The Organism and the Environment

Edexcel asks for these definitions almost every year. Get the wording precise.

Term	Definition	Example
Population	All members of ONE SPECIES in the same area at the same time	All rabbits in one field
Community	All POPULATIONS (different species) living together in the same area	Rabbits + grass + foxes + insects in the field
Habitat	The PHYSICAL PLACE where an organism lives	The meadow; the pond; the rocky shore
Ecosystem	A community of living organisms TOGETHER with the non-living (abiotic) environment	The meadow ecosystem = all organisms + soil + climate + water

Common confusions to avoid:

• 'Population' is ONE species only — not 'all the organisms in the area' (that's community).
• 'Habitat' is just the PLACE; the ecosystem includes the place AND its inhabitants AND the abiotic surroundings.

Wider terms (helpful for context, less often tested):

• Niche = the role an organism plays in its ecosystem (what it eats, where it lives, when it's active).
• Biome = a large geographic region with similar climate / vegetation (e.g. tropical rainforest, desert, tundra).
• Biosphere = all the ecosystems of Earth combined.

An adaptation is a feature that helps an organism survive and reproduce in its habitat. There are three types:

• Anatomical adaptations (structural features) — e.g. thick fur, sharp teeth, deep roots.
• Behavioural adaptations — e.g. hibernation, migration, courtship displays.
• Physiological adaptations — internal biochemistry, e.g. ability to lower metabolic rate, antifreeze proteins.

Three classic Edexcel examples:

1. Polar bear — Arctic environment.

Anatomical:

• Thick white fur → insulates against extreme cold AND camouflages against snow.
• Layer of subcutaneous fat (blubber) → insulation + energy store during food shortage.
• Small ears, short tail → low surface area : volume → less heat loss.
• Large feet with rough pads → distribute weight on ice; partly webbed for swimming; rough grip.
• Sharp claws and powerful jaws → catch and kill seals.

Behavioural:

• Females dig snow dens → insulate cubs from external cold.
• Long-distance swimming between ice floes.

Physiological:

• Lowered metabolic rate during food shortages → reduces energy use.
• Concentrated urine (water-conserving kidneys) → little drinking water available.

2. Cactus — hot dry desert.

Anatomical:

• Spines instead of leaves → tiny surface area → reduces water loss by transpiration. Also deter herbivores.
• Thick waxy cuticle on the stem → waterproof → reduces evaporation.
• Sunken stomata in pits → trap humid air → less water diffuses out.
• Wide shallow roots → catch surface rainwater quickly when it does rain.
• Deep tap roots → reach underground water.
• Thick fleshy stem → stores water for dry periods; green stem also photosynthesises (replaces leaves).

Physiological:

• CAM photosynthesis → stomata open at night (cooler) and store CO₂ → reduces daytime water loss.

3. Fish — aquatic environment.

Anatomical:

• Streamlined body → reduces water resistance → efficient swimming.
• Gills → very large surface area → effective gas exchange with water (low dissolved O₂).
• Scales + mucus → reduce friction with water.
• Fins → manoeuvring and propulsion.
• Swim bladder (bony fish) → gas-filled organ → maintain neutral buoyancy.
• Lateral line → detects pressure / vibration in water → spots predators and prey.

Behavioural:

• Schooling → protection from predators.

Physiological:

• Counter-current gas exchange in gills → maximum O₂ uptake (similar concept to mammal alveoli).

The pattern: each adaptation matches a specific environmental challenge. Cold → insulation. Dry → water conservation. Aquatic → buoyancy + gas exchange.

A quadrat is a square frame (commonly 0.5 m × 0.5 m or 1 m × 1 m) used to sample plants and slow-moving / sessile animals (e.g. limpets, barnacles).

Method:

1. Choose the sample area (e.g. a field, a meadow).
2. Use a random-number generator (or random coordinates from a grid) to pick 10-30 quadrat positions — RANDOM placement avoids bias.
3. Place the quadrat at each position.
4. Inside each quadrat, COUNT the number of individuals of each species, OR estimate the % COVER of plants.
5. Repeat 10-30 times across the habitat.

Calculating population size:

$\text{Total population} = \frac{\text{Mean count per quadrat}}{\text{Area of quadrat}} \times \text{Total habitat area}$

Worked example: 10 quadrats of 0.5 m × 0.5 m (= 0.25 m²) give a mean count of 5 dandelions per quadrat. Field is 50 m × 40 m = 2000 m². Density = 5 / 0.25 = 20 / m². Total = 20 × 2000 = 40 000 dandelions.

Strengths:

• Quick, simple, cheap.
• Random placement gives an unbiased estimate.
• Suitable for plants and slow-moving organisms.

Limitations:

• Not suitable for mobile animals (they move out / back in).
• In NON-uniform habitats (e.g. a sloping shore), random placement may miss patterns.
• Sample size must be large enough to be statistically reliable (≥ 20 quadrats typically).

% cover vs counts. Counts are more accurate for discrete plants (e.g. dandelions); % cover is used for plants that are hard to count individually (grass, moss).

When the question is 'how does species distribution CHANGE along a gradient?' (e.g. low to high tide; pond edge into woodland), use a belt transect.

Method:

1. Lay a tape measure in a STRAIGHT LINE across the gradient — usually PERPENDICULAR to the change (e.g. low-tide mark → high-tide mark on a shore).
2. Place a quadrat at REGULAR intervals along the tape (e.g. every 1 m or 2 m). These are NOT random — they are systematic.
3. At each position, record: species present, count or % cover, AND abiotic factors (light, temperature, soil moisture, pH).
4. Repeat with 2-3 PARALLEL transects to improve reliability.

Belt vs line transect:

• Line transect = note species touching the tape itself.
• Belt transect = use a quadrat at each interval point — more data.

Recording data: Plot a kite diagram or bar chart showing the abundance of each species at each distance along the transect. Plot abiotic measurements as line graphs alongside to spot correlations.

Strengths:

• Clearly shows zonation / how distribution changes with position.
• Links biotic distribution to abiotic factors.

Limitations:

• Only samples a narrow strip — may miss patterns elsewhere.
• More time-consuming than a single set of quadrats.
• The line itself is chosen by the investigator — choice of line may bias the result.

Application examples:

• Rocky-shore zonation: limpets, barnacles, seaweeds zoned by tidal height.
• Pond to woodland succession: aquatic plants → reeds → marsh grasses → woodland herbs → shrubs → trees.

For mobile animals (woodlice, fish, butterflies), quadrats don't work — the animals move in and out. Instead use mark-release-recapture (also called the Lincoln index).

Method:

1. Capture a sample of size N₁. Mark each animal with a small spot of waterproof paint, ear tag, ring, fin clip, etc.
2. Release them back into the same habitat.
3. Leave enough time for the marked animals to MIX EVENLY with the unmarked population (e.g. 24-72 hours).
4. Capture a second sample of size N₂.
5. Count the number of MARKED animals in the second sample: M.

The Lincoln index formula:

$\text{Population} (N) = \frac{N_1 \times N_2}{M}$

Worked example. Captured N₁ = 24 woodlice, marked, released. Two days later captured N₂ = 30; 8 were marked (M = 8). N = (24 × 30) / 8 = 90 woodlice.

Assumptions (often examined!):

1. Marks do not affect survival or behaviour — paint not toxic, not visible to predators, not heavy.
2. No births, deaths, immigration or emigration between the two samples — i.e. a CLOSED population.
3. Marked individuals mix evenly with unmarked — wait long enough.
4. Equal catchability — marked and unmarked are equally likely to be caught in the second sample.

If these assumptions don't hold, the estimate is inaccurate.

Strengths:

• Only practical method for mobile animals.
• Can be done in the field without disturbing the habitat.

Limitations:

• Assumptions may fail (paint may make organisms more visible to predators).
• Animal may avoid recapture (e.g. learn to avoid the trap).
• Time and effort needed (multiple captures).

Whether a species can live in a habitat depends on the conditions — both NON-LIVING (abiotic) and LIVING (biotic).

ABIOTIC factors (non-living):

Factor	Effect on organisms	Measured with
Temperature	Enzymes work fastest at optimum; extremes denature them	Thermometer
Light intensity	Plants need light for photosynthesis; shaded areas favour fern/moss	Light meter (lux)
Soil pH	Acid-loving (heather) vs chalk-loving (orchids) plants	pH probe / Universal indicator
Soil moisture / humidity	Dry tolerant (succulents) vs marsh plants	Soil moisture probe
Oxygen concentration	Fast streams support trout (high O₂); stagnant ponds only carp	Dissolved O₂ probe
Salinity	Freshwater vs marine vs estuarine species	Refractometer / salinity probe
Wind	Exposed coasts vs sheltered woodland	Anemometer
CO₂	Plant growth limited by low CO₂; ocean acidification at high CO₂	CO₂ probe

BIOTIC factors (living):

Factor	Effect
Predators	High predator pressure → reduces prey numbers
Competition	Intraspecific (within species) — for mates, food, territory. Interspecific (between species) — for the same resource
Food availability	No food → no population. Cycle of producers → herbivores → carnivores
Disease / pathogens	Can wipe out populations (Dutch elm disease, panama disease in bananas)
Parasites	Reduce host fitness
Symbiosis / mutualism	Can ENHANCE populations (e.g. legumes + Rhizobium)
Human influence	Habitat destruction, hunting, pollution — increasingly the dominant biotic factor

Tolerance ranges and limiting factors.

• Each species has a range of tolerance for each abiotic factor — some growth possible, fastest at the OPTIMUM, none at the extremes (lethal limits).
• The factor in shortest supply (or most extreme) is the limiting factor — even if all others are perfect, this one determines distribution.

Edexcel exam tip. When asked 'why does species X live here but not there?', cover BOTH abiotic AND biotic factors. e.g. coral reefs need: warm water (abiotic), shallow + clear water (light for symbiotic algae — abiotic + biotic), and freedom from predators / disease (biotic).