Nutrient Cycling in Rainforests and Savannas

This document provides detailed notes on nutrient cycling in rainforests and savannas, covering Gershmehl diagrams, soil fertility, energy flows, and trophic levels. These notes are designed for Cambridge A-Level Geography (9696).

Gershmehl Diagrams

Gershmehl diagrams are conceptual models that illustrate the flow of matter and energy through ecosystems. They are particularly useful for comparing nutrient cycling in different environments.

Rainforest Gershmehl Diagram

The rainforest Gershmehl diagram emphasizes rapid nutrient cycling and low nutrient availability in the soil. Key features include:

• Rapid decomposition of organic matter
• High rates of nutrient uptake by plants
• Minimal nutrient loss through leaching
• Nutrients are primarily stored in the biomass (living organisms)

Savanna Gershmehl Diagram

The savanna Gershmehl diagram highlights slower nutrient cycling and nutrient loss through leaching. Characteristics include:

• Slower decomposition rates compared to rainforests
• Nutrient loss through leaching, especially in areas with seasonal rainfall
• Nutrients are more concentrated in the soil than in biomass
• More significant nutrient loss through fire

Soil Fertility

Soil fertility refers to the capacity of soil to support plant growth. Nutrient availability is a crucial factor determining soil fertility.

Rainforest Soil Fertility

Rainforest soils are generally nutrient-poor despite the lush vegetation. This is because:

• Heavy rainfall leaches nutrients from the soil
• Rapid decomposition quickly releases nutrients, but they are immediately taken up by plants
• Nutrients are largely held within the biomass

Savanna Soil Fertility

Savanna soils are also often nutrient-poor, particularly in areas with heavy rainfall. Leaching is a significant factor.

• Seasonal rainfall can lead to significant nutrient loss through leaching
• Fire can deplete soil nutrients
• Nutrient availability is often linked to the decomposition of grasses and other herbaceous plants

Energy Flows and Trophic Levels

Energy flows through ecosystems in a unidirectional manner, typically starting with the sun and passing through trophic levels.

Trophic Levels

Trophic levels represent the position an organism occupies in a food chain. Common trophic levels include:

1. Producers: Autotrophs (e.g., plants) that convert light energy into chemical energy through photosynthesis.
2. Primary Consumers: Herbivores that eat producers.
3. Secondary Consumers: Carnivores that eat primary consumers.
4. Tertiary Consumers: Carnivores that eat secondary consumers.
5. Decomposers: Organisms (e.g., bacteria, fungi) that break down dead organic matter and waste, returning nutrients to the soil.

Energy Flow Efficiency

The 10% rule states that only approximately 10% of the energy stored at one trophic level is transferred to the next. This is due to energy loss as heat during metabolic processes.

Rainforest Energy Flow

Rainforests have high primary productivity due to abundant sunlight and warm temperatures. However, energy flow is relatively efficient because of the large biomass and rapid nutrient cycling. There are typically multiple trophic levels.

Savanna Energy Flow

Savannas have lower primary productivity than rainforests due to seasonal rainfall and grazing. Energy flow can be less efficient, and there may be fewer trophic levels compared to rainforests.

Trophic Level	Example Organism	Energy Input
Producers	Trees, Grasses	Sunlight
Primary Consumers	Herbivores (e.g., Deer, Zebra)	Producers
Secondary Consumers	Carnivores (e.g., Lions, Eagles)	Primary Consumers
Tertiary Consumers	Top Predators	Secondary Consumers
Decomposers	Bacteria, Fungi	Dead Organic Matter

Comparison Table

Feature	Rainforest	Savanna
Nutrient Availability	Low in soil, high in biomass	Low in soil, more concentrated in soil
Decomposition Rate	Rapid	Slower
Leaching	Minimal	Significant, especially in wet seasons
Primary Productivity	High	Lower
Biomass	Very high	Lower
Gershmehl Diagram Emphasis	Rapid nutrient cycling	Slower nutrient cycling, nutrient loss