Chasing Beavers

by Hari Balasubramanian

A selection of facts, research and personal encounters involving beavers and their habitat.

Longest-dam-GE-liIn October 2007, an 835-meter long beaver dam was discovered on Google Earth. It remains the longest one found so far. The dam was in the “thick wildness of Northern Alberta”, in Wood Buffalo National Park. In July 2014 someone called Rob Mark, an amateur explorer from New Jersey, managed to reach the dam. He reports that it was incredibly difficult terrain to get through. The mosquitoes in Alberta were much worse than the Amazonian rain forest; they sounded like helicopters and bit through his clothes. When Mark finally got to the dam, a resident beaver announced its displeasure with angry slaps of its flat tail on the water.

It was wonderful and somehow liberating to hear this last detail. To the beaver of course, the effort that had gone into this journey of discovery – the sort that seems to matter a lot of us humans – meant absolutely nothing; it only counted as an intrusion.

But I do understand why Mark made the journey. I've been chasing beavers myself in the conservation areas of Amherst, Massachusetts (where I live). Last year, I designed my summer and fall hikes so as to cover as many beaver ponds as possible: like a traveling salesman trying to cover all customer locations efficiently. One evening, with light fading fast, I was walking along the Fort River, a tributary of the Connecticut. Suddenly, there was a tremendous splash as if a boulder had been thrown from a considerable height into the water. It was October, and with winter fast approaching, the beavers were trying to dam the river. A red maple tree, leaves still clinging to its branches, had been felled. But it wasn't the tree that had caused the splash; the tree had been brought down perhaps a couple of days ago. The deep, explosive noise – impossible though that seemed – was the flat tail of a beaver hitting against the running water! As if to dissuade me from exploring further, the beaver produced yet another equally noisy warning.

Intrigued, I visited Amherst town offices a couple of days later, to ask if someone there had information on beavers in conservation areas. A town official heard me out, but he was concerned: “It would be unacceptable if the Fort River was being dammed as you say. This would flood nearby homes. Beavers change the ground water level so even people with homes that are far away from beaver dams notice flooding in their basements and are puzzled. I need to send my land manager out immediately.” A bearded stranger, who happened to be passing by and had overheard, stopped and said eagerly: “Do you need to take care of beavers? Because I know someone who does a very good job.” In effect he was claiming he knew a Beaver Hitman.

These reactions left no doubt about the beaver's modern status as a pest in residential areas. But there is another kind of status this natural engineer has, and it has to do, among other things, with how well it retains water on the landscape even in periods of drought and creates conditions where diverse types of wildlife can thrive. Let's take a closer look.


American_BeaverThe North American beaver, Castor Canadensis, is a large, furry rodent. Most beavers are dark-brown, though some have lighter coats. The darker ones I sighted early on in Amherst reminded me of the big, scary rats I used to spot back in India, disappearing into the gutters and plumbing tunnels of railway stations. But beavers are actually closer to squirrels and marmots than to rats. They diverged 90-100 million years ago from their closest living relatives [1] and have since then charted a unique course as ecosystem engineers.

Those of you who are unfamiliar, here are some important details about the North American beaver (you can also check out this info-graphic):

1. A busy, mostly nocturnal and semi-aquatic creature, the beaver brings down trees by chewing on their trunks with its powerful teeth. Living in the vicinity of rivers and streams, the beaver builds dams and lodges with mud, material collected from the fallen trees, and stone. A vegetarian, the beaver feeds on barks, twigs, leaves and aquatic vegetation. Dams are usually not as large as the one discovered in Alberta: it's more common to spot dams like this one I found last October in Amherst.


2. Dams allow the beaver to create deep ponds which predators have a hard time accessing. To further protect themselves, beavers build multi-chambered lodges in the ponds that only have entrances underwater. The lodge is where a family stays. A family consists of two parents that typically mate for life, and their offspring. In places where the water freezes, temperatures in the lodge stay in a narrow range between 0.8-1.6 degree Celsius, even as the outside maximum and minimum fluctuate between -21 and -6.8 degree Celsius [1]. While in the lodge, the beavers live off a nearby cache of twigs and branches collected before winter begins.


3. Once widespread in North America, beavers were trapped and their fur used to create hats fashionable in Europe from the 17th-19th centuries. The fur trade between Native Americans and Europeans brought numbers down dramatically. In many states, like Massachusetts and New York, beavers disappeared completely. In the 20th century, they were reintroduced in many wilderness areas in the United States and Canada, and have made a very strong recovery.

Mitigating the Effects of Drought

In 1941, beavers were re-introduced into Elk Island National Park in south-central Alberta. Park wardens kept detailed records and maps of how many beavers were active and in which ponds. Here was a long term natural experiment, a unique opportunity to quantify the impact of beavers. A researcher named Glynnis Hood began looking at this data spanning 54 years, from 1948-2002 [2].

In the years following the reintroduction, there were very few beavers. But fifty years later, the number of active lodges in the ponds had gone up significantly. And so, it turned out, had the area of open water. When Hood her co-author Suzanne Bayley looked at the area of open water – measured by digitizing aerial photographs taken in the park over the 54 year period – they found that it was strongly positively correlated with the number of active beaver lodges.

The authors also looked at a host of other variables, such as mean maximum annual temperature, precipitation and rainfall in the months and years leading up to a particular year. These variables, along with the number of active and inactive beaver lodges, were tried as inputs to a statistical model, a regression. The goal was to identify which variables best explained the area of open water. The authors found that the number of active beaver lodges was by far the most significant factor, comfortably surpassing temperature and precipitation.

What's so special, you might ask: beavers simply chase water, which might explain the correlation. But there was an important catch: 2002 was the driest year on record, with the lowest precipitation, yet ponds with active beaver in them had nine times more open water compared to the exact same ponds in 1950, a year that had 47% more precipitation compared to 2002 but no beaver activity! If we take the mean annual precipitation 2-years prior to a particular year we get the following results for 1948, 1950, 1996 and 2001. The numbers here are based my visual inspection of the graphs in the Hood and Bayley paper [2] (I do not have access to the raw numbers, so these are only approximately accurate values). The data below is for ponds that did not have active beaver in them in 1948 and 1950, but did have active beaver colonies in 1996 and 2001.

Notice that while the precipitation levels in 1948 and 1950 are fairly similar to those in 1996 and 2001, differences in open water area are huge!

Hood explains the implications in the PBS Nature documentary, Leave It To Beavers: “In 2002, we had the worst drought on record. The only places where we had water in natural areas was where we had beaver. And farmers were actually seeking out neighbors who had beavers on their landscape to water their cattle. So with beavers back on the land, even during the worst drought on record, they were mitigating the effects of drought and keeping water on the landscape.”

How exactly did beavers manage this? “In part, they were digging these channels,” Hood says. “The bottom of a beaver pond is really, really convoluted, it's flying through the Grand Canyon, where you've got these deep, furrowed valleys and dynamic pond bottoms. Deeper ponds keep more water because you have less evaporation coming off of them. Beavers were using that to their advantage, digging deeper and deeper and allowing water to focus in here, so the ponds with beaver had water, and ponds without beaver didn't, plain and simple.” [short video]

Enabling a Diversity of Wildlife

When a species engineers the retention and flow of a natural resource as important as water, its presence naturally influences the well-being and prevalence of a host of other species. For millions of years, beavers have co-evolved with a wide variety of organisms that take advantage of the engineered habitat. “Minks, muskrats, and bats forage in and around beaver ponds. Salamanders, frogs, turtles, water snakes, herons, grebes, ducks, rails, swallows, hawks, owls, flycatchers, and kingfishers all rely quite heavily on beaver-created habitats” [3]. It's no surprise that the beaver is considered a keystone species.

Beavers eventually abandon their ponds and move on to create new ones. The abandoned ponds then turn into meadows where, after many years, trees of the type the beaver felled now may take root again. This means that at any time there are a patchwork of different habitats, each benefiting a different set of species. Beaver landscapes thus produce results that are far more robust and interesting from a species diversity viewpoint compared to reservoirs and ponds created by human dams, which are not as dynamic.


Enter a beaver pond and you'll notice immediately that something is different. To me, the most striking thing about these ponds is how disorderly they are; they are the opposite of manicured, symmetric lawns and well kept gardens. The still water of the pond sets up the tranquil mood, but otherwise there are logs and broken stumps haphazardly strewn and poking out of the water at odd angles; dead but still standing trees with no leaves, trees that died when the beaver flooded the area. My favorite pond is adjacent to a relatively busy road in south Amherst. In late summer and early fall, a great variety of birds are active there. Woodpeckers, cedar waxwings, wood thrushes, blue jays, goldfinches, cardinals, chickadees, warblers: I saw them all regularly and easily, and always returned from my walks thrilled and rejuvenated.

Most striking of all, a Great Blue Heron would fly in and perch itself on one the dead but standing trees. It happened so often, that I started counting: on 5 successive visits to the same pond, I saw a heron 4 times. I wondered if there was an explicit connection. Turns out that the Great Blue Heron made a convincing recovery in Massachusetts in the 20th century thanks to habitat created by beavers, who were making their own 20th century comeback: “As trees are flooded by rising waters they provide nesting habitat for colonies of great blue herons.” [3]. In the book, Beaver: Its Life and Impact [1], I found this sentence: “The great blue heron exists in the Adirondack Mountains [of New York State] because of the dead standing trees that have been killed by beavers.”


All this chasing of beavers in the summer and fall had an interesting personal consequence. One day my wife and I were set to drive to a theater to watch the much acclaimed Bollywood movie, Haider. Just before we left, I noticed a tick entrenched in my upper back. We brought out a magnifying glass: it was a deer tick, the kind which can transmit bacteria that cause Lyme disease. In seeking beavers, I had walked through so many trails thick with vegetation brushing constantly against me that a tick bite at some point was inevitable. So no movie that day – and I still haven't seen Haider! — instead, I went to a walk-in clinic to get the tick removed. Fortunately, I'd spotted the tick early and there have been no symptoms of Lyme so far. But it did deter, to some extent, my beaver-pond tours for the rest of the season; and maybe this year too. The beavers could use a little privacy.


1. Muller-Schwarze, D. (2011). The beaver: its life and impact. Cornell University Press.

2. Hood, G. A., & Bayley, S. E. (2008). Beaver (Castor canadensis) mitigate the effects of climate on the area of open water in boreal wetlands in western Canada. Biological Conservation, 141(2), 556-567. pdf

3. Scott Jackson and Thomas Decker, 2004, Beavers in Massachusetts, pdf. The lovely black and white illustrations used in this essay are from this document. The illustrator's name is Nancy Haver. Scott Jackson, a professor at the University of Massachusetts, Amherst, was kind enough to speak to me for 45 minutes.