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The Ecology of Invasions by Animals and Plants
By Charles S. Elton
University of Chicago Press
Copyright © 2003
University of Chicago
All right reserved.
ISBN: 0-226-20638-6
Chapter One
Ecological Explosions
Nowadays we live in a very explosive world, and while we may not know
where or when the next outburst will be, we might hope to find ways of
stopping it or at any rate damping down its force. It is not just nuclear
bombs and wars that threaten us, though these rank very high on the list
at the moment: there are other sorts of explosions, and this book is about
ecological explosions. An ecological explosion means the enormous increase
in numbers of some kind of living organism-it may be an infectious virus
like influenza, or a bacterium like bubonic plague, or a fungus like that
of the potato disease, a green plant like the prickly pear, or an animal
like the grey squirrel. I use the word "explosion" deliberately, because
it means the bursting out from control of forces that were previously held
in restraint by other forces. Indeed the word was originally used to
describe the barracking of actors by an audience whom they were no longer
able to restrain by the quality of their performance.
Ecological explosions differ from some of the rest by not making such a
loud noise and in takinglonger to happen. That is to say, they may
develop slowly and they may die down slowly; but they can be very
impressive in their effects, and many people have been ruined by them, or
died or forced to emigrate. At the end of the First World War, pandemic
influenza broke out on the Western Front, and thence rolled right round
the world, eventually, not sparing even the Eskimos of Labrador and
Greenland, and it is reputed to have killed 100 million human beings.
Bubonic plague is still pursuing its great modern pandemic that started at
the back of China in the end of last century, was carried by ship rats to
India, South Africa, and other continents, and now smoulders among
hundreds of species of wild rodents there, as well as in its chief
original home in Eastern Asia. In China it occasionally flares up on a
very large scale in the pneumonic form, resembling the Black Death of
medieval Europe. In 1911 about 60,000 people in Manchuria died in this
way. This form of the disease, which spreads directly from one person to
another without the intermediate link of a flea, has mercifully been
scarce in the newly invaded continents. Wherever plague has got into
natural ecological communities, it is liable to explode on a smaller or
larger scale, though by a stroke of fortune for the human race, the train
of contacts that starts this up is not very easily fired. In South Africa
the gerbilles living on the veld carry the bacteria permanently in many of
their populations. Natural epidemics flare up among them frequently. From
them the bacteria can pass through a flea to the multimammate mouse; this
species, unlike the gerbilles, lives in contact with man's domestic rat;
the latter may become infected occasionally and from it isolated human
cases of bubonic plague arise. These in turn may spread into a small local
epidemic, but often do not.
Most people have had experience of some kind of invasion by a foreign
species, if only on a moderate scale. Though these are silent explosions
in themselves, they often make quite a loud noise in the Press, and one
may come across banner headlines like "Malaria Epidemic Hits Brazil,"
"Forest Damage on Cannock Chase," or "Rabbit Disease in Kent." This
arrival of rabbit disease-myxomatosis-and its subsequent spread have made
one of the biggest ecological explosions Great Britain has had this
century, and its ramifying effects will be felt for many years.
But it is not just headlines or a more efficient news service that make
such events commoner in our lives than they were last century. They are
really happening much more commonly; indeed they are so frequent nowadays
in every continent and island, and even in the oceans, that we need to
understand what is causing them and try to arrive at some general
viewpoint about the whole business. Why should a comfortably placed virus
living in Brazilian cotton-tail rabbits suddenly wipe out a great part of
the rabbit populations of Western Europe? Why do we have to worry about
the Colorado potato beetle now, more than 300 years after the introduction
of the potato itself? Why should the pine looper moth break out in
Staffordshire and Morayshire pine plantations two years ago? It has been
doing this on the Continent for over 150 years; it is not a new
introduction to this country.
The examples given above point to two rather different kinds of outbreaks
in populations: those that occur because a foreign species successfully
invades another country, and those that happen in native or
long-established populations. This book is chiefly about the first
kind-the invaders. But the interaction of fresh arrivals with the native
fauna and flora leads to some consideration of ecological ideas and
research about the balance within and between communities as a whole. In
other words, the whole matter goes far wider than any technological
discussion of pest control, though many of the examples are taken from
applied ecology. The real thing is that we are living in a period of the
world's history when the mingling of thousands of kinds of organisms from
different parts of the world is setting up terrific dislocations in
nature. We are seeing huge changes in the natural population balance of
the world.
The larger ecological explosions have helped to alter the course of world
history, and, as will be shown, can often be traced to a breakdown in the
isolation of continents and islands built up during the early and middle
parts of the Tertiary Period.
In order to focus the subject, here are three case histories of species
which were brought from one country and exploded into another. About 1929,
a few African mosquitoes accidentally reached the north-east corner of
Brazil, having probably been carried from Dakar on a fast French
destroyer. They managed to get ashore and founded a small colony in a
marsh near the coast-the Mosquito Fathers as it were. At first not much
attention was paid to them, though there was a pretty sharp outbreak of
malaria in the local town, during which practically every person was
infected. For the next few years the insects spread rather quietly along
the coastal region, until at a spot about 200 miles farther on explosive
malaria blazed up and continued in 1938 and 1939, by which time the
mosquitoes were found to have moved a further 200 miles inland up the
Jaguaribe River valley. It was one of the worst epidemics that Brazil had
ever known, hundreds of thousands of people were ill, some twenty thousand
are believed to have died, and the life of the countryside was partially
paralysed. The biological reasons for this disaster were horribly simple:
there had always been malaria-carrying mosquitoes in the country, but none
that regularly flew into houses like the African species, and could also
breed so successfully in open sunny pools outside the shade of the forest.
Fortunately both these habits made control possible, and the Rockefeller
Foundation combined with the Brazil government to wage a really astounding
campaign, so thorough and drastic was it, using a staff of over three
thousand people who dealt with all the breeding sites and sprayed the
inside of houses. This prodigious enterprise succeeded, at a cost of over
two million dollars, in completely exterminating Anopheles gambiae on the
South American continent within three years.
Here we can see three chief elements that recur in this sort of situation.
First there is the historical one:-this species of mosquito was confined
to tropical Africa but got carried to South America by man. Secondly, the
ecological features-its method of breeding, and its choice of place to
rest and to feed on man. It is quite certain that the campaign could never
have succeeded without the intense ecological surveys and study that lay
behind the inspection and control methods. The third thing is the
disastrous consequences of the introduction. One further consequence was
that quarantine inspection of aircraft was started, and in one of these
they discovered a tsetse fly, Glossina palpalis, the African carrier of
sleeping sickness in man, and at the present day not found outside Africa.
The second example is a plant disease. At the beginning of this century
sweet chestnut trees in the eastern United States began to be infected by
a killing disease caused by a fungus, Endothia parasitica, that came to be
known as the chestnut blight. It was brought from Asia on nursery plants.
In 1913 the parasitic fungus was found on its natural host in Asia, where
it does no harm to the chestnuts. But the eastern American species,
Castanea dentate, is so susceptible that it has almost died out over most
of its range. This species carries two native species of Endothia that do
not harm it, occurring also harmlessly on some other trees like oak; one
of these two species also comes on the chestnut, C. sativa, in Europe.
Even by 1911 the outbreak, being through wind-borne spores, had spread to
at least ten states, and the losses were calculated to be at least
twenty-five million dollars up to that date. In 1926 it was still
spreading southwards, and by 1950 most of the chestnuts were dead except
in the extreme south; and it is now on the Pacific coast too. So far, the
only answer to the invasion has been to introduce the Chinese chestnut, C.
mollissima, which is highly though not completely immune through having
evolved into the same sort of balance with its parasite, as had the
American trees with theirs; much as the big game animals of Africa can
support trypanosomes in their blood that kill the introduced domestic
animals like cattle and horses. The biological dislocation that occurs in
this trypanosomiasis is the kind of thing that presumably would have
happened also if the American chestnut had been introduced into Asia. The
Chinese chestnut is immune both in Asia and America. Already by 1911 the
European chestnuts grown in America had been found susceptible. In 1938
the blight appeared in Italy where it has exploded fast and threatens the
chestnut groves that there are grown in pure stands for harvesting the
nuts; it has also reached Spain and will very likely reach Britain in the
long or short run. Unfortunately the Chinese chestnut will not flourish in
Italy, and hopes are placed solely on the eventual breeding of a resistant
variety of hybrid.
The third example given here concerns the sea lamprey, Petromyzon marinus,
in the Great Lakes region of North America. This creature is a North
Atlantic river-running species, mainly living in the sea, and spawning in
streams. But in the past it established itself naturally in Lake Ontario,
as well as in some small lakes in New York State. But Niagara Falls formed
an insurmountable barrier to further penetration into the inner Great
Lakes. In 1829 the Welland Ship Canal was completed, providing a by-pass
into Lake Erie. But it was a further hundred years or so before any sea
lampreys were observed in that lake. Then the invasion went with explosive
violence. By 1930 lampreys had reached the St Clair River, and by 1937
through it to Lake Huron and Lake Michigan, where they began to establish
spawning runs in the streams flowing to these lakes. In 1946 they were in
Lake Superior. Meanwhile the lampreys were attacking fish, especially the
lake trout, Salvelinus namaycush, a species of great commercial
importance. The sea lamprey is a combination of hunting predator and
ectoparasite: it hangs on to a fish, secretes an anticoagulant and lytic
fluid into the wound, and rasps and sucks the flesh and juices until the
fish is dead, which may be after a few hours or as long as a week. The
numbers of lake trout caught had always fluctuated to some extent, and the
statistics of the fishery since 1889 have been thoroughly analysed. But
never before the recent catastrophe had the catch collapsed so rapidly: in
ten years after the lamprey invasion began to take effect, the numbers of
lake trout taken in the American waters of Lake Huron and Lake Michigan
fell from 8,600,000 pounds to only 26,000 pounds. On the Canadian side
things were little better. This was not caused by change in fishing
pressure. Other species besides the lake trout have also been hard hit.
Among these are the lake whitefish, burbot, and suckers, all of which
declined in numbers. So, the making of a ship canal to give an outlet for
produce from the Middle West has brought about a disaster to the Great
Lakes fisheries over a century later. But in Lake Erie lampreys did not
multiply, partly because there are not many lake trout there, but probably
also because the streams are not right for spawning in.
These three examples alone illustrate what man has done in deliberate and
accidental introductions, especially across the oceans. Some idea of what
this can mean for the spread of animals can be got from the results of an
ecological survey done by Myers, a noted tropical entomologist, while
traveling on a Rangoon rice ship from Trinidad to Manila in 1929. He
amused himself by making a list of every kind of animal on board, from
cockroaches and rice beetles to fleas and pet animals. Altogether he found
41 species of these travelers, mostly insects. And when he unpacked his
clothes in the hotel in Manila, he saw some beetles walk out of them. They
were Tribolium castaneum, a well-known pest of stored flour and grains,
which was one of the species living among the rice on the ship.
A hundred years of faster and bigger transport has kept up and intensified
this bombardment of every country by foreign species, brought accidentally
or on purpose, by vessel and by air, and also overland from places that
used to be isolated. Of course, not all the plants and animals carried
around the world manage to establish themselves in the places they get to;
and not all that do are harmful to man, though they must change the
balance among native species in some way. But this worldwide process,
gathering momentum every year, is gradually breaking down the sort of
distribution that species had even a hundred years ago.
To see the full significance of what is happening, one needs to look back
much further still, in fact many millions of years by the geological
time-record. It was Alfred Russel Wallace who drew general public
attention to the existence of great faunal realms in different parts of
the world, corresponding in the main to the continents. These came to be
known as Wallace's Realms, though their general distribution had already
been pointed out by an ornithologist, P. L. Sclater. Wallace, however, did
the enormous encyclopaedic work of assembling and classifying information
about them. He supposed these realms to have been left isolated for such
long periods that they had kept or evolved many special groups of animals.
When one was a child, this circumstance was very simply summed up in books
about animals. The tiger lives in India.
Continues...
Excerpted from The Ecology of Invasions by Animals and Plants
by Charles S. Elton
Copyright © 2003
by University of Chicago.
Excerpted by permission.
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