2.4 Biomes, zonation and succession Notes

Biomes and Climate

Biomes are large-scale ecological zones characterized by distinct plant and animal communities adapted to specific climate conditions. The distribution of biomes across the Earth is primarily determined by climate factors, particularly temperature and precipitation patterns.

Climate as the Primary Determinant

The type of biome in a given area is largely dictated by its climate. However, it's important to note that individual ecosystems within a biome may vary due to local abiotic and biotic factors. For instance, while the Amazon rainforest is broadly classified as a tropical rainforest biome, specific areas within it may have unique characteristics due to factors like soil composition, elevation, or the presence of rivers.

Key Climatic Factors

The main factors governing the distribution of biomes are:

1. Insolation: The amount of solar radiation received by an area
2. Precipitation: The quantity and pattern of rainfall or snowfall
3. Temperature: Average temperatures and temperature ranges

These factors interact in complex ways to create the conditions that support different biome types.

Tricellular Model of Atmospheric Circulation

The tricellular model of atmospheric circulation is crucial for understanding the global distribution of biomes. This model explains how air circulates in three distinct cells in each hemisphere, influencing precipitation and temperature patterns worldwide.

How It Works

1. Hadley Cell: Near the equator, warm air rises, creating a low-pressure zone. This air moves towards the poles at high altitudes and descends around 30° latitude, creating high-pressure zones.
2. Ferrel Cell: Between 30° and 60° latitude, air circulates in the opposite direction to the Hadley cell.
3. Polar Cell: Cold air descends at the poles and moves towards the equator at the surface, rising again around 60° latitude.

This circulation pattern leads to distinct precipitation and temperature zones:

• Tropical rainforests near the equator (high precipitation, warm temperatures)
• Deserts around 30° latitude (low precipitation, high temperatures)
• Temperate forests and grasslands in mid-latitudes
• Tundra and ice caps near the poles (low precipitation, cold temperatures)

Climate Change and Biome Shifts

Climate change is causing significant alterations in the distribution of biomes worldwide. As global temperatures rise and precipitation patterns change, many biomes are experiencing shifts in their boundaries and characteristics.

Examples of Biome Shifts

1. Arctic Tundra: Warming temperatures are causing the tree line to move northward, encroaching on tundra ecosystems.
2. Tropical Rainforests: Changes in rainfall patterns are affecting the extent and composition of rainforests in some areas.
3. Deserts: Some arid regions are expanding due to increased temperatures and reduced rainfall.

Zonation in Ecosystems

Zonation refers to the change in community composition along an environmental gradient. This concept is crucial for understanding how species distribute themselves within a larger ecosystem.

Key Factors Influencing Zonation

1. Altitude: Changes in elevation lead to variations in temperature, precipitation, and air pressure.
2. Latitude: Similar to altitude, but on a global scale.
3. Tidal level: In coastal ecosystems, the degree of exposure to seawater creates distinct zones.
4. Distance from shore: In aquatic ecosystems, depth and light penetration change as you move away from the shore.

Succession in Ecosystems

Succession is the process of change in the species structure of an ecological community over time. It involves a series of predictable changes that occur in an ecosystem following a disturbance.

Types of Succession

1. Primary Succession: Occurs in areas where no soil or previous ecosystem existed, such as newly formed volcanic islands or areas exposed by retreating glaciers.
2. Secondary Succession: Takes place in areas where an ecosystem previously existed but was disrupted, such as after a forest fire or agricultural abandonment.

Stages of Succession

1. Pioneer Communities: The first organisms to colonize an area, typically hardy species adapted to harsh conditions.
2. Intermediate Communities: As conditions improve, more complex species begin to establish themselves.
3. Climax Communities: The final stage of succession, characterized by a stable, self-perpetuating community of organisms.

Changes During Succession

As succession progresses, several ecosystem characteristics change:

1. Energy Flow: Typically increases and becomes more complex.
2. Productivity: Often increases as the community becomes more established.
3. Diversity: Generally increases, peaking in intermediate stages.
4. Mineral Cycling: Becomes more efficient and closed as the ecosystem matures.

Habitat Diversity and Biodiversity

A key principle in ecology is that greater habitat diversity leads to greater species and genetic diversity. This relationship is fundamental to understanding biodiversity patterns and conservation strategies.

Mechanisms

1. Niche Differentiation: More diverse habitats provide a wider range of niches, allowing more species to coexist.
2. Edge Effects: The boundaries between different habitat types often support unique species assemblages.
3. Microhabitats: Complex habitats offer a variety of microenvironments, each potentially supporting different species.

Reproductive Strategies: r- and K-Selection

Species have evolved different reproductive strategies that are better suited to different stages of succession and environmental conditions.

r-Selected Species

• Characteristics: Rapid reproduction, short lifespan, small body size, early maturity
• Adapted to: Unstable or unpredictable environments, pioneer communities
• Examples: Bacteria, insects, weeds

K-Selected Species

• Characteristics: Slower reproduction, longer lifespan, larger body size, later maturity
• Adapted to: Stable environments, climax communities
• Examples: Elephants, whales, trees

Alternative Stable States

The concept of alternative stable states challenges the traditional view of a single climax community. It suggests that ecosystems can exist in multiple stable configurations under the same environmental conditions.

Key Points

1. Multiple Equilibria: An ecosystem can have more than one stable state, each with its own characteristic species composition and ecological processes.
2. State Shifts: Ecosystems can shift between these states due to disturbances or changes in environmental conditions.
3. Hysteresis: The path an ecosystem takes to return to a previous state may be different from the path it took to leave that state.

Human Impact on Succession and Ecosystem Stability

Human activities can significantly influence succession processes and ecosystem stability, often diverting ecosystems to alternative stable states.

Ways Humans Modify Ecosystems

1. Land Use Changes: Deforestation, urbanization, and agriculture can reset succession or create novel ecosystems.
2. Species Introductions: Invasive species can alter succession pathways and ecosystem dynamics.
3. Climate Change: Altering temperature and precipitation patterns can affect the trajectory of succession.
4. Pollution: Chemical pollutants can change soil or water conditions, influencing which species can thrive.

Ecosystem Resilience and Diversity

An ecosystem's capacity to survive change and recover from disturbances (its resilience) is closely linked to its diversity.

Relationship Between Diversity and Resilience

1. Functional Redundancy: More diverse ecosystems are likely to have multiple species that can perform similar ecological roles, providing a buffer against species loss.
2. Response Diversity: Different species may respond differently to environmental changes, increasing the chances that some will survive and maintain ecosystem functions.
3. Adaptive Capacity: Greater genetic diversity within species populations can enhance their ability to adapt to changing conditions.

In conclusion, understanding biomes, zonation, and succession is crucial for comprehending the complex dynamics of ecosystems. These concepts are interconnected and influenced by both natural processes and human activities. As climate change continues to impact global ecosystems, the ability to analyze and predict changes in biome distribution, succession patterns, and ecosystem stability becomes increasingly important for ecological management and conservation efforts.