Overexploitation in Conservation Biology

The Historical Context of Overexploitation and Megafauna Extinction

After the most recent glaciation, the grasslands of North America sustained a rich diversity of large mammals including mammoths, mastodons, giant ground sloths, horses, and cheetahs. However, approximately $11,000$ years ago, a massive wave of extinction resulted in the loss of $34$ genera of mammals within a span of only $1,000$ years. In contrast, only $20$ genera had become extinct in the preceding $3,000,000$ years. This period coincides exactly with the arrival of humans in North America via the land bridge from modern Russia. While there has been academic debate regarding the exact cause, evidence strongly suggests these extinctions were human-driven, a pattern observed globally as Homo sapiens expanded into new continents. For example, the Moas—large flightless birds in New Zealand—went extinct shortly after the Maori people settled there. Africa is the primary exception to this trend, as Homo sapiens and the local megafauna evolved together over time.

Documented Cases of Overharvesting Leading to Extinction

There are numerous historical examples of species being harvested to the point of extinction. The Steller sea cow, once found near the Aleutian Islands and Baring Island (part of the Russian Commander Islands), is a prominent example. Another is the great auk of North America, scientifically named $Pinguinus\,impennis$. Although they are the original "penguin," they belong to the family Alcidae rather than the modern penguin family. They were the largest of the auks and the only flightless species in the group, found on islands around Newfoundland such as Funk Island. These birds were hunted by sealers and whalers for food; because they could not fly, hunters could simply club them to death. Similarly, the Moas in New Zealand were easily hunted for their meat until they disappeared. The passenger pigeon, once among the most abundant bird species in North America, was also systematically hunted to extinction for food.

The Atlantic Cod Fisheries and Technological Shifts

The Atlantic cod fishery off the coast of Newfoundland represents a more recent collapse. For centuries, cod were so numerous that it was said one could almost catch them by dropping a barrel into the sea. The primary spawning ground was the Grand Banks (near where the Titanic sank), where the cold Labrador current meets the warm Gulf current. This mixing of currents creates high biomass but also heavy fog, making Newfoundland fishing a dangerous profession historically characterized by rough seas and drownings. Originally, fishermen used small wooden boats called dories and used methods like "jigging" or setting nets in shallow nearshore waters close to port.

As nearshore stocks were depleted, fishermen moved further offshore into deeper waters, aided by significant technological advances. Wooden boats were replaced by metal ones, and natural fiber ropes were replaced by modern plastic nets that are lighter, stronger, and more durable. The purse seine method is often used for schooling fish; fishermen use sonar to locate a school, encircle it with a net, and pull a bottom line (pursing the net) to trap the fish before hauling the catch onto the deck using specialized machinery like cranes.

Deep-Sea Fishing Methods and Environmental Impacts

Several methods are used to target demersal (bottom-dwelling) species. Bottom set gill nets are positioned in the water column with weights keeping them on the floor to entangle fish. A more controversial method is the bottom trawl, which involves dragging heavy weights and large nets across the ocean floor. This is highly destructive, destroying habitats and generating significant bycatch. Despite these impacts, the practice continues largely because the damage occurs out of sight of the general public. Long line fishing is used for deep-sea predatory fish such as the orange roughy (fished near the Chatham Islands) and the Patagonian or Antarctic toothfish. These lines can be set in waters exceeding $1,000\,m$ in depth. These steady advances in technology allow for the exploitation of previously inaccessible fish stocks, such as yellowfin tuna and bluefin tuna, many of which are currently harvested at unsustainable levels.

The History and Sequence of Commercial Whaling

Whaling was originally driven by the demand for meat and blubber, the latter of which was processed into oil for lamps before the widespread use of electricity. Early whaling utilized small rowing boats and handheld harpoons, targeting "right whales" (so named because they were the "right" ones to hunt due to their slow speed) and gray whales. In $1870$, the Norwegian shipping magnate Svend Foyn patented the modern exploding whaling harpoon, which featured a grenade tip. Combined with the steam engine, this allowed whalers to hunt faster species known as rorquals, including blue, fin, sei, and minke whales. Because rorquals tend to sink when dead, later harpoon versions injected air into the carcass to keep it afloat.

By $1935$, hunting right and gray whales was illegal as they had been reduced to less than $5\%$ of their original populations. The International Whaling Commission (IWC) was established in $1946$, initially to manage and sustain whaling. However, populations continued to collapse in a predictable sequence: hunters targeted the largest species (blue whales) until they were scarce, then moved to the second largest (fin whales), followed by sei whales, and finally the smallest rorquals, minke whales. In $1982$, the IWC passed a global moratorium on commercial whaling. While some populations like humpback whales in the Antarctic Peninsula have since recovered, countries such as Japan, Russia, Norway, and Iceland continue whaling by exploiting a legal loophole for "scientific purposes."

Overexploitation in Terrestrial Environments

Overexploitation is not limited to the marine environment. In the Himalayas of Nepal, medicinal plants are declining due to high harvesting pressure from local populations who lack access to expensive formal medical services. An experiment on harvesting intensity showed that when $25\%$ of the plants were harvested (light harvesting), the population density could recover over a three-year period. However, at harvesting intensities of $50\%$, $75\%$, or $100\%$, the populations failed to return to original levels even after four years. For example, plots with $100\%$ harvest reached only $0.2$ (or $20\%$) of their original density after four years, demonstrating that current high levels of harvesting in these regions are unsustainable.

The Tragedy of the Commons

The management of shared resources is often plagued by the "Tragedy of the Commons." The term originates from the old English system of the "common," a patch of grassland accessible to everyone for grazing stock. As long as the total use remains below the carrying capacity, it benefits the entire community. However, self-interest often drives individual users to increase their exploitation of the common. When the resource is used beyond its carrying capacity, it becomes degraded. While the individual user gains the immediate profit from overexploitation, the cost of the degradation is shared by all users. This dilemma applies to modern resources such as fish stocks, whales, clean water, and air. Unless environmental costs are accounted for in land-use and harvesting practices, the common resource eventually becomes unviable, depriving future generations of their rights to these natural assets.