Saving Pennsylvania’s Allegheny Woodrat

While the invasive rats seen in city subways and in trash cans don’t have many fans, Pennsylvania’s Appalachian mountains are home to a unique native rodent rarely seen. The squirrel-sized Allegheny woodrat is native to the Appalachian mountains and is listed as threatened in Pennsylvania due to declining populations across its historic range. Living in small groups on boulder fields and rocky outcrops located in deep forest, their unique population structure and habitat requirements make them particularly vulnerable to habitat fragmentation – and yet also a great candidate for innovative wildlife corridors.

A unique species in decline

Several factors make this species vulnerable. First, the Allegheny woodrat requires very specific habitat, occupying exposed rock ridges with deep and complex tunnels where they can avoid predators, store nuts and seeds for the winter, and enjoy a constant temperature throughout the year. Another factor that makes the Allegheny woodrat vulnerable is the fact that it’s a solitary and territorial species, so they also live at comparatively low densities. The woodrat also has small litters of pups, with a maximum of only four newborns each birthing cycle – and fewer when resources are scarce – making population growth slower than mice, or our invasive city rats, that have litters with as many as twelve pups[1].

Finally, their population is naturally structured as a metapopulation – meaning they live in small, distinct subpopulations that cannot persist indefinitely on their own, but instead require individuals to continuously disperse from one subpopulation to another in order to replenish them – otherwise the population will go locally extinct. This population structure makes them particularly vulnerable to habitat fragmentation. While the woodrat will travel several miles to disperse, it will only do so through intact forest that protects it from predators like owls, bobcats and fishers, meaning that common types of habitat fragmentation in Pennsylvania where forest is permanently lost, like strip mines and pipeline corridors, disrupt the Allegheny woodrat’s dispersal. Without replenishment of new arrivals from other nearby subpopulations, these newly- isolated populations eventually go extinct.

The range for Allegheny woodrats has significantly constricted in the last century, along with a 68% population loss.Photo by Colton Moyer | Used by permission

Since 1930, the Allegheny woodrat has declined by 68% in Pennsylvania with several causes of population loss over the last century[1]. Researchers point to fragmentation preventing woodrats from dispersing across the landscape as a major cause of this loss. When a woodrat population is isolated and stops receiving new individuals, inbreeding increases, which can lend itself to fewer healthy pups being born[2]. Because they exist as metapopulations, even if a large amount of habitat remains – i.e. plenty of suitable rocky outcrops in our forests – the number of woodrat populations across the state will continue to get smaller and smaller because a local populations will be lost if they can’t be replenished by new individuals arriving[3].

The impact of fragmentation has been exacerbated by other threats as well, including less food and a new disease. The near-extinction of the American Chestnut following the arrival of chestnut blight in the early 20th century reduced the amount of food available[4]. Woodrats are also facing the arrival of a new disease – racoon roundworm – which is often, which is often fatal to these rodents[5]. Isolated woodrat populations are less resilient to these new challenges, further endangering the species.

Building stepping stones

Like elsewhere in the state, woodrat metapopulations in Pennsylvania’s Huntingdon County have been fragmented by permanent loss of forest through activities like strip mining, pipelines and the creation of new rural housing developments[6]. This has coincided with the loss of local subpopulations in the county, and the habitat that is ideal for the Allegheny woodrat–exposed rock and boulder fields–now sits empty. In an effort to save this unique native species, the Pennsylvania Game Commission is experimenting with building “stepping stone” style habitat corridors to reconnect nearby woodrat populations at State Gamelands 67, located in remote Huntingdon County 20 miles north of Breezewood. Excitingly, this effort has already seen some local success.

Existing woodrat habitat, phases of added structures.Photo by Justin Vreeland, Pennsylvania Game Commission | Used by permission

In the first phase of the project, three large rock structures approximately 80-100 feet long and 15 feet tall (blue in map) were constructed. The pile of rocks included deep holes, complex and large enough to fit woodrats, but small enough to exclude most of their predators. Located between two habitats already occupied with Allegheny woodrat populations, these new rock structures were close enough to facilitate dispersal from the two adjacent habitats. Within four months, these new rock structures saw woodrats move in.

The second phase (pink in map) was to create 6 smaller den sites as “stepping stones” among three large sites in another area of the metapopulation. These smaller “bachelor pad” dens were created with young males in mind, who have to travel to find new habitat once they outgrow their mother’s territories. These rock structures, placed at the edge of a cleared area, would provide shelter to help them reach the next population with breeding females. So far, monitoring has found woodrats using three of the six new sites.

An ongoing experiment

The third phase is now underway. Building off of the successes of the previous two phases, this next set of stepping stones (red and orange in map) create a bridge of rock structures between two high-quality habitats across the open, inhospitable former strip mine. These small structures provide stepping stone habitat for the woodrats, allowing them to duck out of view of predators while moving across the open terrain.

It’s clear the new rock structures are useful to the woodrats: camera monitoring shows them moving into and through many of the new sites. But continued camera monitoring, trackers, and genetic testing of the populations are needed to better understand how they’re using the modified landscape. This will also be critical to understand best practices to help protect and restore Allegheny woodrat populations as efforts expand to connect their habitat here and elsewhere.

One place these lessons are already being applied is the path of the Mariner East gas pipeline. In this case, widening the pipeline path to accommodate the second Mariner East II pipeline and the accompanying permanent removal of forest increased fragmentation in two Allegheny woodrat habitats in another Huntingdon County game land, State Game Land 71 near Mount Union, Pennsylvania. Advocacy by researchers for the Pennsylvania Game Commission prevented the originally proposed path of the pipeline from destroying the actual dens, but the fragmentation remained. Creating rock structure habitat that stretches into the clear-cut path of the pipeline facilitates crossing for local woodrat populations by creating safe cover for much of the distance between the two adjacent forests[7].

On their own, these stepping stone corridors are not enough to save the Allegheny woodrat – the Game Commission is pairing these efforts with planting food species like serviceberry, elderberry, hickory and oak, as well as working on efforts to eradicate the threat posed by raccoon roundworm[8]. But it’s clear from the early successes in Huntingdon County that restoring the metapopulation structure is critical to ensuring the species survives long term, and that habitat corridors for the Allegheny woodrat will play a key role.

References

1. Pennsylvania Game Commission, Allegheny Woodrat
Wildlife Note (fact sheet), accessed at: https://www.pgc.pa.gov/Wildlife/EndangeredandThreatened/Pages/AlleghenyWoodrat.aspx, 5 February 2025.

2. Justin Vreeland, Pennsylvania Game Commission
“Creating Denning Habitat & Enhancing Connectivity for the
Allegheny Woodrat,” [PowerPoint Presentation] 23 October 2023.

3. Justin Vreeland, Wildlife Management Supervisor, Pennsylvania Game Commission, personal communication, 12 December 2024.

4. Justin Vreeland, Pennsylvania Game Commission
“Creating Denning Habitat & Enhancing Connectivity for the
Allegheny Woodrat,” [PowerPoint Presentation] 23 October 2023.

5. Pennsylvania Game Commission, Allegheny Woodrat
Wildlife Note (fact sheet), accessed at: https://www.pgc.pa.gov/Wildlife/EndangeredandThreatened/Pages/AlleghenyWoodrat.aspx, 5 February 2025.

6. Ibid.

7. Justin Vreeland, Wildlife Management Supervisor, Pennsylvania Game Commission, personal communication, 28 January 2025.

8. Pennsylvania Game Commission, Allegheny Woodrat
Wildlife Note (fact sheet), accessed at: https://www.pgc.pa.gov/Wildlife/EndangeredandThreatened/Pages/AlleghenyWoodrat.aspx, 5 February 2025.