9.10 - Human Impacts on Biodiversity

Learning Objectives

I. Major Threats to Biodiversity (HIPPCO)

• H (Habitat Destruction): Humans destroy habitat through logging, agriculture, urbanization, and damming rivers, which affects water levels downstream.

• I (Invasive Species): The introduction of non-native species like the zebra mussel, emerald ash borer, and Kudzu vine is largely a result of human global transport. Invasives typically outcompete natives and consume resources faster, which reduces biodiversity.

• P (Population Growth): An expanding human population makes every other problem worse, driving more habitat loss, more urbanization, and more demand for food.

• P (Pollution / Pollutants): Specific pollutants like oil spills and pesticides (glyphosate, atrazine) kill non-target species through runoff or wind transport.

• C (Climate Change): Climate change shifts habitat locations and biomes, removes certain habitats entirely, and causes sea level rise that floods coastal ecosystems like estuaries.

• O (Overexploitation): Over-hunting or over-harvesting. It happens when humans harvest species faster than they can repopulate.

II. Habitat Fragmentation and Consequences

• Causes of Fragmentation: Habitat gets broken up by huge networks of roads and pipelines for oil transport, by agricultural land development, and by logging and clearcutting.

• Consequences: Fragmentation disrupts animal movement, hunting (especially for large predators), and breeding. Large K-selected organisms and predators are hit disproportionately hard because they need a large range.

• Scale Depends on the Species: how much fragmentation hurts depends on how far a species moves and how much space it needs. A two-lane road means nothing to a hawk, but it can split a population of salamanders or beetles that won't cross open pavement. A woodlot that's plenty for a white-footed mouse is far too small for a Florida panther, whose males roam home ranges of about 200 square miles.

• Metapopulations: Isolated fragments reduce gene transfer, which means lower genetic diversity and a higher likelihood of inbreeding depression. Wildlife corridors, like highway overpasses, connect these fragmented habitats, enabling movement and improving genetic diversity.

• Real Example (Banff, Canada): along the Trans-Canada Highway in Banff National Park, 44 wildlife crossings (6 overpasses and 38 underpasses) plus fencing cut wildlife-vehicle collisions by more than 80%. Bears, wolves, elk, and cougars all use them.

• Edge Effect: The meeting point of two ecosystems, like a forest and a grassland. Edge habitats can support higher levels of biodiversity, but they also let species uniquely adapted to edges expand. One example is the brown-headed cowbird, a brood parasite that lays its eggs in the nests of songbirds, which often drives songbird population decline.

III. Climate Change and Habitat Shifts

• As global temperature warms, higher latitudes warm faster, which pushes habitats like coniferous and boreal forests northward and shrinks the tundra and Alpine ecosystems.

• The tropics are projected to expand poleward.

• Warming leads to decreased precipitation in some areas, causing soil desiccation and desertification. That dry soil is prone to erosion, which creates a positive feedback loop.

• Sea level rise floods estuary habitats and coastal ecosystems like salt marshes, permanently inundating them with seawater and destroying the habitat.

IV. Impact of Domestication

• Domestication of plants (crops) and animals (livestock) generally decreases species diversity and decreases genetic variability.

• Selective breeding and clonal propagation (growing new plants from cuttings, so every plant is genetically identical, as with bananas and potatoes) mean fewer varieties of plants get grown and fewer breeds of livestock get raised. GMO crops add to this when a few engineered varieties get planted over huge areas.

• That decline in genetic diversity makes these crops and livestock more vulnerable to disease and to changing environmental conditions.

• Real Example (bananas): almost all bananas sold in stores are the Cavendish variety, and every Cavendish plant is a clone. The earlier favorite, the Gros Michel, was wiped out as a commercial crop by a soil fungus (Panama disease) by the 1950s, and a new strain of that fungus (TR4) now threatens the Cavendish.

• Honeybees (managed for economic returns): honeybees are kept in managed colonies for honey and crop pollination, and commercial queens come from a fairly small number of breeding lines, which lowers genetic diversity. Less diverse colonies have a harder time fighting off Varroa mites and disease. Managed colonies trucked from farm to farm can also spread diseases to wild native bees and compete with them for flowers.

• Livestock: most US dairy cows are Holsteins, bred heavily from a few top bulls through artificial insemination, so the breed's genetic diversity is low for a population of millions of cows. Worldwide, many traditional livestock breeds are at risk of disappearing.

V. Mitigation Strategies

• Protecting and Connecting Habitats: Establish wildlife preserves, national parks, and nature centers to prevent human disruption. Use wildlife corridors like highway overpasses to connect fragmented stands and restore metapopulations.
• Trade-offs: corridors and preserves are expensive, take years to set up, and work only if animals actually use them. Corridors can also let diseases, invasive species, or fire spread between patches.

• Sustainable Land Use: Implement urban growth boundaries and use infill building, meaning building up instead of out, to reduce the total amount of land developed. Expanding urban gardens and green roofs within cities increases urban habitat space.

• Sustainable Agriculture: Reduce global meat consumption to cut down the total acres of land needed for agriculture.

• Restore Lost Habitats: Replant clearcut forests, or convert nonproductive agricultural fields and areas like old golf courses back into native prairies.

• Global Goal (30x30): in 2022, the parties to the Convention on Biological Diversity (nearly every country except the US) adopted the Kunming-Montreal Global Biodiversity Framework, which aims to protect 30% of land and 30% of ocean by 2030. As of the 2024 Protected Planet Report, about 17.6% of land and 8.4% of the ocean were protected.