Ecology & Food Chains

2. Biotic and abiotic factors

Every organism is affected by two kinds of factor in its environment:

• Biotic factors are the living parts of the environment: other organisms.
• Abiotic factors are the non-living parts of the environment: physical and chemical conditions.

Whether a particular species can survive in a given habitat depends on both kinds. A polar bear cannot live in a tropical rainforest, not just because it has no penguins to eat, but because the temperature is too high.

Common abiotic factors

Common biotic factors

Case study: red squirrels vs grey squirrels

A famous case study of biotic factors. Native red squirrels lived across Britain for thousands of years. In the 1800s, grey squirrels were introduced from North America. Today red squirrels are confined to small parts of Scotland and a few islands, while greys dominate.

Three biotic factors explain the decline of the red squirrel:

• Competition for food and habitat. Grey squirrels are larger and can eat a wider range of food (including unripe acorns, which reds cannot digest). When the two species share an area, greys outcompete reds for the available food.
• Disease. Grey squirrels carry a virus called squirrelpox. Greys are immune to it; reds are not. Reds catching the virus from grey squirrels usually die within two weeks.
• Habitat preference. Greys do better in mixed broadleaf woodlands (which dominate southern Britain); reds do slightly better in coniferous forests (more common in the Scottish Highlands). This is why reds have hung on in those areas.

This is not an example of natural selection making one species better; it is the introduction of an invasive competitor changing the biotic factors faced by the natives.

3. Biodiversity

Biodiversity is the range and variety of different species of organisms found on Earth, or in a particular ecosystem

A coral reef has very high biodiversity (thousands of species per hectare). An intensive wheat field has very low biodiversity (one main species, plus a few weeds).

Why biodiversity matters

• Stability: ecosystems with high biodiversity are more resilient. If one species is wiped out by disease, other species can fill the gap. In a low-biodiversity system, losing a single species can collapse the whole food web.
• Food security: most food crops are descended from wild varieties. Wild relatives still hold genetic diversity that crop breeders may need in future (e.g. resistance to new diseases).
• Medicine: many drugs were originally discovered in wild plants, fungi or animals. Aspirin came from willow bark; penicillin from a mould; many cancer drugs from rainforest plants.
• Climate: forests, oceans and peat bogs all absorb large amounts of CO₂. Reducing biodiversity (especially through deforestation) accelerates climate change.
• Cultural and recreational value: people enjoy seeing wildlife. Many livelihoods depend on tourism around biodiversity hotspots.

Many human activities reduce biodiversity (covered in topic 16: Human Impact on the Environment): deforestation, pollution, overfishing, climate change, and the introduction of invasive species.

4. Investigating populations: sampling

You cannot count every individual organism in a large area, so ecologists sample: count what is in a small area, then scale up to estimate the whole population.

Quadrats

A quadrat is a square frame, often 0.5 m × 0.5 m or 1 m × 1 m, placed on the ground. You count all the individuals of the species you are interested in inside the quadrat.

Quadrats are mainly used for plants and slow-moving animals (snails, limpets). They are no use for fast animals like birds or mammals, which would run away.

Using a quadrat to estimate population size

1. Mark out the area to be sampled (e.g. a 10 m × 10 m square) using two tape measures.
2. Place the quadrat at a random position by generating two random numbers (one for each coordinate). Random placement is essential: a biased choice of position (e.g. only sunny patches) would give a biased result.
3. Count and record the number of individuals of the target species inside the quadrat.
4. Repeat at least 10 times at different random positions, and take a mean count per quadrat.
5. Calculate the total population using:

estimated total population = (mean count per quadrat) × (total area ÷ area of one quadrat)

Two ways to measure with a quadrat

• Count individuals: works well for things like dandelions or daisies that you can clearly count.
• Percentage cover: works for plants like grass, moss or clover that you cannot count individually. Estimate the percentage of the quadrat area covered by the species.

Line transects (showing how a species varies along a gradient)

If a species is not evenly distributed but instead changes along a gradient (e.g. seaweed species change from the high tide line down to the sea), a quadrat placed randomly is not the right tool. Instead use a line transect:

1. Lay a tape measure in a straight line across the area, along the gradient (from one extreme to the other).
2. At regular intervals along the line (e.g. every 1 m, or every 5 m, depending on the length), place a quadrat.
3. Count the species in each quadrat as before.
4. Plot the results as a graph of abundance vs distance along the transect.

A line transect is perfect for studying changes from a sunny field edge into a shaded woodland, or from a polluted river bank along the gradient of pollution.

Capture-recapture (for mobile animals)

Quadrats do not work for animals that move (mice, fish, butterflies). Instead use the capture-recapture method:

1. Capture a sample of the animals using a humane trap. Count them (let's say you catch n₁ individuals).
2. Mark each one (e.g. small dot of paint, plastic tag).
3. Release them back into the habitat.
4. Wait long enough for the marked animals to mix back in with the rest of the population.
5. Capture a second sample (count = n₂). Among them, count how many are marked from the first sample (m).
6. Estimate the total population using:

estimated total population = (n₁ × n₂) ÷ m

Assumptions and limitations of capture-recapture

The capture-recapture method only gives an accurate estimate if:

• The population size has not changed between the two captures (no births, deaths, or migration)
• The mark does not harm the animal or make it easier for predators to see
• Marked animals mix back in randomly (don't avoid the traps the second time)
• Sampling is random

In real ecological studies, these assumptions are never perfectly true, but with care the estimate is usually within 10–20% of the true value.

5. Trophic levels and food chains

A food chain is a diagram showing the transfer of energy from one organism to the next as one organism eats another. Each step in a food chain is called a trophic level.

The trophic levels

The first trophic level is always a producer because producers are the only organisms that can convert light energy from the Sun into the chemical energy needed by everything else.

Reading a food chain

A typical food chain is written as a sequence of organisms with arrows pointing in the direction of energy transfer:

grass → rabbit → fox

• Grass (1st trophic level, producer) is eaten by the rabbit
• Rabbit (2nd, primary consumer) is eaten by the fox
• Fox (3rd, secondary consumer) is the top of this chain

Arrows point from the eaten organism to the eater (i.e. in the direction energy flows). The arrow means "is eaten by".

Most natural food chains have four trophic levels at most, because too much energy is lost between levels for a fifth one to be supportable.

Food webs

In any real ecosystem, organisms eat many different things, and most are eaten by many different predators. A food web is a diagram showing the network of all the food chains in an ecosystem. The same organism can occupy different trophic levels in different chains.

Species removal: knock-on effects in a food web

Removing a species from a food web can affect every other species in it, sometimes dramatically:

• If a predator is removed, its prey species can multiply unchecked, eventually overgrazing the producers and crashing the whole system.
• If a prey species is removed, its predators have less food and decline; species that depended on it for some other role (e.g. seed dispersal) are also affected.
• If a producer is removed (e.g. an invasive species kills all the grass), every herbivore that ate it suffers, and every carnivore that ate the herbivore is hit too.

A well-known example: sea otters in the Pacific eat sea urchins. Sea urchins eat kelp. When sea otters were hunted to near-extinction in the 19th century, sea urchin numbers exploded, the urchins ate all the kelp, and the kelp forests collapsed. Restoring sea otters has restored the kelp.

6. Pyramids of numbers, biomass, and energy

Food chains can be drawn as ecological pyramids, where each trophic level is shown as a horizontal bar. The width of the bar represents the amount of something at that level. There are three types of pyramid.

Pyramid of numbers

A pyramid of numbers shows how many organisms are at each trophic level. The width of each bar is proportional to the number of individuals.

For most food chains, this gives a true pyramid shape:

grass (millions of plants) → vole (thousands) → owl (few)

But pyramids of numbers are not always pyramid-shaped. If many small organisms feed on one large organism, the producer level is narrower than the consumer level:

oak tree (1) → caterpillar (thousands) → robin (a few) → flea (many)

This gives an awkward shape with a narrow base.

Pyramid of biomass

A pyramid of biomass shows the total mass of living matter at each trophic level (in g/m² or kg per area).

Pyramids of biomass are almost always pyramid-shaped, because the biomass at each level is always less than the level below. The example above looks normal in biomass terms:

oak tree (huge mass) → caterpillars (small mass) → robins (smaller mass) → fleas (tiny mass)

Pyramid of energy

A pyramid of energy shows the energy stored in the biomass at each trophic level (in kJ/m²/year).

Pyramids of energy are always pyramid-shaped. Energy is lost at every level, so there is always less stored energy at higher trophic levels than at lower ones. This is true for every food chain without exception.

Why bigger animals are rarer

The pattern of "many small organisms supporting fewer large ones" appears everywhere because of the energy loss between levels. A square kilometre of grassland can feed thousands of rabbits, but only a few foxes can live off the rabbits. This is why top predators are rare in most ecosystems.

7. Energy transfer along a food chain

Energy enters every ecosystem as sunlight captured by producers. From there it passes through each trophic level by feeding.

The 10% rule

Only about 10% of the energy at one trophic level is transferred to the next. The other ~90% is lost in various ways. This is the famous "10% rule" of ecology, although the actual figure varies between 5% and 20% in real food chains.

Where the lost energy goes

The 90% of energy that does not make it to the next trophic level is lost in several ways:

Why food chains are short

Because so much energy is lost at each step, the chain runs out of usable energy after only a few levels. Most food chains end at the third or fourth trophic level. You almost never see a fifth-level predator (something that eats top predators) because there is too little energy left.

Calculating efficiency of energy transfer

The efficiency of energy transfer between two trophic levels is:

percentage efficiency = (energy at higher level ÷ energy at lower level) × 100

The same formula works for biomass, which is closely linked to energy.

Why farming is more efficient if you eat plants

The 10% rule has a practical implication. If 1000 kJ of plant energy is grown:

• Eat the plants yourself → you get ~100 kJ (after digestion losses).
• Feed the plants to cattle and eat the cattle → cattle get ~100 kJ from plants; you get ~10 kJ from cattle.

A vegetarian diet uses far less land, water and energy per person than a meat-based diet. This is why feeding the world's growing population is partly about shifting some meat consumption to plants.

8. Bioaccumulation and biomagnification

When toxic chemicals enter a food chain, they often build up in living tissue rather than being broken down. This causes two related problems:

• Bioaccumulation is the build-up of a toxic chemical inside a single organism over time. The organism absorbs the chemical faster than it can excrete it, so the concentration in its body rises.
• Biomagnification is the increase in concentration of a toxic chemical at successive trophic levels of a food chain. By the time you reach top predators, the concentration may be thousands of times higher than in the water or soil.

How biomagnification works

1. A small amount of a toxin (e.g. a pesticide, or a heavy metal like mercury) enters the water or soil.
2. Producers absorb it. Each producer ends up with a small dose.
3. A herbivore eats many producers, accumulating the doses from all of them.
4. A carnivore eats many herbivores, accumulating their doses.
5. At each trophic level the concentration in body tissues climbs.

A famous example: DDT and birds of prey

In the 1950s and 60s, the pesticide DDT was used heavily to kill insects on farmland. DDT does not break down easily; it persists in soil and water for decades and dissolves in fat. The chain:

1. Water had tiny amounts of DDT (a few parts per billion).
2. Plankton absorbed it as they grew.
3. Small fish ate plankton and accumulated DDT in their fat.
4. Big fish ate small fish, accumulating ever more.
5. Birds of prey (ospreys, falcons, bald eagles) ate the big fish, ending up with millions of times higher DDT concentration than the water.

The high DDT levels made the birds lay eggs with thin, fragile shells that broke under the parent's weight. Populations of bald eagles, peregrine falcons and ospreys crashed. After DDT was banned in many countries in the 1970s, these populations have recovered.

Other examples

• Mercury in fish: industrial mercury pollution accumulates in fish. Large predator fish (tuna, swordfish) have higher mercury levels than smaller fish, which is why pregnant women are advised to limit their intake.
• Lead: built up in wildlife from leaded petrol and lead shot, contributing to declines in many bird species. Now mostly phased out.
• PCBs (polychlorinated biphenyls): industrial chemicals that biomagnify; banned in most countries but still persist in the environment.

9. The carbon cycle

Carbon is the backbone of every biological molecule. Living things constantly move carbon between four major reservoirs: the atmosphere (as CO₂), the bodies of living organisms (as carbohydrates, proteins, lipids, etc.), dead organic matter, and fossil fuels (coal, oil, gas, formed over millions of years from undecomposed organisms).

The four key processes

The full cycle

1. Photosynthesis in plants takes CO₂ from the air and uses it to make glucose. Carbon is now inside the plant.
2. Plants respire, returning some of the carbon to the atmosphere as CO₂.
3. Animals eat plants (or other animals), taking the carbon into their own bodies.
4. Animals respire, returning more CO₂ to the atmosphere.
5. When plants and animals die, their bodies are broken down by decomposers (bacteria and fungi). The decomposers respire too, releasing CO₂.
6. If dead organic matter is buried in conditions where decomposers cannot get to it (mud, swamps, deep sediment), the carbon stays locked up. Over millions of years it forms fossil fuels.
7. When humans burn fossil fuels (combustion), the carbon that has been locked away for millions of years returns to the atmosphere as CO₂ in a few decades.

Why the carbon cycle matters

For most of Earth's history, the carbon cycle was roughly balanced: photosynthesis removed about as much CO₂ as respiration, decomposition and natural combustion added back. Atmospheric CO₂ levels stayed close to constant.

In the last 250 years, humans have drastically tipped the balance by:

• Burning fossil fuels at a huge rate (releasing fossilised carbon back into the atmosphere)
• Deforestation (which both releases carbon stored in trees and reduces future photosynthesis)

The result is a rise in atmospheric CO₂ from around 280 ppm in 1750 to over 420 ppm today. This extra CO₂ is the main driver of climate change (covered in topic 16).

10. The nitrogen cycle

Nitrogen is needed by every living thing to make proteins (and DNA, which contains nitrogen-rich bases). The atmosphere is 78% nitrogen gas (N₂), but this form is chemically unreactive and most organisms cannot use it directly. The nitrogen cycle shows how nitrogen is converted into usable forms and back again.

The four key processes

The full cycle

1. N₂ gas in the atmosphere is converted to nitrate in the soil by nitrogen-fixing bacteria. These bacteria come in two main types:
   • Free-living bacteria in the soil (e.g. Azotobacter)
   • Mutualistic bacteria (e.g. Rhizobium) living in root nodules of leguminous plants (peas, beans, clover, lentils). These plants make excellent crops in rotation because they enrich the soil with nitrogen.
2. Plants absorb nitrate from the soil through their roots by active transport.
3. Plants use the nitrate to build amino acids and then proteins.
4. Animals eat plants (or other animals), taking nitrogen-containing molecules into their own bodies and building their own proteins from them.
5. Animals excrete nitrogenous waste (mostly urea) and eventually die.
6. Decomposers (bacteria and fungi) break down dead bodies and waste, releasing ammonia into the soil.
7. Nitrifying bacteria convert ammonia → nitrites → nitrates, returning the nitrogen to a form plants can use.
8. In waterlogged or compacted soils, denitrifying bacteria convert nitrates back into N₂ gas that returns to the atmosphere.

Named bacteria you should know

Why farmers manage the nitrogen cycle

Crops remove nitrogen from the soil with every harvest. If farmers do nothing, the soil eventually runs out of nitrogen and crop yields fall. Solutions include:

• Adding fertiliser that contains nitrate ions (industrially fixed nitrogen)
• Crop rotation with legumes, which restore nitrogen via their root-nodule bacteria
• Ploughing waterlogged fields to introduce oxygen and reduce denitrification (denitrifying bacteria need anaerobic conditions)
• Spreading manure, which decomposers convert to ammonia and then nitrate

11. Decomposition

Decomposition is the breakdown of dead organic matter and waste by decomposer organisms (mainly bacteria and fungi). Decomposition is what recycles the carbon and nitrogen tied up in dead bodies back into the soil and atmosphere, where they can be used by new organisms.

Without decomposers:

• Dead bodies would pile up
• The minerals (especially nitrogen) inside them would never be released back to the soil
• Plants would run out of nitrate within a few years and growth would collapse
• The entire ecosystem would grind to a halt within a generation

Factors that affect the rate of decomposition

Like all enzyme-controlled processes (decomposers digest their food using extracellular enzymes), decomposition is sensitive to environmental conditions:

Designing an investigation of decomposition

A typical experiment to investigate factors affecting decomposition rate:

1. Choose an organic substance to decompose (leaf litter, apple slices, slices of bread, cotton fabric). Make sure each replicate has the same starting mass.
2. Set up several samples in identical containers, each with different conditions for the variable being tested:
   • Different temperatures (fridge at 4 °C, room temperature at 20 °C, incubator at 30 °C)
   • Different moisture levels (dry, damp, soaked)
   • Different oxygen levels (open container vs sealed container, optionally with water cover to exclude air)
3. Leave for a fixed time (a few days for fast-decomposing substances, longer for slower ones).
4. Re-weigh each sample. The greater the mass loss, the faster the decomposition.
5. Repeat several times and take a mean to reduce error.

Applying CORMS

For an investigation into the effect of temperature on decomposition:

• C (change): the temperature of incubation
• O (organism): same species and amount of dead organic matter
• R (repeat): at least three replicates at each temperature
• M (measure): the change in dry mass of the sample after a fixed time
• S (same): control moisture, light, container size, and how the samples are handled

Real-world applications

• Compost heaps are designed to maximise decomposition: garden waste is mixed with kitchen scraps, kept moist, and turned regularly to introduce oxygen. A well-managed heap reaches 60 °C and breaks down material in weeks.
• Sewage treatment uses controlled decomposition by microbes in aeration tanks to break down organic waste into safe products.
• Refrigeration slows decomposition by lowering temperature, which is why food keeps longer in a fridge.
• Freezing stops decomposition entirely (water is needed for enzymes; ice is not available water).
• Drying and salting stop decomposition by removing water. This is why beef jerky and salted fish keep without refrigeration.