Ecology and Ecosystems

Ecology is the study of the interactions between organisms and their environment. At A-Level, you need to understand how energy flows through ecosystems, how nutrients are recycled, how populations change over time, and how human activities affect ecological systems. This topic draws together concepts from photosynthesis and respiration and applies them to the scale of whole communities and ecosystems.

Key Definition: An ecosystem is a community of living organisms (biotic components) interacting with each other and with their non-living (abiotic) environment, within a defined area. It includes all the energy flows and nutrient cycles that sustain it.

---

Key Ecological Terms

Before exploring ecological concepts in depth, it is essential to define the core terminology precisely, as examiners frequently test whether students can distinguish between these terms.

Term	Definition
Population	All the organisms of one species living in a particular area at a particular time
Community	All the populations of different species living and interacting in a particular area at a particular time
Habitat	The place where an organism lives; the physical environment in which a species is found
Ecological niche	The role of an organism within its ecosystem, including its habitat, its feeding relationships, its interactions with other species, and the abiotic conditions it requires
Biotic factors	Living components that affect an organism, such as predation, competition, disease, food availability, and symbiotic relationships
Abiotic factors	Non-living components that affect an organism, such as temperature, light intensity, water availability, pH, mineral ion concentration, and wind speed

Key Definition: An organism's ecological niche describes not just where it lives, but how it lives — its role in the community, including what it eats, what eats it, and its interactions with both biotic and abiotic factors.

The competitive exclusion principle (Gause's principle) states that no two species can occupy exactly the same ecological niche in the same habitat at the same time. If two species compete for identical resources, one will inevitably outcompete the other, leading to competitive exclusion — the weaker competitor will be eliminated from that habitat. In practice, species often reduce competition through resource partitioning, dividing the niche so that each species uses a slightly different subset of resources.

---

Food Chains, Food Webs, and Trophic Levels

Key Definition: A food chain is a linear sequence showing the transfer of energy from one organism to the next, beginning with a producer. A food web is a network of interconnected food chains within an ecosystem.

Trophic Levels

Energy enters an ecosystem as sunlight and is converted into chemical energy by producers (autotrophs, primarily photosynthetic organisms). Energy then flows through a series of trophic levels:

Trophic Level	Description	Examples
Producer (T1)	Organisms that synthesise organic molecules from inorganic sources using light energy (photosynthesis) or chemical energy (chemosynthesis)	Green plants, algae, cyanobacteria
Primary consumer (T2)	Herbivores that feed on producers	Rabbits, caterpillars, zooplankton
Secondary consumer (T3)	Carnivores that feed on primary consumers	Foxes, small birds, frogs
Tertiary consumer (T4)	Carnivores that feed on secondary consumers	Hawks, large predatory fish
Decomposers	Organisms that break down dead organic matter (detritus) and waste products, releasing inorganic nutrients back into the soil or water	Bacteria, fungi (saprotrophs)

A described diagram of a food web in a British woodland might show oak trees as producers, with arrows pointing from oak to caterpillars (primary consumers), from caterpillars to blue tits (secondary consumers), and from blue tits to sparrowhawks (tertiary consumers). Additional chains would branch off: oak acorns to squirrels to foxes; leaf litter to earthworms to blackbirds to sparrowhawks. Arrows always point in the direction of energy transfer — from the organism being eaten to the one doing the eating.

Exam Tip: Arrows in food chains and webs show the direction of energy transfer, not "what eats what". The arrow points FROM the organism being consumed TO the consumer. Getting this the wrong way round is a very common mistake that will lose marks.

---

Energy Transfer Through Ecosystems

Gross Primary Productivity (GPP) and Net Primary Productivity (NPP)

Key Definition: Gross primary productivity (GPP) is the total rate of energy fixation (photosynthesis) by producers in an ecosystem, measured in kJ m⁻² yr⁻¹. Net primary productivity (NPP) is the rate of energy storage in plant biomass, available to the next trophic level. NPP = GPP − R (where R is the energy lost through plant respiration).

The relationship is:

NPP = GPP − R

• GPP represents the total energy fixed by photosynthesis.
• R represents the energy used by the plant itself for respiration (growth, maintenance, reproduction, active transport).
• NPP is the energy that remains stored in plant biomass and is therefore available for consumption by primary consumers.

Similarly, at each subsequent trophic level, the energy available to the next level is reduced because organisms use energy for their own respiration.

Efficiency of Energy Transfer Between Trophic Levels

Only a small proportion of energy is transferred from one trophic level to the next — typically 5–20% in most ecosystems (often quoted as approximately 10% as a rough average).

Energy is lost between trophic levels because:

1. Respiration — organisms use a large proportion of the energy they assimilate for metabolic processes (movement, maintaining body temperature in endotherms, active transport, biosynthesis). This energy is ultimately released as heat.
2. Not all biomass is consumed — parts of organisms are not eaten (e.g., roots of plants, bones and fur of animals).
3. Not all consumed biomass is assimilated — some passes through the gut and is egested as faeces (uneaten material that was ingested but not absorbed across the gut wall).
4. Excretory losses — urea (from deamination of excess amino acids) and other metabolic waste products contain chemical energy that is lost from the organism.

These losses explain why food chains rarely have more than four or five trophic levels — by the fourth or fifth level, too little energy remains to support a viable population.

Worked Example 1 — Energy Transfer Efficiency:

In a grassland ecosystem, the gross primary productivity (GPP) of the grass is 20,000 kJ m⁻² yr⁻¹. The grass uses 12,000 kJ m⁻² yr⁻¹ in respiration. Primary consumers (rabbits) ingest 2,000 kJ m⁻² yr⁻¹ of plant biomass, of which 500 kJ m⁻² yr⁻¹ is lost in faeces and 1,100 kJ m⁻² yr⁻¹ is lost in respiration.

(a) Calculate the NPP of the grass. (b) Calculate the percentage of NPP consumed by the rabbits. (c) Calculate the energy available to the secondary consumer (fox). (d) Calculate the efficiency of energy transfer from producer to primary consumer.

Solution:

(a) NPP = GPP − R = 20,000 − 12,000 = 8,000 kJ m⁻² yr⁻¹

(b) Percentage consumed = (2,000 / 8,000) × 100 = 25%

(c) Energy assimilated by rabbits = ingested − faeces = 2,000 − 500 = 1,500 kJ m⁻² yr⁻¹ Energy available to fox = assimilated − respiration = 1,500 − 1,100 = 400 kJ m⁻² yr⁻¹

(d) Efficiency = (energy available to next trophic level / energy available at current trophic level) × 100 Efficiency = (400 / 8,000) × 100 = 5.0%

Exam Tip: Energy transfer calculations are very common in A-Level exams. Always show each step clearly and state the formula you are using. The most common error is confusing GPP with NPP, or forgetting to subtract faecal losses before calculating respiration losses. Remember: efficiency = (energy in to next level / energy in from previous level) × 100.

---

Nutrient Cycling

Unlike energy, which flows through an ecosystem in one direction (from sunlight to heat), nutrients are recycled — the same atoms are used again and again.

The Carbon Cycle

A described diagram of the carbon cycle would show several interconnected processes. At the centre, CO₂ in the atmosphere connects to all other compartments. Arrows would show:

• Photosynthesis removes CO₂ from the atmosphere and fixes it into organic molecules in producers (plants, algae).
• Respiration by all living organisms (producers, consumers, decomposers) releases CO₂ back into the atmosphere.
• Feeding transfers carbon-containing organic molecules from one trophic level to the next.
• Death and excretion transfer organic carbon to the soil/water, where decomposers (bacteria and fungi) break it down through saprotrophic nutrition, releasing CO₂ by respiration.
• Fossilisation — over millions of years, dead organisms that are not fully decomposed may form fossil fuels (coal, oil, natural gas), locking carbon away in geological stores.
• Combustion of fossil fuels releases CO₂ back into the atmosphere (a process greatly accelerated by human activity since the Industrial Revolution).
• Dissolving — CO₂ dissolves in oceans and other bodies of water, forming carbonic acid and carbonate ions. Marine organisms use these to build calcium carbonate shells (e.g., coral, molluscs), which eventually form limestone — another geological carbon store.

flowchart TD
    ATM["CO₂ in Atmosphere"]
    ATM -->|"Photosynthesis"| Producers["Producers
(Plants, Algae)"]
    Producers -->|"Feeding"| Consumers["Consumers"]
    Producers -->|"Respiration"| ATM
    Consumers -->|"Respiration"| ATM
    Producers -->|"Death"| Dead["Dead organic matter"]
    Consumers -->|"Death & excretion"| Dead
    Dead -->|"Decomposition
(bacteria & fungi respire)"| ATM
    Dead -->|"Fossilisation
(millions of years)"| Fossil["Fossil Fuels
(coal, oil, gas)"]
    Fossil -->|"Combustion"| ATM
    ATM -->|"Dissolving"| Ocean["Oceans
(carbonate ions, shells → limestone)"]

The Nitrogen Cycle

Nitrogen is essential for the synthesis of amino acids, proteins, nucleic acids, and ATP. Atmospheric nitrogen (N₂) makes up 78% of the atmosphere but cannot be used directly by most organisms because the triple bond (N≡N) is extremely stable and requires a great deal of energy to break.

A described diagram of the nitrogen cycle would show the following processes:

1. Nitrogen fixation — the conversion of atmospheric N₂ into ammonia (NH₃) or ammonium ions (NH₄⁺):

• Biological nitrogen fixation by nitrogen-fixing bacteria:
  • Free-living bacteria in the soil (e.g., Azotobacter)
  • Mutualistic bacteria in root nodules of leguminous plants (e.g., Rhizobium in clover, peas, beans). The plant provides the bacteria with organic nutrients; the bacteria provide the plant with fixed nitrogen.
  • The enzyme nitrogenase catalyses this reaction: N₂ + 8H⁺ + 8e⁻ + 16ATP → 2NH₃ + H₂ + 16ADP + 16Pi
• Industrial fixation — the Haber process converts N₂ + 3H₂ → 2NH₃ (used to manufacture fertilisers).
• Lightning — provides enough energy to oxidise N₂ to nitrogen oxides, which dissolve in rainwater to form nitrates.

2. Ammonification — the decomposition of organic nitrogen compounds (proteins, nucleic acids, urea) in dead organisms and excretory products by saprotrophic bacteria and fungi, releasing ammonia (NH₃) or ammonium ions (NH₄⁺) into the soil.

Key Definition: Ammonification is the conversion of organic nitrogen (in proteins, nucleic acids, and urea) into ammonium ions (NH₄⁺) by decomposer microorganisms.