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ECOLOGY. A most unusual (super)predator

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Modern humans evolved as cooperative hunter-gatherers whose cultural and technological evolution enabled them to slay prey much larger than themselves, across many species groups. One might think that those hunting skills have faded since the advent of agriculture and animal husbandry almost 10,000 years ago. Yet, as Darimont et al. show in a global analysis on page 858 of this issue ( 1 ), we are still the unique superpredator that we evolved to be. Analyzing an extensive database of 2135 exploited wild animal populations, the authors find that humans take up to 14 times as much adult prey biomass as do other predators. Our trophic dominance is most pronounced outside our own habitat, in the oceans (see the chart).
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INSIGHTS
Unnatural selection. Human hunters and fishers (such as Ernest Hemingway, pictured with a marlin) specialize in adult prey and often target large, healthy individuals.
PERSPECTIVES
Modern humans evolved as coopera-
tive hunter-gatherers whose cultural
and technological evolution enabled
them to slay prey much larger than
themselves, across many species
groups. One might think that those
hunting skills have faded since the advent of
agriculture and animal husbandry almost
10,000 years ago. Yet, as Darimont et al. show
in a global analysis on page 858 of this issue
(1), we are still the unique superpredator that
we evolved to be. Analyzing an extensive da-
tabase of 2135 exploited wild animal popula-
tions, the authors find that humans take up
to 14 times as much adult prey biomass as
do other predators. Our trophic dominance
is most pronounced outside our own habitat,
in the oceans (see the chart).
Several recent studies have tracked the
impacts of people on past ( 2, 3) and contem-
porary ( 4) wildlife populations, as well as
their knock-on effects across many ecosys-
tems ( 5). Darimont et al. go beyond this pre-
vious work to compare land and sea animals
across various trophic levels. They show that
on land, hunters put much greater pressure
on top carnivores than on herbivores. In
contrast, fishing pressure appears similarly
A most unusual (super)predator
By Boris Wor m
Effects of human hunting and fishing differ fundamentally from those of other predators
ECOLOGY
Biology Department, Dalhousie University, Halifax, Nova
Scotia, Canada, B3H4R2. E-mail: bworm@dal.ca
PHOTO: CORBIS
Published by AAAS
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21 AUGUST 2015 • VOL 349 ISSUE 6250 785SCIENCE sciencemag.org
high across different trophic groups (see
the chart), a pattern that has been dubbed
“fishing through marine food webs” ( 6).
Consistent with this hypothesis, the rate of
population collapse in small fish low in the
food chain, such as herring or anchovies,
matches or exceeds that of higher trophic
level predators such as sharks and tuna ( 7).
One reason for this imbalance between land
and sea is likely that fishing is now mainly a
mechanized industry, much like agriculture
(but unlike hunting) on land. Total marine
fish catch (including unreported catch and
discards) likely exceeds 100 million tons (Mt)
per year ( 8), whereas the terrestrial take is
estimated to be less than 5 Mt per year ( 9).
Historical shifts from hunting to fishing can
locally reverse where fisheries are depleted:
Coastal overfishing off West Africa, for exam-
ple, has caused food scarcity that intensified
again the hunt for wild meat on land ( 10).
Why do human hunters and fishers focus
so heavily on adults rather than juveniles,
the preferred prey for most nonhuman
predators? Probably this relates again to our
technological means, which, for example, al-
low killing from a safe distance, and specific
culture, for example, hunting for trophy and
status (see the photos). This unique pref-
erence, however, has implications for the
sustainability of exploitation and even the
course of evolution. Adult individuals pro-
vide the “reproductive capital” of a popula-
tion, akin to the financial capital in a bank
account or retirement fund. The interest
that is generated by annual growth is repre-
sented by the juveniles produced every year,
as well as the physical growth
of individuals. Depleting the
capital is risky, particularly in
long-lived, late-maturing organ-
isms. Trophy hunters and fish-
ers, in particular, often target
the largest, healthiest, and fit-
test organisms (see the photos).
This produces a strong selec-
tion pressure away from certain
traits, such as the ability to grow
rapidly to large size. As a conse-
quence, the gene pool of many
exploited populations changes
in ways that could compromise
their potential to recover from
previous depletion ( 11).
Two potential biases are asso-
ciated with research into human
effects on contemporary wildlife.
First, there is survivorship bias:
We only measure what is left.
Many vulnerable wildlife species
on land have already disappeared
during the past 40,000 years in
successive waves of extinction on
continents and islands that were
colonized by people ( 3). Related to that is ob-
server bias: The data-rich populations that
are scientifically observed and monitored
likely also experience some form of manage-
ment, which may motivate data collection
in the first place. Both biases render the re-
sults of Darimont et al. conservative. More
worrying are populations that are hunted or
fished essentially unobserved; at least in the
oceans, there is clear evidence ( 12) that these
are worse off than the assessed stocks repre-
sented in the chart below.
What does this general body of work ( 15)
tell us then, about our own species? There
are three key insights. First, the hunting of
large prey is deeply embedded in our iden-
tity and remains a powerful ecological and
evolutionary force. Second, the ability to
target mostly adult individuals across ma-
rine and terrestrial prey groups makes us
unique among all other predators. And third,
we have the unusual ability to analyze and
consciously adjust our behavior to minimize
deleterious consequences. This final point, I
believe, will prove critical for our continued
coexistence with viable wildlife population
on land and in the sea.
REFERENCES
1. C. T. Darimon t, C. H. Fox, H. M . Bryan, T. E. Reim chen ,
Science 349, 858 ( 2015) .
2. H. K. Lot ze, B. Worm, Trends Ecol. Evol. 24, 254 (2009).
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D. E. MacPhee, Ed. (Kluwer Academic/Plenum, New York,
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Natl. Acad. Sci. U.S.A. 103, 3171 (2006).
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Natl. Acad. Sci. U.S.A. 108, 8317 (2011).
8. B. Worm , T. A. Branc h, Trends Ecol. Evol. 27, 594 (2012 ).
9. E. J. Milner-Gulland, E. L. Bennett, S. A. M. W. M. Group,
Trends Ecol. Evol. 18, 351 (2003).
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S. Wap le s, Trends Ecol. Evol. 27, 542 (201 2).
12. C. Cost ello et al. , Science 338, 517 (201 2).
0
5
10
15
20
Nonhuman
predators
Herbivore
Carnivore
Top predator
H
C
TP
HAll C TP H C TP
Human
hunters
Type of prey
Type of predator
Human
shers
Relative rate of exploitation
10.1126/science.aac8697
Wildlife under pressure. Darimont et al. show that the rates at which
humans exploit adult land mammals and marine fish vastly exceeds
the impacts of other predators ( 1). Marine fish experience “fishing
through marine food webs,” with different trophic groups similarly
affected. In contrast, land predators are exploited at much higher rates
than herbivores.
PHOTO: PAULA FRENCH/SHUTTERSTOCK; ILLUSTRATION: ADAPTED BY P. HUEY/SCIENCE
Trophy hunt. On land as
well as on sea, humans
exploit their ability to hunt
from a safe distance and
hunt for trophies or status.
Published by AAAS
DOI: 10.1126/science.aac8697
, 784 (2015);349 Science
Boris Worm
A most unusual (super)predator
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Recently, the global state of marine fisheries and its effects on ecosystems have received much scientific (and public) scrutiny. There is little doubt that global limits to exploitation have been reached and that recovery of depleted stocks must become a cornerstone of fisheries management. Yet, current trends appear to be diverging between well-assessed regions showing stabilization of fish biomass and other regions continuing to decline. This divergence can be explained by improved controls on exploitation rates in several wealthy countries, but low management capacity elsewhere. Here, we identify an urgent need to direct priorities towards 'fisheries-conservation hotspots' of increasing exploitation rates, high biodiversity, and poor management capacity, and conclude that the future of fish depends, at least in part, on redoubling science, co-management and conservation efforts in those regions.
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Conservation biology research exhibits a striking but unhelpful dichotomy. Analyses of species decline, extinction risk, and threat mitigation typically encompass broad taxonomic and spatial scales. By contrast, most studies of recovery lack generality, pertaining to specific species, populations, or locales. Narrowly focused analyses offer a weak empirical basis for identifying generic recovery correlates across species, particularly in cases where recovery is not effected by an abatement of threats. We present a research framework for multi-species meta-analyses to identify early-warning signals - 'red flags' - of impaired recovery that can be used as predictors of recovery potential before recovery efforts are initiated. An empirically comprehensive understanding of the demographic, ecological, evolutionary, and threat-related factors affecting the rate and trajectory of species recovery will strengthen conservation efforts to set recovery priorities, targets, and timelines.