Pennsylvania Wildlife Corridors

Pennsylvania is blessed to be home to incredible native species. From our elk, bobcats and black bear to the millions of songbirds that cross the Commonwealth as they migrate north and south each year, we have stunning wildlife that call Pennsylvania home. Yet each year, more and more of the state’s wildlife habitat is criss-crossed by roads, energy infrastructure, sprawling development and other projects that fragment our wilderness, making it harder for Pennsylvania’s native species to roam, migrate, mate, hunt and forage.

And the problem of habitat loss is accelerating. For example, despite no net population growth statewide, Pennsylvania ranks eighth in the nation for loss of agricultural land to low-density housing sprawl[1], and the loss and fragmentation of habitat is now the number one threat to wildlife in the commonwealth[2].

What is habitat fragmentation?

Habitat fragmentation refers to the carving up of a species’s habitat by barriers that prevent wildlife from accessing resources – and each other. This can negatively impact the health of wildlife populations in a variety of ways.

Crossing deaths
When roads divide habitats, wildlife risk being struck by vehicles when they attempt to cross. According to the U.S. Department of Transportation, over one million animals are killed on our nation’s roads every year, with Pennsylvania ranking among the top five states for animal-vehicle collisions year in and year out. Besides being lethal for wildlife, it’s also dangerous – and expensive – for drivers. Incredibly, over 150,000 animal collision claims were filed in Pennsylvania from July 1, 2023 to June 30, 2024 alone[3]. Nationally, wildlife-vehicle collisions kill over 200 and injure an additional 26,000 people, and cost more than $8 billion[4]. These costs are borne by all drivers – even those who never hit an animal – with insurance companies passing along the cost of these pricey collisions to policyholders[5].

And wildlife-vehicle collisions aren’t just a deer problem. In 2021, over 500 black bears perished on Pennsylvania roadways and 60 of our state’s mere 1,400 elk were killed by vehicles[6]. Many of Pennsylvania’s turtle species, whose numbers are declining, are also frequent victims of traffic accidents as they move to the ponds where they breed[7]. Smaller animals killed by vehicles typically go unreported.

Inbreeding
Habitat fragmentation divides wildlife populations, preventing isolated groups from mating with each other. This results in inbreeding, which over generations will result in unhealthier populations that are less resistant to parasites and diseases, more vulnerable to changing environmental conditions, including shifting patterns in temperature or precipitation[8] or a new predator[9], and more likely to suffer from genetic diseases. This endangers the long term health—and potentially survival—of the whole population. For example, as bobcat populations are cut off from one another by freeways, more inbreeding occurs and is followed by increased health problems and lower survival rates[10].

Loss of interior forest habitat
Fragmentation of woodland eliminates interior forest habitat, defined as large tracts of mature forest unbroken by development. Over 40 of Pennsylvania’s bird species, including the wood thrush, scarlet tanager, northern goshawk and barred owl, depend specifically on interior forest for habitat[11]. The same holds true for native mammal species like the Allegheny woodrat, which is threatened in Pennsylvania[12] and the northern flying squirrel, which is listed as endangered in Pennsylvania[13], as well as the fisher, whose populations are only now returning after being driven to extinction by overharvesting in the 19th century[14]. As Pennsylvania’s interior forests are carved up by roads, pipelines, powerlines and other infrastructure, interior habitat is converted to edge habitat. Edge habitat consists of different plant species, meaning it also supports different animal species. These ecosystems are also less sheltered from direct sunlight, wind, snow and rain, creating harsher conditions that are less favorable for many interior species[15].

Interrupted migration
Pennsylvania is a key part of the Atlantic Flyway, the path along which billions of birds from Canada and the northern United States migrate south to the Gulf of Mexico and beyond[16]. When Pennsylvania’s forest, meadows and wetland habitats are sliced up or destroyed, it threatens not only our resident birds, but species throughout the Western Hemisphere. The same is true for the multi-generational migration of the monarch butterfly from Mexico to northern North America. It is one of the global marvels of evolution, but it’s fragile and depends on suitable habitat all along the way, including here in Pennsylvania. As monarch butterfly habitat—namely its main food source, milkweed—is lost, populations of the eastern monarch have declined 80% since the 1990s[17].

Migration is also critical for the many fish species that travel upstream to spawn, such as Pennsylvania’s native shad and striped bass. In 2023, the National Low Head Dam Inventory Task Force estimated that there are over 531,000 dams in the United States[18]. According to the National Oceanic and Atmospheric Administration, the barriers to fish migrations created by these dams are the leading cause of decline of American migratory fish populations[19].

Decreased climate resilience
As the climate warms, the geography for a given species’ ideal climate is shifting, with both plant and animal ranges chasing cooler temperatures northwards in latitude and higher in altitude. For example, the Audubon Society projects northward range shifts for almost all North American bird species over the coming decades[20]. Areas that are predicted to remain more favorable are called climate refugia, providing a ‘refuge’ for species to persist as other regions become more hostile. However, these refugia can only aid in a species’ resiliency insofar as they are accessible[21]. If fragmentation cuts off the population from migrating to these favorable habitats, these locations can’t aid in the species’ survival.

Wildlife corridors: a proven solution

Wildlife corridors are a proven method of addressing habitat fragmentation. This is a broad term for a number of different strategies used for connecting habitats and helping ensure that wildlife can safely move and migrate, to decrease vehicle-wildlife collisions, and to keep our wildlife populations healthy[22].

Engineered crossings
Wildlife crossings are built structures that allow an animal to safely cross our infrastructure. These include overpasses, also known as “green bridges,” usually for larger mammals and herd animals, as well underpasses that range from large tunnels that can accommodate bears and deer to small amphibian tunnels. Recent research has found that wildlife-vehicle crashes can be reduced by as much as 97% with a combination of crossings and fencing[23].

A Nevada wildlife crossingPhoto by Nevada Department of Wildlife | Public Domain

A marten uses a culvert during dry weather.Photo by Show Us Your Togwotee | CC-BY-2.0

Culverts, a subset of tunnels that allow water to flow and are often used to direct rain water and runoff, can be designed with animals in mind to aid in the passage of mammals as well as water-loving reptiles, amphibians and fish. A culvert’s design will influence which species use it – the size of the culvert limits what size species it can accommodate, while choices in substrate, shape and lighting in the culvert are also influential in who is able to use it. Research from Pennsylvania’s neighboring states has demonstrated what some of the key considerations are when designing for mid-Atlantic species. A study from University of Maryland found that gray squirrels avoided arch-shaped tunnels, that deer won’t utilize a cobbled floor, and that great blue herons prefer a box shape with a sandy floor[24]. For semi-aquatic animals like frogs and turtles, the ongoing presence of water in or near the culvert is critical, and for reptiles and amphibians it’s important to incorporate a grate on the top that keeps humidity and temperature in the tunnel consistent with the ambient conditions[25].

A fish ladderPhoto by NOAA | Public Domain

In aquatic ecosystems, where dams are the biggest barrier to connectivity, fish ladders can help aquatic species move over or around an existing dam and continue upstream[26]. However, a growing movement is calling for the removal of many dams entirely, especially those structurally deteriorating and/or no longer serving their original purpose. According to American Rivers, Pennsylvania led the nation in dam removals in 2023 with 15[27].

Travel corridors linking habitat
Another strategy to ensure connectivity is linking disconnected habitats by planting additional habitat as bridges. For example windbreaks are a common tool used by farmers, planting lines of trees to reduce windspeeds and for protecting crops and livestock from the elements[28]. If native tree species are used and the windbreak is sufficiently wide, it can not only create wildlife habitat but serve as a protected travel corridor across an otherwise exposed landscape for species attempting to avoid open areas because of the threat of predators or exposure to the elements[29].

A riparian buffer amidst farmlandPhoto by USDA | Public Domain

Riparian buffers can play a similar role. A long- established method for protecting the water quality in a stream, riparian buffers consist of trees and other native vegetation planted along a waterway to stabilize the bank and prevent erosion, to filter out pollutants and sediment, and to mitigate flash flooding. However, they have the additional benefit of creating a travel corridor for wildlife along the stream bank, providing cover and foraging opportunities along the stream and river valleys that already often serve as the highways of terrestrial migration[30].

Stepping stones
Habitat doesn’t need to be contiguous to benefit wildlife movement: a type of corridor conservation practice known as “stepping stones” has shown to be effective. This is when habitat patches are used for both local connectivity and migrating wildlife. On a local level, stepping stone-style patches of habitat help an animal cross a hostile environment from one side to another. For example, while forest-dwelling mammals like mink and or the Allegheny woodrat often won’t cross a wide exposed expanse that leaves them vulnerable to predators like owls, they can dash across small gaps to forest islands that offer shelter and protection.

Photo by USDA | Public Domain

These stepping stones of habitat are also critical within the larger landscape for migrating birds, who need frequent stops in suitable habitat to rest and forage all along a journey of a thousand miles or more. As a songbird flies from the Caribbean to northern North America, much of the landscape below – be it agricultural, suburbanized or industrial – may not provide the specific resources they need to rest and eat along their journey. Many birds will stop for multiple days at a time, sometimes doubling their body weight at a rest stop as they refuel for the next leg of their flight[31], so to maintain connectivity throughout the Atlantic Flyway, there need to be rest stops throughout the route. Green spaces in urban areas are particularly critical, with spaces like John Heinz National Wildlife Refuge in Philadelphia and even New York City’s Central Park serving as critical stopovers for migrating birds every summer and fall[32].

Necessary barriers
Finally, reducing animal-vehicle collisions and reducing wildlife mortality on our roads means not only providing safe crossings, but also preventing animals from entering the roadway by deploying fencing and other barriers. For example, when fencing was utilized along the Trans-Canada highway, it resulted in an 80-97% reduction in collisions between vehicles and herd animals like deer and elk[33]. These barrier projects can either stand alone, or ideally are paired with a wildlife crossing, preventing animals from entering the roadway at unsafe locations and instead funneling them towards a bridge, tunnel or other piece of crossing infrastructure.

Putting solutions into practice

Wildlife corridors are a proven method of addressing habitat fragmentation. As the case studies in this report demonstrate, work is already being done by Pennsylvania state agencies and commissions, and by private land conservancies, to stitch together fragmented habitats and create the crossings and corridors needed to reduce crossing deaths, and to make sure our wildlife can move, mate and migrate.

Examples of this in Pennsylvania include:

• The Pennsylvania Department of Transportation
  (PennDOT) has already constructed 35 wildlife
  crossings across the commonwealth;
• The Pennsylvania Game Commission is prioritizing lands contiguous to their existing holdings for future acquisition[34]
• The Pennsylvania Fish and Boat Commission are
  retrofitting dozens of existing culverts to better serve
  fish and other aquatic animals[35].
• The Pennsylvania Department of Conservation and Natural Resources (DCNR), through its Conservation Landscape Program, works with conservation nonprofits, landowners, local governments and the business community toward regional conservation goals[36].

But as the 2024 report The Current Status of Conservation Connectivity in Pennsylvania by the Pennsylvania Legislative Budget and Finance Committee lays out, the commonwealth lacks both a state-wide plan to maximize habitat connectivity and protect against future fragmentation, as well as a structure within our state government to ensure interagency cooperation and a shared set of goals around wildlife corridors.

To that end, the PennEnvironment Research & Policy Center recommends the creation of an interagency working group which, joined by conservation nonprofits already doing much of this work, should identify the highest priority areas for habitat connectivity, and establish common goals across state agencies.

Learn more: Download a pdf of the report, “Wildlife Corridors: How reconnecting habitats is protecting Pennsylvania’s native species”

Recommendations

Wildlife corridors are a proven method for addressing habitat fragmentation, reducing crossing deaths, facilitating migration and making sure our native species can access one another and as well as the resources they need to survive and thrive. From far-reaching regional plans that create habitat connectivity at the landscape-level to wildlife crossing projects such as strategically placed highway underpasses or rock structures across a pipeline path, this set of case studies shows that there are many tools in the toolbox to prevent or repair fragmentation and reconnect wildlife habitat.

Right now there are many opportunities to implement best practice like the ones highlighted in this report that can enhance habitat connectivity in Pennsylvania. But as laid out in the report put out by the state’s Legislative Budget and Finance Committee in 2024 entitled The Current Status of Conservation Connectivity in Pennsylvania, there is a lack of existing policy around wildlife corridors in Pennsylvania that would support these best practices. While many of our state agencies and commissions are already independently pursuing conservation goals, including habitat connectivity, there isn’t yet a statewide habitat connectivity plan or list of priority wildlife crossings to drive a common interagency agenda.

Yet this collaborative work has precedent in the commonwealth. Pennsylvania’s conservation landscapes – regional partnerships like the Kittatinny Ridge Conservation Landscape – demonstrate the efficacy of bringing state agencies, scientific experts, county governments and local nonprofits together under a common goal. To truly protect Pennsylvania’s wildlife, we need a commonwealth-wide plan to maximize habitat connectivity, protect against future fragmentation and use both large and small approaches to repair disconnections between habitats that already exist.

To that end, the PennEnvironment Research & Policy
Center recommends the following:

These administrative actions and legislative steps would go a long way towards implementing best practices for habitat connectivity in Pennsylvania, protecting the health of the commonwealth’s wildlife and our natural heritage for generations to come.

References

1.  Ad Crable, “Large-lot housing popping up across PA farmland,” The Bay Journal, 3 August 2020.

2. Pennsylvania Legislative Budget and Finance
Committee, A Study Pursuant to House Resolution 2023 – 87: The Current Status of Conservation Connectivity in Pennsylvania, June
2024.

3. State Farm Mutual Automobile Insurance, New State
Farm data reveals the likelihood of hitting an animal while driving in every state (press release), 24 September 2024.

4. Wan Yan Siu, The Center for Growth and Opportunity
at Utah State University, Wildlife Crossing Ahead: Costs and Benefits of Avoided Collisions, 4 April 2023.

5. Maryland Insurance Administration, Effects of Wild
Animal Collisions on Premium Increases for Private Passenger
Automobile Insurance, 2 December 2024.

6. Brian Whipkey, “Ever hit an animal on the road?
Pennsylvania ranks high for animal collisions,” GoErie.Com, 22
November 2022.

7. Marcus Schneck. “Turtle crossing time on Pennsylvania
roadways,” PennLive.com, 26 May 2020.

8. Karen Schiegg et al., “Inbreeding and experience affect response to climate change by endangered woodpeckers,” Proc Biol Sci; 269(1496):1153–1159. doi: 10.1098/rspb.2002.1966, 7 June 2002.

9. Jon Richardson, Pauline Comin and Per T. Smiseth,
“Inbred burying beetles suffer fitness costs from making poor
decisions,” Proc Biol Sci; 285(1881). doi: 10.1098/rspb.2018.0419, 27 June 2018.

10. National Parks Service, Genetic Change in LA Bobcats (press release), 16 March 2015.

11. Western Pennsylvania Conservancy, Species At Risk:
Forest Birds, accessed at https://waterlandlife.org/wildlife-pnhp/species-at-risk-in-pennsylvania/forest-interior-birds/, 18 November 2024.

12. Pennsylvania Game Commission, Wildlife Note: Allegheny Woodrat (fact sheet), May 2021.

13. Sanjay Pyare, Winston P. Smith and Colin S. Shanley, “Den use and selection by northern flying squirrels in fragmented landscapes,” Journal of Mammology, 91(4): 886-896 https://doi.org/10.1644/09-MAMM-A-179.1, August 2010.

14. Pennsylvania Game Commission, Wildlife Note: Fisher (fact sheet), February 2021.

15. Ron Masters, Steve Ditchkoff and Stephanie C Farley, Oklahoma State Extension, Wildlife Management Notes: Edge and Other Wildlife Concepts (fact sheet), April 2017.

16. National Audubon Society, Vulnerable Birds in Atlantic
Flyway, accessed at https://www.audubon.org/climate/survivalbydegrees/flyway/atlantic, 18 November 2024.

17. Xerces Society for Invertebrate Conservation, Monarchs
in Decline, accessed at: https://xerces.org/monarchs/conservation-efforts 4 February 2025.

18. Jessie Thomas-Blate, American Rivers, Saying Adios to 80
Dams in 2023, (blog), 13 February 2024. Accessed at https://www.americanrivers.org/2024/02/saying-adios-to-80-dams-in-2023/, 21 November 2024.

19. National Oceanic and Atmospheric Administration, Barriers to Fish Migration, accessed at https://www.fisheries.noaa.gov/insight/barriers-fish-migration, 20 November 2024.

20. National Audubon Society, Survival By Degrees: 389 Bird Species on the Brink, accessed at https://www
.audubon.org/climate/survivalbydegrees, 18 November 2024.

21. Toni Lyn Morelli et al, “Climate change refugia and
habitat connectivity promote species persistence,” Climate Change Responses 4(8) https://doi.org/10.1186/s40665-0170036-5, December 2019.

22. Rodney Van Der Ree et al. “Wildlife Tunnel Enhances
Population Viability” (PDF). Ecology and Society. 14 (2): 7. https://www.jstor.org/stable/26268295. December 2009.

23. Rob Ament, Renee Callahan and Elizabeth Fairbank,
Center for Large Landscape Conservation, Wildlife-Vehicle Conflict, Crossing Structures, and Cost Estimates. 2020.

24. University of Maryland Center for Environmental
Science, New study uncovers how animals use underground
highway culverts, (press release), 10 November 2011.

25. New Jersey Department of Environmental Protection,
Division of Fish and Wildlife, CHANJ (Connecting Habitat Across New Jersey) Guidance Document Version 1.0, April 2019.

26. National Oceanic and Atmospheric Administration.
What is a fish ladder? 6 June 2024. Accessed at: https://oceanservice.noaa.gov/facts/fish-ladder.html 19 February 2025.

27. Jessie Thomas-Blate, American Rivers, Saying Adios to 80
Dams in 2023, (blog), February 13, 2024. Accessed at https://www.americanrivers.org/2024/02/saying-adios-to-80-dams-in-2023/, November 21, 2024.

28. United States Department of Agriculture, Windbreak-
shelterbelt establishment and renovation (fact sheet), July 2021, accessed: 4 February 2025.

29. University of Nebraska-Lincoln Extension, Windbreaks
and Wildlife (fact sheet), 2006.

30. PennState Extension, Riparian Buffers for Wildlife, 5 August 2024, accessed at: https://extension.psu.edu/riparian-buffers-for-wildlife, 4 February 2025.

31. Ken Rosenberg, Nick Bayly, and Camila Gómez,
American Bird Conservancy, Stopover Habitats and What Birds
Need on Migration (blog post), 27 April 2017.

32. Rachel Fritts, American Bird Conservancy, Avian Superhighways: The Four Flyways of North America (blog post), 16 May 2022.

33. Marcel P. Huijser et al., “Effectiveness of short sections
of wildlife fencing and crossing structures along highways in
reducing wildlife–vehicle collisions and providing safe crossing
opportunities for large mammals,” Biological Conservation 197:
61-68, https://doi.org/10.1016/j.biocon.2016.02.002, May 2016.

34. Pennsylvania Legislative Budget and Finance Committee, A Study Pursuant to House Resolution 2023 – 87: The Current Status of Conservation Connectivity in Pennsylvania, June 2024.

35. Ad Crable, “Stream crossings in Pennsylvania gamelands will get upgrade,” The Bay Journal, 18 January 2024.

36. Pennsylvania Department of Conservation and Natural
Resources, Conservation Landscape Program, accessed 5 February 2025.