47.3 Threats to Biodiversity

Threats to Biodiversity

Threats to Biodiversity

Habitat loss is the single biggest driver of biodiversity decline. It happens when natural environments are destroyed or changed so much that native species can no longer survive there.

• Deforestation clears forests for timber, agriculture, or development. The Amazon rainforest alone loses thousands of square kilometers per year, destroying habitat for roughly 10% of all known species on Earth.
• Urbanization replaces diverse ecosystems with roads, buildings, and infrastructure. Urban sprawl pushes outward from cities into surrounding wildlands.
• Agricultural expansion converts wild lands into monoculture crops or pastures. Palm oil plantations in Southeast Asia, for example, have replaced vast stretches of tropical rainforest.
• Habitat fragmentation divides continuous habitats into smaller, isolated patches. This is especially damaging because of three compounding effects:
  • Smaller patches support fewer individuals, making populations more vulnerable to random events like storms or disease outbreaks
  • Isolated populations can't exchange individuals, so gene flow drops and inbreeding depression becomes more likely (this connects to concepts from island biogeography)
  • Migration corridors get severed, preventing species from reaching breeding grounds or seasonal resources

Invasive species are non-native organisms that establish themselves in a new environment and spread, often at the expense of native species. They arrive through both intentional introductions (ornamental plants, pets, biological control agents) and unintentional ones (zebra mussels hitchhiking in ship ballast water, seeds stuck to cargo).

What makes invasive species so destructive is that they typically lack natural predators in their new range. The brown tree snake on Guam, for instance, was introduced accidentally after World War II and drove most of the island's native forest birds to extinction. Invasive species cause harm through several mechanisms:

• Competition with natives for food, shelter, and breeding sites (Asian carp outcompeting native fish in North American rivers)
• Alteration of ecosystem processes like nutrient cycling and fire regimes (cheatgrass in the western U.S. increases fire frequency, which then favors more cheatgrass over native plants)

Overexploitation means harvesting species from the wild faster than they can reproduce. This takes several forms:

• Commercial exploitation driven by market demand can rapidly crash populations. Atlantic bluefin tuna populations have declined by over 70% due to overfishing.
• Poaching and illegal wildlife trade target rare or protected species. African elephant populations have been decimated for ivory, and pangolins are the most trafficked mammals on Earth, hunted for their scales and meat.
• Overharvesting of plants for food, medicine, or other uses can push species toward extinction (wild ginseng, for example).

The collapse of Atlantic cod fisheries off Newfoundland in the early 1990s is a classic case: decades of overfishing caused the population to crash, and the fishery was closed entirely in 1992. It still hasn't fully recovered.

Anthropogenic disturbance is a broad category covering other human activities that degrade ecosystems. Pollution from industrial, agricultural, and urban sources can harm species directly (pesticide runoff killing aquatic invertebrates) or indirectly (nutrient pollution causing algal blooms that create oxygen-depleted dead zones).

Human Impacts on Species Extinction

Current extinction rates are estimated to be 100 to 1,000 times higher than the natural background rate. This accelerating loss is often called the sixth mass extinction, or the Holocene extinction, and human activities are the primary driver.

Disruption of ecosystem services is one of the most consequential results of biodiversity loss. Ecosystems provide functions that humans depend on:

• Pollination by insects and other animals supports the reproduction of roughly 75% of flowering plants, including many food crops
• Nutrient cycling through decomposition and nitrogen fixation maintains soil fertility (mycorrhizal fungi, for example, help plants absorb nutrients)
• Water purification by wetlands and aquatic ecosystems filters contaminants and regulates water flow (mangroves also buffer coastlines from storms)
• Carbon sequestration by forests and peatlands stores atmospheric $CO_2$, helping mitigate climate change

Cascading effects on food webs occur when losing one species triggers a chain reaction through the ecosystem. Three patterns to know:

• Keystone species loss: Sea otters eat sea urchins, which graze on kelp. When otters were hunted to near-extinction, urchin populations exploded and destroyed kelp forests along the Pacific coast.
• Trophic cascades: The reintroduction of wolves to Yellowstone in 1995 reduced elk overgrazing, which allowed streamside vegetation to recover, which stabilized riverbanks. Changes at one trophic level rippled through the whole system.
• Foundation species loss: Coral reefs support roughly 25% of all marine species. When corals die, the entire community that depends on reef structure collapses.

Reduction in genetic diversity happens when populations shrink due to habitat loss, fragmentation, or selective pressures. Cheetahs, for example, have extremely low genetic variation, making them less able to adapt to new diseases or environmental changes. Tasmanian devils face a similar problem: a transmissible facial tumor disease has spread rapidly through their genetically uniform population.

Biodiversity hotspots are regions with exceptionally high concentrations of endemic species (species found nowhere else) that have already lost at least 70% of their original habitat. These areas, like the tropical Andes and Madagascar, are conservation priorities because protecting them yields outsized benefits for global biodiversity.

Climate Change Effects on Biodiversity

Climate change threatens biodiversity through several interconnected mechanisms.

Shifts in species distributions are already underway as organisms track suitable climate conditions. Many species are moving poleward or to higher elevations. Some butterfly species in Europe have shifted their ranges northward by 35–240 km over recent decades. These range shifts create problems when interacting species move at different rates, creating asynchrony. The great tit in Europe, for example, times its breeding to coincide with peak caterpillar abundance, but warming temperatures are causing caterpillars to emerge earlier, creating a mismatch.

Phenological changes refer to shifts in the timing of life cycle events like flowering, migration, and breeding. Cherry blossoms in Japan and Washington, D.C. are blooming earlier than historical records show. When these shifts happen unevenly among interacting species, mismatches arise. Pied flycatchers in Europe migrate based on day length (which hasn't changed), but their caterpillar prey now peaks earlier due to warmer springs, so the birds arrive too late for peak food availability.

Increased extinction risk is especially high for three groups:

• Species with limited dispersal abilities that can't relocate fast enough (the American pika, a small mammal in western mountain ranges, is losing habitat as temperatures rise at high elevations)
• Specialists with narrow niches, like polar bears that depend on sea ice for hunting seals
• Species in isolated or fragmented habitats with nowhere to migrate to (cloud forest species on tropical mountaintops)

Alteration of ecosystem structure occurs as species assemblages reshuffle. Kelp forests, for instance, are declining in warming waters, replaced by less productive ecosystems. Biome boundaries are shifting too: Arctic tundra is experiencing "greening" as shrubs and grasses expand into formerly frozen ground.

Ocean acidification is a distinct but related threat. As oceans absorb more atmospheric $CO_2$, seawater pH drops. This reduces the ability of shell-forming organisms like corals, mollusks, and pteropods to build their calcium carbonate structures. Coral bleaching occurs when heat stress and acidity cause coral polyps to expel their symbiotic algae (zooxanthellae), turning white and often dying. The Great Barrier Reef has experienced multiple mass bleaching events since 2016. Because coral reefs are the structural foundation of their ecosystems, their degradation cascades through entire marine food webs.

Conservation and Restoration Efforts

Conservation biology is an interdisciplinary field that combines ecology, genetics, and social sciences to understand and protect biodiversity. It applies scientific research to practical management decisions, from designing protected areas to managing endangered species recovery programs.

Ecosystem restoration aims to repair degraded habitats and reestablish ecological processes. Projects range widely in scale: reforestation of cleared land, reintroduction of locally extinct species, removal of invasive species, and reconnection of fragmented habitats through wildlife corridors.

Preserving biocultural diversity recognizes that biological and cultural diversity are deeply linked. Indigenous and local communities often hold traditional ecological knowledge that is valuable for conservation, and their land management practices have shaped many of the ecosystems we now seek to protect.

Enhancing ecological resilience is a core goal of modern conservation. Resilient ecosystems can better absorb disturbances (including climate change impacts) and recover afterward. Strategies include maintaining genetic diversity within populations, protecting habitat connectivity, and restoring degraded areas to functional condition.