Rucete ✏ Campbell Biology In a Nutshell
Unit 8 ECOLOGY — Concept 54.4 Biogeographic Factors Affect Community Biodiversity
Large-scale geographic patterns—such as latitude and area—play a major role in shaping the diversity of ecological communities. These biogeographic factors influence species richness across the globe, and island ecosystems in particular offer unique insights into how immigration and extinction affect biodiversity.
1. Latitudinal Gradients in Species Richness
- Biodiversity is greatest in the tropics, decreasing toward the poles
- Example: tropical Malaysia (711 tree species in 6.6 ha) vs Michigan (10–15 in 2 ha)
- Two main hypotheses explain this pattern:
- Evolutionary history: Tropical regions are older and more stable, allowing more time for speciation
- Climate: High sunlight and precipitation in the tropics promote diversity
- Evapotranspiration (combined evaporation and plant transpiration) correlates with species richness in terrestrial vertebrates
2. Area Effects
- Species-area curve: larger geographic areas support more species
- Due to more diverse habitats, microclimates, and niches
- Example: larger Sunda Islands of Malaysia host more bird species than smaller islands
- General rule: bigger = more species, assuming similar habitats
3. Island Biogeography and the Equilibrium Model
- Islands (including isolated habitat patches) are ideal for studying species diversity
- MacArthur and Wilson’s Island Equilibrium Model:
- Species number on an island reaches an equilibrium where immigration = extinction
- Immigration decreases as more species are present
- Extinction increases with more species
- Island size and distance from mainland matter:
- Large, near islands have more species (higher immigration, lower extinction)
- Small, far islands have fewer species (lower immigration, higher extinction)
4. Experimental Evidence
- In the Florida Keys, scientists tested the model on six mangrove islands
- All arthropods were removed via fumigation
- Over time, species recolonized to pre-disturbance levels
- Closer islands recovered faster than distant ones
- Confirmed the model’s prediction: island size and distance strongly influence species richness
In a Nutshell
Large-scale geographic factors—like latitude, area, and isolation—play critical roles in determining community diversity. Tropics are biodiversity hotspots due to their age and favorable climate, while island studies reveal how colonization and extinction balance to shape species richness. These principles help predict patterns of diversity and inform conservation efforts.