Ecology and Island Biogeography

Island of Krakatau Story

• Location: Between Java and Sumatra, Indonesia

  • Volcanic history, major eruption in August 1883

  • Eruption classified as VEI-6 (volcanic explosivity index)

  • Equivalent power: 200 megatons of TNT, 13,000 times Hiroshima bomb

  • Ejected $21\text{ km}^3$ of material; loudest sound recorded (3,000 miles away)

  • Casualties: 36,000 died, many more injured, tsunamis caused destruction

  • Island loss: Approximately two-thirds destroyed

• Aftermath of Eruption

  • 1884: island lifeless except for one spider

  • Formation of Anak Krakatau (Child of Krakatau)

  • Repopulated after 1884, early observations showed rapid biodiversity. Krakatau serves as a classic example of island colonization and ecological succession.

  • First recorded species: 43 species within 3 years; 10 years later, over 100 recorded. Early colonizers included species like floating coconuts.

  • By 25 years after eruption, 243 species of animals documented; dominated by birds

Ecological and Evolutionary Insights

• Understanding community formation and species interaction

• Factors determining species arrival and survival on islands: immigration and extinction rates

• Theory of Island Biogeography by Robert MacArthur and E.O. Wilson (1967)

  • Immigration rates: Measured by the number of species arriving on an uninhabited island. Initially high, immigration decreases over time as the island accumulates species and available niches fill.

  • Extinction rates: Impacted by small population sizes, limited resources, competition. Extinction increases over time as the number of species on the island grows, due to increased competition and smaller individual population sizes.

  • Balance between immigration and extinction leads to equilibrium species diversity ($S*$)

  • At equilibrium, the number of species remains stable despite ongoing immigration and extinction. The time to equilibrium varies by species group, depending on their dispersal and life history characteristics.

  • Island size correlates with extinction rates: Larger islands lower risk of extinction due to more resources

  • Isolation impacts immigration rates: Islands closer to the mainland have higher immigration potential

  • Rationales for new inhabitants: The dynamic balance predicted by the Theory of Island Biogeography, driven by island size and isolation, explains the patterns of species colonization and persistence observed on islands like Krakatau.

Predictions Based on Island Size and Isolation

• Larger, less isolated islands support higher species diversity

• Isolation leads to reduced diversity due to lower immigration

• Factors from studies show predictions align with terrestrial impacts, similar conservation strategies for fragmented habitats. Terrestrial islands, such as isolated mountain tops, fragmented forests (e.g., spruce forests), or urban parks, function similarly to oceanic islands regarding species diversity, but present different dispersal challenges. For oceanic islands, dispersal is often over water (e.g., via wind, ocean currents for seeds like coconuts, or flying for birds), whereas for terrestrial islands, dispersal might be through unsuitable terrestrial habitats.

Impact of Habitat Fragmentation

• As habitat is fragmented, extinction rates increase, leading to "relaxation" of biodiversity

  • Previous species diversity declines towards a new lower threshold

• Community studies indicate isolation leads to decreased species diversity

• Some studies showed no relationship between diversity and isolation in certain fragmented habitats due to barriers hindering immigration

Designing Effective Wildlife Preserves

• Essential lessons for conservation biology:

  • Large reserves preferable over multiple small ones

  • Consideration of corridors to facilitate species migration and connectivity between habitats. Immigration must be possible to prevent extinction and maintain genetic diversity, particularly in fragmented landscapes.

• Impact of ecological truncation: larger and specialist species lost first during fragmentation

  • Emphasis on protecting charismatic megafauna (e.g., tigers, pandas) over smaller species

Conclusion and Considerations for the Future

• Importance of understanding ecological principles for conservation

• Overall goal: Preserving biodiversity and habitat integrity in the face of human development

• Final thoughts: Ensure to apply ecological insights effectively for a sustainable future

• Introduction of final interactive question to consolidate learning.