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The Antarctic blue whale was exploited to near extinction because it continued to be hunted opportunistically alongside other whale species. Isabel Beasley

How to hunt a species to extinction

Exploitation is one of the major causes of extinction. More than 120 species have become extinct at least in part because of hunting, fishing and logging, including the famous Passenger Pigeon.

Wildlife continues to be threatened on land and in the oceans, from whales to elephants.

Theoretically, hunting a species to extinction should be really hard. But our new study suggests reckless exploitation isn’t the only threat.

What is opportunistic exploitation?

Modern economists argue that actually shooting, catching, or chopping down every last individual would be economically unfeasible. Profits from harvesting rare species would fail to compensate for the escalating search costs as a species becomes rare.

Our study reveals a path to extinction that we’ve called “opportunistic exploitation”. It is widespread but has been largely overlooked by ecologists as a cause of extinction.

Usually more than one species is exploited by humans, even if one species is the main target. Desirable, high-value species are harvested first to maximise profits.

The sea cucumber Holothuria nobilis is Endangered in the Indian Ocean. Steven Purcell

The problem starts when these high-value species become scarce, and hunters switch to more abundant, lower-value species. But when hunters encounter a rare, high-value species, they are more than happy to snare the profits.

Opportunistic exploitation occurs when loggers, hunters or fishers encounter the rare, high-value species while searching for lower-value species and take it as an unplanned windfall.

Why does it matter anyway?

In our field of fisheries we’ve seen this effect in whaling, trawl fisheries and even sea cucumber fisheries.

In the 1950s and 1960s, Antarctic blue whale populations were hunted to 0.15% of the numbers found before whaling. This hunt would have been economically infeasible is it weren’t for fin and sperm whales. These whales were more common and found in the same waters as Antarctic blue whales, allowing the hunt to continue even as the blue whales became sparse.

On land opportunistic exploitation can occur in scenarios such as logging, and hunting for bush meat and large game. Other exploitation modes could include collecting butterflies, shells and corals, and recreational fishing.

So have any species actually gone extinct from opportunistic exploitation? We suspect so. Steller’s sea cow, a large sea mammal related to dugongs, and species of New Zealand moa may have become extinct this way.

So what do we do about it?

Accidental exploitation occurs when a species with no economic value is taken. Incidental exploitation occurs when species of lower value are taken while pursuing target species. In contrast, opportunistic exploitation involves high-value species taken as a bonus.

Black rhinoceros, now classed as Critically Endangered, are still killed opportunistically by hunters despite their rarity. Rob Wilkinson (

There are ecological parallels in which a predator kills a prey species that might not be its primary target. However, unlike natural predators, humans can maintain and even increase exploitation of high-value species when they are scarce. Effectively, opportunistic exploitation removes any refuge a species might gain in being rare.

Published studies of the factors that increase a species’ risk of extinction focus on attributes of each species without considering how that species relates to others.

In contrast, our article highlights that extinction risk depends not only on the value of the species in question, but also on other lower-value species that could subsidise continued exploitation.

So what can resource managers do to mitigate opportunistic exploitation?

We need to recognise the circumstances that allow people to exploit valuable species opportunistically. Regulatory measures could include shortlists of allowable species or species-specific catch quotas, where they can be implemented and enforced.

This article was co-authored by Aaron S. Lobo from the Nature Conservation Foundation.

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