6.2 Island colonization

Island colonization is a fascinating process that shapes biodiversity patterns. Species arrive on islands through various dispersal methods, facing challenges like isolation and limited resources. Successful colonizers adapt to new environments, potentially leading to unique evolutionary paths.

Understanding island colonization helps explain current species distributions and community structures. Factors like island size, distance from mainland, and environmental conditions influence colonization success. This knowledge is crucial for predicting biogeographical patterns and informing conservation strategies.

Characteristics of islands

• Islands play a crucial role in understanding biogeographical patterns and processes in World Biogeography
• Isolated ecosystems on islands serve as natural laboratories for studying evolution, speciation, and ecological dynamics
• Island characteristics significantly influence biodiversity, colonization rates, and species distributions

Types of islands

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• Oceanic islands formed by volcanic activity (Hawaii)
• Continental islands separated from mainland by sea level rise (British Isles)
• Barrier islands created by sediment deposition along coastlines (Outer Banks)
• Coral atolls developed on submerged volcanic islands (Maldives)
• Artificial islands constructed by humans for various purposes (Palm Jumeirah)

Island isolation factors

• Distance from nearest landmass affects colonization rates and gene flow
• Ocean currents influence dispersal patterns of marine organisms and seeds
• Prevailing wind directions impact dispersal of airborne organisms and propagules
• Geographical barriers like mountain ranges or deep ocean trenches limit species movement
• Temporal isolation due to geological events or sea level changes

Island size considerations

• Larger islands generally support higher species richness and diversity
• Smaller islands often have limited resources and carrying capacity
• Island area affects habitat diversity and niche availability
• Size influences vulnerability to environmental disturbances and extinction risks
• Relationship between island size and species diversity described by species-area curve

Colonization processes

• Colonization of islands is a fundamental concept in World Biogeography
• Understanding colonization processes helps explain current species distributions and community structures
• Colonization events can lead to unique evolutionary trajectories and adaptations

Dispersal mechanisms

• Wind dispersal carries lightweight seeds and small organisms (dandelion seeds)
• Ocean currents transport floating seeds, fruits, and rafting organisms
• Animal-mediated dispersal includes:
  • External transport on fur or feathers (burrs)
  • Internal transport through digestive systems (fruit seeds)
• Human-assisted dispersal intentionally or unintentionally introduces species
• Volcanic eruptions eject spores and seeds over long distances

Founder effects

• Small number of individuals establish new population on an island
• Limited genetic diversity in founding population leads to:
  • Reduced adaptive potential
  • Increased susceptibility to genetic drift
• Founder effects can result in rapid divergence from source population
• Genetic bottlenecks may occur due to limited founding individuals
• Unique traits or alleles may become fixed in island populations

Adaptive radiation

• Rapid diversification of species from a common ancestor
• Occurs when colonizers encounter diverse ecological niches
• Results in evolution of new species adapted to different habitats
• Classic examples include:
  • Darwin's finches in Galapagos
  • Hawaiian honeycreepers
• Adaptive radiation leads to increased biodiversity and endemism on islands

Island biogeography theory

• Island biogeography theory provides a framework for understanding species diversity on islands
• Developed by Robert MacArthur and E.O. Wilson in the 1960s
• Explains patterns of species richness and turnover on islands

Species-area relationship

• Positive correlation between island area and number of species
• Described by power function: $S = cA^z$
  • S represents number of species
  • A represents island area
  • c and z are constants
• Larger islands support more species due to:
  • Greater habitat diversity
  • Larger population sizes
  • Reduced extinction risk
• Species-area relationship applies to habitat islands as well (mountain tops, lakes)

Equilibrium model

• Proposes dynamic balance between immigration and extinction rates
• Immigration rate decreases as more species colonize island
• Extinction rate increases as more species establish on island
• Equilibrium reached when immigration rate equals extinction rate
• Factors influencing equilibrium:
  • Island size
  • Distance from source population
  • Habitat diversity

Turnover rates

• Measures changes in species composition over time
• Influenced by colonization and extinction events
• Higher turnover rates observed on:
  • Smaller islands
  • More isolated islands
• Turnover rates affect community stability and resilience
• Can be measured using species presence/absence data over time

Factors influencing colonization

• Multiple factors affect the success and rate of island colonization
• Understanding these factors is crucial for predicting biogeographical patterns
• Colonization processes shape island ecosystems and biodiversity

Distance from mainland

• Inverse relationship between distance and colonization rate
• Closer islands receive more colonizers due to:
  • Reduced dispersal barriers
  • Higher probability of successful arrival
• Distance effect varies among different taxonomic groups
• Stepping stone islands can facilitate colonization of distant islands
• Distance influences genetic similarity between island and mainland populations

Environmental conditions

• Climate similarity between source and island affects colonization success
• Habitat availability determines establishment of colonizing species
• Extreme environments may limit colonization to pre-adapted species
• Presence of suitable food resources impacts survival of colonizers
• Environmental stability influences long-term persistence of colonizing populations

Resource availability

• Nutrient availability affects plant colonization and growth
• Presence of pollinators crucial for establishment of flowering plants
• Availability of prey species impacts colonization by predators
• Water resources influence colonization success in arid island environments
• Soil characteristics determine suitability for plant establishment

Succession on islands

• Succession processes on islands shape ecosystem development over time
• Understanding succession helps predict long-term changes in island biodiversity
• Succession patterns on islands may differ from mainland due to isolation and limited species pool

Primary vs secondary succession

• Primary succession occurs on newly formed or bare substrates
  • Volcanic islands (Surtsey)
  • Newly exposed land after glacial retreat
• Secondary succession follows disturbance of existing ecosystems
  • After natural disasters (hurricanes, landslides)
  • Human-induced disturbances (deforestation, agriculture)
• Primary succession typically slower due to lack of soil and established organisms
• Secondary succession benefits from existing soil and nearby seed sources

Pioneer species

• First organisms to colonize bare or disturbed areas
• Characteristics of pioneer species include:
  • Rapid growth and reproduction
  • Efficient dispersal mechanisms
  • Tolerance to harsh conditions
• Examples of pioneer species:
  • Lichens and mosses on rocky substrates
  • Wind-dispersed plants (fireweed, cottonwood)
• Pioneer species modify environment, facilitating establishment of other species

Climax communities

• Relatively stable ecological communities at end of succession
• Characterized by:
  • Dominant, long-lived species
  • Complex community structure
  • Efficient resource utilization
• Climax communities vary based on environmental conditions
• May take centuries to develop on isolated islands
• Examples include:
  • Tropical rainforests on older volcanic islands
  • Mature coral reef ecosystems

Evolutionary processes

• Islands serve as natural laboratories for studying evolutionary processes
• Isolation and unique environmental conditions drive evolutionary change
• Understanding island evolution provides insights into broader evolutionary patterns

Speciation on islands

• Allopatric speciation common due to geographical isolation
• Adaptive radiation leads to rapid diversification (Galapagos finches)
• Founder effects and genetic drift contribute to genetic divergence
• Sympatric speciation possible in diverse island habitats
• Hybridization between closely related species can occur

Endemism patterns

• High rates of endemism often observed on isolated islands
• Factors contributing to endemism:
  • Long-term isolation
  • Unique selective pressures
  • Limited gene flow with mainland populations
• Examples of island endemics:
  • Lemurs in Madagascar
  • Komodo dragon in Indonesia
• Endemism rates vary among taxonomic groups and island characteristics

Adaptive strategies

• Island species often develop unique adaptations to island environments
• Examples of adaptive strategies:
  • Dwarfism in large mammals (pygmy elephants)
  • Gigantism in small animals (Galapagos tortoises)
  • Loss of flight in birds (kiwi, dodo)
  • Changes in plant dispersal mechanisms
• Adaptive strategies reflect trade-offs between survival, reproduction, and resource utilization

Human impacts on islands

• Human activities significantly affect island ecosystems and biodiversity
• Islands often more vulnerable to anthropogenic disturbances due to isolation and limited resources
• Understanding human impacts crucial for effective conservation and management

Introduced species

• Intentional or accidental introduction of non-native species
• Invasive species often thrive due to lack of natural predators or competitors
• Examples of problematic introduced species:
  • Brown tree snake in Guam
  • Rabbits in Australia
• Introduced species can cause:
  • Native species extinctions
  • Habitat alterations
  • Changes in ecosystem processes

Habitat destruction

• Deforestation for agriculture, urbanization, or resource extraction
• Coastal development impacts marine and terrestrial habitats
• Mining activities can cause extensive environmental damage
• Habitat fragmentation reduces connectivity and gene flow
• Loss of critical habitats threatens endemic species

Conservation challenges

• Limited land area increases competition between conservation and development
• Isolation makes recolonization of extinct species difficult
• Small population sizes increase vulnerability to stochastic events
• Climate change poses significant threats to island ecosystems
• Balancing economic development with biodiversity conservation

Case studies

• Examining specific island systems provides valuable insights into biogeographical processes
• Case studies allow for comparison of patterns across different island types and locations
• Understanding unique characteristics of island systems informs conservation strategies

Galapagos Islands

• Volcanic archipelago in the Pacific Ocean
• Famous for Charles Darwin's observations on evolution
• Unique adaptations observed in various species:
  • Marine iguanas
  • Giant tortoises
  • Darwin's finches
• Challenges include introduced species and increasing tourism
• Ongoing research on evolutionary processes and conservation

Hawaiian archipelago

• Isolated volcanic island chain in the Pacific
• Exhibits high levels of endemism and adaptive radiation
• Notable examples of island evolution:
  • Hawaiian honeycreepers
  • Silversword alliance plants
• Severe impacts from human activities and introduced species
• Active conservation efforts to protect remaining native biodiversity

Caribbean islands

• Diverse archipelago with various island types and sizes
• Mix of continental and oceanic islands
• High biodiversity and endemism rates
• Unique fauna includes:
  • Anolis lizards
  • Eleutherodactylus frogs
• Threatened by habitat loss, invasive species, and climate change
• Important area for studying island biogeography and evolution

Island biodiversity patterns

• Island biodiversity patterns provide insights into ecological and evolutionary processes
• Understanding these patterns is crucial for effective conservation and management
• Biodiversity on islands often differs from mainland systems in composition and structure

Species richness gradients

• Species richness generally increases with:
  • Island size
  • Habitat diversity
  • Proximity to mainland
• Latitudinal gradients in species richness observed on island chains
• Elevational gradients in species richness on mountainous islands
• Mid-domain effect may influence species richness patterns on islands

Taxonomic composition

• Islands often have disharmonic faunas compared to mainland
• Over-representation of certain taxonomic groups:
  • Birds and plants with good dispersal abilities
  • Reptiles on remote islands
• Under-representation of other groups:
  • Large mammals on oceanic islands
  • Freshwater fish on isolated islands
• Taxonomic composition reflects colonization history and adaptive radiation

Functional diversity

• Island ecosystems often have simplified food webs
• Functional redundancy may be lower on islands
• Keystone species play crucial roles in island ecosystems
• Loss of important functional groups can have cascading effects
• Functional diversity patterns influenced by:
  • Island size and age
  • Isolation and colonization history
  • Environmental heterogeneity

Island ecosystems

• Island ecosystems exhibit unique characteristics due to isolation and limited resources
• Understanding island ecosystem dynamics is crucial for effective management and conservation
• Island ecosystems often serve as models for studying ecological processes

Trophic structures

• Simplified food webs compared to mainland ecosystems
• Limited number of trophic levels due to energy constraints
• Top predators often absent on small or isolated islands
• Mesopredator release can occur in absence of top predators
• Trophic cascades may have stronger effects in island systems

Nutrient cycling

• Closed nutrient cycles on isolated islands
• Limited external nutrient inputs except from:
  • Atmospheric deposition
  • Marine sources (seabirds, marine mammals)
• Efficient nutrient recycling mechanisms often develop
• Nutrient limitations can affect ecosystem productivity
• Human activities may disrupt natural nutrient cycles

Energy flow

• Energy flow in island ecosystems influenced by:
  • Primary productivity
  • Consumer diversity and abundance
  • Ecosystem size and complexity
• Marine subsidies important for many island food webs
• Energy constraints may limit population sizes and diversity
• Efficient energy use adaptations often observed in island species

Geological influences

• Geological processes play a crucial role in shaping island biogeography
• Understanding geological history helps explain current biodiversity patterns
• Geological influences affect island formation, persistence, and habitat diversity

Volcanic vs continental islands

• Volcanic islands formed by hotspots or plate boundaries
  • Often more isolated and have unique geological features
  • Examples include Hawaii and Galapagos
• Continental islands separated from mainland by sea level changes
  • Share geological history with nearby continent
  • Examples include British Isles and Madagascar
• Differences in geology affect:
  • Soil composition and fertility
  • Topographic diversity
  • Habitat types and distribution

Island age effects

• Newly formed islands have limited biodiversity
• Species richness and endemism generally increase with island age
• Very old islands may experience reduced diversity due to:
  • Erosion and habitat loss
  • Subsidence
• Island age influences:
  • Soil development
  • Habitat complexity
  • Time available for colonization and evolution

Subsidence and uplift

• Subsidence gradually reduces island area and elevation
  • Can lead to habitat loss and species extinctions
  • Affects older volcanic islands (Hawaii)
• Tectonic uplift can increase island area and elevation
  • Creates new habitats and opportunities for speciation
  • Examples include New Zealand and Caribbean islands
• Balance between subsidence and uplift influences long-term island persistence

Climate and colonization

• Climate plays a significant role in shaping island biodiversity and colonization patterns
• Understanding climate influences is crucial for predicting future changes in island ecosystems
• Climate interacts with other factors to determine species distributions and community composition

Microclimates on islands

• Islands often have diverse microclimates due to:
  • Topographic variation
  • Coastal influences
  • Rain shadow effects
• Microclimates create habitat diversity within small areas
• Species may adapt to specific microclimatic conditions
• Microclimate diversity can support higher species richness

Climate change impacts

• Islands particularly vulnerable to climate change effects
• Sea level rise threatens low-lying islands and coastal habitats
• Changes in temperature and precipitation patterns affect:
  • Species distributions
  • Phenology
  • Ecosystem processes
• Increased frequency and intensity of extreme weather events
• Climate change may facilitate establishment of new colonizing species

Seasonal variations

• Seasonal climate patterns influence:
  • Timing of reproduction and migration
  • Resource availability
  • Species interactions
• Some islands experience pronounced wet and dry seasons
• Seasonal variations can create temporal niche partitioning
• Species may develop adaptations to cope with seasonal changes

Dispersal barriers

• Dispersal barriers play a crucial role in island biogeography
• Understanding barriers helps explain patterns of species distribution and endemism
• Barriers can be physical, chemical, or biological in nature

Oceanic currents

• Influence dispersal of marine organisms and floating propagules
• Can facilitate long-distance dispersal to remote islands
• Major ocean currents create biogeographic regions
• Upwelling zones may act as barriers for some species
• Changes in ocean currents can affect colonization patterns

Wind patterns

• Important for dispersal of airborne organisms and propagules
• Trade winds influence dispersal patterns in tropical regions
• Storms and hurricanes can lead to long-distance dispersal events
• Wind patterns affect:
  • Seed and spore dispersal
  • Insect and bird movements
• Seasonal changes in wind patterns may create temporal dispersal windows

Geographical obstacles

• Mountain ranges create barriers within islands
• Deep ocean trenches limit dispersal of marine organisms
• Large distances between islands reduce colonization success
• Inhospitable environments (deserts, ice sheets) act as barriers
• Human-made structures can create new barriers or corridors

Genetic considerations

• Genetic factors play a crucial role in island biogeography and evolution
• Understanding genetic processes helps explain patterns of diversity and adaptation
• Genetic considerations are important for conservation and management of island species

Genetic drift in populations

• Stronger effect in small island populations
• Can lead to loss of genetic diversity over time
• May result in fixation of neutral or slightly deleterious alleles
• Genetic drift interacts with natural selection in shaping island populations
• Can contribute to rapid divergence from mainland populations

Bottleneck effects

• Occur when population size is drastically reduced
• Common during initial colonization of islands
• Results in reduced genetic diversity
• Can lead to increased susceptibility to diseases or environmental changes
• Recovery from bottlenecks may take many generations

Inbreeding depression

• More likely in small, isolated island populations
• Results from mating between closely related individuals
• Can lead to:
  • Reduced fitness
  • Expression of deleterious recessive alleles
  • Decreased adaptability to environmental changes
• Some island species may evolve mechanisms to avoid inbreeding
• Management strategies may be necessary to maintain genetic diversity

Extinction risks

• Islands often have higher extinction rates compared to mainland ecosystems
• Understanding extinction risks is crucial for effective conservation planning
• Multiple factors contribute to increased vulnerability of island species

Island vulnerability factors

• Small population sizes increase susceptibility to stochastic events
• Limited habitat availability restricts population growth
• Specialized adaptations may reduce ability to cope with changes
• Lack of predator recognition in naive island species
• Vulnerability to introduced diseases due to limited immunity

Extinction debt

• Time lag between habitat loss and eventual species extinctions
• Occurs when populations persist but are no longer viable long-term
• Factors contributing to extinction debt:
  • Habitat fragmentation
  • Population isolation
  • Disruption of ecological processes
• Difficult to quantify and predict
• Important consideration for conservation planning

Rescue effects

• Immigration from other populations can prevent local extinctions
• Less common on isolated islands due to dispersal limitations
• Stepping stone islands may facilitate rescue effects
• Human-assisted translocations can mimic rescue effects
• Consideration of genetic factors important in rescue attempts

Management and conservation

• Effective management and conservation strategies are crucial for preserving island biodiversity
• Unique challenges of island ecosystems require tailored approaches
• Integration of ecological, social, and economic factors necessary for successful conservation

Invasive species control

• Priority for many island conservation programs
• Methods include:
  • Physical removal
  • Biological control
  • Chemical treatments
• Prevention of new introductions through biosecurity measures
• Eradication programs successful on some islands (rats on South Georgia)
• Challenges include reinvasion and impacts on non-target species

Habitat restoration

• Aims to recover degraded or lost ecosystems
• Techniques include:
  • Native species reintroduction
  • Removal of invasive plants
  • Soil remediation
• Often requires long-term commitment and monitoring
• Success stories include:
  • Aldabra Atoll seabird restoration
  • New Zealand forest regeneration projects
• Challenges include limited reference ecosystems and climate change impacts

Sustainable development strategies

• Balancing economic development with biodiversity conservation
• Eco-tourism as alternative to extractive industries
• Sustainable resource management practices
• Community-based conservation initiatives
• Integration of traditional ecological knowledge
• Development of renewable energy sources to reduce fossil fuel dependence