Lec 21

Biogeography: The Study of Species Distribution

Introduction to Biogeography

Biogeography is the scientific study of the distribution of organisms, both living and extinct, across geographic space and through geological time. It seeks to understand the factors that influence where species are found and why certain regions are more diverse than others.

• Endemism: The restriction of a species' range to a specific geographic area.

• Disjunctions: The division of a species' range into separate, non-contiguous areas.

• Species Richness: The number of different species present in a region; varies greatly across the globe.

• Biogeographic Regions: Large areas with distinct assemblages of organisms, often shaped by historical and environmental factors.

Historical Biogeography and the Importance of History

Lost Continents, Vicariance, and Biotic Interchange

Historical events such as continental drift, the formation and closure of seaways, and the rise of mountain ranges have profoundly influenced the distribution of species. Vicariance (the splitting of populations by barriers) and biotic interchange (the mixing of faunas between regions) are key processes in shaping biogeographic patterns.

Determinants of Species Richness

Latitudinal Gradients and Environmental Factors

Species richness is not evenly distributed across the globe. Several hypotheses explain why the tropics are more diverse than temperate regions:

• Species-Energy Hypothesis: Greater energy input (sunlight) in the tropics leads to higher primary productivity, more ecological niches, and thus more species.

• Climatic Harshness: Tropical climates are less stressful, allowing more species to persist.

• Climatic Stability: Stable climates over time promote long-term persistence and diversification of species.

Species richness is also influenced by latitude, with higher richness near the equator and lower richness toward the poles.

Resource Partitioning and Elevational Gradients

Within regions, species may partition resources along environmental gradients, such as elevation, to reduce competition and promote coexistence.

Patterns of Endemism and Disjunction

Case Studies: Cacti and Marsupials

Some groups, such as cacti, are endemic to the New World, while similar-looking succulents are found in the Old World due to convergent evolution. Marsupials are primarily found in the southern hemisphere, with distinct lineages in South America and Australia.

• Endemic Areas of Cactaceae: Cacti are native only to the Americas.

• Marsupial Distribution: Marsupials are most diverse in Australia and South America, with few species in North America.

Phylogeny of Marsupials

Phylogenetic studies reveal that marsupials in Australia and South America share a common ancestry, reflecting ancient connections between these continents.

Biogeographic Regions and Barriers

Wallace's Line and Biogeographic Provinces

Biogeographic regions are separated by barriers such as oceans, mountains, and deserts. Wallace's Line is a famous boundary separating Asian and Australian faunas in the Malay Archipelago.

• Species on either side of major barriers tend to be more different from each other than those separated by smaller barriers.

• Species are often more closely related to others in the same region than to those in distant regions with similar habitats.

Biogeographic Processes

Vicariance, Dispersal, Extinction, and Range Expansion

Several processes shape the geographic ranges of species:

• Vicariance: The separation of populations by geological barriers, leading to divergence.

• Dispersal: Movement of individuals to colonize new regions.

• Local Extinction and Recolonization: Loss and subsequent return of populations to regions.

• Range Expansion: Continuous movement across favorable habitats.

Quantifying Diversification and Biogeographic Events

Phylogenies and Diversification Rates

Phylogenetic trees can be used to estimate diversification rates within clades. A constant rate of diversification predicts a linear increase in the number of lineages over time, while deviations indicate bursts of speciation or extinction events.

• Lineage-Through-Time Plots: Graphs showing the accumulation of lineages over time; non-linear plots suggest changes in diversification rates.

Case Study: Marine and Terrestrial Biotic Interchange

The closure of the Central American Seaway during the Miocene had profound effects on marine and terrestrial biotas, leading to the Great American Biotic Interchange (GABI), where species migrated between North and South America.

Summary Table: Key Biogeographic Processes

Process	Definition	Example
Vicariance	Separation of populations by geological barriers	Formation of the Isthmus of Panama dividing marine species
Dispersal	Movement of individuals to new regions	Colonization of islands by birds
Local Extinction	Loss of populations from a region	Extinction of large mammals in North America
Range Expansion	Continuous movement across suitable habitats	Spread of invasive species

Conclusion

Biogeography integrates ecological, evolutionary, and geological perspectives to explain the distribution of life on Earth. Understanding these patterns and processes is essential for interpreting biodiversity, speciation, and the effects of historical events on present-day ecosystems.