Overharvesting is the practice of exploiting a renewable biological resource at a rate that surpasses its natural capacity for regeneration. This unsustainable removal prevents the population from recovering, leading to a diminished supply over time. Biodiversity represents the total variety of life on Earth, encompassing differences at the genetic, species, and ecosystem levels. The connection between overharvesting and biodiversity loss operates through several interconnected ecological pathways, ranging from the immediate numerical collapse of the target species to broad disturbances that restructure entire ecosystems.
Direct Population Collapse
The most immediate consequence of overharvesting is the numerical reduction of the targeted species population. When harvest rates continuously exceed the biological production rate, the population shrinks toward unsustainable levels. This removal rate surpasses the theoretical Maximum Sustainable Yield (MSY)—the largest amount that can be taken without long-term resource decline. Once the harvest surpasses this threshold, the population is driven into a rapid decline.
Targeting species for their size or age, such as large fish or mature trees, accelerates this collapse. Removing the largest individuals often eliminates the most reproductively successful members of the population. This selective pressure reduces the reproductive capacity of the remaining stock, making it harder for the species to rebound. The 1992 collapse of the Atlantic cod fishery off Newfoundland provides an example, where intensive fishing drove the population to less than one percent of its historical biomass.
The Ripple Effect: Disrupting Trophic Levels
The removal of a single species triggers widespread changes throughout the food web, a phenomenon known as a trophic cascade. This occurs when the loss of a top predator or dominant herbivore releases lower trophic levels from their natural controls. The resulting overabundance or depletion of intermediate species alters the structure and composition of the ecosystem.
When top-level consumers, such as sharks or large tuna, are overfished, their mid-level prey populations can surge. This mid-level population then over-consumes its own food source, causing a dramatic decline in that lower trophic level. In the Aleutian Islands, the decline of sea otters led to an explosion in their sea urchin prey. These unchecked urchins subsequently overgrazed the kelp forests, transforming a complex habitat into barren underwater landscapes.
On land, the overhunting of large herbivores or seed dispersers can prevent the regeneration of specific plant species. This alters the vegetative structure of the habitat, which then affects countless other species that rely on those plants for shelter and food. This demonstrates a loss of ecosystem-level biodiversity.
Erosion of Genetic Resilience
Beyond the reduction in numbers, overharvesting severely compromises the genetic health of the remaining population. When a species undergoes a rapid population decline, it experiences a genetic bottleneck. During this bottleneck, a large number of the population’s unique alleles, or variations of genes, are lost.
The surviving individuals possess a smaller, non-representative sample of the original gene pool. This loss of allelic diversity reduces the species’ capacity to adapt to new environmental pressures, such as disease or climate change. For example, the northern elephant seal population was reduced to around 30 individuals in the late 19th century due to hunting. Although their numbers have rebounded, their genetic variation remains extremely low, making them highly susceptible to widespread threats.
Collateral Damage and Habitat Destruction
Biodiversity loss is driven by destructive harvesting methods that harm non-target species and their habitats. Bycatch refers to the unintentional capture and death of animals not meant for harvest. Millions of non-target marine animals, including sea turtles, dolphins, and seabirds, are killed annually after being accidentally entangled in fishing gear like longlines and gillnets.
Furthermore, certain harvesting techniques physically destroy the foundational structure of ecosystems. Bottom trawling involves dragging heavy, weighted nets across the ocean floor, effectively plowing the seabed. This practice devastates slow-growing habitats such as cold-water coral reefs and sponge beds that provide shelter and nursery grounds for countless species. Similarly, clear-cutting forests removes the entire ecosystem structure, leading to soil erosion and the loss of the canopy layer essential for numerous plant and animal communities.