Decline of fish species shows much about
By Mark Buchanan
It's a small world: take anybody else
on earth, and you are probably linked
through six acquaintances. What's scary is that
a similar rule applies to natural life.
An international team of marine ecologists
recently completed an exhaustive
historical study of coastal ecosystems, ranging
from coral reefs and tropical seagrass beds to river estuaries and continental
shelves. Their findings were
disturbing. In every case, fish numbers had declined
precipitously with the
onset of modern methods of industrial fishing.
As the researchers concluded: "Everywhere, the magnitude of losses was
enormous in terms of biomass and
abundance of large animals that are now effectively
The situation has become especially critical
in the past few decades. Stocks
of Atlantic cod have reached historic lows, while
haddock and other species have been declared commercially extinct. Thriving
food webs that were stable
for millions of years have in the past 20 been
radically altered, and almost three-quarters of the world's commercially important
marine fish stocks are
now fully fished, overexploited or depleted.
This is just one illustration of the
trouble facing the global ecosystem.
Biologists estimate that the rate of species extinction
worldwide is at least a thousand times greater now than it was before
human beings walked the
earth, and that one-quarter of all species could
be obliterated in 50 years.
But does it really matter to us? The
political scientist Bj0rn Lomborg, in
The Skeptical Environmentalist, has argued that
much of what environmentalists
have said is overstated - that fears of ecosystem collapse
are irrational and largely the result of scare
tactics. On a strict cost-benefit
analysis, he says, the consequences of species extinction,
like those of global
warming, are not serious enough to warrant the expense
of trying to stop
them. We are better off trying to adapt - by seeking other
sources of fish to eat, for example. And many others
think the extinction of species is of interest and concern only to nature
Any ecosystem, however, is a staggeringly
complex network in which many
species interact with one another in delicate and
all but unfathomable patterns.
Indeed, it is our inability to understand how these living
networks hang together
- and consequently, how they might fall apart - that has seriously undermined efforts to assess the vulnerability
of the global ecosystem.
But in the past few years, researchers
have discovered that ecological
networks are not unique in their complexity. In
their basic architecture and pattern of assembly, ecosystems turn out to
be in many ways identical to
other complex networks such as the internet, and
even to our webs of social acquaintances.
What emerges from this new science is
anything but reassuring. The biological
world turns out to be a remarkably small one, with
the predator-prey links between species arranged in such a way that
no species is more than a handful
of steps away from any other. More than anyone
suspected, the global ecosystem is an intimately connected whole,
and we should indeed be very
worried about what we are doing to it.
Most of us have run into a friend of
a friend far away from home and felt
that the world is somehow smaller than we thought.
We usually put such encounters
down to coincidence even though they happen with disconcerting
frequency. Recent scientific work suggests that
this "small world" phenomenon is by no means limited to social relations.
In the social setting, the "small
world" experience is closely linked to the
notion of "six degrees of separation"
- the idea that each of us is linked to
everyone else on the planet by a chain of no more
than six intermediary acquaintances. Amazingly, this seems to be roughly
true. In the 1960s, the
American social psychologist Stanley Milgram sent
letters to random people living in Nebraska and Kansas, asking each to
forward the letter to a
stockbroker friend of his living in Boston. He
stipulated that they were to send the letter only to someone they knew personally
and whom they thought
might be socially "closer" to this man.
Even though the US then had a population of around 200 million, most of the
letters made it to the
stockbroker in just five or six mailings.
Researchers have found similarly small
worlds in many other settings. The
worldwide web is a network of more than one billion
sites connected by hypertext
links. Take two sites at random, and it needs only about
19 clicks to get
from one to the other. Other studies have come upon a
in the layout of the world's electrical power grids, in
the patterns of
neural connections in the mammalian brain, and in the
web of chemical
reactions within the living cell. The world's ecosystems
- or more precisely,
the food webs that underlie them - appear to share this
"small world" character.
How many species-to-species links does
it take to link any two organisms in
some chain of cause and effect? In the ecological
setting, two species are linked if one feeds upon the other, be it a
fox eating a rabbit or a beetle
munching an oak leaf. Last year, a Spanish physicist,
Ricard Sole, and an ecologist, Jose Montoya, studied Silwood Park,
an ecosystem in the UK for
which researchers know the fairly complete food
web. They found the number of degrees of separation to be only two or three.
The tapestry of life is made
of a truly dense cloth.
Silwood Park does not represent the global
ecosystem; it is certainly more
than two steps from a woodpecker in Illinois to
a shrimp in the South China Sea. Even so, whales and many species of fish
populate the oceans as a whole,
and numerous birds migrate between the continents.
Bacteria, algae, tiny spiders and other creatures fly round the world
in storm systems. These
organisms provide links that tie the biological
world together. For the global ecosystem, the number of degrees of separation
may not be two, but it
is probably not much higher than ten.
This discovery is not comforting. It
suggests that the extinction of one
species will affect not only everything that the
species eats, competes with, or is eaten by, but will send out fingers of
influence which, in a few steps,
will reach most other species in the entire system.
It suggests that any belief in our capacity to control the effects
of ecological destruction is
badly misplaced. That lesson becomes clearer as
one delves more closely into the small world phenomenon and into exactly
how large networks - such as the
human social network - can be so remarkably small.
As first suggested by the American sociologist
Mark Granovetter in the 1970s,
the answer can be seen by making a distinction
between "strong" and "weak"
social ties. Strong ties bind us to family members
and good friends, or to colleagues at work. These links form the threads
of a dense fabric of social
structure, and are socially most important to us.
But these are not the ties
that make for a small world.
Each of us also has "weak"
links to people we see rarely, or may never see
again. Think of some of your friends from the past
- long-lost college mates, say. Or someone you met when travelling. Perhaps
you went to Japan and
briefly made friends with a fellow tourist from
Australia. Your links to this person, or to those friends now out of touch,
are weak social links.
What makes them especially important
is that they connect you to people who
otherwise belong to quite distinct social spheres.
Your link to the Australian
tourist, for example, establishes a social bridge that
connects you in
just two steps to every person this man knows. Not only
that, but this single
link connects each of your local acquaintances, in London,
say, to every one
of his local acquaintances in Australia. In this way,
weak links act
like short cuts through the social world.
Mathematics backs up this insight. In
1998, in a paper published in Nature,
two mathematicians from Cornell University showed
that the effect of weak ties in a social network really does explain
six degrees of separation. In a
large network - even one of six billion people
- just a few weak links running between people from distant places will
indeed make for an extremely
small world, with every pair of persons linked
by a short chain of intermediaries.
The small-world character of the world's
ecosystems can be traced to
similarly weak links - that is, to links between
species that interact only
occasionally. Perhaps just one bird in an English
wood migrates long distances,
and, en route, settles briefly in southern Spain. This
is enough to link
the organisms of these two food webs together by short
chains of cause
But ecologists are beginning to suspect
that weak links within food webs also
play an important role in maintaining ecosystem
stability. Their argument is subtle, but important, as it could help us to
protect the world's food webs
If a predator eats just one other species,
it will do so frequently, having
no other options. Consequently, the link between
these species will be strong. Conversely, if a predator feeds upon
15 different prey, it may eat
each species only occasionally. It will then have
relatively weak links with these species.
Suppose that, after a climate change
or some human intrusion, the numbers of
a predator's favourite prey have been severely
depleted. What will happen? If this particular predator feeds on only this
one prey - if they share a strong
link, that is - then the predator must continue
to seek that prey even though its numbers are vanishing, driving this species
even closer to extinction.
When this happens, the population of predators
may then fall precipitously as well. As a paper in Nature pointed out a few
years ago, this should be a
general tendency: the loss of a strong link within
a food web will be destabilising, tending to stir up large and
dangerous fluctuations in species
But weaker links can save the day. Consider
a predator with 15 different
prey. If the numbers of one of these species become
very low, for whatever
reason, the natural response of the predator is
to shift its attention to
another species that is more numerous and easier
to catch. As a result, the
predator would continue to find food, while the
prey in danger of extinction
could revive its numbers. In this way, weak links
between species not only
make for a small ecological world but also act
as natural pressure valves,
playing a central role in guaranteeing the health
of an ecosystem.
You might expect that all species would
have roughly the same number of links
with other species. Not so. Nature doesn't dole
the links out equitably. Studies in Silwood Park and elsewhere reveal
that a few species always play
the role of superconnected hubs: they "own"
a high fraction of the links in
the food web, far and away more than the average
By simple logic, most of these links
will be weak links. So these hub species
provide the network with an ability to redistribute
stress and prevent one species from wiping out another by uncontrolled
predation or competition. And
that explains why we should be so worried about
Half the tropical forests, where two-thirds
of all species find their
habitat, have now been logged or burned to clear
land for human development,
with another one million square kilometres disappearing
every five to ten years.
If healthy ecosystems are small worlds characterised by
a few hub species,
with a preponderance of weak links providing their stability,
then the global
depletion of species numbers is truly alarming. As species
continue to disappear, the remaining species will
necessarily be linked more
strongly - if only by simple arithmetic. If some
predator preys on only six
species where before it preyed upon ten, its links
with the six will be stronger,
and ecosystem stability can only suffer. As one ecologist,
Kevin McCann, argues,
the lesson is that, if we wish to preserve an ecosystem,
or any species
within it, we had best proceed "as if each species
is sacred". What's more, the consequences of removing just
one of the "superconnected"
species can be dramatic, as a huge number of weak
stabilising links would go
with it. Ecologists have long talked about "keystone"
species, crucial organisms whose removal might bring the web
of life tumbling down like a
house of cards. A recent study has demonstrated
just how crucial their preservation may be.
Suppose you begin removing species from
an ecosystem. Slowly but surely, the
food web should fall apart. But how? First the
good news. Sole and Montoya have used a computer to mimic the loss of species
from a food web and have
found that real communities stand up relatively
well when the species to be removed are selected at random. Now the bad
news. Suppose instead that the
most highly connected species get knocked out first.
In this case, ecological disaster ensues quickly. Removing even 20 per
cent of the most highly
connected species fragments the web almost entirely,
splintering it into many tiny pieces. As the web falls apart, the disintegration
triggers numerous secondary
extinctions as some species lose all their connections
to others and become
The obvious answer is to take special
care to preserve the highly connected
"hub" species. But it is not easy to
predict which species will be the hubs for any particular food web. In the past, ecologists
have suspected that the
hubs would tend to be large predators, but this
does not seem to be true. Sole and Montoya found that they were often
inconspicuous organisms in the
middle of the food chain, or were sometimes basic
plants at the very bottom.
Most species now going extinct are ants,
beetles and other kinds of insect.
Some take comfort in this, but they are wrong to
do so. These species may well be linchpins of the living fabric.
What Sole and Montoya achieved on their
computer, human activity is achieving
in reality - the methodical dismantling of the
world's ecosystems. The leaders of many governments and large corporations
find it convenient to
suppose that worries about the ecosystem are overstated,
and anyway, that it would
be demented to carry out reforms that are not politically
popular. But we
are disassembling the web of life that supports our existence,
with little understanding
of what we are doing. That is truly demented.
Mark Buchanan's Small World: uncovering
nature's hidden networks has just
been published by Weidenfeld & Nicolson (£18.99)