Ecology Nov. 11th

Biogeography

Introduction

• Definition: Biogeography is the study of patterns of species composition and diversity across geographic locations.
• Key Regulators: Physical factors and species interactions are important at local scales; however, global and regional processes significantly influence species distributions and diversity, largely driven by continental drift, global climate patterns, and major evolutionary events.

Key Observations in Biogeography

• Species Richness Example:
  • Amazon Rainforest: Most species-rich forest globally with approximately 1,300 tree species.
  • Boreal Forests, Canada: Contains only two dominant tree species, despite vast coverage, due to harsh climatic conditions and limited resource availability.

General Patterns of Species Distribution

• Latitude Effects:
  • Lower latitudes boast higher species richness compared to higher latitudes, often linked to greater climatic stability and energy availability.

Forest Locations and Tree Species Richness

• Tree Species Richness by Location & Latitude:
  • Amazon, Brazil: 4°S, 60°W — 1,300 species
  • Southern California, USA: 35°N, 125°W — 57 species
  • Boreal Forest, Canada: 64°N, 125°W — 742 species
  • New Zealand Beech Forest: 45°S, 170°E — 20 species
  • Flowering Tree Forest, New Zealand: 35°S, 170°E — 100 species
• Source References: Laurance et al. (2001), Allen et al. (2007), Franklin and Dyrness (1988), Kricher (1998), Dawson and Lucas (2000).

Variability in Species Richness

• Species richness and composition vary by continent and are influenced by geographic locations. Communities of similar types differ in richness and composition depending on their location. Ecologists investigate the processes influencing these patterns through various spatial scales.

Hierarchical Interconnection of Spatial Scales

• Patterns of species diversity and composition at larger scales affect conditions at smaller scales.

Spatial Scales:

1. Global Scale:
   • Characteristics: Involves entire world where species are isolated by long distances over extended periods.
   • Influenced by rates of speciation, extinction, and dispersal affecting species diversity and composition, largely driven by continental drift, global climate patterns, and major evolutionary events.
2. Regional Scale:
   • Definition: Areas with uniform climates, limiting species dispersion to that area. These climatic and geographic boundaries define the potential pool of species able to exist within a broad region.
   • Regional Species Pool: Represents all species within a region (gamma diversity) providing the raw material for local assemblages and establishes a theoretical upper limit on community diversity.
3. Landscape Scale:
   • Definition: Considers topographic and environmental features of a region.
   • Shapes migration and extinction rates affecting species composition and diversity.
4. Local Scale:
   • Definition: Equivalent to a community, where species physiology and interspecies interactions affect species diversity (alpha diversity).
5. Beta Diversity:
   • Definition: The turnover of species from one community to another, serving as a link between local and regional scales.

Spatial Scale Examples

• Local and Regional Richness Examples:- Variations depend on the species and communities of interest. Local scales for terrestrial plants may be around $10^2 - 10^4 m^2$, whereas for bacteria, it could be $10^2 cm^2$.

Global Biogeography

• Control Factors: Global species diversity and composition are influenced by geographic area and isolation, evolutionary history, and global climate.
• Alfred Russel Wallace (1823–1913): Known as the father of biogeography for exploring species distribution at large scales. He independently conceived the theory of evolution by natural selection and is renowned for his extensive fieldwork and contributions to the understanding of species distribution patterns across the globe.
• Two Global Patterns Identified:
  1. Gradient of species diversity with latitude.
  2. Division of land masses into six biogeographic regions corresponding to Earth's tectonic plates.

Biogeographic Regions

• Regions:
  • Nearctic: North America, Greenland, and Central Mexican highlands; characterized by vast forests and deserts.
  • Neo-Tropical: South America, Central America, and the Caribbean; known for its exceptionally high biodiversity, including the Amazon rainforest.
  • Ethiopian: Africa south of the Sahara, including Madagascar and much of the Arabian Peninsula; home to unique savanna ecosystems and megafauna.
  • Palearctic: Europe, North Asia, parts of North Africa, and the Middle East; the largest biogeographic region, encompassing diverse temperate and boreal biomes.
  • Oriental: The Indian subcontinent and Southeast Asia; rich in tropical forests and diverse Asian wildlife.
  • Australasian: Australia, New Zealand, New Guinea, and surrounding islands; notable for its unique marsupial and monotreme fauna, a result of long geological isolation.

Continental Drift and Its Legacy

• Pangaea: The supercontinent that existed during the Permian period (251 mya) and its subsequent division into Laurasia and Gondwana during the Cretaceous period.
• Vicariance: Evolutionary separation of species due to geographical barriers (e.g., continental drift).
• Example: Flightless birds (ratites) share a common ancestor from Gondwana, evolved unique features but retained large size and flightless characteristics. Examples include ostriches, emus, rheas, and kiwis, whose distribution across southern continents provides strong evidence for Gondwanan vicariance.

Latitudinal Gradient in Diversity

• Historical Documentations: The latitudinal gradient showing increased species diversity towards lower latitudes has been consistent over 200 years.
• Study Findings (Willig et al. 2003): Analyzed 162 studies across taxonomic groups, revealing that negative relationships between latitude and diversity are most common.

Study Breakdown:

• Negative relationship (higher latitude = lower diversity)
• Positive relationship (higher latitude = higher diversity)
• No relationship (consistent diversity across latitudes)
• Unimodal relationship (diversity peaks at middle latitudes).

Examples of Species Diversity Variation

• Global Seabird Diversity by Latitude: Reported number of species shows sharper decline at higher latitudes.
• Gaston et al. (1995) Study Findings: Indicated that species families rise at lower latitudes, influenced by longitudinal factors, identifying biodiversity hotspots.

Hypotheses on Patterns of Species Richness

1. Species Diversification Rate:
   • Tropics have large land areas and stable temperatures reducing extinction; species speciation is more likely due to geographic isolation. The larger land area in the tropics provides more habitat diversity and greater opportunities for allopatric speciation due to geographic isolation. Stable temperatures reduce extinction rates, contributing to higher net diversification.
   • Land Area: Appears to influence how species diversity corresponds with climatic zones.
2. Species Diversification Time:
   • The climatic stability in the tropics fosters longer evolutionary timelines for species diversification compared to temperate/polar regions. The consistent, warm, and moist climates in tropical regions have provided longer, uninterrupted periods for species to evolve and diversify compared to temperate and polar regions which have experienced more frequent and severe climatic fluctuations, such as glacial cycles. This longer evolutionary time allows for more speciation events to accumulate.
   • Species may originate in the tropics and migrate to higher latitudes during warm climatic periods, but the tropics often act as an 'evolutionary cradle' and a 'museum' where high speciation and low extinction rates allow diversity to accumulate over long periods.
3. Productivity or Carrying Capacity:
   • High productivity in tropics leads to larger populations, decreasing extinction rates and fostering higher species richness. High primary productivity in the tropics supports larger populations, which tend to have lower extinction probabilities. This leads to an accumulation of species over time, filling available niches.
   • Notably, some productive habitats like estuaries show low species diversity due to other ecological factors; however, factors such as disturbance regimes, nutrient availability, and specific physiological constraints can override the effect of high productivity in certain systems.

Regional Biogeography

• Species–Area Relationship: Species richness increases with area sampled, influenced by area and distance affecting immigration and extinction rates.
• Species–Area Curves: The relationship between species richness $S$ and area $A$ is typically described by the power law: $S = cA^z$, where $c$ is a constant and $z$ represents the slope of the curve on a log-log plot, indicating how species richness increases with area. This relationship can be linearized for statistical analysis using a logarithmic transformation: $\log(S) = \log(c) + z \log(A)$, allowing for estimation through linear regression.

Island Biogeography Theory

• Definition: Any isolated habitat (including fragments) behaves like an island.
• Species Diversity Trends: Larger islands host more species than smaller islands.
• MacArthur and Wilson Theory: Equilibrium theory indicates species number on an island balances immigration rates and extinction rates; factors include island size and distance to mainland sources of species and the dispersal ability of species.

Observational Studies

• Data from volcanic island Krakatau showed predictions of migration and extinction possible; after eruptions, re-colonization was observed. The equilibrium species count reached close to theoretical predictions.
• Simberloff and Wilson (1969) Manipulation of Mangrove Islands: Removal of insects showed subsequent recolonization patterns aligned with proximity to species sources.

Consequences of Habitat Fragmentation

• Amazon Rainforest Fact: One hectare contains more plant species than all of Europe. The number of fish species in the Amazon River exceeds those in the Atlantic Ocean.
• Deforestation Impacts: Around 15% of rainforest has been cleared, leading to direct habitat loss, reduced population sizes, genetic isolation, and increased susceptibility to stochastic extinction events. Fragmentation particularly affects species that require large, contiguous habitats.
• Biological Dynamics of Forest Fragments Project (BDFFP): Initiated to explore minimum rainforest size necessary for species diversity maintenance.- Importance of forest sizes and proximity in maintaining species diversity was highlighted; even larger fragments showed significant species loss.

Edge Effects in Fragmented Habitats

• Edge effects: Increased exposure to light, temperature fluctuations, wind, and predation from generalist species, as well as disturbances like fire, penetrates into forest fragments from their edges. This microclimatic alteration and increased disturbance can lead to significant changes in species composition, favoring disturbance-tolerant species and contributing to the local extinction of sensitive interior species.
• Regeneration leads to secondary succession, potentially altering edge effects, but often towards a more degraded or altered state, rather than a full return to original forest conditions.
• Southern Amazon Issues: Non-native plantations impacting edge effects due to practices such as burning, which promote disturbance-sensitive species, further exacerbating the impacts of fragmentation and increasing fire risk within fragments.