Bowdoin Science Journal

Environmental Science and EOS

Living in Beaverland: The Ecology and Biogeochemistry of Beavers

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Next time you’re flying up to the Portland Jetport and the third rerun of Endgame just isn’t cutting it, look out the window. Much of the northern half of the continent is a sprawling landscape dotted with kettle lakes, winding rivers, and other vestigial scars of a world once drowned in ice. Certainly, of the processes that shaped the bedrock Bowdoin sits on, none are as immediately evident as the glaciers that covered it for seventy thousand years. Today, however, a much tinier (and cuter) force gnaws away at the landscape of modern Maine. Our waterways are a record of glaciers, true, but they’re just as much a record of the furry engineers that now inhabit them: Castor canadensis, the North American beaver. 

 

NPS / Neal Herbert, Public Domain.

 

Calling beavers “ecosystem engineers” isn’t science mumbo-jumbo: beavers quite literally show an understanding of the forces of hydrology that backdrops their dams. When a stream encounters a thin opening in bedrock that constricts its flow, its waters gurgle and bubble as it narrows through the gap. It is this gurgling that first allures a beaver to build its dam. In essence, beavers can “‘hear’ the geometry of the river basin.” There, a pioneering beaver colony lays the first branches of speckled alder that they consider too bitter for food, pointing the branches upstream to catch and anchor sediment as part of this keystone layer of wood. Layer upon layer of mud and stick convexes upstream, like the shape of the Hoover Dam, to combat increasing pressure from the pond when the dam plugs more water. As the colony gets larger, beavers build secondary and tertiary dams upstream to relieve pressure on their lodge, so that within generations, the colony will have terraformed their entire forest environment with ponds and meadows along a stairway of rivers and dams.

This process of familial expansion has impacted nearly all of the waterways in the northern United States, especially following their bounce back to pre-colonial populations. For instance, in the North Woods of Minnesota, 90% of streams flow through at least one dam, and overall, 15% of land is covered by beaver ponds or meadows. In the process, beavers unknowingly change the ecology, hydrology, and chemistry of their ponds— often by simply slowing down water. 

At its simplest, slower water cannot carry as much sediment, and in beaver habitats, this has incredible repercussions. Regions that experience alarming rates of erosion benefit from dams because streams cannot carry away soil. So, if rivers cannot carry sediment, they must deposit it instead of erode: beaver ponds carry much more sediment than other streams, increasing the amount of organic matter stored in its pond bottoms. When abandoned dams are broken through, this standing stock of nutrients encourages plant growth and a more biodiverse wet meadow after flooding settles. In the Colorado River, for instance, the distribution of sediment deposited in a dam flood influenced where a diverse plant community was able to grow.

By the same token, beaver habitats are essentially wetlands, critically changing the biogeochemical conditions around dams. Deeper waters and deeper soils foster denitrification, a form of bacterial activity that filters waters affected by nitrate pollution from things like fertilizers. Depositing more sediment also increases the carbon found in beaver ponds. Altogether, these wetland conditions increase biodiversity by a third of what’s found without beaver habitation. Because slower, sediment-laden streams downstream of dams are more likely to curve and branch, more beaver colonies can use them. With these prolific changes, the continent might truly, as environmental journalist Ben Goldfarb puts it, “better be termed Beaverland.”

Scientists have kept log (pun intended) of these benefits for quite some time— the foundational study on their role as geomorphic agents was published in 1938— but attempts to work together with the rodents in structures called beaver dam analogs (BDAs) have only caught on in recent years. In one of its earlier uses, ecosystems analyst Michael Pollock worked on the restoration of a stream that steelhead trout used in their migration inland. Before long, “beavers came and set up shop” on Pollock’s somewhat ad-hoc dams, and the results his team saw were incredible: BDAs increased habitat and reared more than three times more steelhead than an undammed stream nearby.

Despite its happy ending, Pollock argues that their study isn’t the paper to end all papers. How effectively BDAs can mimic and aid beaver dam construction still requires much more testing, and whether it’s worthwhile to reintroduce beavers at all is still debated. Nonetheless, some local governments and farmers alike who benefit from their application have begun to consider the idea of a true BDA Beaverland, so long as regulators get on board too. Whether the costs of building and maintaining BDAs are worth a beaver dam’s biogeochemical and ecological benefits are still up for talk among officials hesitant to rely on these furry rodents. But with all its controversy, you can’t deny that the impacts of Beaverland truly seem to be giving glaciers a run for their money.

Seismic Songs and Slinkies

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            In 1970, a 34-minute album was released by bio-acousticians Katharine and Roger Payne composed entirely of humpback whale songs. A few minutes into the album, phrases begin to coalesce into conversations with voices rising and falling in a strangely familiar rhythm. Some phrases sound like whining teenagers. Others like an elder beginning a story or a family on a trip. At times, though, the songs sound completely otherworldly.  The album is called Songs of the Humpback Whale, and it turned people’s ears towards the sounds of the oceans.

           Today, whale songs have caught the attention of a different audience: geophysicists studying the ocean floor. Our modern understandings of bathymetry – or underwater topography – have largely come from studies using instrumental acoustic waves onboard ships. As a ship travels through the ocean, an instrument called a transducer sends sound waves through the water and receives the signals that bounce back. Imagine standing at the railing of a ship and flinging one end of a slinky down to the seafloor: when the slinky hits the bottom, it sends a pulse back up to your hand. Now imagine your slinky reaches a layer of mud on the seafloor. It will send a very different signal back to your hand than if it penetrated to a denser material, like the ocean crust. That return pulse can be analyzed to produce profiles of seafloor texture and thickness.

           As powerful as this method – called the echo sounder method – is to the study of bathymetry, it comes with a major problem. Roger Payne and others helped us understand that marine mammals use sounds to socialize, navigate, find food, and identify mates. These are just a few possible interpretations of marine mammal vocalizations; it’s still a language we have yet to fully translate. What happens, though, when the ambient noise of the ocean from shipping increases? Global ship density increased by a factor of four from 1992 to 2012 which was linked to a 3 decibel per decade increase in ambient marine noise. That might not seem like a lot, but studies have found that maritime noise negatively interferes with marine mammals.  As some biologists describe it, maritime noise is like smog; “it shrinks the perceptual world of whales, fish, and other marine life.” And it turns out, the echo sounder method and other acoustic methods used to study the seafloor can also be detrimental to marine mammals and cause temporary hearing loss, disorientation, and behavioral changes.

           Is there a solution? Can geophysicists and oceanographers simultaneously study the seafloor while minimizing the impacts on marine life? From Science, authors Václav Kuna and John Nábĕlek introduce an innovative and less invasive technique for studying ocean seafloor structure by harnessing fin whale songs. 

Fin whales (Balaenoptera physalus) are the second largest whale in our oceans, growing up to 75-85 feet long. Fin whales use baleen – hair-like plates made of keratin that hang inside whales’ mouths – to filter feed on krill and small fish. The most important characteristic of fin whales for Kuna and Nábĕlek’s research was their powerful vocalizations – often reaching up to 189 dB, which is louder than a jet engine. These vocalizations have been picked up at ocean-bottom seismometer (OBS) stations which monitor underwater earthquakes. Kuna and Nábĕlek discovered that these OBS stations were not only recording songs directly from fin whales but also the secondary waves reflected up from the seafloor.

           Kuna and Nábĕlek studied six fin whale song recordings collected from three OBS stations located near an active fault in the north east Pacific, about 150 km off the coast of Oregon. The authors determined the whales’ paths and distances from the stations based off of the difference between the arrival times of two wave types – the direct wave, registering first at a higher velocity, and the waterborne multiple wave, a series of waves that bounce off the seafloor and sea surface before registering at an OBS station.

           In addition to sound waves, the authors identified four distinct seismic waves based on their unique subsurface reflection patterns. The arrival times of these seismic phases at the OBS stations varied because their velocities differed depending on the composition of the layers they penetrated. Again, think of the slinky interacting with seafloor layers that have differing densities. Based on the arrival times of the seismic phases relative to the arrival times of the waterborne wave types, Kuna and Nábĕlek determined that the seismic velocities of the whales’ calls corresponded with wave interactions at sediment, basaltic, and lower crustal layers. These findings aligned well with geophysicists’ previous maps of the bathymetry in the region.

           The significance of these results is nothing short of remarkable. Kuna and Nábĕlek harnessed a natural phenomenon – fin whale songs – to better understand geologic features of the seafloor that are completely unrelated to whales. They created a link between two distinct spheres of Earth that no one knew existed. Now as Songs of the Humpback Whale comes to an end, I’m left with an image of whales sharing secrets of the seafloor as they travel across ocean basins. 

 

 

 

An Ozone Success Story

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As a child, news of the hole in the ozone layer terrified me. I pictured green aliens covered in slime sliding through the dark hole into the skies above us, casting murky shadows over entire continents. Standing below the ozone hole I wanted to know could you see their tentacles reach through the clouds?  I was convinced the ozone hole was a portal to a frightening and unknown world. 

The stratospheric ozone layer, 15-35 km above us, acts like sunscreen. Ozone refers to O3 gas – three oxygen atoms bonded together. The layer protects our (and every other living thing’s) DNA from harmful levels of ultraviolet (UV) light from the sun that pelt our planet. Here’s the catch: ultraviolet light also kickstarts chemical reactions in our atmosphere that cause oxygen atoms to break off from ozone. This leaves ozone continually crumbling and combining in a balanced cycle that maintains the layer’s UV repellency. In the 1980s, however, a geophysicist and two meteorologists began noticing that the ozone layer was thinning each spring, particularly over Antarctica. 

Thus, the ozone “hole” was discovered. Here’s what the scientists concluded. The thinning ozone layer represented a history of accumulating aerosols and chlorofluorocarbons – a kind of gas known as CFCs. At the time, CFCs were used as refrigerants in air-conditioners and cars, as cleaning products, and as foaming agents for insulation. Even hairspray aerosol cans contained CFCs. One ingredient in CFCs is chlorine which, as a gas, erodes away the ozone layer faster than it can be created. As gaseous CFCs accumulated in the upper atmosphere, the ozone layer was gradually eaten away and higher levels of ultraviolet light began to be recorded in the Southern Hemisphere. These findings mobilized scientists to act. 

The Montreal Protocol from 1987 is one of a few examples of multinational cooperation on environmental regulations. The treaty banned production and international trade in a number of ozone-depleting substances, CFCs included. As the only UN Treaty in history to receive universal ratification, the signatories collaborated around a common objective to protect the ozone layer and the animals and plants who live beneath it. The ozone hole contributed to an awareness of the capacity of human behavior to mangle natural processes. 

This story does not end in 1987. In 2013, the decline of CFCs unexpectedly slowed from 0.85% between 2002-2012 to 0.4% after 2013. This was a sign that newly produced CFCs — 13,000 tons, to be precise — were entering the atmosphere, and scientists didn’t know where it was coming from. That might not sound like that much – given that in 2019 the US alone emitted 5.1 billion tons of CO2 – but any new CFC production contributes to a reservoir of CFCs that still exists in discarded items, like refrigerators and air conditioners. That pool hasn’t been released yet, but its effect on the atmosphere has been accounted for. These newly produced CFCs contribute to a 6-45% increase in the global reservoir and future cumulative emissions. We might see aliens yet. 

Eventually the CFCs were traced to eastern mainland China through monitoring stations in Japan and Korea. The Chinese government questioned the source of the emissions but agreed that improved atmospheric monitoring was needed. China, as a signatory of the Montreal Protocol, was obligated to take action in the face of these findings and they did just that. Their atmospheric testing stations now record CFC levels in the atmosphere, labs were built to test for CFCs in suspect consumer products, and hefty fines were placed on any factories producing CFCs. 

Like so many emissions, the effects of CFCs aren’t limited to the current moment but to many years in the future. That being said, the Montreal Protocol worked. The damage to the ozone layer will be negligible and China took firm action. We can apply our sunscreen peacefully knowing the thinned ozone layer is still on track to recover fully with no aliens in sight.