🐼Conservation Biology Unit 7 – Extinction: Causes and Prevention

What is Extinction?

• Extinction is the permanent loss of a species from Earth
• Occurs when the last individual of a species dies without producing offspring
• Results in the irreversible loss of unique evolutionary lineages and genetic diversity
• Can be caused by natural factors (climate change, asteroid impacts) or human activities (habitat destruction, overhunting)
• Differs from extirpation, which is the local disappearance of a species from a specific area while still existing elsewhere
• Background extinction rate is the natural rate at which species go extinct over geological time
  • Estimated at 1-5 species per year based on fossil record
• Mass extinctions are periods of significantly elevated extinction rates compared to background levels (End-Permian extinction, K-Pg extinction)

Historical Extinctions

• Extinctions have occurred throughout Earth's history, with five major mass extinction events in the past 540 million years
  • End-Ordovician (444 million years ago): 85% of species lost
  • Late Devonian (375 million years ago): 75% of species lost
  • End-Permian (252 million years ago): 96% of species lost
  • End-Triassic (201 million years ago): 80% of species lost
  • End-Cretaceous (66 million years ago): 76% of species lost, including non-avian dinosaurs
• Pleistocene megafauna extinctions occurred during the last ice age (130,000-11,700 years ago)
  • Large mammals like mammoths, ground sloths, and sabertooth cats went extinct
  • Causes debated, but likely a combination of climate change and human hunting
• Holocene extinctions have accelerated in the past 10,000 years due to human activities
  • Examples include the dodo (Mauritius), passenger pigeon (North America), and thylacine (Tasmania)

Major Causes of Extinction

• Habitat loss and fragmentation due to human land use (agriculture, urbanization, deforestation)
  • Reduces available space and resources for species
  • Isolates populations, reducing gene flow and increasing vulnerability to stochastic events
• Overexploitation through hunting, fishing, and collecting
  • Can drive species to critically low population levels (whales, sharks, elephants)
• Invasive species introduced by humans can outcompete or prey upon native species
  • Examples include brown tree snakes (Guam), Nile perch (Lake Victoria), and feral cats (islands worldwide)
• Pollution and environmental toxins can cause direct mortality or reduce reproductive success
  • Pesticides (DDT), oil spills, and plastic debris are common pollutants affecting wildlife
• Climate change driven by anthropogenic greenhouse gas emissions
  • Alters temperature, precipitation, and sea level, forcing species to adapt or shift ranges
  • Ocean acidification from absorbed CO2 threatens coral reefs and marine organisms with calcium carbonate shells
• Synergistic effects of multiple threats can amplify extinction risk

Extinction Rates and Patterns

• Current extinction rates are estimated to be 100-1000 times higher than background rates
  • Described as the "sixth mass extinction" or Anthropocene extinction
• Species-area relationship predicts that habitat loss will lead to predictable levels of extinction
  • Losing 90% of habitat is expected to result in 50% of species going extinct
• Extinction debt is the time lag between habitat loss and the eventual extinction of species
  • Populations may persist for a time in remnant patches before ultimately disappearing
• Extinction risk is not evenly distributed across taxa
  • Amphibians, reptiles, and freshwater fish are particularly threatened
  • Island species are more vulnerable due to limited ranges and naive to introduced predators
• Extinction cascades can occur when the loss of a keystone species triggers further extinctions
  • Sea otters maintain kelp forests by controlling sea urchin populations

Endangered Species and Red Lists

• Endangered species are those at risk of extinction in the near future
• IUCN (International Union for Conservation of Nature) maintains the Red List of Threatened Species
  • Categorizes species based on extinction risk: Least Concern, Near Threatened, Vulnerable, Endangered, Critically Endangered, Extinct in the Wild, Extinct
  • Assessments based on population size, geographic range, and rate of decline
• National and regional endangered species lists also exist (U.S. Endangered Species Act, China's National Key Protected Wild Plants List)
• Red Lists help prioritize conservation efforts and allocate resources to the most threatened species
• Limitations of Red Lists include taxonomic bias (charismatic megafauna), data deficiency, and political boundaries
• Shifting baseline syndrome can mask long-term declines and lead to underestimation of extinction risk

Conservation Strategies

• Protected areas like national parks and wildlife reserves
  • Safeguard critical habitat and reduce human pressures
  • Ex-situ conservation maintains captive populations in zoos, aquariums, and seed banks as a backup
• Habitat restoration and connectivity
  • Reestablishing degraded ecosystems and creating wildlife corridors between fragments
• Captive breeding and reintroduction programs
  • Recovering species from brink of extinction (California condor, black-footed ferret)
  • Genetic management to minimize inbreeding and maintain adaptive potential
• Invasive species control and eradication
  • Removing or suppressing invasive populations to relieve pressure on native species
• Sustainable use and community-based conservation
  • Engaging local communities in conservation efforts and providing economic incentives (ecotourism, sustainable harvesting)
• International agreements and legislation
  • CITES (Convention on International Trade in Endangered Species) regulates wildlife trade
  • Migratory Bird Treaty Act and Endangered Species Act in the United States

Case Studies in Extinction Prevention

• Arabian Oryx rescued from extinction through captive breeding and reintroduction
  • Hunted to extinction in the wild by 1972, but saved by coordinated ex-situ and in-situ efforts
• Black-footed Ferret recovery in North America
  • Captive breeding program established from last 18 individuals, reintroduced to prairie dog colonies
• Mauritius Kestrel and Pink Pigeon saved from brink of extinction
  • Intensive management and habitat restoration on Mauritius, a biodiversity hotspot
• Yellowstone Wolf reintroduction and trophic cascades
  • Extirpated from the park in 1926, reintroduced in 1995 with far-reaching ecological effects
• Seychelles Magpie-Robin recovered through translocation and habitat management
  • Population bottlenecked at 12-15 individuals, but successful conservation efforts led to downlisting

Future Challenges and Opportunities

• Addressing the underlying drivers of extinction (human population growth, overconsumption, climate change)
• Incorporating climate change projections into conservation planning and assisted migration
• Expanding the protected area network and improving management effectiveness
• Filling knowledge gaps through biodiversity inventories and monitoring, especially in understudied regions and taxa
• Harnessing new technologies like eDNA, remote sensing, and gene editing for conservation
• Engaging the public and fostering a culture of conservation through education and outreach
• Developing innovative funding mechanisms (carbon credits, biodiversity offsets) and partnerships with the private sector
• Integrating conservation with sustainable development goals to balance human well-being and biodiversity protection