Ecosystems¶

Part of Module 6: Genetics, evolution and ecosystems.

An ecosystem is the complete system of living organisms and their physical environment in a particular place. Understanding ecosystems requires tracking two things simultaneously: the flow of energy through trophic levels, and the cycling of materials (particularly carbon and nitrogen) between living organisms and the abiotic environment. This topic also covers succession — how communities change over time — and the sampling methods used to study them.

Learning Objectives¶

What Is an Ecosystem?¶

No two species can occupy exactly the same niche: if they do, one will outcompete and exclude the other (competitive exclusion principle).

Succession¶

Succession is the directional change in the composition of a community over time. As each community of organisms modifies the environment, different species become better suited to the changed conditions, and the community changes in composition.

Primary Succession¶

Primary succession occurs on land that has never been colonised before, or where all life has been completely removed (bare rock, newly formed volcanic island, sand dunes, a new glacial outwash area).

Stages:

1. Pioneer community: Harsh, nutrient-poor conditions allow only highly adapted species to survive. Pioneer species (e.g. lichens, cyanobacteria on bare rock) colonise first. They can fix nitrogen, tolerate extreme conditions, and need no existing soil.

2. Soil formation: When pioneers die, their organic matter is decomposed by microorganisms. This, combined with weathering of the rock, begins to form a thin humus layer. Humus improves moisture retention and mineral availability.

3. Colonisation by more complex species: Better soil conditions allow mosses and small herbaceous plants to establish. These shade the pioneers and compete them out.

4. Progressive change: As soil deepens and improves, larger plants (shrubs, then trees) can establish. Each stage modifies the soil and microclimate, allowing the next successional stage to replace it.

5. Climax community: Eventually a stable, self-sustaining climax community is established — the final seral stage. In the UK, the climax community on most fertile soils would be temperate deciduous woodland (dominated by oak, ash or beech). The climax community is characterised by:

6. High species diversity
7. Complex food webs
8. Relatively stable populations
9. Organic matter input balanced by decomposition

Deflected Succession and Plagioclimax¶

Human activity can interrupt succession and prevent an ecosystem from reaching its natural climax community. This is called deflected succession. The final community that results is known as a plagioclimax — it is maintained by continued human intervention (e.g. grazing, burning, cultivation).

Examples of activities that cause deflected succession: - Continuous grazing by livestock preventing tree growth - Regular mowing of grassland preventing scrub development - Burning land to clear forest

Remove the human activity and succession will resume toward the climax community.

Secondary Succession¶

Secondary succession occurs in a previously colonised area where an existing community has been partially or wholly destroyed, but the soil remains (e.g. after forest fire, flood, agricultural abandonment, clear-felling).

Because soil is already present, succession starts at a later seral stage and proceeds faster than primary succession. Annual weeds colonise first, followed by perennial plants, shrubs and eventually trees.

General Trend During Succession¶

Energy Flow Through Ecosystems¶

Trophic Levels and Energy Transfer¶

Organisms are grouped into trophic levels based on how they obtain energy: - Producers (trophic level 1): Photosynthetic organisms (plants, algae) that fix light energy into organic molecules - Primary consumers (TL2): Herbivores that eat producers - Secondary consumers (TL3): Carnivores that eat herbivores - Tertiary consumers (TL4): Carnivores that eat secondary consumers - Decomposers: Bacteria and fungi that break down dead organic matter at all levels; return mineral ions to the soil

Energy is lost at each trophic level transfer: - Not all of the prey's biomass is consumed (uneaten parts) - Not all consumed material is digested and absorbed (egested as faeces) - Much absorbed energy is used in respiration (heat is lost) - Some is lost in excretion (urine, urea)

Only approximately 10% of energy is typically transferred to the next trophic level (efficiency varies from ~5–20%). This is why food chains rarely exceed 4–5 links (there is insufficient energy to support a 6th trophic level).

Measuring Energy Stored in Biomass: Calorimetry¶

Biomass is the total mass of living material present at a particular time in a specific place. It serves as an indicator of energy content, measured in g m⁻² on land or g m⁻³ in aquatic environments.

To estimate the chemical energy stored in dry biomass, calorimetry is used:

1. Dry the biomass sample until its mass no longer changes (removes water, which contributes no energy)
2. Weigh the dry mass
3. Burn the sample in a calorimeter
4. Measure the volume and temperature change of the surrounding water
5. Calculate the heat energy released: Q = mcΔT (where m = mass of water, c = specific heat capacity, ΔT = temperature change)

This gives an estimate of the energy content per unit dry mass (e.g. kJ g⁻¹).

Gross and Net Primary Productivity¶

Gross primary productivity (GPP) is the total rate of photosynthesis — the total chemical energy fixed by producers per unit area per unit time.

Net primary productivity (NPP) is the energy remaining in plant biomass after the plant has used some for its own respiration:

NPP = GPP − R

Where R = energy used by the plant in respiration.

NPP represents the energy available for all other trophic levels (herbivores, decomposers). It is the true measure of a habitat's productivity.

Units: kJ m⁻² year⁻¹ or g dry mass m⁻² year⁻¹

Worked example: A forest has a GPP of 22,500 kJ m⁻² year⁻¹ and respiratory losses of 9,500 kJ m⁻² year⁻¹. NPP = 22,500 − 9,500 = 13,000 kJ m⁻² year⁻¹.

Net production of consumers (N):

N = I − (F + R)

Where: - I = chemical energy in ingested food - F = chemical energy lost in faeces (undigested material; egestion, not excretion) - R = respiratory losses (heat) - N = net production available for growth and reproduction

Ecological Efficiency¶

Ecological efficiency is the percentage of energy (or biomass) transferred from one trophic level to the next:

Ecological efficiency = (energy available after transfer ÷ energy available before transfer) × 100%

To compare trophic levels, use their net production or NPP values.

Worked example: The NPP of plants in a grassland is 36,000 kJ m⁻² year⁻¹. Rabbits in the same area have a net production of 4,800 kJ m⁻² year⁻¹.

Ecological efficiency = (4,800 ÷ 36,000) × 100% = 13.3%

This is consistent with the approximate 10% rule (efficiency ranges from ~5–20% depending on the ecosystem).

Agriculture and Trophic Level Manipulation¶

Human agriculture manipulates the transfer of energy through ecosystems by: - Creating simple food chains with fewer trophic levels (e.g. eating crops directly rather than feeding them to livestock first) - Reducing energy lost to non-food organisms by removing competing species, using pesticides, and controlling disease - Keeping livestock in heated or confined conditions to minimise energy lost to thermoregulation and movement

Every additional trophic level reduces the energy available to humans by ~90%. Eating plant crops directly is therefore far more efficient than eating animals fed on those crops.

The Nitrogen Cycle¶

Nitrogen is an essential element in amino acids, proteins, nucleotides and ATP. Although the atmosphere is ~78% nitrogen gas (N₂), most organisms cannot use N₂ directly. Nitrogen must first be fixed — converted into a usable form (NH₄⁺ or NO₃⁻).

Key Processes and Organisms¶

Rhizobium and Mutualism¶

Rhizobium bacteria live in root nodules of leguminous plants (peas, beans, clover, soya). The relationship is mutualistic: - The bacteria benefit by receiving glucose (from plant photosynthesis) as their carbon and energy source - The plant benefits by receiving fixed nitrogen compounds (ammonium ions) from bacterial nitrogen fixation

The bacteria use an enzyme called nitrogenase to reduce N₂ to NH₃. Nitrogenase is sensitive to oxygen, so the root nodules contain leghaemoglobin (a red oxygen-binding protein produced jointly by plant and bacteria) to maintain a low oxygen environment inside the nodule while still supplying enough oxygen for aerobic respiration.

The Carbon Cycle¶

Carbon is a component of all organic molecules. It cycles between the atmosphere (as CO₂), living organisms and geological stores:

Human activities (burning fossil fuels, deforestation) are increasing the rate of CO₂ release faster than the natural cycle can remove it, leading to increasing atmospheric CO₂ and climate change.

Fluctuations in Atmospheric CO₂¶

Atmospheric CO₂ levels fluctuate on three timescales:

Daily fluctuations: - Respiration occurs continuously throughout the day and night, releasing CO₂ - Photosynthesis only occurs during daylight hours, removing CO₂ - CO₂ levels therefore fall during the day and rise at night

Seasonal fluctuations: - In summer, long days and warm temperatures increase photosynthesis rates → CO₂ levels fall - In winter, shorter days and cold temperatures reduce photosynthesis → CO₂ levels rise - In the northern hemisphere, atmospheric CO₂ peaks in spring and troughs in late summer

Annual (long-term) increase: - Greenhouse gas emissions (including CO₂) have been rising year on year - Deforestation removes photosynthesising biomass, reducing the capacity to absorb CO₂ - Burning cleared biomass releases additional CO₂ - Rising CO₂ accelerates global warming, which reduces the solubility of CO₂ in oceans (less gas dissolves in warmer water), releasing still more CO₂ into the atmosphere — a positive feedback loop

Sampling Methods¶

To measure the distribution and abundance of organisms in a habitat, statistical sampling techniques are used.

Quadrats¶

A quadrat is a square frame (commonly 1 m² or 0.25 m²) placed randomly in the sampling area. Within each quadrat, organisms are identified and counted (or percentage cover is estimated for plants).

• Random placement: Random number tables or coordinates are used to choose positions; avoids sampling bias
• Abundance: Expressed as number per unit area, or as frequency (proportion of quadrats in which species appears)
• Percentage cover: Useful for plants where individual counting is impractical

Transects¶

A line transect is a line (tape measure or rope) stretched across the habitat. Organisms in contact with the line are recorded at each point. Useful for showing changes in community composition across a habitat gradient (e.g. from high shore to low shore; from clearing to forest edge).

A belt transect places quadrats at regular intervals along a line transect, recording all organisms within the quadrat rather than just those touching the line. This gives quantitative data about species abundance across a gradient.

Mark-Release-Recapture (Lincoln Index)¶

For mobile animals (where quadrats are impractical): 1. Capture a sample of organisms; mark them (e.g. paint spot, leg ring) 2. Release them and allow them to mix with the unmarked population 3. After an interval, capture a second sample; count total captured and number that are marked

Population estimate = (number in first capture × number in second capture) ÷ number of marked individuals in second capture

Assumptions: the mark does not affect survival or behaviour; marked individuals mix randomly; population size does not change between captures; marks remain detectable.

Key Terms¶

• Ecosystem: all the organisms in an area together with the abiotic components of their environment.
• Niche: the role of an organism in its environment, including how it uses resources and interacts with other organisms.
• Succession: directional change in community composition over time.
• Pioneer species: first species to colonise an area during succession.
• Climax community: relatively stable end-stage community in a succession sequence.
• Deflected succession: interruption of succession by human activity, preventing the ecosystem from reaching its natural climax community.
• Plagioclimax: a stable community maintained by continued human intervention, preventing natural succession to the climax community.
• Gross primary productivity (GPP): total chemical energy fixed by photosynthesis in an ecosystem.
• Net primary productivity (NPP): energy remaining in plant biomass after respiratory losses; NPP = GPP - R.
• Calorimetry: technique for measuring the energy content of a substance by burning a dried sample and measuring the heat released.
• Ecological efficiency: the percentage of energy transferred from one trophic level to the next; typically ~10%.
• Ammonification: conversion of organic nitrogen in dead material or waste into ammonia or ammonium compounds.
• Nitrification: oxidation of ammonium compounds to nitrites and then nitrates by nitrifying bacteria.
• Denitrification: reduction of nitrates to nitrogen gas by anaerobic bacteria.
• Quadrat: square frame used to sample plants or slow-moving organisms in a measured area.
• Line transect: sampling method in which organisms touching a line are recorded along a gradient.

Connected Pages¶

• 6.3.2 Populations and sustainability
• 5.2.1 Photosynthesis (primary productivity)
• 5.2.2 Respiration (respiratory losses from productivity)
• 4.2.1 Biodiversity (sampling techniques; biodiversity measurement)
• Photosynthesis vs respiration
• Module 6: Genetics, evolution and ecosystems