Biomes and Ecosystems

Sonoran Desert and Saguaro National Park

The Sonoran Desert is a major North American desert ecosystem, characterized by unique plant and animal communities adapted to arid conditions. Saguaro National Park in Arizona is a representative site within this biome, known for its iconic Carnegiea gigantea (saguaro cactus) and diverse desert flora and fauna.

• Desert Biome: Defined by low precipitation, high temperature variability, and specialized vegetation such as cacti and succulents.

• Geographic Location: The Sonoran Desert spans parts of Arizona, California, and northwestern Mexico, typically between 25-35 degrees latitude.

• Ecological Importance: Supports unique adaptations for water conservation and temperature regulation.

• Example: Saguaro cacti store water in their tissues and have shallow, widespread roots to maximize water uptake after rare rainfalls.

Speciation and Biogeography

Allopatric Speciation in Freshwater Fishes

Allopatric speciation is the process by which new species arise due to geographic isolation. In North American freshwater fishes, this often occurs when populations are separated by physical barriers such as rivers, mountains, or glaciation events.

• Allopatric Speciation: The formation of new species from populations that are geographically isolated from each other.

• Mechanisms: Isolation prevents gene flow, allowing populations to diverge genetically through mutation, natural selection, and genetic drift.

• Applications: Understanding speciation helps explain patterns of biodiversity and endemism in freshwater ecosystems.

• Example: Glacial cycles in North America have repeatedly isolated and reconnected fish populations, leading to speciation events.

Major Biomes of Earth

Definition and Classification

Biomes are large ecological regions defined by their dominant vegetation, climate, and characteristic animal communities. They are shaped primarily by abiotic factors such as temperature and precipitation.

• Key Abiotic Factors: Average annual temperature, average annual precipitation, and their seasonal variation.

• Examples of Biomes: Tropical wet forests (rainforests), deserts, temperate forests, grasslands, tundra.

• Vegetation: Each biome is characterized by specific plant growth forms adapted to the local climate.

• Net Primary Productivity (NPP): The rate at which plants convert solar energy into biomass; highest in tropical rainforests, lowest in deserts and tundra.

Climate and Biome Distribution

The distribution of biomes is determined by global climate patterns, which are influenced by latitude, seasonality, and geographic features.

• Latitude: Equatorial regions receive more solar energy and have higher biodiversity.

• Seasonality: Some biomes, like tropical rainforests, have little temperature variation but may have wet and dry seasons.

• Precipitation: Determines whether a region supports forests, grasslands, or deserts.

Species Distribution and Biogeography

Island Biogeography Theory

The theory of island biogeography, developed by MacArthur and Wilson, explains how species richness on islands (or isolated habitats) is determined by immigration and extinction rates, which are influenced by island size and distance from the mainland.

• Immigration Rate: Higher on islands closer to the mainland; decreases as the number of species increases.

• Extinction Rate: Lower on larger islands; increases as the number of species increases due to competition.

• Equilibrium Number of Species: The point where immigration and extinction rates balance.

• Application: The theory applies to any isolated habitat, such as lakes, mountaintops, or forest fragments.

Latitudinal Gradients in Biodiversity

Biodiversity is generally highest at the equator and decreases toward the poles. This pattern is influenced by both abiotic and biotic factors.

• Abiotic Drivers: Greater solar energy and stable climates near the equator promote higher productivity and less harsh conditions.

• Biotic Drivers: More intense species interactions and niche availability in the tropics lead to greater species diversity.

• Hypotheses: High speciation rates, low extinction rates, and high immigration contribute to equatorial diversity.

Key Terms and Concepts

• Biome: A large ecological community defined by climate, vegetation, and animal life.

• Allopatric Speciation: The formation of new species due to geographic isolation.

• Net Primary Productivity (NPP): The amount of biomass produced by plants per unit area per year.

• Island Biogeography: The study of species composition and richness on islands or isolated habitats.

• Latitudinal Gradient: The pattern of increasing biodiversity from the poles to the equator.

Summary Table: Factors Affecting Species Richness

Key Equations

• Net Primary Productivity:

Where: = Net Primary Productivity = Gross Primary Productivity = Respiration by plants

• Island Biogeography Equilibrium:

= Equilibrium number of species (where immigration rate equals extinction rate)

Additional info: These notes synthesize key concepts from ecology and evolutionary biology, focusing on biomes, biogeography, and mechanisms of speciation, as presented in the provided materials.