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What is an Ecosystem?

Every time you step into a forest or even look closely at a garden pond, you are witnessing a complex web where nothing survives entirely on its own. An is a biological community made up of interacting living organisms, known as components, and their physical, non-living environment, known as components.

factors include plants, animals, fungi, and bacteria.
factors include the climate, soil, water, light, and underlying rock.

The Web of Interdependence

The most crucial concept in an is . This means every component relies on the others to function, and a change in one will create a "knock-on" effect throughout the entire system.

The Impact of Change: Trophic Cascades

When one part of an is altered, it triggers a . For example, if a primary consumer (such as a caterpillar) is removed from a woodland :

Producers (grass and leaves) will increase in volume because they are no longer being eaten by the caterpillars.
Secondary Consumers (sparrows) will decrease because their primary food source has disappeared.
Tertiary Consumers (hawks) will decrease as well, or be forced to find alternative prey, potentially disrupting other food chains.

Nutrient Cycling and Energy Transfer

is the continuous movement of organic and inorganic matter between three main stores, often illustrated in a :

: Living plants and animals.
: Dead organic matter (e.g., fallen leaves) on the surface.
Soil: Nutrients held within the earth.

Decomposers like fungi and bacteria break down the store, releasing nutrients back into the soil as humus so plants can absorb them again. The relative sizes of these stores vary by :

Tropical Rainforest: is the largest store because of the massive volume of vegetation. Nutrients are recycled rapidly in the heat but are easily lost to if the trees are removed.
Hot Desert: Soil is the largest store (though it is nutrient-poor). The store is tiny because there is very little vegetation to die and decay.
Temperate Forest: The store is significant due to deciduous trees shedding leaves annually.

Energy enters the via sunlight and moves through : Producers $\rightarrow$ Primary Consumers $\rightarrow$ Secondary Consumers $\rightarrow$ Tertiary Consumers. At each stage, approximately 90% of the energy is lost through heat, respiration, and movement.

Worked Example

If the producers in a woodland generate 25,000 kJ of energy, how much energy will reach the secondary consumers? (Assume only 10% of energy is transferred to the next trophic level)

Step 1: Calculate the energy passed to primary consumers.

Energy at primary consumer level = $25{,}000 \times 0.10 = 2500$ kJ

Step 2: Calculate the energy passed from primary to secondary consumers.

Energy at secondary consumer level = $2500 \times 0.10 = 250$ kJ

Answer: 250 kJ reaches the secondary consumers.

Global Biome Distribution

Large-scale global are known as , and their locations are primarily driven by latitude and .

Low Pressure Belts: Where air rises, it cools and condenses, causing heavy rainfall. This occurs at the Equator (Tropical Rainforests) and at 60° N/S (Temperate Forests).
High Pressure Belts: Where air sinks, it creates dry, arid conditions. This occurs at 30° N/S (Hot Deserts) and at the poles at 90° N/S (Polar Regions).
: Inland areas far from the sea experience more extreme temperatures and lower rainfall than coastal regions, influencing the location of grasslands.

Biome Characteristics: Forests and Grasslands

Tropical Rainforests: Located at 0°–10° N/S (e.g., Amazon, Congo Basin). They have a constant heat of ~27°C and over 2,000mm of annual rainfall. The soil, called , is nutrient-poor due to heavy . Plants feature buttress roots, lianas, epiphytes, and drip tips.
Temperate Deciduous Forests: Found at 40°–60° N/S (e.g., UK). They experience four distinct seasons with 750–1,500mm of rain. Flora includes oak and beech trees that lose their leaves in autumn to conserve moisture.
Tropical Grasslands (Savanna): Located 15°–30° N/S (e.g., Kenya). Distinct wet and dry seasons with 800–900mm of rain. Flora includes water-storing baobab trees and fire-resistant acacia trees.
Temperate Grasslands: Found at ~40° N/S in continental interiors (e.g., Prairies). Extreme seasonal temperatures with moderate rainfall (500–900mm). Flora consists of short grasses and Big Bluestem.

Biome Characteristics: Extremes and Oceans

Hot Deserts: Located 15°–30° N/S (e.g., Sahara). <250mm of rain and massive diurnal temperature range. Soils are saline due to . Plants are , featuring succulents, tap roots, waxy cuticles, and ephemerals.
Polar Regions: Located at 66.5° to 90° N/S. Precipitation is low (<500mm Arctic / 50mm Antarctic). Extremely cold temperatures. Flora includes lichens, mosses, and the Arctic poppy.
Coral Reefs: Found between 30° N and 30° S. Require shallow (<60m), warm (23–29°C), clear, and saline conditions. Formed by living in a symbiotic relationship with algae.

For 'Interdependence' questions, you must provide a causal link. Don't just say species are connected; explain that if X decreases, Y will increase/decrease because of the specific relationship (e.g., loss of food source).

Common Exam Question: Compare nutrient cycling in two biomes. Use the Gersmehl diagram terms (Biomass, Litter, Soil) and state which store is largest and WHY (e.g., 'Biomass is largest in Rainforests due to high biodiversity').

Calculation Tip: In energy transfer questions, remember that energy is lost at each level. Ensure you apply the percentage reduction (usually 90% loss or 10% transfer) for every step between the producer and the target consumer.

When outlining biome distribution, always mention three things: latitude (e.g., 30° N/S), pressure belts (e.g., high pressure), and an example continent or region.

A biological community of interacting living organisms (biotic) and their physical, non-living environment (abiotic).

The living components of an ecosystem, such as plants, animals, fungi, and bacteria.

The non-living components of an ecosystem, such as climate, soil, water, light, and rock.

The reliance of every component within an ecosystem on the others, where a change in one triggers a knock-on effect elsewhere.

A process where a change at one trophic level of a food web results in a series of knock-on effects for other levels.

The exchange of organic and inorganic matter back into the production of living matter via three stores: Biomass, Litter, and Soil.

A diagram representing the three main nutrient stores in an ecosystem and the flow of nutrients between them.

The store of nutrients found in living plants and animals.

The store of nutrients found in dead organic matter on the soil surface.

The process where heavy rainfall washes essential minerals and nutrients down through the soil profile.

The hierarchical levels in an ecosystem comprising organisms that share the same nutritional relationship to the primary sources of energy.

A very large-scale global ecosystem characterized by its distinct climate, flora, and fauna.

The large-scale movement of air that distributes heat and moisture across the Earth via cells (Hadley, Ferrel, Polar).

The climatic effect of being far from the sea, resulting in more extreme seasonal temperatures and lower rainfall.

Deep, red, nutrient-poor soils found in tropical rainforests.

Plants that are specially adapted to survive in extremely dry, arid environments.

The accumulation of salts in the soil as water evaporates, common in arid environments.

Tiny marine invertebrates that build coral reefs and live in symbiosis with algae.

Microscopic algae that live inside coral polyps and provide energy via photosynthesis.