Climate Change and Migratory Species

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” …  animal populations ‘can never increase beyond the supply of food in
the least favourable season.'”

“His alarming message is that around the world great animal migrations are
disappearing. Thus, international conservation efforts are urgently needed to
save the migrants from the devastating effects of over- exploitation, habitat
destruction, human-created obstacles, and climate change.”
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SCIENCE
1 FEBRUARY 2008 VOL 319

BOOKS ET AL.

No Way Home
The Decline of the World’s Great Animal Migrations
by David S. Wilcove
Island Press, Washington,
DC, 2008. 253 pp. $24.95.
ISBN 9781559639859.

Give Way to the Migrants
Thomas Alerstam

Migration represents a spectacularly successful strategy
among animals, providing access to a richness of ephemeral
and seasonal resources that can sustain large populations.
Its importance for promoting abundance was stressed by
Alfred Russel Wallace in his 1858 paper that set forth the
fundamentals of natural selection in biological evolution
and stirred Darwin to finally publish his long-considered
ideas (1). Wallace pointed to the example of the passenger
pigeon, which–in spite of its limited fecundity and
flagrant exposure to predation–reached its immense
abundance through rapid long-distance movements from
depleted to fresh feeding grounds. The example illustrates,
he argued, that animal populations “can never increase beyond
the supply of food in the least favourable season.” What he
did not realize at that time was the passenger pigeon’s great
vulnerability to human exploitation. Within Wallace’s lifetime
(1823-1913), the species plummeted from tens of millions of
birds. The last-known individual died in captivity in 1914.

The view of animal migration as a phenomenon of abundance and
vulnerability forms the central theme of David Wilcove’s No Way
Home. His alarming message is that around the world great animal
migrations are disappearing. Thus, international conservation
efforts are urgently needed to save the migrants from the
devastating effects of over-exploitation, habitat destruction,
human-created obstacles, and climate change.

Animals traveling thousands or tens of thousands of kilometers
in the air, on land, or in water inspire much awe. To complete
its annual return journey between northerly breeding latitudes
and tropical winter regions, a tiny songbird must keep to
seasonal and daily timetables, change its physiological
machinery between phases of fuel consumption and fuel deposition,
vary flight steps and fuel loads in relation to the crossing
of benign or hostile regions, find its way by compass and
navigation systems, negotiate weather and winds, and correctly
adjust flight speed and altitude. The bird’s endowment with all
necessary instructions represents a striking manifestation of
the accomplishments achieved by biological evolution.

Wilcove, an ecologist at Princeton University, presents elegant
and informed accounts of migrations in various taxa: birds (the
New and Old World systems of billions of songbirds traveling to
and from tropical winter quarters, red knots flying between the
latitudinal extremes of the American continents, and bellbirds
moving down and up the slopes in Central American cloud forests),
insects (dragonflies that behave like migrating birds; monarch
butterflies that depart each spring from high-altitude fir forests
in Michoacán, Mexico, to start a multigenerational annual cycle of
movement across North America; and now-extinct Rocky Mountain
locusts that once moved in swarms of millions), terrestrial mammals
(wildebeest of the Serengeti, springbok of South Africa,white-eared
kob of Sudan, and bison and pronghorn of North America), sea mammals
(right whale in the Atlantic and gray whale in the Pacific), sea
turtles, and fish (Atlantic and Pacific salmon).

For each case, Wilcove takes us into the field to
meet the animals (or to the scene of now-extinct
migrations), often in company with researchers
conducting exciting projects. Migration studies
are currently in a phase of dynamic development,
with novel tracking, physiological, and molecular
techniques (2). In addition, the author provides
fascinating stories of the animals’ natural history,
glimpses of recent scientific discoveries about migration
performance and navigation mechanisms, and historical
sketches. He also describes population trends and describes
the threats and conservation efforts. These strands are
skillfully woven together, making his comprehensive
perspective on animal travelers a delight to read.

Some of the migrants’ predicaments stem from the complexity in
seasonally and spatially shifting uses of resources. Increased
specialization often goes hand in hand with increased vulnerability.
However, the picture is not altogether dark.

Some migratory populations, such as the gray
whale, have shown encouraging recoveries. In
recent decades, reduced persecution and the
banning of toxins have led to the comeback of
many birds of prey, including both short- and
long-distance migrants. Changing their migration
routes to exploit new resources provided by
farming, some populations of geese and cranes
have dramatically expanded. Their opportunistic
flexibility is facilitated by learning; knowledge
of migration routes is transferred between
generations that travel together in families or
mixed flocks (3). For still other species,
migration may promote range expansion, leading to
the establishment of new travel routes and the
colonization of new breeding destinations.

I would have appreciated more discussion of
factors that differ between declining and
expanding migratory populations. How important in
this respect are cultural versus genetic
evolution of migratory routes, short versus long
migration distances, and levels of complexity in
the annual cycle and habitat requirements?

Absorbing and thought provoking, No Way Home
deserves to be widely read and used to promote
conservation action. It illustrates the
importance of science for deepening our
appreciation of animal migrations and for guiding
our efforts to preserve them. There is no
conflict between scientific exploration of
migratory mechanisms and connectivity and
aesthetic marveling at the superb arrangements of
nature. The investigation of animal migration is
a major challenge in biology, more fascinating
and urgent than ever. Wilcove urges us to
proactively protect threatened migration systems
while the migrants are still abundant.

References
1. A. R. Wallace, Proc. Linn. Soc. London3, 53 (1858).
2. M. S. Webster, P. P. Marra, S. M. Haig, S. Bensch, R. T.
Holmes, Trends Ecol. Evol.17, 76 (2002).
3. W. J. Sutherland, J. Avian Biol.29, 441 (1998).

The reviewer is at the Department of Animal Ecology, Lund
University, Ecology Building, Lund, SE-22362, Sweden.

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