🌾World Biogeography Unit 6 – Island biogeography

Key Concepts and Definitions

• Island biogeography studies the factors that affect the species richness and biodiversity of isolated natural communities
• Species richness refers to the number of different species represented in an ecological community, landscape or region
• Biodiversity encompasses the totality of genes, species and ecosystems in a region
• Equilibrium theory proposes that the number of species on an island is determined by a balance between immigration and extinction rates
  • Immigration rate is influenced by the distance of the island from the mainland source of species
  • Extinction rate is influenced by the size of the island, with larger islands generally having lower extinction rates
• Habitat diversity describes the variety of different habitats or ecological niches available on an island
• Adaptive radiation is the process by which a single ancestral species gives rise to multiple descendant species, each adapted to utilize a different ecological niche

Historical Context and Development

• The theory of island biogeography was developed in the 1960s by ecologists Robert MacArthur and E.O. Wilson
• It built upon earlier work by biologists studying the distribution of species on islands, such as Charles Darwin's observations in the Galapagos
• MacArthur and Wilson's 1967 book "The Theory of Island Biogeography" was a seminal work that helped establish the field
• The theory was initially developed to explain species richness on actual islands, but has since been applied to other isolated habitats (mountaintops, lakes, forest fragments)
• It has been influential in the development of conservation biology and the design of nature reserves
  • The theory provides a framework for understanding the impacts of habitat fragmentation on biodiversity
• Advances in molecular genetics and phylogenetics have provided new tools for testing and refining the theory

Island Biogeography Theory

• The equilibrium theory of island biogeography proposes that the number of species on an island represents a dynamic equilibrium between immigration and extinction
• The rate of immigration to an island depends on the distance of the island from the mainland source of colonizing species
  • Islands closer to the mainland will receive more immigrants and thus have higher species richness
• The rate of species extinction on an island is determined by the size of the island
  • Larger islands can support larger populations, making species less vulnerable to stochastic extinction events
• The theory predicts a positive relationship between island size and species richness, and a negative relationship between island isolation and species richness
• Islands are also predicted to have an equilibrium number of species, where the rate of new species arriving is balanced by the rate of species going extinct
• Turnover, the rate at which species are replaced by new immigrants, is higher on islands closer to the mainland

Factors Influencing Species Distribution

• Island size is a key determinant of species richness, with larger islands generally supporting more species
  • Larger islands have more resources, can support larger populations, and often have greater habitat diversity
• The degree of isolation of an island from the mainland or other source areas influences the rate of species immigration
  • More isolated islands receive fewer immigrants and thus have lower species richness
• The habitat diversity of an island can influence its carrying capacity for different species
  • Islands with a greater variety of habitats tend to support more species
• The geological age of an island can affect its biodiversity
  • Older islands have had more time for species to colonize and adapt, and may have higher endemism
• Anthropogenic factors, such as habitat destruction or the introduction of invasive species, can greatly impact island biodiversity

Colonization and Extinction Dynamics

• Islands are colonized by species from mainland source populations
  • Colonization often occurs through chance long-distance dispersal events (birds blown off course by storms, floating vegetation carrying small organisms)
• Successful colonization depends on a species' ability to survive and reproduce in the island environment
• Once established, populations on islands are vulnerable to extinction due to their small size and limited resources
  • Small populations are more susceptible to stochastic events (disease outbreaks, extreme weather, genetic drift)
• Extinction rates are influenced by island size, with populations on smaller islands being more prone to extinction
• Equilibrium is reached when the rate of new species immigrating is balanced by the rate of species going extinct
• Turnover of species is a feature of the equilibrium, with new immigrants replacing species that have gone extinct
  • Turnover is higher on islands nearer the mainland due to higher immigration rates

Case Studies and Examples

• The Galapagos Islands are a famous example of island biogeography and adaptive radiation
  • Darwin's finches diversified into many species, each adapted to different food sources (seeds, insects, cactus flowers)
• The Hawaiian Islands demonstrate the effects of isolation on species richness and endemism
  • Their extreme isolation has resulted in high levels of endemic species found nowhere else (silversword plants, honeycreeper birds)
• The islands of the Caribbean have been impacted by human activities, including habitat destruction and the introduction of invasive species (rats, mongoose)
• Mountaintops can be considered "sky islands" that show similar biogeographic patterns to true islands
  • The Sierra Nevada mountains of California have isolated populations of alpine species on their peaks
• Lakes and ponds can also function as freshwater islands for aquatic organisms
  • The cichlid fish of the African Rift lakes have undergone spectacular adaptive radiations in isolated lake "islands"

Applications in Conservation

• Island biogeography theory has been applied to the design of protected areas and nature reserves
  • The theory suggests that larger reserves will preserve more species, and that reserves should be clustered rather than dispersed to facilitate species migration
• It has also been used to predict the impacts of habitat fragmentation on biodiversity
  • Fragmentation creates isolated habitat "islands" vulnerable to species loss, particularly if the fragments are small
• The theory can guide strategies for conserving rare or endangered species on islands
  • Focusing on the largest islands or habitat fragments can protect the most species
  • Creating corridors between fragments can facilitate species migration and gene flow
• Invasive species management is a key concern in island conservation
  • Islands are particularly vulnerable to the impacts of introduced species, which can disrupt the equilibrium and cause extinctions of native species
• Restoration of degraded island habitats can help boost native species populations
  • Removing invasive species and replanting native vegetation are common restoration strategies

Current Research and Future Directions

• Advances in molecular genetics are providing new insights into the colonization history and evolutionary relationships of island species
  • Phylogenetic studies can reveal the order and timing of species radiations on islands
• Climate change is predicted to have major impacts on island biogeography
  • Rising sea levels may reduce island size or submerge low-lying islands entirely
  • Changing climate patterns may alter habitat suitability and species distributions
• Anthropogenic habitat alteration and fragmentation are creating "islands" in previously continuous habitats
  • Applying island biogeography theory to these anthropogenic islands can help predict and mitigate biodiversity loss
• Metapopulation dynamics, which consider multiple island-like habitat patches, are an area of active research
  • Understanding how species migrate between habitat patches can inform conservation strategies
• Integration of island biogeography with other fields, such as community ecology and ecosystem ecology, is providing a more holistic understanding of island systems