The Beaver: Natural History of a Wetlands Engineer

Preface

The beaver plays an enormously important role in today’s ecology. In the 21st century, this animal provides ecosystem services such as creating and maintaining wetlands, controlling floods, improving water quality, creating habitat for plants and animals, and preventing soil erosion. As a wetlands engineer, the beaver builds dams and digs channels. As a keystone species, it sustains ponds and wetlands. But when spreading as a non-native to new areas—as in South America—it can become an invasive species, an undesirable trait.

The two species of beaver are the North American beaver, Castor canadensis, and the Eurasian beaver, C. fiber, both of which are covered in this book. Given my field research experience in North America, many examples are drawn from Castor canadensis. This animal dazzles with its extensive constructions, demands respect as a survival artist, and—when in conflict with humans—challenges the best minds in wildlife management. Many citizens’ groups and professionals strive to resolve those conflicts and find ways for us to coexist with beavers.

In recent times, the North American beaver and the Eurasian beaver have posed opposite problems for wildlife managers. In North America the managers sought to limit the damage to human infrastructures by burgeoning beaver populations. In the countries that make up the region of Eurasia, however, they worked to protect beavers and build up viable populations.

Fur trapping has declined in North America, leading to rapid saturation of suitable beaver habitat. As North American beavers move back into their former habitat, they find it developed and conflicts ensue: beavers flood roads, parking lots, and golf courses, and cut down ornamental and forest trees. The beaver becomes a political symbol, pitting landowners against animal rights’ advocates, and wildlife managers against concerned citizens.

In Europe, in a spectacular comeback, Eurasian beaver populations have rebounded from near extinction. They were reduced to about 1200 beavers one hundred years ago, but now count about half a million, with over one hundred thousand in the small country of Lithuania alone. All this happened in a developed landscape, although Lithuania’s human population does not have the same density as elsewhere in Europe. Reintroductions are continuing, with the latest occurring in Scotland.

Now both continents face the same issue: how to find a balance between maintaining healthy beaver populations and minimizing the damage they inflict by flooding and tree-cutting. This remarkable species has come full circle: from seemingly limitless abundance, to near extinction in vast areas of North America via the relentless and ruinous fur trade and habitat change, to a new abundance today in a profoundly changed landscape.

The first edition of this book (The Beaver: Natural History of a Wetlands Engineer, 2003) was co-authored by Dr. Lixing Sun, one of my former graduate students. It aimed to fill a serious gap in the wildlife literature, because a modern, comprehensive book on beaver biology for college and high school students and the general reader did not exist at the time. The scientific literature was scattered over a vast universe of academic journals. This called for a compilation of contemporary information on beaver biology, accessible to scientist and non-specialist alike.

This second edition incorporates the latest research. Previously not seen behavior is described, such as the stick display. Satellite photography revealed the longest beaver dam yet discovered. Climate change appears to impact beavers: At Yellowstone National Park, the growing season now lasts four weeks longer, boosting willow growth, which in turn benefits beavers. Beaver populations in places as far apart as Eastern Europe and Tierra del Fuego are rapidly changing. We are gaining more experience in managing beavers for habitat improvement and protection of rare species. Finally, recent years have seen sophisticated genetic studies which benefit conservation of beaver populations.

Many colleagues and assistants have helped to collect the original field data reviewed in this book. I thank Tim Schwender, technician, for his dedication. Undergraduate assistants, particularly Richard Corradi, Susan Heckman, Linda Morehouse, and Brent Speicher spent countless hours sitting quietly at beaver ponds, suffering attacking blackflies and mosquitoes. Our work was supported by staff at Allegany State Park, the late Dick Sage at Huntington Wildlife Forest in Newcomb, and Larry Rathman at Cranberry Lake Biological Station in the Adirondacks. My wife Christine Müller-Schwarze and daughter Annette Müller-Schwarze assisted in collecting data in the field. Ideas were exchanged with former graduate students Drs. Peter Houlihan, Bruce Schulte, and Lixing Sun; Stasia Bembenek, Meaghan Boice-Green, Heather Brashear, Kristen Buechi, Brett Mosier, Rebecca Coleman Quail, Maryann Schwoyer, and Robert G. Welsh. My colleague and friend, the late Dr. Robert M. Milt Silverstein, collaborated on many experiments, joined us in the field and asked penetrating questions. Our work was supported in part by grants from the U.S. National Science Foundation.

Uncounted callers with beaver problems helped us to apply the results of our field research to beaver management. Through conversations with colleagues, wildlife managers, property owners, highway superintendents, and citizens’ groups I began to think about ways to apply biological and ecological knowledge for better and wiser management. Basic and applied research work hand in glove. It is my hope that the information in this book will help readers to understand the environmental role of the beaver worldwide and contribute to finding ways to coexist with this extraordinary ecosystems engineer.

Introduction

How many pelts in a beaver coat?

A beaver as large as a bear?

Beaver declared a fish?

What is beaver fever?

The beaver in a court of law?

Do otters prey on beavers?

Engineers lose jobs to beavers?

Beavers a menace to trout streams?

Find the answers in the pages ahead.

Two beaver species inhabit our world: the North American and the Eurasian beaver. Both had been extirpated over large areas by the beginning of the 20th century. But during the past 50 years, and continuing today, each of the species has traveled along a different trajectory. In the United States, reintroduction of the North American beaver in its former range has been so successful that burgeoning populations have no choice but to move into developed land. Such nuisance beavers make headlines by flooding land and downing trees. Like deer and Canada geese, the beaver has joined the ranks of overabundant wildlife.

In Europe, meanwhile, reintroductions have given some countries their first beavers in decades. Still small in numbers, these new populations are being carefully nurtured. Given that much of the landscape is developed, the carrying capacity will be reached there much sooner than in North America.

Viewable wildlife, as an alternative to hunted and trapped game species, is gaining in importance with increasing urbanization and antitrapping and antifur sentiment. Beavers are singularly suited as viewable wildlife because they can be found at the same place year-round. First-hand experience with these animals leads to an understanding of the profound, but not easily visible, ecosystems services they provide. This represents a sea change in our attitude toward wildlife. Historically valued for their fur, meat, and perfume and medical ingredients, beavers are now appreciated alive more than dead.

Watching beavers for the first time or finding some moving into or through one’s property raises many questions about their natural history. In our unpublished survey in middle and high schools in upstate New York, for example, children listed more harmful effects of beavers than benefits. Ecosystem services beavers provide, such as creating wetlands that purify water and increase biodiversity, need explaining and education. This is what this book is all about. The following chapters illuminate what beavers are doing and provide insight into what they could be up to next.

The text is organized into five parts. It progresses from the beaver as a single organism in Part I to the behavior of family units and the celebrated artifacts such as dams and lodges that result from this behavior in Part II. Part II also discusses communication by smell, sound, and visual signals, as well as the feeding behavior that is so obvious to even a casual observer by the sightings of felled trees and remaining stumps. Part III expands our view to whole populations, while Part IV explains specific aspects of the beaver’s relationships to particular components of its habitat, such as water, vegetation, predators, and diseases. The beaver, in turn, creates habitat for other organisms, also covered in Part IV. Finally, Part V turns to the relationship between humans and beavers, to many readers perhaps the most timely topic. Taking the historical approach, this section of the book reviews fur trapping, the fur trade, and its historical impact: trapping depleted populations; reintroductions replenished the stocks. Part V also discusses the problem of nuisance beavers, how to deal with them by proactive management, and how we might harness the beavers’ ecosystem services, to serve us as ecosystems engineers.

Part I

The Organism

Chapter 1

Now and Then: The Species, Including Fossils

In creation stories of many northern native Americans, at the time of beginning and transformation, giant animals roamed the world and ate people.

The culture hero Saya of the Athapaskan-speaking Beaver Indians at the Peace River in British Columbia and Alberta took on, and transformed the giant, people-eating animals to their current size. Now the roles are reversed: people eat the animals.

The Amikwa on the north shore of Lake Huron have the Great Spirit Manitou shrink the giant animals to their present size.

R. Ridington, 1981

Comment: The sight of large bones of Pleistocene mammals might have spawned such narratives to link the past to the present. Ethnographers have also often invoked a collective memory stemming from times when giant animals coexisted with people.

Author

Living Beavers

The beaver is the second largest rodent after the South American capybara (Hydrochoerus hydrochaeris). Beavers belong to the family Castoridae in the suborder Sciuromorpha of the order Rodentia. They are more closely related to squirrels and marmots than to mouselike rodents (Muridae). Beavers split from their closest living relatives 90–100 million years ago.¹ For expediency, clever politicians have maneuvered the beaver into strange taxonomic neighborhoods. In 1760, the College of Physicians and the Faculty of Divinity in Paris permitted beaver meat on fasting days because the beaver’s scaly tail classified it as a fish and not a mammal! By the way, the beaver is not alone in this regard. The pope declared the similarly semiaquatic capybara of South America, the largest rodent in the world, a fish, when petitioned by Venezuelans and Colombians in the 16th century. So every year observant parishioners eat 400 tons of capybara during the fast of Lent.²

Two species of beaver live today: the North American Castor canadensis and Castor fiber in Eurasia. Efforts to find unequivocal genetic tests to distinguish the two species are continuing. Fused chromosomes, specifically monobrachial centric fusions of G-banded chromosomes, can be diagnostic.³ The North American beaver occurs from coast to coast and ranges from Alaska, Hudson Bay, and northern Labrador in the North to the U.S.-Mexico border, Gulf Coast, and Florida state line in the South (Fig. 1.1).

In the Old World, the beaver used to occur throughout Europe to the Mediterranean and far into northern Asia. In the early 20th century only about 1200 beavers remained in this vast area. In a spectacular comeback, by 2010 the numbers had increased nearly 900-fold! There are at least 1.05 million Eurasian beavers today. Beavers now populate most countries they formerly roamed in. In Europe, only southern countries lack them. These include Portugal, Italy, Greece, Albania, Bulgaria, Macedonia, Kosovo, and Montenegro.⁴ Figure 1.2 shows today’s distribution of Eurasian beavers in Europe, and Figure 1.3, for Russia and Asia.⁴ Chapter 18 deals with beaver populations in specific countries.

The North American beaver cuts almost all tree species and builds elaborate freestanding lodges. The like-sized Eurasian beaver, on the other hand, cuts mostly willow, even in mixed forest stands, and lives in bank burrows. For instance, along the Danube near Vienna, Austria, researchers found only one freestanding lodge; all others were bank lodges (J. Sieber, personal communication, 1982). The Eurasian beaver appears more ancient and conservative than its New World cousin, thought to be a younger and more progressive species.

Figure 1.1 | Geographical range of the North American beaver.

Figure 1.2 | Map of beaver distribution in Europe. Many isolated small populations have been coalescing by natural migration, and the process is continuing. The numbers 1–5 designate the five traditionally recognized subspecies of Castor fiber in Europe: 1 = C. f. fiber; 2 = C. f. albicus; 3 = C. f. galliae; 4 = C. f. belarusicus; 5 = C. f. osteuropaeus. Dark grey: current range of Castor fiber; black: relict populations of C. fiber; light grey: range of Castor canadensis in Europe, notably in Finland. Squares (e.g. Spain): reintroduction sites where beavers have not yet spread significantly. From: Halley, Rosell, and Saveljev, in press. Kindly supplied by Duncan Halley.

The two beaver species differ in their number of chromosomes: C. canadensis has 40 chromosomes (2n), and C. fiber has 48 (2n) (see Table 1.1).⁴ The two species can also be readily distinguished by alleles of the esterase-D locus (Es-d): on horizontal starch gel electrophoresis, the Eurasian beaver exhibits a fast-migrating allele, while the North American beaver has a slowly migrating allele.⁵

Figure 1.3 | Map of beaver distribution in Asia (Russia, Kazakhstan, Mongolia, and China). Dark grey: present range of Castor fiber; black: relict populations of C. fiber; light grey: range of C. canadensis in Finland is indicated. Numbers indicate traditionally recognized subspecies: 1 = C. fiber fiber; 6 = C. f. pohlei; 7 = C. f. tuvinicus; 8 = C. f. birulai. Kindly supplied by Duncan Halley.

Fossil Beavers

Enough fossils exist to illuminate the ancestry of modern beavers. The smallest extinct beavers were as large as a muskrat, and the largest reached 100 kg. Table 1.2 summarizes the sequence of beaver fossils found in North America and Eurasia.

Fossils show that beavers occurred from the early Oligocene to the Holocene in the New World, the late Oligocene to the Holocene in Europe, and the late Miocene to the Holocene in Asia.⁶ The first clear castorid was Palaeocastor from the Upper Oligocene of North America. The combination of swimming and exploitation of trees in a mammal probably appeared about 24 million years ago in the early Miocene.⁷ Tree exploitation evolved in high latitudes, possibly as adaptation to hard winters, as Rybczynski points out. In the middle and late Tertiary beavers were very numerous. It has been suggested that beavers of the Tertiary migrated between Eurasia and North America in both directions: Steneofiber from Eurasia to America, and Eucastor from the New World to the Old. The genus Castor may have migrated from Eurasia to North America during the Pliocene.

Beaver-gnawed wood occurs often in the fossil record. A former beaver pond from the early Pliocene on Ellesmere Island in the Canadian Arctic still contains beaver-cut branches and saplings and bears witness to the higher temperatures during that period.¹⁰ The plant and beetle remains in the accumulated peat tell of winter temperatures 15°C higher than today, and summer temperatures 10°C higher. The beaver pond deposits also contained remains of the first North American meline badger, Arctomeles. Other fossils included a shrew, a three-toed horse, a bear, a musk deer, and a wolverine.¹⁰ At the Ellesmere Island site, the beaver-gnawed sticks were found intertwined, suggesting some kind of nest, lodge, or dam.⁷

Table 1.1 | Some Differences between the Two Beaver Species

Dipoides from about 5 million years ago (late Pliocene) is thought to have given rise to Procastoroides. Procastoroides was first found in Nebraska and occurs in layers about 3 million years old. Procastoroides idahoensis, the Idaho beaver, about two thirds the size of Castoroides, in turn, is thought to be the ancestor of the giant Castoroides.

During the Pleistocene, the age of giant mammals, huge beavers such a Trogontherium in Europe, lived side by side with other colossal mammal species. It is anybody’s guess what their lodges must have looked like, if they built any. Trogontherium ranged from France to China.¹¹ Castoroides ohioensis in North America was even larger. Recent calculations have refuted earlier claims that Castoroides was as large as a black bear. The earlier conclusion had been reached by extrapolating from the size relationships of skulls and femurs to body size in living beavers to those in fossil ones. Because the skull evolves in response to special needs, such as gnawing and chewing, it is a poor predictor for body mass. One obtains a different formula for the relationship between these measures and body mass by using the skull and femoral lengths of a broader taxonomic group, such as 19 rodent species.¹² According to new modeling, the body mass of Castoroides was probably 60–100 kg.¹² (By comparison, Palaeocastor weighed 0.8–1.2 kg). Fig. 1.4 compares the skull sizes of Castoroides, the North American and the Eurasian beaver.

Table 1.2 | Fossil beavers

Figure 1.4 | Comparison of the skulls of the fossil giant beaver Castoroides ohioensis (left) and the extant Eurasian (Castor fiber) (middle) and North American (Castor canadensis) (right) beaver. Proportions (e.g. length of nasal bones in relation to length of entire skull) are more diagnostic than any size differences. Modified from Hinze, G. 1953. Der Biber.

In North America, most giant beaver fossils stem from the eastern part, notably in the region south of the Great Lakes. Castoroides fossils were contemporaneous with the mastodon, as for instance in Wayne County, New York. Castoroides lived in the Pleistocene as early as about 1 million years ago, and has been found from Alaska to Florida. Castoroides, up to 2.75 m long and weighing 60–100 kg, was about 5 times as large as the North American beaver today. In 1867 beaver-gnawed wood was found near Albany, New York, in the same cavity as a skeleton of a mastodon, and 5 feet (~ 150 cm) above it. Imbedded in clay and river ooze, it rested on gravel, and was covered by peat. In the Pleistocene, Castoroides and C. canadensis coexisted.

In Europe, Trogontherium (gnawing animal) fossils surfaced first in the Sea of Azof, and later in England. Fossil beavers are commonly found in peat bogs of England and Ireland, together with the famous Irish elk, Megaceros. Remains of Castor fiber rested next to those of hippos, rhinos, and hyenas in a Pleistocene formation of Italy’s Val d’Arno. Giant beaver fossils occurred close to those of the smaller Castor species as we know them today, as was the case with fossils of Trogontherium and Castor fiber near Liège in Belgium. The giant beavers died out near the end of the last glaciation about 10,000 years ago.

Although clear evolutionary lines cannot be established, some specialists speak of a sequence Dipoides-Procastoroides-Castoroides in North America. In Eurasia, some have thought of a sequence Steneofiber-Palaeomys-Castor.

Fossil beaver incisor teeth, because they grow continuously, can reveal paleotemperatures. The ¹⁸O concentration in the enamel of the teeth tracks local precipitation, which in turn is controlled by temperature. Ongoing investigations of 8-million-year-old teeth of Dipoides stirtoni and 2.5-million-year-old Castoroides idahoensis in Nebraska apply our knowledge for living beavers to determine historical patterns of climate variation.¹³

Very early in mammalian evolution, a semiaquatic carnivore had a beaver-like tail. Accordingly named Castorocauda lutrasimilis, this early mammal from the Middle Jurassic in China had tail vertebrae similar to those of the beaver. In both animals these vertebrae have butterfly-like transverse processes, supporting the paddle-like tail, a striking example of convergence in connection with a semiaquatic lifestyle.¹⁴

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