Back9.1 iogeography and the Equilibrium Theory of Island Biogeography
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Biogeography
Definition and Scope
Biogeography, also known as geographic ecology, is the study of ecological structure and processes at large geographic scales. It focuses on the variation in species composition and diversity among geographic locations, examining how physical geography influences biological diversity.
Key Point: Biogeography investigates patterns of species richness and distribution across regions, islands, and other isolated habitats.
Key Point: It is fundamental for understanding biodiversity, conservation, and ecosystem function.
Example: Comparing species diversity on oceanic islands versus mainland areas.
Species Richness and Area
Relationship Between Area and Species Richness
Species richness refers to the number of different species present in a given area. Empirical data, especially from islands, show that species richness increases with the area sampled, but this relationship can reach a saturation point.
Key Point: Larger islands typically support more species than smaller islands due to greater habitat diversity and resources.
Key Point: The relationship between area and species richness is often described by the species-area curve.
Formula: The species-area relationship can be mathematically expressed as: where is species richness, is area, is a constant, and is the slope of the log-log plot.
Example: Freshwater benthos species richness increases with area, as shown in empirical studies.
Equilibrium Theory of Island Biogeography
MacArthur and Wilson's Model (1967)
The Equilibrium Theory of Island Biogeography explains species richness on oceanic islands as a balance between immigration and extinction rates.
Key Point: Closer islands have higher immigration rates due to proximity to source populations.
Key Point: Larger islands have lower extinction rates because they support larger populations and more diverse habitats.
Key Point: The equilibrium number of species is reached when immigration and extinction rates balance.
Formula: Immigration and extinction rates are functions of the number of species present: where is immigration rate, is extinction rate, is current species number, and are maximum rates, and is maximum possible species.
Empirical Tests: Krakatau and Mangrove Islands
The theory has been tested through natural experiments, such as the recolonization of Krakatau after volcanic eruption and controlled experiments on mangrove islands in Florida.
Key Point: After disturbance, islands show predictable patterns of species accumulation and equilibrium.
Example: Mangrove islands were experimentally defaunated and monitored for recolonization rates.
Factors Affecting Extinction and Immigration
Species Biology and Extinction Rates
Extinction rates depend on the biology of the species present.
Key Point: Primary producers and smaller organisms tend to have lower extinction rates.
Key Point: Species at higher trophic levels, those with mutualistic partners, and larger organisms have higher extinction rates.
Species Biology and Immigration Rates
Immigration rates are influenced by dispersal ability.
Key Point: Good dispersers include birds, plants with wind-dispersed seeds, marine mammals, and large land mammals.
Key Point: Poor dispersers include amphibians, plants with heavy seeds, and freshwater fish.
Island Formation and Types
How Islands Form
The formation of islands affects the number and types of species present. Most oceanic islands form in two ways:
Continental islands: Form when land breaks away from continents.
Volcanic islands: Form when new land is created from volcanic activity.
Other Island Formation Mechanisms
Key Point: Islands can also form through sediment accumulation (corals, mangroves), landslides, or falling boulders.
Other Types of "Islands"
Key Point: Isolated habitats such as lakes, mountaintops, and forest patches can function as "islands" with unique species compositions.
Example: Lakes formed by filling a depression versus being cut off from a water source; mountaintops formed by erosion versus uplift.
Species Richness and Composition: Other Factors
Regional Processes, Time, and Geographic History
Biodiversity can vary even at similar latitude, longitude, biome, or distance from mainland due to regional processes, time, and geographic history.
Key Point: Historical events, regional climate, and evolutionary processes contribute to differences in species richness and composition.
Example: Two islands at the same latitude may have different species due to past geological events or regional climate differences.
Summary Table: Factors Influencing Island Biodiversity
Factor | Effect on Species Richness | Example |
|---|---|---|
Island Area | Larger area increases richness | Large oceanic islands |
Distance from Mainland | Closer islands have higher immigration | Channel Islands vs. Mainland France |
Species Biology | Good dispersers increase immigration; high trophic level increases extinction | Birds vs. amphibians |
Island Formation | Type affects initial species pool | Volcanic vs. continental islands |
Regional Processes | Can override local factors | Historical climate events |
Conclusion
Biogeography provides a framework for understanding how geographic and ecological factors shape biodiversity. The Equilibrium Theory of Island Biogeography remains a cornerstone for explaining patterns of species richness, emphasizing the dynamic balance between immigration and extinction, and the influence of island characteristics and species biology. Additional info: The notes expand on the original lecture slides by providing definitions, formulas, and examples for clarity and completeness.
