BOOK REVIEW
Battling Bioinvaders
Harold Mooney
Out of Eden: An Odyssey of Ecological Invasion. Alan
Burdick. viii + 324 pp. 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, species.
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 chain.
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 problem.
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.