Local Weasel Population Difficult to Assess as Nationwide Study Finds Species in Decline
By TODD McLEISH/ecoRI News contributor
A national study of weasels found across much of the United States has revealed significant declines in all three species evaluated, which has a local biologist wondering about the status of the animals in Rhode Island.
Long-tailed weasel by Keith and Kasia Moore, Flickr CC BY-SA 2.0
The study by scientists in Georgia, North Carolina, and New Mexico found an 87-94 percent decline in the number of least weasels, long-tailed weasels, and short-tailed weasels harvested annually by trappers over the past 60 years.
While a drop in the popularity of trapping and the low value of weasel pelts is partially to blame for the declining harvest, the researchers still detected a significant drop in the populations of all three species.
“Unless you maybe have chickens and you’re worried about a weasel eating your chickens, you probably don’t think about these species very often,” said Clemson University wildlife ecologist David Jachowski, who led the study. “Even the state agency biologists who are charged with tracking these animals really don’t have a good grasp on what is going on.”
The three weasel species are small nocturnal carnivores that feed primarily on mice, voles, shrews, and small birds, often by piercing their preys’ skull with their canine teeth. 
Fishers (a.k.a. fisher "cats") are members of the weasel family. Photo by Will Collette
The weasels prefer dense brush and open woodland habitats, where they search for prey among stone walls, wood piles, and thickets.
Because of their secretive nature and cryptic coloring, they are difficult to find and observe.
By assessing trapper data, museum collections, state statistics, a nationwide camera trapping effort, and observations reported on the internet portal iNaturalist, the scientists found the animals to be increasingly rare across most of their range.
“We have this alarming pattern across all these data sets of weasels being seen less and less,” Jachowski said. “They are most in decline at the southern edges of their ranges, especially the Southeast. Some areas like New York and the Canadian provinces can still have some dense populations in localized areas.”
Jachowski noted weasel populations in southern New England are likely facing similar declines as the rest of the country. He believes, however, there is the potential for some areas of the Northeast to still have robust numbers of weasels, especially long-tailed weasels, which are considered the most common of the three species.
Charles Brown, a wildlife biologist at the Rhode Island Department of Environmental Management, who contributed data to the national study, said in his 20 years of monitoring mammal populations in the Ocean State, the only one of the three weasel species he has found is the long-tailed weasel.
“I’ve had a few infrequent encounters with them over the years and seen a few dead ones on the road,” he said. “A mammal survey done in the 1950s and early ’60s documented two short-tailed weasels, and those are the only records I’ve found for the species.”
Least weasels are not found within 300 miles of Rhode Island.
Brown has contributed 19 or 20 long-tailed weasel specimens to the Museum of Comparative Zoology at Harvard University over the years from many mainland communities, including Little Compton, East Providence, Warwick, and South Kingstown. Weasels are not known to inhabit any of the Narragansett Bay islands.
“It’s hard to say what their status is here,” he said. “Trappers might bring in one or two a year, and some years none, and we don’t have any other indexes to monitor them because they’re cryptic and we rarely see them.”
Brown said a monitoring program could be developed for weasels in the state using track surveys and camera traps, but because the animals have little economic value and do not cause significant damage, they have not been a priority to study.
“In a perfect world, I’d certainly like to try to find a specimen of a short-tailed weasel to see if they’re still around here, but I have nothing to go on about them from a historic perspective,” he said.
Data from the University of Rhode Island is helping to provide a current perspective of the species’ distribution. URI scientists recently concluded a five-year study of bobcats and the first year of a study of fishers, each using 100 trail cameras scattered throughout the state. Among the 850,000 images collected so far are about 150 photos of long-tailed weasels.
According to Amy Mayer, who is coordinating the studies, the weasel images were collected at numerous locations around the state, suggesting the population does not appear to be concentrated in any particular area of Rhode Island.
It is uncertain what could be causing the national decline in weasel numbers, though Jachowski and Brown believe the increasing use of rodenticides, which kill many weasel prey species, could be one factor. A recent study of fishers collected from remote areas of New Hampshire found the presence of rodenticides in the tissues of many of the animals.
“How it’s getting into the food chain in these remote areas, we don’t know,” Brown said. “There was some discussion that a lot of people go up there to summer camps, and when they close the camp up for the season, they bomb it with rodenticides to keep the mice out. That’s just speculation, but it makes sense.”
The decline of weasels may also have to do with changes to available habitat, the scientists said. The maturing of forests and decline of agricultural land has caused a reduction in the early successional habitats the animals prefer. Brown also believes the recovery of hawk and owl populations, which compete with weasels for mice and voles and which may occasionally kill a weasel, could also be a factor.
Jachowski said the findings from his national study have led to the formation of what he is calling a “weasel working group” to share data and discuss how to monitor the animals around the country. Brown is among the state biologists and academic researchers who are members of the group.
“We’re hoping the public will become involved, too, by reporting their sightings to iNaturalist,” Jachowski said. “We need to see where they persist, and then we can tease out what habitats they’re still in, what regions, and then do our studies to figure out what kind of management may be needed.”
Rhode Island resident and author Todd McLeish runs a wildlife blog.
