Chapter 21 Part 2: Biodiversity—Hotspots, Fragmentation, Threats, Extinction & Conservation

Biodiversity Hotspots

  • Definition: Geographical areas containing exceptionally high numbers of endemic species that are also extremely threatened.
  • Map Highlights
    • California Floristic Province (entire state + parts of Baja & Nevada) is an officially recognized hotspot—“right outside the front door” for anyone in Sacramento.
    • Lecturer’s personal count: has visited 11 hotspots; encourages travel to regions such as Madagascar where >80\% of species are endemic.
  • Conservation Payoff
    • If humanity fully protected the hotspots, we would safeguard ≈42\% of terrestrial vertebrates and 50\% of plant species.

Primary Human-Driven Threats to Biodiversity

  • Human population growth ⇒ escalating resource consumption.
  • Five major mechanisms (plus climate change):
    1. Habitat loss – outright destruction or severe alteration.
    2. Habitat fragmentation – breaking continuous habitat into isolated patches.
    3. Overharvesting – removal of organisms faster than they can reproduce.
    4. Invasive (exotic) species – non-natives that out-compete, prey upon, or otherwise disrupt local biota.
    5. Climate change – rapid, human-induced warming & ocean acidification.

Habitat Loss & Fragmentation

  • Satellite Example: Amazon Basin Landsat images (1975 ▸ 1986 ▸ 1992 ▸ 2001) show progressive clear-cutting; large blocks of dark-green forest shrink to thin ribbons within 26 years.
  • Social dilemma: local farmers clear land to feed families, yet cumulative impact devastates ecosystems.
  • Differential Impact
    • Large predators & wide-ranging species suffer most (need extensive territory to hunt/reproduce).
    • Small species may persist yet lose micro-habitats if clear-cutting hits their exact niche.
  • “How much is too much?”
    • Scientific uncertainty regarding thresholds for irreversible damage; lack of full species inventory complicates regulation.

Island Biogeography Applied to Fragments

  • Species richness in a patch (“island”) relates to patch size and distance from source habitat.
  • Dissertation topic of lecturer: measured species density & richness across forest fragments.
  • Buffer Zones
    • Core fragment left \approx100\% intact; surrounding buffer left \approx60\% intact to soften edges and enlarge functional area.
  • Umbrella Species Concept
    • Charismatic focal species (e.g.A0tiger, elephant, polar bear) attract funding; protection umbrellas all co-occurring organisms.
  • Wildlife Corridors / Land Bridges
    • Engineered overpasses in Banff (Canada), Singapore, Montana (USA).
    • Fencing funnels animals; camera traps confirm multi-species use; roadkill dramatically reduced.

Invasive Species

  • General Traits: lack of native predators/parasites; life-history traits that give competitive edge; often introduced intentionally (biocontrol) or accidentally.
  • Case Studies
    • Kudzu vine (SE USA) – covers houses & trees in <1 yr, halts photosynthesis underneath.
    • Burmese python (Florida Everglades) – released pets now apex predators; eradication hunts estimated 10\text{ snakes day}^{-1} for \ge100 yrs would still not eliminate them.
    • Cane toad (Australia) – introduced 1940s; large mouth eats mammals, reptiles, birds; considered a pest worth deliberately running over.
    • Jamaican mongoose – biocontrol for snakes, but switched to native birds.

Overexploitation

  • Industrial fishing: single net can haul “millions” of fish.
    • Removes adults before reproduction ⇒ collapses age structure.
    • Bycatch: dolphins, turtles, sharks die incidentally.
  • Historical crash: North-Atlantic cod—near extinction; multi-year bans & strict quotas required for partial recovery.

Climate Change

  • Keeling-type atmospheric CO_2 curve: 1880-2005 shows mild rise until \approx1940, slight plateau, then near-exponential climb after \approx1980.
  • Pandemic traffic drop produced brief dip, but levels rebound; oceans absorb CO_2 ⇒ acidification threatens marine life, coral reefs, etc.

Extinction: Concepts & Metrics

  • Working definition: no confirmed sighting in \ge50 yrs (exceptions for well-monitored species).
  • Rediscoveries: coelacanth (1938), Cuban bird (70 yrs unseen).
  • Background rate: 1{-}5 species yr$^{-1}.
  • Human impact graph: post-1850 extinction rates for birds & mammals skyrocket; anecdote of passenger pigeon going from sky-darkening flocks to extinction within 20 yrs.

Geographic Patterns

  • Map (last 500 yrs) clusters extinctions where human population density high: eastern USA, Caribbean, Brazil, Andes, Madagascar, SE Asia, etc.
  • United States leads in documented extinctions.

Taxonomic Risk Snapshot

  • IUCN-style categories: Green = Least Concern, Red = Extinct/Extinct in Wild/Endangered.
  • Notable values
    • Amphibians: \approx41\% threatened (and rising).
    • Cycads (ancient gymnosperms): >60\% at least threatened.

Critically Endangered Flagships

  • Amur leopard: \approx35 individuals.
  • Rhinos: black, white (northern subspecies functionally extinct), Javan, Sumatran all <$100$.
  • Mountain gorilla: \approx782.
  • Ivory-billed woodpecker: status contested; probable extinct.
  • Vaquita porpoise: \approx10 left (2023 estimate); gill-net bycatch primary cause.

Conservation Successes

  • American alligator – hunting bans & wetland protection.
  • Nēnē (Hawaiian goose) – captive breeding.
  • Bald eagle – DDT ban + captive rear & release.
  • Peregrine falcon – urban adaptation + pesticide bans.
  • El Segundo blue butterfly – habitat restoration.
  • Sea otter – legal protection; populations rebounding but fragile.
  • California condor – extinct in wild (1980s) → captive breeding → releases in CA, AZ, NV, WY; new fungal threat emerging.

Mass Extinction Events

  • Definition: >50\% of species lost in geologically short interval.
  • Earth’s history: 5 events; last (Cretaceous–Paleogene) \approx67 Mya wiped out non-avian dinosaurs.
  • Trend: dominant group perishes each event.
  • Evidence suggests 6th mass extinction under way, driven by humans; dominant species (Homo sapiens) likely to disappear if pattern holds.

Key Takeaways & Action Points

  • Biodiversity hotspots are efficient conservation targets—protect half the planet’s plant species by securing <3\% of land area.
  • Habitat fragmentation theory (island biogeography, buffer zones, corridors) provides practical design principles for reserves.
  • Addressing invasives, overexploitation, and climate change requires coordinated global policy, local enforcement, and sustainable livelihoods.
  • Success stories prove recovery is possible with science-based management, funding, and public support.