6.1 Theory of island biogeography

Island biogeography theory explains species diversity on islands through immigration and extinction rates. It considers island size, distance from mainland, and habitat diversity to predict species richness and turnover.

The theory, developed by MacArthur and Wilson in the 1960s, revolutionized ecology. It applies to both actual islands and habitat fragments, influencing conservation strategies and our understanding of biodiversity patterns in isolated ecosystems.

Origins of the theory

• Theory of Island Biogeography emerged as a fundamental concept in ecology and biogeography, providing insights into species distribution patterns on islands
• Developed in the 1960s, this theory revolutionized understanding of ecological processes and biodiversity dynamics in isolated ecosystems
• Applies to both literal islands and habitat islands, influencing conservation strategies and ecosystem management worldwide

MacArthur and Wilson's contribution

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• Robert MacArthur and E.O. Wilson jointly formulated the theory in 1963, publishing their seminal work "The Theory of Island Biogeography" in 1967
• Proposed a mathematical model explaining species richness on islands as a dynamic equilibrium between immigration and extinction rates
• Introduced the concept of species turnover, challenging the prevailing notion of static island ecosystems
• Emphasized the importance of island size and distance from mainland in determining species diversity

Historical context

• Built upon earlier biogeographical observations, including those of Charles Darwin and Alfred Russel Wallace
• Addressed limitations of existing equilibrium theories in ecology, which failed to explain species diversity patterns on islands
• Emerged during a period of rapid advancement in ecological research and quantitative modeling techniques
• Influenced by developments in population genetics and evolutionary biology, incorporating principles of species adaptation and colonization

Key principles

• Theory of Island Biogeography explains biodiversity patterns on islands through a balance of colonization and extinction processes
• Emphasizes the dynamic nature of island ecosystems, with species composition constantly changing over time
• Provides a framework for predicting species richness based on island characteristics and geographical context

Species equilibrium

• Postulates that islands reach a dynamic equilibrium in species number over time
• Equilibrium occurs when immigration rate of new species equals extinction rate of existing species
• Species composition may change while total number remains relatively stable
• Equilibrium point varies depending on island size, distance from mainland, and other ecological factors

Immigration vs extinction rates

• Immigration rate decreases as more species colonize an island due to fewer available niches
• Extinction rate increases with more species present due to increased competition and limited resources
• Curves of immigration and extinction rates intersect at the equilibrium point
• Factors affecting these rates include island area, isolation, habitat diversity, and species characteristics

Island size effect

• Larger islands support more species due to greater habitat diversity and resource availability
• Increased area reduces extinction risk by providing larger populations and more refugia
• Supports MacArthur and Wilson's species-area relationship, expressed as S = cA^z (where S is species number, A is area, c is a constant, and z is the slope)
• Larger islands also have higher immigration rates due to larger "target" area for dispersing organisms

Distance from mainland

• Islands closer to mainland or source populations have higher immigration rates
• Distant islands experience lower colonization rates due to dispersal limitations
• Isolation affects species composition, favoring good dispersers on remote islands
• Distance influences the time lag for species to reach equilibrium after disturbances

Mathematical model

• Theory of Island Biogeography employs a mathematical framework to quantify and predict species richness on islands
• Model incorporates key variables such as island size, distance from mainland, and rates of immigration and extinction
• Provides a basis for empirical testing and refinement of the theory across various island systems

Equation components

• Species richness (S) at equilibrium expressed as a function of immigration (I) and extinction (E) rates
• Immigration rate (I) decreases with increasing species richness and distance from mainland
• Extinction rate (E) increases with species richness and decreases with island area
• General form of the equation: dS/dt = I(t) - E(t), where dS/dt represents the rate of change in species number
• Equilibrium reached when dS/dt = 0, or I(t) = E(t)

Graphical representation

• Model typically depicted using a graph with species number on the x-axis and rates on the y-axis
• Immigration curve slopes downward from left to right, representing decreasing colonization as species accumulate
• Extinction curve slopes upward from left to right, showing increasing extinctions with more species present
• Intersection of immigration and extinction curves indicates the equilibrium point
• Multiple curves can be plotted to compare islands of different sizes or distances from mainland

Factors influencing colonization

• Colonization processes play a crucial role in shaping island biodiversity and species composition
• Various biotic and abiotic factors affect the ability of species to reach and establish on islands
• Understanding these factors helps explain observed patterns of species distribution and predict future changes

Species dispersal abilities

• Organisms with efficient dispersal mechanisms (wind-dispersed seeds, strong flyers) more likely to colonize distant islands
• Adaptations for long-distance dispersal (coconuts, floating seeds) enhance colonization success in marine environments
• Variation in dispersal abilities leads to disharmonic island biotas, with over-representation of good dispersers
• Some taxa develop reduced dispersal abilities over time on islands (flightless birds, large-seeded plants)

Ocean currents and wind patterns

• Prevailing ocean currents influence the direction and frequency of species arrivals on islands
• Wind patterns affect dispersal of airborne organisms and propagules (spores, seeds, small insects)
• Seasonal variations in currents and winds can create temporal windows for colonization
• Extreme weather events (hurricanes, storms) may facilitate long-distance dispersal of organisms

Stepping stone islands

• Intermediate islands between source and target islands facilitate colonization of distant areas
• Allow for gradual range expansion and genetic exchange between populations
• Reduce effective isolation, increasing immigration rates to more distant islands
• Important for conservation planning, particularly in designing marine protected area networks

Factors affecting extinction

• Extinction processes on islands are influenced by various ecological and environmental factors
• Understanding these factors is crucial for predicting species persistence and implementing effective conservation strategies
• Island characteristics and species interactions play key roles in determining extinction rates

Resource availability

• Limited resources on islands increase competition and vulnerability to extinction
• Fluctuations in resource abundance (seasonal changes, El Niño events) can lead to population crashes
• Specialized species more susceptible to extinction due to narrow resource requirements
• Human activities often reduce resource availability, exacerbating extinction risks (habitat destruction, overharvesting)

Habitat diversity

• Greater habitat diversity supports more species and reduces extinction risk
• Islands with varied topography and microclimates provide more niches and refugia
• Habitat diversity buffers against environmental fluctuations and disturbances
• Loss of key habitats (wetlands, native forests) can trigger cascading extinctions

Interspecific competition

• Increased species richness leads to higher competition for limited resources
• Competitive exclusion may cause extinctions, particularly among ecologically similar species
• Introduced species often outcompete native island species, lacking coevolutionary history
• Competition intensity varies with island size, with stronger effects on smaller islands

Island characteristics

• Physical and ecological attributes of islands significantly influence their biodiversity patterns
• Island biogeography theory considers these characteristics to explain and predict species richness
• Understanding island features helps in comparing and categorizing different island ecosystems

Area vs species richness

• Positive correlation between island area and species richness, known as the species-area relationship
• Larger islands support more species due to increased habitat diversity and reduced extinction risk
• Relationship often expressed as power function: S = cA^z, where S is species number and A is area
• z-value (slope) typically ranges from 0.2 to 0.35, varying with taxonomic group and island type

Habitat heterogeneity

• Islands with diverse habitats support more species due to niche partitioning
• Topographic complexity (mountains, valleys) creates varied microclimates and soil conditions
• Habitat diversity often correlates with island area but can vary independently
• Heterogeneous habitats provide refugia during environmental fluctuations, reducing extinction risk

Isolation effects

• More isolated islands have lower immigration rates and often support fewer species
• Isolation promotes endemism through adaptive radiation and reduced gene flow
• Remote islands often have disharmonic biotas, lacking certain taxonomic groups
• Isolation effects can be modified by factors like ocean currents, wind patterns, and human-mediated dispersal

Applications in conservation

• Theory of Island Biogeography provides valuable insights for conservation planning and management
• Principles applied to both literal islands and fragmented terrestrial habitats
• Informs strategies for preserving biodiversity and mitigating impacts of habitat loss

Marine protected areas

• Design of marine reserves incorporates island biogeography principles to maximize biodiversity protection
• Size and spacing of protected areas influence species persistence and recolonization rates
• Network design considers connectivity through larval dispersal and adult movement
• Larger reserves support more species and provide better protection against local extinctions

Habitat fragmentation

• Fragmented landscapes viewed as habitat islands within a matrix of unsuitable area
• Species-area relationship applied to predict biodiversity loss in fragmented habitats
• Edge effects and isolation increase extinction risk in small fragments
• Conservation strategies focus on preserving large, contiguous habitat patches where possible

Corridor design

• Wildlife corridors connect isolated habitat patches, facilitating species movement and gene flow
• Corridor width and quality influence their effectiveness as dispersal routes
• Stepping-stone habitats can function as corridors for some species
• Corridor networks aim to reduce effective isolation and maintain metapopulation dynamics

Criticisms and limitations

• Theory of Island Biogeography, while influential, has faced various criticisms and identified limitations
• Ongoing research addresses these concerns and refines the theory's applications
• Understanding limitations is crucial for appropriate use of the theory in research and conservation

Oversimplification concerns

• Model assumes all species are ecologically equivalent, ignoring functional differences
• Does not account for species interactions beyond simple competition
• Neglects historical factors and evolutionary processes in shaping island biotas
• Simplifies complex ecological dynamics into a few parameters (immigration, extinction rates)

Non-equilibrium scenarios

• Some island systems may not reach equilibrium due to frequent disturbances or ongoing environmental changes
• Theory less applicable to recently formed islands or those undergoing rapid ecological transitions
• Difficulty in determining if observed patterns represent true equilibrium or transient states
• Non-equilibrium dynamics may be more common than originally assumed, especially in human-impacted systems

Human impact considerations

• Original theory did not explicitly account for human-induced changes to island ecosystems
• Anthropogenic factors (habitat destruction, invasive species) often override natural biogeographic processes
• Challenges in applying the theory to heavily modified landscapes and urban environments
• Need for incorporating human activities into modern applications of island biogeography

Extensions of the theory

• Researchers have expanded upon the original Theory of Island Biogeography to address limitations and new ecological insights
• Extensions incorporate additional factors and processes to better explain observed patterns in nature
• These developments enhance the theory's applicability to a wider range of ecological systems and scenarios

Metapopulation dynamics

• Extends island biogeography concepts to networks of habitat patches with varying connectivity
• Considers local extinctions and recolonizations within a larger regional population
• Incorporates patch quality, size, and isolation in determining population persistence
• Applies to both natural habitat mosaics and fragmented landscapes

Nested subset hypothesis

• Proposes that species assemblages on smaller or more isolated islands are nested subsets of larger, less isolated islands
• Suggests a predictable order of species loss as island area decreases or isolation increases
• Influenced by species-specific traits (dispersal ability, habitat requirements) and island characteristics
• Provides insights into community assembly processes and extinction vulnerability

Rescue effect

• Describes the reduction in extinction rate due to immigration of individuals from other populations
• Particularly important for small islands or habitat patches near source populations
• Modifies the original theory by linking immigration and extinction processes
• Influences the shape of species-area curves and equilibrium species richness

Case studies

• Empirical studies of island systems have been crucial in testing and refining the Theory of Island Biogeography
• Case studies provide real-world examples of how biogeographic principles operate in diverse ecological contexts
• These examples offer insights into the theory's strengths and limitations when applied to specific island groups

Galápagos Islands

• Volcanic archipelago that inspired Charles Darwin's work on evolution and natural selection
• Demonstrates principles of adaptive radiation and endemism (Darwin's finches, giant tortoises)
• Shows effects of island age, size, and isolation on species diversity and composition
• Highlights human impacts on island ecosystems, including introduced species and habitat alteration

Hawaiian archipelago

• Exhibits extreme isolation effects, with high endemism rates across various taxonomic groups
• Demonstrates the influence of island age and area on biodiversity (island chronosequence)
• Illustrates concept of disharmonic biotas, lacking many taxonomic groups found on continents
• Provides examples of evolutionary phenomena like adaptive radiation (Hawaiian honeycreepers) and loss of dispersal ability (flightless birds)

Caribbean islands

• Diverse archipelago with complex geological history and varying degrees of isolation
• Shows effects of island size and distance from mainland on species richness (Anolis lizards)
• Demonstrates importance of stepping-stone islands in facilitating colonization
• Illustrates impact of historical sea-level changes on island connectivity and species distributions

Modern developments

• Recent advancements in technology and scientific understanding have led to new applications and refinements of island biogeography theory
• These developments enhance our ability to study and predict biodiversity patterns in island and island-like systems
• Integration of multiple disciplines provides a more comprehensive approach to biogeographical research

Molecular techniques in biogeography

• DNA sequencing and genetic analysis reveal colonization histories and evolutionary relationships
• Molecular clock methods help estimate timing of island colonization events
• Population genetics studies provide insights into gene flow between islands and source populations
• Metabarcoding techniques allow for rapid biodiversity assessments of island ecosystems

Remote sensing applications

• Satellite imagery and LiDAR technology enable detailed mapping of island habitats and land cover changes
• Remote sensing data used to quantify habitat heterogeneity and fragmentation at various scales
• Allows for monitoring of environmental changes (deforestation, urbanization) affecting island biogeography
• Facilitates study of marine island systems, including coral reefs and seamounts

Climate change implications

• Shifting climate zones affect species distributions and migration patterns on islands
• Sea-level rise threatens low-lying islands and coastal habitats, altering biogeographic patterns
• Changes in ocean currents and wind patterns may affect species dispersal and colonization rates
• Climate-induced habitat changes on islands may lead to novel species assemblages and extinction events