You know what a stream looks like. It
has a pair of steep banks that have been scoured by shifting
currents, exposing streaks and lenses of rock and old sediment.
At the bottom of this gully—ten to fifty feet down—the
water rushes past, and you can hear the click of tumbling rocks
as they are jostled downstream. The swift waters etch soil from
first one bank, then the other as the stream twists restlessly
in its bed. In flood season, the water runs fast and brown with
a burden of soil carried ceaselessly from headwaters to the
sea. At flood, instead of the soft click of rocks, you can hear
the crack and thump of great boulders being hauled oceanward.
In the dryness of late summer, however, a stream is an algae-choked
trickle, skirted by a few tepid puddles among the exposed cobbles
and sand of its bed. These are the sights and sounds of a contemporary
You don’t know what a stream looks
like. A natural North American stream is not
a single, deeply eroded gully, but a series of broad pools,
as many as fifteen per mile, stitched together by short stretches
of shallow, braided channels. The banks drop no more than a
foot or two to water, and often there are no true banks, only
a soft gradation from lush meadow to marsh to slow open water.
If soil washes down from the steep headwaters in flood season,
it is stopped and gathered in the chain of ponds, where it spreads
a fertile layer over the earth. In spring the marshes edging
the ponds enlarge to hold floodwaters. In late summer they shrink
slightly, leaving at their margins a meadow that offers tender
browse to wildlife. An untouched river valley usually holds
more water than land, spanned by a series of large ponds that
step downhill in a shimmering chain. The ponds are ringed by
broad expanses of wetland and meadow that swarm with wildlife.
Until the arrival of Europeans in North America, this
second vision was, almost without exception, what streams looked
like. They were transformed into the gullied channels we mistake
for the natural state of streams soon after the killing of millions
of beaver. Most European settlers never saw the original
condition of our watersheds, because the trappers came before
them, a deadly colonial avant-garde that swept relentlessly
from Atlantic to Pacific coast and hunted the beaver to near
extinction. Deeply gullied ravines had been the norm in
an anciently beaver-cleared Europe, and they quickly became
the norm here too. Removing the beaver drastically altered
and simplified the landscape.
Before Europeans arrived, there were an estimated 100
to 400 million beaver in North America. Today there are roughly
9 million, with their numbers having rebounded from an even
lower nadir at about 1900. Early records show that beaver
lived in nearly every body of water in New England.
The first white settlement in New England began with the arrival
of the Mayflower in 1620, and in the decade following, 100,000
beaver were skinned in Massachusetts and Connecticut. Having
quickly depleted the coastal stocks, trappers moved west into
New York and killed another 800,000 beaver from 1630 to 1640.
In 1638 England’s Charles II declared beaver fur to be
mandatory in the manufacture of hats, to the animal’s
As the slaughter spread westward, the numbers increased:
The French port of Rochelle received 127,080 beaver pelts in
1743 alone (beaver were not the sole target—1267 wolves
and a staggering 16,512 bears were also shipped to Rochelle
that year). By 1850, beaver were nearly extinct from the Atlantic
to the Oregon Territory. Entire deciduous riparian forests disappeared
from the west coast. Without the beaver’s omnipresent
influence, streams in every watershed eroded into the deep channels
we know today, and soil washed to the sea.
of the Watershed
As Bill Mollison has observed, everything gardens. The beaver,
however, goes far beyond simple gardening to feats of complex
ecosystem transformation. Beaver don’t merely build
dams that create ponds. They control the flow of vast amounts
of energy and material. With tough incisors and instinct, beavers
create a shifting mosaic of moist and dry meadows, wet forests,
marshes, bogs, streams, and open water that change the climate,
nutrient flow, vegetation, wildlife, hydrology, and even geology
of entire watersheds.
One of permaculture’s core principles advises
that we intervene at the point of maximum effectiveness—achieve
the greatest result with the least effort—and beaver epitomize
that axiom. The beaver understood how to hold water and
soil on the land long before Keyline originator P.A. Yeomans,
and the stunning increases in diversity and sheer biomass achieved
by the beaver serves to confirm the wisdom of Yeomans’s
vision. We can learn much that is useful to permaculturists
from a closer look at how the beaver works, and how their actions
reach deep into the heart of ecosystem health and function.
When a beaver fells an aspen—their favorite food
and building stock—the tree sends up suckers. The new
shoots respond to the cutting of their parent tree by producing
bitter alkaloids that beaver don’t like. This promotes
a dynamic balance between aspen growth and beaver felling. However,
the young suckers are just right for moose and elk, and these
large mammals prosper in the tasty browse where inedible treetrunks
Tree-cutting by beaver changes the course of ecological
succession by opening the canopy and removing certain plant
species. Light-loving plants, such as alders, hazels, and spruces,
thrive and multiply. The chips and abandoned brush from the
felled trees offer shelter and food to insects, small mammals,
and birds. Most of the tree, though, is used by the beaver for
dams and lodges.
Beaver choose the gently sloping lower reaches of valleys
for their work. A small dam on flat land impounds more water
behind it than one on a steep slope, doing the least work to
create a large pond. The water that backs up behind the dam
saturates the soil beneath it, creating a blend of anaerobic
and aerobic pockets, varying with water depth, vegetation, soil
type, and distance from the pond edge. Decomposition at the
anaerobic sites is slow, preserving organic matter. Dead trees
and snags left by the beaver or killed by flooding become home
to a wide array of animals and microbes. The structural, biological,
and chemical complexity of the region increases.
Vegetation drowned by the pond rots, releasing vast flows
of nutrients into the water. The pond bubbles methane into the
atmosphere. Erosion caused by the lapping of the expanding upstream
shoreline pulls more nutrients into the water. In the pond and
downstream from the dam, biomass now surges because of the water’s
increased fertility. The growing plants and animals trap these
nutrients and begin to cycle them.
Ecosystems that retain nutrients recover more easily
from disturbance than nutrient-losing ones. This means the pond
communities and those around it are likely to persist for a
Because the pond
has slowed the once-rushing water, it can’t carry as much
sediment. The released burden settles onto the pond bottom.
The small dam’s ability to collect sediment is enormous:
An average beaver dam, containing four to eighteen cubic meters
of wood, will eventually retain 2000 to 6500 cubic meters of
sediment behind it. That’s tremendous leverage, and
very effective use of resources! Paleoecological evidence shows
that entire valley floors have been raised many meters by beaver
These sediments contain carbon, potassium, phosphate,
and other nutrients, which are slowly released into the pond,
or provide food for burrowers and other burgeoning denizens
of the soft bottom. The burrowing worms and other creatures
alter nutrient flows as well. They stir up the sediment, releasing
soluble chemicals into the water, but they also trap and retain
nutrients, storing them as bodies and food, and coating their
burrows with organic matter.
Huge numbers of tubeworms and clams are nurtured by the
slow water-speeds and the sediments that result, as well as
abundant dragonflies and other predatory insects. Because of
these predators, fewer blackflies and mosquitoes infest beaver
ponds than man-made ponds.
Sediments in beaver
ponds and wet meadows at their margins are warmer than those
in dry meadows and forests, which means faster growth of plants
and soil organisms. In many cases, beaver ponds also raise the
water table, making moisture more available to roots and soil
life. Shrews, voles, and other small mammals thrive in the warm,
More fish species are found in and near beaver ponds
than in open streams. Overall, the diversity and biomass
of plants and animals in beaver ponds is two to five times that
of riffling streams.
The ponds themselves can vary hugely, creating many different
habitats. Some ponds are squeezed into deep, narrow uplands,
and others spread across broad, low valleys. Downstream ponds
are closer to permanent aquatic habitats at river mouths, and
thus trade species with them. Dams regularly collapse, and some
are not repaired, so ponds are often in various stages of conversion
to dryer habitats.
But just as significant are the varied habitats that
ring beaver ponds. Upstream and down are open stretches of flowing
water, home to stream species. At the pond edges the beaver
have created bogs, marshes, wet meadows, and riparian forests.
The new wetlands and meadows contain more nutrients than the
older uplands, and so support more types and numbers of living
beings. Edging the wetlands are dry meadows and woodlands. And
beaver meadows are very persistent, because their previous flooding
has acidified the soil, helping them resist invasion by shrubs
All these habitats are flooded in a very complex pattern
that varies with both the flow of water over the seasons and
the beaver’s activity. This means the conditions in all
these communities vary widely over time, allowing yet more biodiversity.
Beaver create a stunningly diverse mosaic of habitats
that shift over both space and time. Scientists in Minnesota
found that returning beaver transformed a section of uniform
deciduous forest into 32 different aquatic, emergent, shrub,
and forested wetland communities at various successional stages.
A beaverless watershed will most likely contain a deeply
gullied stream with a dry edge. A watershed with beaver will
have open, shallow streams, many ponds both active and abandoned,
wet and dry meadows, drowned, riparian, and dry forests, and
different wetlands of all sizes, types, and successional phases.
This whole network and the many species living there will shift
and repattern as beaver move out of ponds or return to abandoned
dams. These animals and the work they do are the key to biodiversity
in the watershed.
The importance of the beaver hasn’t gone unnoticed by
ecologists, and these creatures also offer both conceptual tools
and affirmation to permaculturists as well. Recently, ecologists
have coined a phrase to describe animals like the beaver: Ecosystem
engineers. These are organisms that directly affect and
regulate the availability of resources to other species, by
causing physical changes in biotic and abiotic materials. In
doing this they create and/or modify habitats.
wild about calling animals “engineers,” as my personal
view of engineering is that it is not as creative, inspiring,
or appropriate as what nature does—I’d rather call
engineers “retarded beavers”—but the term
is well established and will have to do here.
Ecosystem engineers fall into two camps. In the first
are creatures like the beaver and earthworm, which work their
magic by manipulating living and non-living materials (they
are called allogenic engineers, for those who like fancy terms).
The second group are those which alter the environment
by changes in their own bodies (autogenic engineers). Trees
are the consummate example of autogenic engineers, and Mollison
has written brilliantly of the way trees interact with and affect
their environment. However, he focuses mainly on the effects
of trees on the non-living world: how they affect rainfall,
hydrology, soil, clouds, and wind. One could deepen his essays
by describing how trees regulate the other species around them.
They create habitat for many species amidst their trunks, branches,
water-filled crotches, leaves, and roots. The roots provide
cavities and aeration, and change soil texture and infiltration
rates, which affect both underground and surface dwellers. Leaf
litter changes the drainage, moisture level, and gas and moisture
exchange rates in soil habitats, and creates barriers to or
protection for microbes, seeds, seedlings, and animals. Trunks,
branches, and leaves drop into streams, altering flow and otherwise
providing new habitat. This list could go on: The ways that
trees “engineer” habitat are multifold.
The principal point to grasp about ecological engineers
is that they act at points of maximum leverage to change the
flow, availability, and pattern of energy, nutrients, and other
resources that are used by other species. They often are
not part of these flows themselves, thus their interactions
are on a very different level from the predator/prey relations
(trophic level) upon which so many of ecology’s precepts
Ecosystem engineers “design” their own
habitats and those of others, and exert a great deal of control
over them. This means they create stable, predictable conditions
for themselves and for the ever-increasing numbers of creatures
who become dependent on them, and for ecosystem processes. They
damp the wild flows passing through their homes. They usually
enhance biodiversity and make environments more complex.
Sound familiar? The whole idea of ecosystem engineers
drops neatly into the permaculture toolbox. These species, like
good designers, create and improve habitat for many species
as a by-product of enhancing their own environment. They cooperate
with ecosystem processes and energy and matter flows, directing
them with minimal, efficient intervention, and they benefit
themselves and others by doing so.
By understanding ecosystem engineers like the beaver,
we can shine a bright, critical light on many of the practices
and principles of permaculture. The effects of beaver on a watershed
sound to me like nature’s application of P.A. Yeomans’
Keyline concepts, and support permaculture’s belief that
earthworks and ponds are critical for restoring ecosystem health.
In sites where beaver have returned after a century or more
of absence, we have natural models that demonstrate the hugely
beneficial effect of holding water on the land.
Trees, as Mollison
understood, are another ecosystem engineer to learn from. Others
that could be integrated into the permaculture corpus of knowledge
• Earthworms and other burrowers (the whole class are
called bioturbators for their churning of sediments)
• Certain key fungi and other microbes, which mobilize
• Algae, which change how light and nutrients are distributed
• Elephants, which uproot, trample, and eat whole forests
and then deposit huge manure loads elsewhere, stimulating
• Woodpeckers, which alter insect abundance and create
nest sites and shelter in trees for many species
• Alligators, which dig wallows that create new habitats
The final and most drastic ecosystem engineer is, of
course, Homo sapiens. We’re not very good at it. Usually
the effect of our ecosystem engineering is to reduce the possibilities
for every other species, rather than to enhance them. But
by looking more carefully at the many ways in which nature’s
ecosystem engineers improve their own homesites while boosting
the productivity and diversity of the larger environment, we
can become wiser in our own manipulations.
Jones, CG, JH Lawton,
M Shachak (1997). Positive and Negative Effects of Organisms
as Ecosystem Engineers. Ecology 78:1946-1957.
Matthiessen, P (1959) Wildlife in America. Viking Press, New
Naiman, RJ (1988). Animal Influences on Ecosystem Dynamics.
Naiman, RJ, CA Johnston, JC Kelley (1988). Alteration of North
American Streams by Beaver. BioScience 38:753-762.
Toby Hemenway is
associate editor of Permaculture Activist and the author of
Gaia’s Garden: A Guide to Home-Scale Permaculture (Chelsea