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
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 further misfortune.
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 once grew.
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 long time.
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 pond
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
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.
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, verdant growth.
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
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 and trees.
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
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.
not 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 are based.
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