American Scientist Online
October 06, 2005

Harold A. Mooney

Book review of Alan Burdick's Out of Eden: An Odyssey of Ecological
Invasion. Farrar, Straus and Giroux, 2005. $25.

In Out of Eden, Alan Burdick embarks on a journey to investigate the
massive biological disruptions that nonnative species often bring
about, traveling first to what might be considered ground zero of
ecological invasion, Guam.

There he engages with scientists fighting to control the brown tree
snake, Boiga irregularis, which first appeared on this Pacific island
shortly after World War II, most likely having arrived in military
transport from an area near New Guinea. Now as many as a hundred of
these snakes inhabit each hectare on Guam, where they have had
dramatic effects. For one thing, the sound of birds is principally
gone -- 9 of the 14 species of native forest birds have been locally
extirpated, and three of those species are now extinct, because Guam
was the only place they were found. Every year the snakes bite more
than 200 people and cause as many as 100 electrical outages as they
cross power lines.

So the brown tree snake is a pretty nasty creature. But Burdick can't
help marveling at its capacity to penetrate barriers designed
specifically to keep it out. Thus he introduces a recurring theme:
that how one views the alteration of the biotic face of the Earth is a
matter of values and focus -- and, I might add, a matter of the lens
one uses to view the natural world. For example, some of the citizens
and businesspeople of Guam have taken the attitude that there's
nothing wrong with cosmopolitan fauna, saying, "Well, if we can't have
native birds here anymore, why not introduce [other] tropical birds,
parrots, because tourists will like it."

On leaving Guam, Burdick travels to the Hawaiian Islands, where
authorities are intently guarding against being invaded by the brown
tree snake because of the damage it could do to resident biota and to
the popular image of the islands as a paradise.

However, the main battle against invasive species in Hawaii is in many
ways already over. Every year some five new types of plants and about
20 new kinds of insects become established -- plus other critters,
including pathogens. Nearly one-half of all of the flowering plant and
mammal species now in Hawaii are relative newcomers, having arrived
after humans showed up about 1,500 years ago. All of the amphibians
and most of the reptiles are in this sense new to the islands, as are
more than 2,000 species of insects.

What about those organisms that evolved in place from those few
ancestors that arrived via currents of wind and water? Many were
driven to extinction by the early Polynesian settlers, and much later
the introduction of avian malaria and its vector, the mosquito, took a
devastating toll. Other human-driven perturbations include habitat
loss and purposeful introductions of birds and mammals.

Burdick has thoroughly acquainted himself with what biologists have
learned about invasive species. In the course of his encounters with
scientists and other residents of Hawaii, he refers repeatedly to
Charles Elton's pioneering and eloquent 1958 book, The Ecology of
Invasions. Elton, the most respected animal ecologist of his time, was
highly qualified to write about species interactions. "We must make no
mistake," he said. "We are seeing one of the great historical
convulsions of the world's fauna and flora."

Some of the conclusions Elton came to in his early work are still,
even after decades of empirical investigation, subject to considerable
debate and continue to be modified and refined. Most controversial is
the question of whether diverse communities are more resistant to
invasions, as Elton believed. Experiments conducted on small plots
support this conclusion, but at larger scales, different processes
come into play.

Burdick does a nice job of telling the story of the development of the
science of invasion biology, blending it with his depictions of the
everyday work of researchers and of the concerns of others who benefit
from the bounties of nature. These narratives illustrate the conflicts
that occur when groups of people with different values and objectives
share the same landscapes and resources. The early colonizers of the
Hawaiian Islands brought with them useful animals and plants from
their native lands, including pigs, which have thrived and are now
feral. Park managers, to protect native biota, have constructed fences
to keep the pigs out and to control them -- objectives that bring the
managers into conflict with people who hunt the pigs and see them as
an important part of the cultural heritage of native Hawaiians.

Burdick tags along with ecologist David Foote, who is studying the
impact of the feral pigs. Foote is interested in such questions as
whether soil tilled by feral pigs contains fewer native micropods and
whether nonnative weeds are more likely to grow in it. Burdick, in
translating what he learns from Foote about biogeochemistry, notes
rather poetically that "at the lowest trophic levels... the animate
and inanimate come... close to converging: geology melts into soil
chemistry; soil chemistry merges with biochemistry; biochemistry
begets biology. There is a river of nutrients flowing up and down the
scales of perspective."

Foote's colleague Peter Vitousek observes that ecological processes in
Hawaii are being significantly altered by such nonnatives as Myrica
faya, the fire tree. This plant enriches the soil by fixing nitrogen,
but in doing so completely changes the dynamics of plant colonization
on the volcanic flows. Myrica faya, interacting with native as well as
introduced species, has created a whole new biotic world on the
islands. Similarly, an invading grass species can change the fire
regime of an ecosystem and in time inhibit the recovery of native
forest species after a conflagration. So scientists must consider not
just one invading species at a time, but many, and how they interact.

This complexity, and uncertainty, appears to trouble Burdick. Do we
know enough -- about pig exclusion, for example -- to take expensive
action? Acting in the face of incomplete knowledge, or uncertainty, is
the great challenge to policy makers everywhere. But ecological
systems are complex, often with nonlinear responses to change.
Inaction at an early stage of an invasion can result in irreversible
damage, so applying the precautionary principle is appropriate.

As he describes the current status of the ecosystems of Hawaii,
Burdick reflects on Elton's conclusions. These older tenets form a
counterpoint to our growing knowledge of the dynamics of invasive
species and of newer ideas, such as Daniel Simberloff's invasional
meltdown theory, which posits that new introductions continue to
increase rather than leveling off as in the Eltonian world.

Burdick also brings up biocontrol, the use of other species to attack
target invasive organisms. Once an introduced species has built up a
large population, control of it by mechanical or chemical means is
costly and may harm the environment. Thus it is tempting to try
biocontrol instead. Unfortunately, in some cases the creatures used
have attacked not only the invader but also native, and often rare,

Burdick comments that some of the biologists seeking to limit invasive
species are motivated by a personal wish to maintain the uniqueness of
a given place. He notes the difficulty of achieving this goal in a
world where Darwinian competition reigns and massive global exchanges
of biotic material occur. Burdick does not, however, dwell on another
motive of scientists and the public alike, that of "controlling"
nature to benefit society (by providing food, beauty or water
purification, for example). The battles against invasive species are
generally waged to reduce massive economic losses or to mitigate
possible harm to human health.

Among the last stops in Burdick's journey is the San Francisco Bay.
With more than 250 nonindigenous species known to be established and
more pouring in, it is one of the most highly modified estuaries in
the world. Many of the invaders come on ships, which account for 80
percent of all global transport. These vessels carry ballast water
laden with living animals from foreign places and dump it into distant
ports. The resulting changes in marine systems are not visible to the
casual observer, but they are dramatic nonetheless: The whole way
these systems function is transformed, with effects all along the food

Here Burdick's book changes in tenor as he becomes captivated by the
inspirational passion with which leading scientists James T. Carlton
and Greg Ruiz pursue their joint work on marine invasions. Burdick
describes well such poster children of aquatic invasion as the green
crab and the zebra mussel, which have not only had major effects on
ecosystems but have also fueled interest in invasive species.

Many biologists working on invasions are concerned only about aliens
that are jeopardizing valuable ecosystem processes or causing harm to
the economy or public health -- a list that is quite long. However,
managers of natural ecosystems who are charged with protecting native
biodiversity may have a mandate to consider all alien species a

At the end of the book, after being tutored by a remarkable collection
of scientists, Burdick is still pondering what nature is and why
people might care about invasions. Of course all living things are
part of nature. But biologists studying invasive organisms are
striving to understand why, when certain species from distant lands
enter a new habitat, their population explodes, injuring ecosystem
processes and native species that humans treasure. To answer this
question, one must understand the whole train of events that leads to
transport, establishment and spread -- a daunting task.

Playing the role of investigative reporter, Burdick does a good job of
describing some major battlegrounds, the scientists involved and the
state of the science. Unfortunately, the book has no notes, references
or index -- not even a hint of the literature he consulted, even
though not all of the facts he cites could have come from personal
communications. And the book does not state explicitly what society
should do to respond to Elton's historic convulsion.

Burdick evidently was saving his opinions on that subject for a
different venue, a cover article in the May 2005 issue of Discover
magazine, where he is a senior editor. This piece, titled "The Truth
about Invasive Species: How to Stop Worrying and Learn to Love
Ecological Intruders," repeats material from Out of Eden and adds new
interpretations. Here Burdick states flatly that "invasions don't
cause ecosystems to collapse." However, the caption for one figure in
the article notes that a "comb jelly, native to American waters,
entered the Black Sea in the 1980s and promptly ate everything -- fish
eggs, fish larvae, and zooplankton -- in it." And the fisheries of the
Black Sea did collapse in the early 1990s, falling in tonnage harvests
from 250,000 to 30,000 tons. If by saying that ecosystems don't
collapse Burdick means that they don't lose all primary producers,
consumers and decomposers, he is right, but such a distinction may not
be very meaningful for people who can no longer depend economically on
what was there before.

The Convention on Biological Diversity specifically targets not
nonnative species per se, but primarily ones with robust, growing
populations -- "those alien species which threaten ecosystems,
habitats or species" and those that disrupt ecological systems that
society values and may even depend on. There are certainly plenty of
invaders of this ilk, as Burdick describes so nicely in Out of Eden.
The fact that in this article he seems to have forgotten many of the
lessons his generous mentors taught him and instead emphasizes that
most intruders can be safely embraced is quite puzzling.

Reviewer Information: Harold A. Mooney is Paul S. Achilles Professor
of Environmental Biology and a senior fellow by courtesy at Freeman
Spogli Institute for International Studies at Stanford University. He
is the author of The Globalization of Ecological Thought (Ecology
Institute, 1998) and is the editor or coeditor of 25 books, including
Invasive Alien Species: A New Synthesis (Island Press, 2005).

Copyright Sigma Xi, The Scientific Research Society