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Natural selection gives way to human selection
Practice of hunting and harvesting the
biggest animals or plants is changing species much faster than nature,
researchers find
MARTIN MITTELSTAEDT
From Tuesday's Globe and Mail
January 13, 2009 at 4:15 AM EST
Humans have become "superpredators," speeding up the
evolution of the species they hunt and harvest at rates far above what is
found in nature, according to new research, some of it conducted by Canadian
biologists.The researchers believe that many
recently observed changes in species, ranging from the shrinkage in the horns
of bighorn sheep in the Rockies to the reduction in the size of caribou in
Scandinavia, are being driven by humans.
The biologists estimate that hunting has caused such
characteristics as body size and reproductive age to change at a rate that is
a staggering 300 per cent above the pace that would prevail in nature. This
figure is even greater than the change attributed to other human
interferences, such as pollution, which was estimated to alter species 50 per
cent faster than what normally happens.
"The implications are pretty wide and profound," said
Paul Paquet, a University of Calgary biologist who dubs humans "superpredators"
for this outsized impact.
A paper outlining the findings was posted yesterday
in the online edition of Proceedings of the National Academy of Sciences.
In it, Dr. Paquet and others argue that current
hunting and harvesting practices are inadvertently causing dramatic changes in
the behaviour and appearance of species. The reason: Humans tend to
"high-grade" nature, taking out the biggest and best in a species.
The approach is clearest in trophy hunting, where
animals with the biggest horns or bodies are killed, while inferior ones are
left. But the practice also occurs in commercial fishing, where mesh openings
in nets capture the biggest fish while smaller ones escape.
The human approach is the opposite to what happens in
nature, where predators kill the easiest-to-catch animals, such as the young,
the old and the sick, but are unable to take out the fittest adults, which
then reproduce and pass their desirable characteristics on to future
generations.
Many hunting rules are based on a view that bagging
mature animals is the best approach, but Dr. Paquet says humans should be
trying to emulate nature, even though at first glance it doesn't seem sporting
to kill younger creatures. "That's clearly the best management regime," he
said.
The researchers, who are affiliated with the
University of Calgary and the University of Victoria as well as two U.S.
universities, looked at 34 studies that examined physical or biological
characteristics in 29 species subject to heavy pressure from human predation.
Besides the bighorn sheep and caribou, they included snails; a pair of
medicinal plants; and various commercially caught fish.
"Harvested organisms are the fastest-changing
organisms of their kind in the wild because we are superpredators and we take
such high proportions of a population and target the largest individuals," Dr.
Paquet said.
The rate of change is staggering from an evolutionary
point of view, where the alteration in species is often thought to occur
slowly. Alberta's bighorn sheep, for instance, have experienced a 20-per-cent
drop in the length and size of their horns in only the past 30 years. The
sheep are prized for the distinctive curve in their horns, making them
sought-after trophies.
Atlantic cod, whose populations have collapsed
because of overfishing, now reproduce at an average age of five years, rather
than six, as was the case previously. The change, which in humans would amount
to children reaching sexual maturity and having offspring at ages of around
nine to 11, occurred in only two decades.
Among the species harvested or hunted by humans and
reviewed in the paper, body size or horn size decreased by an average 18 per
cent. There was also a pattern of reproduction at an earlier age or smaller
size.
*****
THE DIMINISHING PREY
BIGHORN SHEEP
The sheep, found in mountainous areas of Western
Canada and the United States, are famed for the unusual curved horns on the
rams. The rams are hunted as trophies, but researchers believe the practice of
taking the biggest specimens has prompted genetic change leading to a marked
decline in horn size.
CARIBOU
Caribou from southern Norway, the last remaining wild
population in Europe, have shrunk because of hunting. The selective killing of
the biggest animals has led to a reduction in the size of bodies, antlers and
jaws. Researchers suspect the same trend may have occurred in southern
populations of caribou in Canada.
AMERICAN GINSENG
The gnarled root of the plant is prized for its
medicinal properties - collected for illnesses ranging from cancer to
Alzheimer's disease. But extensive harvesting has led to a change in the
composition of wild stands, with an increased number of smaller,
non-reproductive plants.
COD
The destruction of cod stocks has led to altered
reproductive behaviour. The fish produce eggs at a younger age, but this early
breeding has a big drawback. The early breeders are producing abnormally low
numbers of eggs.
Martin Mittelstaedt
++++++++++++++++++++++++++++++++++++++++++++++++++++++++++
Proceedings of the National Academy of Sciences
at:
Human predators outpace other agents of trait
change in the wild
-
Chris T. Darimonta,b,1,
-
Stephanie M. Carlsonc,
-
Michael T. Kinnisond,
-
Paul C. Paquete,
-
Thomas E. Reimchena
and
-
Christopher C. Wilmersb
+Author Affiliations
-
aDepartment of Biology, University of Victoria, P.O. Box
3020, Stn CSC, Victoria, BC, Canada V8W 3N5;
-
bDepartment of Environmental Studies, University of
California, 405 Interdisciplinary Sciences Building, 1156 High Street,
Santa Cruz, CA 95060;
-
cDepartment of Environmental Science, Policy, and
Management, University of California, 137 Mulford Hall #3114,
Berkeley, CA 94720;
-
dSchool of Biology and Ecology, University of Maine, 321
Murray Hall, Orono, ME 04469; and
-
eDepartment of Environmental Design, University of Calgary,
2500 University Drive Northwest, AB, Canada T2N 1N4
-
Edited by Gretchen C. Daily, Stanford University, Stanford,
CA, and approved November 21, 2008 (received for review September 15,
2008)
Abstract
The observable traits of wild populations are continually
shaped and reshaped by the environment and numerous agents of natural
selection, including predators. In stark contrast with most predators,
humans now typically exploit high proportions of prey populations and
target large, reproductive-aged adults. Consequently, organisms subject to
consistent and strong ‘harvest selection’ by fishers, hunters, and plant
harvesters may be expected to show particularly rapid and dramatic changes
in phenotype. However, a comparison of the rate at which phenotypic
changes in exploited taxa occurs relative to other systems has never been
undertaken. Here, we show that average phenotypic changes in 40
human-harvested systems are much more rapid than changes reported in
studies examining not only natural (n = 20 systems) but also
other human-driven (n = 25 systems) perturbations in the wild,
outpacing them by >300% and 50%, respectively. Accordingly, harvested
organisms show some of the most abrupt trait changes ever observed in wild
populations, providing a new appreciation for how fast phenotypes are
capable of changing. These changes, which include average declines of
almost 20% in size-related traits and shifts in life history traits of
nearly 25%, are most rapid in commercially exploited systems and, thus,
have profound conservation and economic implications. Specifically, the
widespread potential for transitively rapid and large effects on size- or
life history-mediated ecological dynamics might imperil populations,
industries, and ecosystems.
Footnotes
 | 1To whom correspondence
should be addressed. E-mail:
darimont@ucsc.edu |
|
Author contributions: C.T.D., M.T.K., P.C.P., and T.E.R.
designed research; C.T.D., S.M.C., and C.C.W. performed research; C.T.D.,
S.M.C., and C.C.W. analyzed data; and C.T.D., S.M.C., M.T.K., P.C.P.,
T.E.R., and C.C.W. wrote the paper. |
|
The authors declare no conflict of interest. |
|
This article is a PNAS Direct Submission. |
| © 2009 by The
National Academy of Sciences of the USA |
http://www.pnas.org/content/early/2009/01/12/0809235106.abstract?sid=86007a16-ac80-469e-8dfc-a93e8c9d521e
++++++++++++++++++++++++++++++++++++++++++++++++++++++
Research Ties Human Acts to
Harmful Rates of Species Evolution
Atlantic cod and Bighorn sheep are generally harvested by
humans as mature adults, whereas predators would target the smaller or weaker.
Published: January 12, 2009
Human actions are increasing the rate of evolutionary change in plants and
animals in ways that may hurt their long-term prospects for survival,
scientists are reporting.
The idea that target species evolve in response to predation is not new.
For example, researchers reported several years ago that after decades of
heavy fishing, Atlantic
cod had evolved to reproduce at younger ages and smaller sizes.
The new findings are more sweeping. Based on an analysis of earlier studies
of 29 species — mostly fish, but also a few animals and plants like bighorn
sheep and ginseng — researchers from several Canadian and American
universities found that rates of evolutionary change were three times higher
in species subject to “harvest selection” than in other species. Writing in
The Proceedings of the National Academy of Sciences, the
researchers say the data they analyzed suggested that size at reproductive
maturity in the species under pressure had shrunk in 30 years or so by 20
percent, and that organisms were reaching reproductive age about 25 percent
sooner.
In Alberta, Canada, for example, where regulations limit hunters of bighorn
sheep to large animals, average horn length and body mass have dropped, said
Paul Paquet, a biologist at the University of Calgary who participated in the
research. And as people collect ginseng in the wild, “the robustness and size
of the plant is declining,” he said.
The researchers said that reproducing at a younger age and smaller size
allowed organisms to leave offspring before they were caught or killed. But
some evidence suggests that they may not reproduce as well, said Chris
Darimont, a postdoctoral fellow in environmental studies at the University of
California, Santa Cruz, who led the work. The fish they studied that are
reproducing earlier “on average have far, far, far fewer eggs than those who
wait an additional year and grow a few more centimeters,” he said in an
interview.
Dr. Darimont said it was unknown whether traits would change back if
harvesting were reduced, or how long that might take.
The researchers also noted that the pattern of loss to human predation like
hunting or harvesting is opposite to what occurs in nature or even in
agriculture.
Predators typically take “the newly born or the nearly dead,” Dr. Darimont
said. For predators, targeting healthy adults can be dangerous, and some
predator fish cannot even open their mouths wide enough to eat adult prey.
Animals raised as livestock are typically slaughtered relatively young, he
said, and farmers and breeders retain the most robust and fertile adults to
grow their herds or flocks.
But commercial fishing nets and other gear that comply with conservation
regulations typically trap large fish while letting smaller ones escape.
Trophy hunters typically seek out the largest animals. And for some fish in
some areas, as much as 50, 60 or even 80 percent of the stock may be caught
every year.
“Targeting large, reproducing adults and taking so many of them in a
population in a given year — that creates this ideal recipe for rapid trait
change,” Dr. Darimont said.
Some fisheries scientists have said their studies of fish stock had not
shown a correlation between fishing intensity and growth rates. And some
wildlife conservationists question the idea that hunting can have harmful
effects on species.
Dr. Paquet said that although he had confidence in the new findings, he
knew there would be questions about the analytical methods he and his fellow
researchers used. “That’s expected,” he said. “That’s how science proceeds.”
He said he had anticipated that the work would be “contentious” among
trophy hunters. “Essentially, we are saying, ‘You should not do this because
it is having effects even you might not like,’ ” he said.
Daniel Pauly, who directs the Fisheries Center at the University of British
Columbia, said the new findings “make sense.”
Though Dr. Pauly said he had not seen the new work, he recalled similar
changes in black chin tilapia, fish that live in brackish water. He said in an
interview that he had studied the fish more than 30 years ago, when he was a
young graduate student doing field work in Ghana.
After decades of heavy fishing, the size of the typical adult fish had
shrunk to about 10 centimeters from about 15 centimeters. But at the time, he
said, “I did not realize what was happening.”
Some fisheries managers are already suggesting that conservation
regulations should be changed to safeguard larger fish in protected species.
“Lots of people argue for that because the big ones are so fecund,” Dr. Pauly
said. But he said customers in fish markets typically prefer larger fish. And
if fishers are not permitted to keep the big ones, they “must catch enormous
quantities of fish to have a good tonnage.”
More Articles in Science » A version of this article appeared in print
on January 13, 2009, on page D3 of the New York edition.
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