Comprehensive Study Notes on Island Biogeography and Biodiversity
Foundations of Island Biodiversity and Resilience
Core Theme of Biodiversity:
Biodiversity is defined as the total direct variety of life forms within a given ecosystem.
Variety provides ecological resilience, which is defined as the capability of an ecosystem to withstand negative environmental impacts and successfully recover from negative consequences.
Significance of Islands:
Islands possess unique isolated characteristics and environmental challenges.
Maintaining diversity on islands is a critical concern because isolated ecosystems are exceptionally sensitive to species loss.
Primary Factors Influencing Island Diversity: Area and Distance
Island Area (Size) Correlation:
: Island area measured in square miles.
: Species richness, defined as the exact number of distinct species present.
Observed Trend: There is a strong positive correlation between island area and species richness. As island size increases, species richness increases.
Ecological Mechanism: Larger islands offer significantly more room, physical space, and available resources. This abundance eases competitive pressures, allowing larger total populations and a wider variety of species to coexist.
Distance from Mainland Correlation:
: Distance from the larger mainland origin.
: Species richness.
Observed Trend: There is a negative correlation between distance from the mainland and species richness. As the distance from the mainland increases, species richness decreases.
Ecological Mechanism: Remote islands are difficult for organisms to reach. High dispersal distances restrict colonization, resulting in lower total species richness.
The Equilibrium Model of Island Biogeography
Graph Dynamics and Axes:
: Total number of species present on the island (ranging from low on the left to high on the right).
: Process rates (immigration rate and extinction rate).
Low Species Richness State (Left Side of Graph):
Immigration Rate: High. When few species are present, resource availability and physical space are maximized. Arriving organisms encounter minimal competition and establish easily.
Extinction Rate: Low. Minimal species density results in little to no competition for resources, preventing local extinctions.
High Species Richness State (Right Side of Graph):
Immigration Rate: Low. As species accumulate, open niches vanish and resources become scarce. Arriving organisms face high competition from established species, preventing successful establishment.
Extinction Rate: High. High species density intensifies competition for limited resources. Organisms that are less adapted die off at an elevated rate.
Equilibrium Point:
Definition: The intersection point between the declining immigration curve and the rising extinction curve.
Species Turnover: At equilibrium, the total number of species remains stable (). Arriving immigrating species continually replace species going locally extinct.
Shift Factors: The exact location of the equilibrium point depends directly on an island's specific area and distance from the mainland.
Comparative Model Variations:
Island Size Shifts (Fixed Distance):
Large islands lower the overall extinction curve and support a higher equilibrium species richness point.
Small islands elevate the extinction curve and result in a lower equilibrium species richness point.
Island Distance Shifts (Fixed Size):
Near islands elevate the immigration curve, raising the equilibrium species richness point.
Far islands lower the immigration curve, lowering the equilibrium species richness point.
Four Island Scenarios:
Big and Close: Highest overall species richness equilibrium.
Big and Far: Intermediate species richness equilibrium.
Small and Close: Intermediate species richness equilibrium.
Small and Far: Lowest overall species richness equilibrium.
Spatial Heterogeneity and Edge vs. Interior Dynamics
Habitat Diversity within Islands:
Except for extremely tiny landmasses, islands contain multiple distinct habitat types.
Islands contain at least two primary ecological zones:
Coastal / Outer Boundary Habitat: Characterized by severe boundary exposure and specialized environmental stresses.
Inner / Interior Zone Habitat: Protected from coastal exposure; tends to be significantly more ecologically productive and stable.
Niche Specialization and Geometry:
Organisms evolve distinct adaptations specialized for either coastal or interior environmental conditions.
Edge-to-Interior Ratio: A higher ratio of interior habitat relative to edge area is optimal for preserving biodiversity. Expanded interior surface area promotes higher productive capacity and ecological stability.
Broadening the Island Concept and Habitat Fragmentation
Ecological Definition of an Island:
An island is fundamentally defined by ecological isolation, not by surrounding water.
Surrounding water is not a requirement for an island ecosystem in environmental science.
Terrestrial / Human-Made Islands:
Linear infrastructure such as streets, roadways, and highways forms impermeable boundaries that wildlife cannot safely cross.
Land patches enclosed by heavy traffic create terrestrial islands isolated from surrounding habitats.
Habitat Fragmentation Process:
Definition: Splitting a large, continuous habitat into multiple smaller, isolated habitat fragments through development.
Drivers: Forest clearing or burning, agricultural development, urban expansion, and highway network construction.
Consequence: Converting a continuous ecosystem into small fragments isolates populations, increases boundary exposure, and lowers overall diversity.
Case Study: Southern California Mountain Lions:
Mountain lion habitat across Southern California is broken up by urbanized centers and major transportation routes.
Inability to safely navigate across populated areas isolates mountain lions into discrete terrestrial habitat islands.
Conservation Design Rules for Diversity:
Size: Large reserves are superior to small reserves.
Distance: Proximity between fragments is superior to far isolation.
Connectivity: Interconnected habitat fragments connected by wildlife corridors are superior to completely isolated patches.
Ecological Vulnerability: Climate Impacts and Specialist Species
Climate Change Impact:
Islands are on the front lines of global climate change and global warming impacts.
Island ecosystems experience severe environmental damage before continental landmasses feel equivalent impacts.
Specialist vs. Generalist Dynamics:
Specialist Species: Characterized by an exceptionally narrow ecological niche. Specialists excel within their specific environmental conditions and outcompete generalists under stable conditions.
Vulnerability: Specialists lack adaptable behavioral or dietary options. If their specific niche parameters are disrupted, specialists cannot adapt and face rapid local extinction.
Island Endemism:
Because islands present unique, isolated environmental conditions, island organisms predominantly evolve into hyper-specialized species.
High concentrations of specialist species make island biodiversity exceptionally fragile when faced with habitat disruption or climate shifts.
Questions & Discussion
Question: What is the trend between species richness and distance from a larger mainland?
Answer (Zach): The further out an island is from the mainland, the less species rich it becomes.
Question: In terms of island habitat geometry, is a high edge-to-interior ratio or high interior area preferred for diversity?
Answer: High interior area relative to edge area is preferred. Maximizing interior surface area provides a larger, stable, and highly productive habitat zone.
Question: How do interior island habitats compare to coastal boundary habitats regarding productivity?
Answer (Ethan): Interior sections of an island tend to be more ecologically productive and provide less variable environmental conditions compared to coastal boundary sections.