Amphibian Crossings in Acadia National Park

Amphibians use multiple types of habitat throughout the year. Most frogs and salamanders spend their winters underground in the forest. When spring arrives, amphibians wake from hibernation and begin to move. They migrate to wetlands, where they will breed and lay their eggs. Roads often intersect the habitat between forests and wetlands, leaving migrating amphibians with no choice but to cross the road. Unfortunately, many amphibians are killed by vehicles when they migrate across roads. These animals' small size and slow movements make them difficult to see from a car.

Acadia National Park protects a variety of habitats for the benefit of humans and wildlife. The park's wetland, forest, and stream habitats are vital to twelve species of amphibians. Bullfrogs and green frogs are a common sight in the park's wetlands. Wood frogs, spotted salamanders, and red-backed salamanders live throughout the park's forest. Two-lined salamanders inhabit many of Acadia's freshwater streams.

Acadia’s scenic roads are one of the park’s most popular attractions. The famous Park Loop Road winds along coastlines, mountains, and forests. The park’s roads intersect many wetlands where amphibians likely breed. But whether amphibians migrate across Acadia's roads has not yet been studied.

Fortunately, Acadia's roads are closed off in the winter and spring. These closures spare any migrating amphibians from the deadly impacts of vehicles. But with visitation to the park steadily increasing throughout the year, Acadia's roads may soon open earlier in the year. It is important to identify areas in Acadia National Park where amphibians may cross the road. Once these crossing sites are identified, we can take action to prevent amphibian roadkill.

Predicting where amphibians might cross Acadia's roads requires an understanding of amphibian ecology. Amphibians have very specific habitat requirements, and they move between habitats throughout the year. Amphibians are more likely to cross roads that border their preferred habitat. Fortunately, GIS gives us the capability to compare different types of habitat data. Then, we can create can create a map of potential amphibian road crossing locations.

Aquatic habitats such as wetlands, vernal pools, and streams are important to amphibians. Most of the amphibian species in Acadia migrate to wetlands and vernal pools in the spring to lay their eggs. The juvenile amphibians remain in the aquatic habitat until they are ready to move onto land. At that point, most species migrate away from the wetlands. The new generation of amphibians will later return to the same wetlands to breed. Because amphibians repeatedly migrate to and from wetlands, we can use the presence of wetlands near the road to predict amphibian crossings.

In the map to the right, bright blue areas represent wetlands, brown spots represent vernal pools, and blue lines represent streams.

Many of Acadia's amphibian species live in the forest as adults. Wood frogs, spotted salamanders, and red-backed salamanders spend most of their lives on the forest floor. Previous research has estimated that forest-dwelling amphibians need a forest cover greater than 40% to thrive (Patrick et al. 2012). When amphibians migrate to the wetlands, they will move away from these dense forests. Therefore, amphibians are more likely to cross roads that intersect both wetlands and dense forests.

In the map on the right, areas where forest cover exceeds 40% are shaded in green. Mountain slopes and summits are excluded since Acadia's amphibians do not inhabit higher elevations.

When amphibians migrate, they tend to move from upland forests to low-lying wetlands. Contour lines show us elevation changes in the landscape. We can strengthen our predictions by comparing contour lines to the earlier habitat data.

When predicting amphibian crossing locations, we should only consider the habitat that occurs close to the road.

With GIS, we can create a buffer zone that shows the habitat around existing roads.

In the map on the right, the yellow zone highlights habitat within 500 yards of roads. The buffer zone is discontinuous because it only includes roads inside the Acadia National Park boundary.

We can overlay all of the data above to create a composite map. In the map below, the red ovals represent potential amphibian crossing zones. These zones were placed in areas where the road intersects both wetland and forest habitats.

We strengthen our predictions by adding amphibian observation data.

 iNaturalist  is a website where citizen scientists submit observations of plants and animals from anywhere in the world. Through iNaturalist, we can obtain amphibian observation data from Acadia National Park. We can then plot these observations as points on our GIS map. The triangles on the map below represent amphibian observations from iNaturalist. The points are color-coded by family. Swipe the map to see how the observation points compare to our predicted crossing zones.

Now that we've made our predictions, we should evaluate each potential amphibian crossing site. How likely is it that amphibians will cross at each of our predicted sites? We can give each site a score based on the following criteria: - What type of habitats are present near the road? - Are there wetlands, vernal pools, streams, and/or forests? - Is there an elevation change between the roadside habitats? - Have amphibians been observed near the predicted crossing zone? - Is the road heavily used by visitors in Acadia? Prior studies suggest that amphibians are more likely to cross in places where wetlands occur on the sides of the road (Langen et al., 2009). So, we should also include that in our criteria: - Are there wetlands on both sides of the road? The following map shows predicted amphibian crossing sites, colored by score. Amphibians are less likely to cross at sites with lower scores. Yellow crossings have lower scores, while orange crossings have higher scores. Red crossings have the highest scores, and are most likely to be true amphibian crossing areas.

My map identifies 24 potential amphibian crossing sites across Acadia National Park. These sites were predicted based on roadside wetland and forest habitat data. Most of these sites occur on the eastern side of Mount Desert Island, where the park's visitation is also concentrated. Potential amphibian crossings occur on major roads within the national park, including Park Loop Road, Route 3, Eagle Lake Road, Seal Cove Road, and Seawall Road. Four sites with the highest potential for amphibian crossings occur near Eagle Lake, Jordan Pond, and the Hadlock Ponds. Acadia National Park should take potential amphibian crossings into consideration when making decisions regarding its roads.

No matter how thorough our predictions are, direct observation is the only way to truly know where amphibians cross the road. We can use our map of predicted crossings to choose locations for future research. In the future, field surveys could identify places in Acadia where amphibians actually cross the road. Targeted research could also reveal which species migrate and the relative number of individuals. Surveys may even locate the breeding sites of amphibians that are uncommonly seen in Acadia, such as the four-toed salamander.

When combined with research, maps of predicted amphibian crossings like this one could help biologists recommend ways to prevent amphibian roadkill in Acadia National Park.

Explore the various layers and landscape features that indicate potential amphibian crossings. Click and drag your mouse to navigate the map.

Select the icons on the sidebar to: 1. View the legend 2. Toggle layers on/off 3. Change the base map 4. Swipe a layer of your choice You may also view the attribute table by clicking on the triangle icon below the map.

This map is best viewed in a desktop browser. This map is intended for educational use only.