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T E A C H E R S
N O T E S
keystone species
> This poster introduces the concept of the ‘keystone
species’, defined as “a species which affects the
survival and abundance of many other species in the
community in which it lives”. As the poster illustrates,
the removal of such species can have a profound
effect on the ecosystem in which they live, and
sometimes even on the physical structure of the
environment. And often the importance of these
species is not evident until they disappear.
> These Teachers Notes expand on this concept,
with further examples. The concept provides a simple
yet powerful illustration of the need for wholeecosystem conservation.
A joint venture between Scottish Natural Heritage and the RSPB
Sea otters (Enhydra lutris) were once found along the
entire Pacific coast of North America, but are now
confined to California and Alaska. They were hunted
for their valuable fur, and the Californian population
was thought to be extinct until a small group was
discovered on a remote coast in 1938. With careful
protection, numbers have built up to over 2,000 along
one stretch of the Californian coast.
Californian
sea otter.
Michael Scott
C H E C K S
A N D
B A L A N C E S
the keystone sea otter
> Edward Wilson, the American scientist who did much to popularise the concept of
biodiversity, is also a strong advocate of the concept of the keystone species. In his book,
The Diversity of Life (Harvard University Press, 1992), Wilson illustrated the idea with reference
to the Californian sea otter, which was hunted almost to extinction for its valuable pelt.
Wilson wrote:–
> “In places where sea otters disappeared completely, an unexpected sequence of events
unfolded. Sea urchins, normally among the major prey of the otters, exploded in numbers and
proceeded to consume large portions of the kelp and other inshore seaweeds. In otter times, the
heavy kelp growth, anchored on the sea bottom and reaching to the surface, was a veritable
forest. Now it was mostly gone, literally eaten away. Large stretches of the shallow ocean floor
were reduced to a desert-like terrain, called the sea urchin barrens.”
> In these sea urchin barrens, few fish could live and, as a result, many inshore fishermen
went out of business. The great fisheries and the coastal canneries of towns like Monterey
virtually disappeared, just as the sea otter had done.
T O P
P R E D A T O R S
> The concept of the ‘keystone species’ is a powerful one, although it is important to note that
it may be an oversimplification of living communities (or ecosystems), in which all species play
some role. It is also not possible to identify keystone species in every ecosystem. However the
idea illustrates powerfully how the loss of a species from an ecosystem (or the addition of a
species that does not “belong” in that ecosystem) can have a profound effect on how that
ecosystem works, often to the detriment of humans.
Red deer stag, Strathspey.
L. Gill
> The keystone species is often the “top predator” in food webs, and so it is not surprising that
it has a profound effect on the balance of numbers in species at lower levels in the webs. There
are good Scottish examples to illustrate this process. For example, the extermination of the wolf
in the late eighteenth century removed the last predator of red deer. Today – helped by the way
we manage the land and by inadequate culling policies – red deer have increased to huge
numbers in many parts of the Highlands, causing problems for farmers, crofters, grouse-moor
owners and for the conservation of our native forests and plant communities.
> project ideas
> The fish canneries of Monterey were
described by John Steinbeck in his 1945 novel
Cannery Row. By reference to this book,
biodiversity quotes
investigate the effect that the loss of the fishery,
resulting from the sea otter’s decimation, might
> “Because scientists have yet to put names on
most kinds of organisms, and because they entertain
European wolf.
Laurie Campbell
have had on Monterey and its citizens.
> The wolf once inhabited Scottish forests,
only a vague notion of how ecosystems work, it is
but it was exterminated because of its perceived
reckless to suppose that biodiversity can be
threat to humans and livestock. Today some
diminished indefinitely without threatening humanity
people advocate the reintroduction of the
itself. . . . As extinction spreads, some of the lost
wolf to Scotland.
forms prove to be keystone species and trigger a
> Develop with the class a simple food web
ripple effect through the demographies of the
for the ecosystem in which wolves and red deer
survivors. The loss of a keystone species is like a drill
co-existed.
accidentally striking a power-line. It causes lights to
> Discuss the implications of such a
go out all over.”
reintroduction, and stage a debate on the
EDWARD WILSON, The Diversity of Life (1992)
merits of the proposal.
marine disruption
> The sea otter story illustrates how easily marine ecosystems are disrupted. We know least
about how these ecosystems work, yet we continue to over-exploit individual species from
the seas.
> There are already good examples of the complex impacts such exploitation may have.
For example, overfishing of cod and other large predatory fish is thought to have led to an
increase in numbers of the smaller fish on which they preyed. Some seabirds, such as the
puffin, which feed on smaller fish, may have benefited initially from these changes, and this
Puffin.
may explain recent increases in the size of some of their breeding colonies.
Laurie Campbell
> Now, however, industrial fisheries increasingly are targeting smaller fish, like sprats and sand
eels. We have seen the impact of such fisheries already on Norwegian islands such as Røst,
where almost a million puffin chicks starved to death during several summers in the 1980s
because their parents could not find enough food to feed them.
B I O D I V E R S I T Y
C O N S E R V A T I O N
the RSPB and
biodiversity
> It could be said that the RSPB’s work is not just
about birds but about biodiversity. By looking after
wild birds and their habitats, the RSPB brings wider
biodiversity benefits; after all, much of a bird’s habitat
is made up of other living things. So, from a focus on
wild birds, many living things, including humans,
benefit. The RSPB conducts campaigns, carries out
research, manages reserves, gives advice and
organises volunteers, all to benefit biodiversity.
By adopting a planned and prioritised approach,
and by working with others, the RSPB aims to
conserve and enhance Scotland’s, and the world’s,
variety of life.
ISBN No. 1 85397 274 6