildlife Service manages more than 85% of it. The rest is privately owned and a lighthouse and a renovated lighthouse keeper’s home, a pair of rental cabins and a few outbuildings.
Foerst dominated by maples, oaks and pines surround a pond in the island’s interior, and its Lake Huron shoreline ranges from marshy to sandy to rocky, depending on water levels and location, the study said.
For a baseline on biodiversity there, scientists led by Giorgia Auteri, a senior research scientist at Bat Conservation International, used the findings of the University of Michigan’s Mershon Expedition which surveyed Charity Island’s wildlife in 1910.
Other members of the research team are affiliated with Eastern Michigan University and the University of Georgia.
The expedition was named for William Mershon, a Saginaw mayor and business owner who sponsored the survey, and led by U-M zoologist Alexander Ruthven.
“Just over 100 years later, we frequented the island during 2013-14 to observe how the mammalian and amphibian assemblages had changed since the Mershon survey,” the authors of the new study wrote in the journal Northeastern Naturalist.
Auteri said, “We were very surprised to find so few bats on the island compared to the mainland, because they could easily fly over. It was often quite windy there, and it was a small island, so maybe it was just too unappealing for them.”
“We were also surprised to not find any shrews or squirrels. It was surprising that the previous study didn’t find any chipmunks, as they were incredibly abundant on the island,” Auteri said.
They found seven species not previously documented there, including the eastern chipmunk, American bullfrog and gray treefrog, which the study described as “genuinely new arrivals.”
“In contrast, the previously detected snowshoe hare, eastern American toad and pickerel frog were not detected in our survey,” the study said.
Among those found in both surveys were the big brown bat, common muskrat, red fox, blue-spotted salamander and northern raccoon.
Overall, the researchers tallied 10 species that were present in both 1910 and 2013-14, seven that were identified for the first time in the latest survey and at least two that “seem newly absent,” the study said.
“These findings indicate that modest but detectable species turnover occurs on isolated, high-latitude islands, even among otherwise common species,” it said. “This difference in diversity is due to species’ inability either to reach the island or to persist there.”
Auteri said, “One implication is that small patches of habitat can still be important, especially if they represent key stopover sites which facilitate connectivity between larger habitat patches – in this case, the mainland.”
At its closest point, the island is about 6/10th of a mile from the mainland, where about 48 mammal species and 16 amphibian species live.
Auteri said, “On the other hand, small habitat patches are limited in how many species they can steadily support, and that is certainly also reflected in our findings — the island has fewer species of mammals and amphibians compared to the nearby mainland.”
The fact that several species were not previously found there “highlights how species can blink on and off for a habitat patch, particularly if it’s isolated and thereby hard to recolonize,” she said.
The study said the absence of common mainland species such as white-tailed deer, voles and mice, “while potentially surprising, make sense in light of the size and isolation of the island.”
The numbers for both surveys are not necessarily precise, it cautions.
For example, field research took place during the summer, while some species are active above-ground only in the spring, and some medium-to-large mammals may visit only in the winter by crossing the frozen water of Lake Huron.
As for the study’s broader significance, the study said documenting the arrival, disappearance and reappearance of species could help biologists better understand patterns of wildlife survival and contribute to effective ecosystem management.”
“Animals living on islands often face heightened susceptibility to (local disappearance) in conjunction with limited recolonization,” the study said, pointing to such factors as an island’s size, distance from the mainland and surrounding habitat.
“Small habitat patches, including islands, may still be important for conserving biodiversity,” it said, and can “facilitate movement of species across the landscape.”
