Beavers Are Ecosystem Engineers

Beavers are ecosystem engineers that build dams. The dams change the way water moves between streams, rivers, lakes, and the land around them. Beaver dams in streams block the flow of water, making ponds that flood nearby soils (Figure 1). Scientific studies of beaver ponds in the western United States and Canada show that beavers can impact water quality. Beavers can change the amounts of nutrients (which act like fertilizers), carbon (from dead plant material), and harmful substances (like mercury) in streams and lakes [1]. Beavers can also cause the spread of diseases like Giardia (sometimes called beaver fever). This disease makes people sick if they drink unfiltered water from streams. Beavers also create habitats for fish and wildlife, and they can also affect landscapes by encouraging growth of vegetation and limiting the spread of wildfires. Given all of this, there is good reason why people use the phrase “busy as a beaver”!

Beavers Are Moving North in Alaska

Beavers typically live in regions with forests. They use trees, branches, mud, and rocks to build dams and lodges. In Alaska, beavers have mainly lived in the boreal forests in the lower part of the state. But scientists recently discovered that beavers are moving north, beyond Alaska’s boreal forests and into the Arctic tundra [2]. Using satellite images, researchers can see that, over the last 30 years, beaver ponds have increased in northwest Alaska (Figure 2). Beavers are moving into new habitat in Alaska’s Arctic national parks, including Bering Land Bridge National Preserve, Cape Krusenstern National Monument, and Noatak National Preserve.

Beaver numbers have changed due to both trapping and climate change. In the 1800s, people used to trap beavers for their valuable fur, which kept the number of beavers in the region low. By the 1900s, new laws slowed beaver trapping, which caused the number of beavers to increase. In the past, the Arctic was too cold and lacked both beaver food and the large sticks needed for building their homes and dams. But the climate is warming rapidly in Alaska—much faster than in the rest of the United States. As it gets warmer, trees and shrubs can grow farther north. This new growth provides the wood beavers can use to build dams and lodges. Other mammals, like snowshoe hares and moose, are also moving north as Earth’s climate warms and shrubs grow bigger. Climate models—or computer simulations—predict that Alaska’s climate will continue to warm for tens of years into the future. As a result, scientists believe that the number of beavers will continue to increase, and they will expand throughout the Alaskan Arctic. Scientists from the US Geological Survey, the National Park Service, and the University of Alaska Fairbanks are currently studying beavers to understand how they are changing Arctic lands and waters.

Do Beavers Cause Permafrost To Thaw?

Beaver ponds in the Arctic are different from beaver ponds in the rest of the US due to the presence of permafrost. Permafrost, or frozen soil, is an important feature of Arctic regions like Alaska. Permafrost forms in cold climates. Most permafrost has remained frozen for hundreds or even thousands of years. But recent climate warming in the Arctic is causing permafrost to thaw. Perhaps permafrost is not permanent after all! When permafrost thaws, the ice melts, water flows away, and the ground surface can collapse. In Arctic towns and cities, permafrost thaw can also cause houses to collapse and roads to break.

New beaver ponds flood the surrounding Arctic permafrost soils. During summer, the relatively warm pond water causes the permafrost to warm and rapidly thaw in a process known as thermokarst [3]. This permafrost thaw can occur beneath ponds, making ponds deeper over time. Permafrost thaw can also happen around the edges of ponds. This increases the surface area of beaver ponds as the pond banks thaw, collapse, and erode over time (Figure 3). It is clear how beaver ponds can change the land and streams. How can they impact water quality and climate change?

Beaver Effects on Water Quality and Fish

Permafrost stores large amounts of organic carbon and nutrients. Think of carbon and nutrients as food for stream plants and algae. Algae grow attached to rocks and wood or they float in the water. Algae and other water plants make up the base of the food web. They support bugs, fish, and all other animals that eat plants. For example, moose are often spotted eating plants out of beaver ponds. When carbon and nutrients are frozen in permafrost, it is like storing food in a freezer. When permafrost thaws, it is like you are transferring the food from the freezer to the refrigerator. When the food thaws, it can be eaten, or it can decompose.

As we discussed earlier, beavers can cause permafrost soils to thaw by building ponds. In that way, beavers cause the release of carbon and nutrients from permafrost into ponds and streams. Water quality is a measure of how safe that water is for people or an ecosystem¹. Scientists do not yet know the consequences of beaver ponds on water quality and stream food webs. A recent study found carbon from thawing permafrost in the muscles of Arctic fish species. Species called Arctic Grayling and Dolly Varden had permafrost carbon in their muscles because it was in their food [4]. Permafrost carbon enters the food web through algae and moves up the food chain through bugs to fish. It is possible that beaver ponds are a great place for some fish species because permafrost carbon can support their growth and energy needs.

One concern of scientists, national park managers, and fishermen is the release of mercury, a metal, from thawing permafrost into beaver ponds. In addition to carbon and nutrients, permafrost stores large amounts of mercury. Some forms of mercury are toxic and can be taken up by stream food webs. To better understand this release of mercury, we visited beaver ponds in northwest Alaska to make scientific observations and collect water and fish samples. Our research on Arctic beaver ponds in national parks shows that beaver ponds can be “hotspots” for toxic mercury. For instance, we found that toxic forms of mercury can account for up to 80% of total mercury in beaver pond sediments. If people eat fish with high amounts of mercury, it can negatively affect their health. Therefore, it is important to determine if beaver ponds are causing mercury to accumulate in Arctic fish.

Can Beavers Impact Earth’s Climate?

When permafrost thaws and carbon moves from the freezer to the refrigerator, this thawed carbon can also move from the soil to the air. Soil bacteria can decompose the thawed carbon, similar to the way animals chew and digest food. By doing so, these bacteria produce the greenhouse gases carbon dioxide (CO₂) and methane (CH₄). These greenhouse gases are the main reason why Earth’s climate has been warming so quickly over the last 40–50 years. Beaver ponds and other shallow Arctic lakes release lots of methane to the atmosphere [5]. We expect that permafrost will continue to thaw and more carbon will be released to the atmosphere as beavers move north. By adding more greenhouse gases to Earth’s atmosphere, beavers may be contributing to Earth’s warming climate! However, scientists do not know how large of an impact beavers will have on climate. We will continue to study these ecosystems to better understand the importance of beavers to the Arctic and to Earth’s climate.

As ecosystem engineers, beavers have a large effect on ecosystems. Now that beavers have moved north into the Arctic tundra, their effects could be even greater. The combination of beavers and permafrost thaw makes tundra streams exciting places to study. It is important to understand the effects of these changes throughout the food web, to the climate, and to people. We are just now beginning to understand all the different things that change when beavers make a tundra stream their home. In the future, we will collect more water and fish samples to better understand the effects of beavers on mercury and greenhouse gases. Now, scientists are as busy as beavers.

Funding

This project was supported by a grant from the USGS-NPS Water Quality Partnership Program.

Glossary

Boreal Forest: ↑ A large, forested region in the Northern Hemisphere. The boreal forest is dominated by conifer trees, like spruce and pine. Boreal trees and soils store lots of carbon.

Arctic Tundra: ↑ The region north of the boreal forest. Arctic tundra ecosystems are cold, with small plants (such as moss and lichen) and permafrost soils.

Climate Models: ↑ Complex computer programs that use math to understand Earth’s climate. Climate models can be used to study how land, air, and oceans interact to affect the climate.

Permafrost: ↑ Soil that has remained frozen for at least two straight years, although most permafrost has been frozen for much longer.

Thermokarst: ↑ When icy permafrost soils thaw, the ground surface can collapse. This process is known as thermokarst.

Organic Carbon: ↑ Carbon that forms from living things, such as plant or animals. In the Arctic, soils store lots of organic carbon.

Algae: ↑ Simple plants that grow in streams and lakes. Unlike many land plants, algae do not have stems, roots, or leaves. “Algae” is plural, and the singular term is “alga.”

Permafrost Carbon: ↑ Carbon, or dead plant material, stored in permafrost. When permafrost thaws, large amounts of carbon can be released into the air as greenhouse gases.

Conflict of Interest

The authors declare that the research was conducted in the absence of any commercial or financial relationships that could be construed as a potential conflict of interest.

Footnote

1. ↑For more information about water quality, see: https://www.usgs.gov/special-topic/water-science-school/science/water-quality-information-topic.