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Opinion
Opportunistic exploitation: an
overlooked pathway to extinction
Trevor A. Branch1, Aaron S. Lobo2, and Steven W. Purcell3
1
School of Aquatic and Fishery Sciences, Box 355020, University of Washington, Seattle, WA 98195, USA
Nature Conservation Foundation, Gokulam Park, Mysore – 570 002, Karnataka, India
3
National Marine Science Centre, Southern Cross University, PO Box 4321, Coffs Harbour, NSW 2450, Australia
2
How can species be exploited economically to extinction? Past single-species hypotheses examining the economic plausibility of exploiting rare species have argued
that the escalating value of rarity allows extinction to be
profitable. We describe an alternative pathway toward
extinction in multispecies exploitation systems, termed
‘opportunistic exploitation’. In this mode, highly valued
species that are targeted first by fishing, hunting, and
logging become rare, but their populations can decline
further through opportunistic exploitation while more
common but less desirable species are targeted. Effectively, expanding exploitation to more species subsidizes the eventual extinction of valuable species at
low densities. Managers need to recognize conditions
that permit opportunistic depletion and pass regulations
to protect highly desirable species when exploitation
can expand to other species.
Pathways to extinction by exploitation
Humans are by far the biggest drivers of extinction, both
directly and indirectly [1]. Yet extinction through exploitation seems paradoxical: how can this be profitable given
the exorbitant costs of targeting rare species? Two previous
hypotheses suggest ways in which this could be profitable,
but here we expand on another exploitation mode termed
opportunistic exploitation (see Glossary) that arguably
offers a much more widespread pathway to extinction.
One previous hypothesis, the ‘economics of overexploitation’ [2], contends that when population growth rates are
low relative to monetary returns on investments, exploiters could maximize their net present value by catching the
entire population, banking the money, and living off the
comparatively high interest. The applicability of this hypothesis has recently been challenged: even for very unproductive species, catching the last individual would be
too costly to maximize net present value [3].
An alternative hypothesis is the ‘anthropogenic Allee
effect’, which notes that humans place exaggerated value
on species as they become rare, allowing profitable exploitation at very low population sizes [4,5]. For example, the
traditional Chinese medicine market generates intense
demand for products from rare fauna such as rhinoceroses
and tigers [6]. One instance of this demand is for the
Corresponding author: Branch, T.A. ([email protected]).
0169-5347/$ – see front matter
ß 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.tree.2013.03.003
Chinese bahaba, Bahaba taipingensis, which possesses a
swimbladder that is highly coveted by Asian consumers of
tonic soups for its supposed medicinal properties [7]. A
single 60.5 kg Chinese bahaba caught in the Fujian and
Guangdon Provinces was once sold for the same price
(US$23,895) as a three-bedroom house [7]. However, the
100–200 Taiping boats seeking this species must make
their living off other species, because only a handful of
Chinese bahaba are caught each year [7].
Glossary
Accidental exploitation: this mode occurs when the exploited species has no
economic value but is killed while a target species is exploited. Cases include the
deaths of sea snakes caught and discarded in trawl fisheries in Australia [47] and
accidental snare captures in Newfoundland of endangered marten (Martes
americana) while snowshoe hares (Lepus americanus) are targeted [48].
Incidental exploitation: this mode involves the exploitation of less desirable
species when commingled with the target species (e.g., Figure 5 in [4]). In
fisheries, it has long been recognized that, in such a situation, maximizing catches
from all species combined will result in the depletion or extirpation of lessresilient species [49–52]. Specific examples include the incidental bycatch of
shark species in longline fisheries for tuna and swordfish [53] and the near
extirpation of common skate (Dipturus batis) in the Irish Sea when caught by
trawlers targeting more valuable and abundant species such as cod [42,54].
Opportunistic exploitation: the exploitation of a scarce but desirable species,
when encountered while targeting other less desirable but more common
species. Exploitation is only profitable because of the presence of the less
desirable species. The term was originally coined as ‘opportunistic depletion’
[8]. Opportunistic exploitation allows for continued exploitation at densities
below the bioeconomic equilibrium [55], when profit (income minus expenses)
is the same as from available alternatives. In the single-species case, humans
should logically cease exploitation at this point. However, where multiple species
can be exploited in the same habitat, humans will often first deplete the most
desirable species, depleting it to the point of bioeconomic equilibrium, before
switching to less desirable species. The crux of opportunistic exploitation is that
at this point the sparse but desirable species can continue to be taken opportunistically whenever it is encountered, providing an unexpected bonus to the
exploiter. The definition of ‘opportunistic’ involves ‘exploiting circumstances or
opportunities to gain immediate advantage, rather than following a predetermined plan. . .; especially with the implication of cynicism or a lack of regard to
principles’ (Oxford English Dictionary, http://www.oed.com). Thus, opportunistic
exploitation carries the dual connotations of being both unplanned and selfishly
targeting an already depleted species.
Targeted exploitation: this is the most common mode, which occurs when one or
a few species are the primary subjects of resource extraction. Examples include
trophy hunting for wild goats (subfamily Caprinae) [5] and diving for black-lip
abalone (Haliotis rubra) in Australia [56].
Key differences between exploitation modes
Exploitation mode
Opportunistic
Targeted
Incidental
Accidental
Value of species
Higher than target
Profitable
Lower than target
No value
Primary target?
No
Yes
No
No
Trends in Ecology & Evolution, July 2013, Vol. 28, No. 7
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Trends in Ecology & Evolution July 2013, Vol. 28, No. 7
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idy
ubs
from
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e
oth
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OE
0
Loss
EB
Stock biomass
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Figure 1. Economics of opportunistic exploitation (OE). Curves show hypothesized
changes in economic rent (revenue minus costs) at varying biomass levels of highvalue species in multispecies (top curve) and single-species (lower curve)
exploitation systems. Subsidy from other species permits opportunistic
exploitation of the high-value stock because there is no bioeconomic equilibrium
(EB) at which exploitation is unprofitable.
Both the economics of overexploitation and the anthropogenic Allee effect hypotheses offer explanations of how
exploitation focused on a single species (target exploitation) can lead to extinction. However, it is much more
common for multiple species to be exploited together,
offering more pathways to extinction. These pathways
include accidental exploitation of species with no value
and incidental exploitation of species with lower value
while pursuing target species. We contend that a further
pathway occurs when rare, high-value species are encountered during the exploitation of more common target species. We term this largely unrecognized mode opportunistic
exploitation [8]. This pathway allows critically sparse but
valuable species to be profitably exploited when they are
encountered while targeting abundant but lower-value
species in the same habitat (Figure 1). Note that opportunistic exploitation is more pernicious than incidental or
accidental exploitation, because the depleted species has
high value, which results in stronger economic incentives
for further exploitation. Opportunistic exploitation examples from disparate habitats, scales of exploitation, and
trophic levels now provide compelling support for this
extinction pathway.
Marine examples of opportunistic exploitation
Antarctic blue whales were used as the key example
exemplifying the economics of overexploitation by Colin
Clark [2], but ironically exemplify opportunistic exploitation. As quoted within the original article [2]: ‘Gulland
[pers. comm.] has pointed out to me that fishing for the
Antarctic blue whale probably would have become uneconomical several years earlier had it not been for the
simultaneous occurrence of finback whales in the same
area.’ This view is even held by Clark himself in a recent
book, where he writes that targeting Antarctic blue whales
was effectively a zero-cost activity subsidized by the exploitation of fin whales [9]. The underlying story is that
Antarctic blue whales were heavily depleted by pelagic
whalers, who caught 28,000 in 1930 but only 7000 in 1950
410
World
War II
ec
r sp
Total catch (thousands)
Profit
si
Sub
Economic rent
rom
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50
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ecie
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e
oth
40
Humpback
Sperm
30
20
10
0
Fin
Antarcc blue
1940
1950
1960
Year
TRENDS in Ecology & Evolution
Figure 2. Opportunistic exploitation of the Antarctic blue whale (Balaenoptera
musculus intermedia) demonstrated by the number of whales caught annually by
pelagic whalers in the Southern Hemisphere from 1932 to 1963. During this period,
whalers shifted their focus from Antarctic blue whales to target lower-value and
more abundant species, particularly fin whales (Balaenoptera physalus) and sperm
whales (Physeter macrocephalus). Although Antarctic blue whales were too scarce
to support a target whaling industry, they were targeted opportunistically
whenever they were encountered, resulted in depletion to near-extinction. Small
catches of other species have been excluded. The comparative scale of the whale
drawings is indicative. Whale drawings reproduced with permission from
Greennature.
and fewer than 200 in 1963 as the population size plummeted [10]. However, this was not due to targeted exploitation, which would never have been profitable at such low
population sizes. By the 1950s, profits flowed from less
sought-after but more abundant species, particularly fin
whales, whereas Antarctic blue whales comprised only 3%
of all catches (Figure 2). Thus, it was opportunistic whaling
that reduced the abundance of Antarctic blue whales to
0.15% of pre-whaling levels [10], before international regulations finally halted the slaughter.
Multispecies trawl fisheries are also prone to opportunistic exploitation. For example, in India, trawl fisheries
began by targeting valuable shrimp, cephalopods, and
large fishes such as snapper and grouper in the 1950s,
primarily for export. After these were depleted, trawlers
shifted focus to lower-value fish such as croakers and
sardines for domestic markets, including the traditionally
discarded bycatch comprising small-bodied animals with
lower consumer preference, called ‘trash fish’. In India and
many other parts of South and Southeast Asia, trash fish
are now processed to fishmeal and used as a protein
supplement in the poultry and aquaculture industry.
The extra subsidy received from these former discards
(Figure 3) has allowed trawlers to further deplete highvalue species whenever they are encountered [11].
Opportunistic exploitation also occurs in small-scale sea
cucumber fisheries where multiple species are targeted [8].
Sea cucumbers feature prominently in Asian medicinal
markets and banquet dishes [12]. In sea cucumber fisheries, one or two high-value species are often fished to low
levels before fishers shift to lower-value species in the same
grounds and then the high-value species continue to be
fished while lower-valued species are targeted. In unfished
Opinion
Trends in Ecology & Evolution July 2013, Vol. 28, No. 7
(A)
(D)
(B)
(C)
(E)
(F)
TRENDS in Ecology & Evolution
Figure 3. Case studies demonstrating opportunistic exploitation. (A) Poachers caused the extinction of the black rhinoceros (Diceros bicornis) in Zambia despite focusing
mainly on elephants. Photograph (from Namibia) by Rob Wilkinson (www.Wilkinsonsworld.com). (B) Formerly discarded, trash fish caught in Indian shrimp trawl fisheries
are now auctioned in piles at the dock, effectively subsidizing continued exploitation of rarer high-value species. Photograph by Aaron Lobo. (C) Illegal trapping of babirusa
(Babyrousa celebensis) while the more common Sulawesi wild pig is targeted has depleted babirusa populations to vulnerable levels. Photograph by Pauli Hien. (D) The
prickly redfish (Thelenota ananas), one of many high-value coral reef sea cucumber species collected in small-scale multispecies fisheries, has been depleted
opportunistically from East Africa to the Central Pacific. Photograph by Steven Purcell. (E) Reductions by 93–95% of big-leaf mahogany (Swietenia macrophylla) by initial
logging are exacerbated by subsequent waves of loggers targeting the remaining species. Photograph by James Grogan. (F) The Antarctic blue whale (Balaenoptera
musculus intermedia) was depleted to less than 1% of its original numbers by opportunistic whaling by whalers targeting more abundant but less valuable fin whales and
other species. Photograph by Isabel Beasley.
areas, high-value sea cucumber species (Figure 3) are
naturally common and widely distributed, but in areas
subject to multispecies sea cucumber fisheries they are
often critically overfished [13]. Conversely, in single-species sea cucumber fisheries in Alaska, Iceland, New Zealand, British Columbia, Newfoundland and Labrador,
Washington, and eastern Russia, where there are no other
sea cucumber species to support opportunistic exploitation, even high-value sea cucumber species have not been
heavily depleted [8]. In other words, economic value must
be coupled with the presence of more abundant but less
desirable species to subsidize the chronic depletion of sea
cucumber stocks.
Given these examples, we argue that this switch from
targeted exploitation to opportunistic exploitation [14] is
probably widespread in marine and freshwater environments. For example, overfishing of Napoleon wrasse (Cheilinus undulatus) [15], white abalone (Haliotis sorenseni)
[16], and freshwater populations of lake trout (Salvelinus
namaycush) and walleye (Stizostedion vitreum) [17] were
all likely to have been exacerbated because fishing could
continue on other species in the same habitat. We also
speculate that examples of opportunistic exploitation
might include the extinctions of Steller’s sea cow (Hydrodamalis gigas) [18] and the Caribbean monk seal (Monachus tropicalis) [19].
Terrestrial case studies
Terrestrial systems also provide examples of opportunistic
exploitation. For instance, black rhinoceroses (Figure 3)
were illegally poached in the Luangwa Valley, Zambia in
the 1980s, despite an economic analysis showing this was
unprofitable due to their rarity [20]. However, poachers
could make a living by targeting elephants, which were ten
times more abundant, while poaching black rhinoceroses
when they were encountered [20]. By the end of the 20th
century, black rhinoceroses were extinct not just in the
Luangwa Valley, but in all of Zambia [21].
Similarly, hunters in the Indonesian island of Sulawesi
use snares to trap abundant Sulawesi wild pigs (Sus
celebensis), but this allows them simultaneously to poach
another wild pig, the rare babirusa (Babyrousa celebensis)
(Figure 3) [22], which would otherwise not be profitable to
hunt. Although the babirusa are legally protected, they
are caught more easily by the snares and thus even a small
amount of trapping of Sulawesi wild pigs can eliminate
babirusa from an area. Rowcliffe et al. make use of wordplay to describe this example as ‘piggyback extinction’
[23]. Bushmeat hunting in Equatorial Guinea and Ghana
also displays classic signs of opportunistic exploitation,
with more abundant smaller animals maintaining profitable hunting while larger, rare, and more valuable animals are still hunted whenever they are encountered
[24,25].
Selective logging of big-leaf mahogany (Swietenia
macrophylla) also exemplifies opportunistic exploitation.
In the Brazilian Amazon, these highly valued lumber trees
are found at low densities, but logging removes 93–95% of
commercial-sized trees [26]. Further waves of logging for
low-value species ensures that big-leaf mahogany missed
during the first wave are eradicated [27], leading to simplified forests containing only low-value species.
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Opinion
There are likely to be many more terrestrial examples of
opportunistic exploitation, from both the past and the
present. Possible candidates include the present-day depletion of Madagascar rosewood (Dalbergia spp.) [28], the
near-extinction of American bison (Bison bison) [29], and
the extinctions of South African blue antelope (Hippotragus leucophaeus) [30] and New Zealand moa species (Aves,
Dinornithiformes) [31].
Parallels in ecological theory
Opportunistic exploitation might thus be a widespread but
overlooked phenomenon wherever humans exploit multiple species together, with some ecological parallels in the
general field of ‘apparent competition’ where the abundance of two prey species becomes linked through shared
predators, affecting predator functional responses [32].
Multispecies models invoking apparent competition can
explain how early hunters caused the mass extinction of
large mammals in North America, whereas models based
on a single prey species cannot [33,34]. For instance,
common forms of functional response in predators can
act to keep one prey species in a low-abundance ‘predator
pit’ [35,36] whereby, if they increase, predators will start
targeting them and keep their numbers depressed, but at
low densities predators no longer maintain a search image
for that species and switch to other prey species [37].
Unlike most natural predators, however, humans maintain and even increase their desire to target high-value
species when they are encountered at low densities [5], so
there is no refuge in rarity.
‘Hyperpredation’ in natural systems is also somewhat
analogous to opportunistic exploitation. Hyperpredation
occurs when one prey source increases, subsidizing higher
predator populations, which in turn cause declines in
alternative prey populations [38]. This is typified by
declines and extinctions of native Australian rodents,
where feral cats and other predators subsidize their diets
with introduced rabbits and mice [38], and by the declines
of island foxes in the California Channel Islands due to
introduced pigs providing a steady food source for predatory golden eagles [39]. However, whereas hyperpredation
requires an increase in alternative prey, opportunistic
exploitation can happen even when the abundance of
alternative prey is constant or declining.
Implications for conservation and policy
Management and conservation policies need to identify
species at risk of opportunistic exploitation and take steps
to avoid their potential depletion and extinction. Past
studies have focused on attributes of each species in isolation to predict their risk of extinction: factors such as lifehistory traits, overexploitation, loss of habitat, disease,
rates of decline, abundance, and geographic range [4,40].
We argue that multispecies attributes are also important;
the risk of opportunistic exploitation not only depends on
the value of the species in question, but is also linked to the
presence of co-occurring lower-value species that could
subsidize continued exploitation.
There are several specific management actions that can
be taken to overcome opportunistic exploitation. One key
realization is that private ownership of multispecies
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Trends in Ecology & Evolution July 2013, Vol. 28, No. 7
resources can lead to economically rational overexploitation. Managers should pay special attention to rare species
that are highly valuable and institute protocols to limit and
monitor trade. For example, Papua New Guinea has an
export ban on seven species of birdwings, the largest
butterflies in the world, due to their rarity and the global
demand from collectors [41]. Another useful policy is to
develop shortlists of allowed species [8], instead of imposing species bans that inevitably are developed too late to
avoid depletion from opportunistic exploitation. The shortlists could include common exploitable species but exclude
threatened high-value species and others that are not
targeted to preserve ecosystem functioning. Used in this
manner, shortlists would prevent the future expansion of
exploitation to new species before they can be assessed.
Naturally, trade restrictions and shortlists should be based
on sound taxonomy, to ensure that the diagnosis of extinction risk is not clouded by cryptic species that are part of
undetected species complexes [42]. Another pitfall to avoid
is a group quota for the exploitation of multiple similar
species. The International Whaling Commission initially
set an annual quota on the weighted catch of all species of
large whales combined, which contributed greatly to the
decline of individual species, including Antarctic blue
whales. Instead, sustainable individual-species quotas
should be set wherever quotas can be practically established, monitored, and enforced.
Related to the idea of individual quotas for species is the
proposal for balanced fisheries exploitation, where catches
are spread broadly across a range of species while care is
taken to ensure that exploitation rates for individual species remain in proportion to their productivity [43,44].
Fishers are capable of applying this approach in multispecies fisheries, provided there are sufficient incentives to
target and avoid particular species, but this requires near100% enforcement and monitoring [45]. The danger of the
balanced-exploitation approach is that expanding catches
to new species acts as a subsidy for the continued exploitation of current species, as seen in our example of multispecies trawl fisheries in India.
In addition to these specific policy ideas to mitigate
opportunistic exploitation, there are a number of familiar
policies that are also needed. For instance, ideally, species
should be regulated by limiting the amount exploited and
not the amount of effort expended (a surprisingly common
regulation in fisheries). Terrestrial and marine reserves
should be set aside where no extractive exploitation is
allowed, to serve as a refuge against opportunistic exploitation. Finally, all regulations are moot without effective
monitoring and enforcement to deter illegal exploitation.
Concluding remarks
Examples from fishing, hunting, and logging demonstrate
that opportunistic exploitation is a pervasive pathway to
depletion and extinction wherever multiple species are
exploited. We anticipate that further examples of opportunistic exploitation, both historically and in the present, will
come to light wherever humans exploit natural resources.
For example, opportunistic exploitation might arise in spearfishing, in trapping, and in the process of collecting bird eggs,
orchids, mushrooms, insects, shells, and ornamental corals
Opinion
[46]. Policy efforts should focus on conserving rare, highvalue species exploited within multispecies systems, to avoid
scenarios where subsidy by lower-value species allows a
profitable pathway that drives species toward extinction.
Trends in Ecology & Evolution July 2013, Vol. 28, No. 7
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Acknowledgments
The authors thank Daniel Schindler, Ray Hilborn, Jim Kitchell, and
Nicholas Dulvy for helpful comments on earlier drafts that greatly
improved the manuscript.
29
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