Overexploitation

Overexploitation, also known as overharvesting or ecological overshoot, occurs when a renewable resource is harvested at an unsustainable rate.

This process happens when the rate of extraction exceeds the natural capacity of a population to replenish itself through reproduction. In ecology, overexploitation is recognized as one of the five primary activities threatening global biodiversity. It can lead to the destruction of a resource or even the extinction of entire species. While the term is often used in conservation biology to describe human economic activity, it can also apply to non-human actors. For example, introduced predators or herbivores can overexploit native flora and fauna in an ecosystem.

To understand the mechanism of overexploitation, consider how a fish stock functions.

A population maintains its biomass, which is the total weight of the living organisms, through reproduction. For a fishery to remain sustainable, the rate of harvesting must be balanced by the rate of reproduction. When humans increase the harvest rate beyond this balance, the total biomass begins to decrease. There is a specific threshold known as the maximum sustainable yield. This is the highest amount of a resource that can be harvested without causing a long-term decline. In practice, this point often occurs when a fishery is reduced to about 30% of its original biomass.

If harvesting continues past the maximum sustainable yield, the consequences become severe. A fishery is officially considered to have "collapsed" if its biomass declines by more than 95% of its maximum historical levels.

This collapse can be abrupt and difficult to reverse. A famous example is the Atlantic cod stocks. These populations were severely overexploited during the 1970s and 1980s. This led to a sudden collapse in 1992. Even after fishing activities ceased, the cod stocks have failed to recover. This shows how removing an apex predator can permanently alter an ecosystem.

Overexploitation manifests in many different sectors of natural resource management. In fisheries, it is often called overfishing. In forest management, the term is overlogging.

When managing livestock, it is referred to as overgrazing.

In hydrology, the term for taking too much water from aquifers is overdrafting. Even when a resource is not completely destroyed, overexploitation can reduce its quality. For instance, the footstool palm in Southeast Asia has seen its leaf size decrease due to overharvesting for food wrapping and thatching.

Historical evidence shows that overexploitation is not a purely modern phenomenon. It has been documented for millennia across various human societies.

In New Zealand, the Māori people hunted the giant, flightless moa birds for food and resources. By 1500 AD, only 200 years after human settlement, ten species of moa had gone extinct. This rapid loss also caused the extinction of the Haast's eagle, which preyed on the moa. Similarly, in Hawaii, the demand for mamo bird feathers for ceremonial cloaks was immense. Crafting just one cloak required the feathers of approximately 70,000 adult mamo birds, contributing to their extinction.

Scientific research has provided deeper insights into the timing and scale of these events. The "overkill hypothesis" suggests that human migration and population growth drove the rapid extinction of megafauna.

In North America, about 80% of large mammal species became extinct within 1,000 years of human arrival. A 2018 study also found that large mammals in Africa began facing extinction from human hunting at least 125,000 years ago. This is 90,000 years earlier than previous estimates. Other studies suggest that extinction rates in large carnivores correlate with increased hominin brain size and changes in vegetation.

One way to conceptualize the social drivers of overexploitation is through the "tragedy of the commons." This concept was popularized by Garrett Hardin in 1968, based on an 1833 parable by William Forster Lloyd.

Imagine a group of herders sharing a common piece of land for grazing. Each individual herder acts in their own self-interest by adding more cows to the land to increase their own profit. However, the benefit of each new cow goes to the individual, while the damage to the shared land is shared by everyone. Eventually, every herder makes the same rational decision, and the land is destroyed. Hardin noted that this tragedy occurs in unregulated commons, such as oceans, rivers, and the atmosphere. To prevent this ruin, these shared resources must be appropriately regulated.