• Producers convert light energy into chemical energy (GPP). Some of that is used up in respiration, and what's left (NPP) is stored as biomass and available to the rest of the food chain.
• Energy is lost at every trophic level — only around 10% passes from one level to the next. This is why food chains rarely have more than 4–5 levels.
• Ecology has a nested vocabulary: habitat → population → community → ecosystem. Each term builds on the one before it.
• Populations are shaped by biotic factors (living things — predators, competition, disease) and abiotic factors (non-living things — temperature, light, pH).
• A species' niche is its full "job" in the habitat. No two species can occupy exactly the same niche — one will always out-compete the other.
• Quadrats (frame and point) and transects (line and belt) are the tools used to measure abundance and distribution of species scientifically.
• Succession is the story of an ecosystem's life — from bare rock colonised by pioneer species, all the way to a stable climax community.

Think of a plant as a tiny solar-powered factory. Sunlight comes in, and the plant uses photosynthesis to turn that light energy into chemical energy — stored in sugars, and eventually in all the other molecules that make up the plant's body (its biomass). Organisms that do this conversion job are called producers.

But here's the catch: not all of that converted energy actually gets "saved" as new plant material. Just like your body burns some of the food you eat simply to stay alive (breathing, moving, keeping warm), a plant uses some of its own newly-made sugar for respiration — releasing energy to fuel its own cellular processes. That energy is spent, not stored, so it can't be passed on to anything that eats the plant.

This gives us two really important terms:

• Gross Primary Productivity (GPP) — the total rate at which the plant converts light energy into chemical energy (sugars) during photosynthesis. This is the "gross income."
• Net Primary Productivity (NPP) — what's left after the plant has paid its own "respiration bill." This is the energy actually stored as new biomass, and it's the only energy available to primary consumers (herbivores) and decomposers.

Roughly 90% of the energy converted during photosynthesis gets burned off in respiration — only about 10% typically ends up as new, storable plant biomass. This 90/10 split is the seed of a pattern you'll see again and again in this chapter.

NPP is measured in units of energy per unit area (or volume) per unit time — e.g. J m⁻² yr⁻¹ for land habitats, or J m⁻³ yr⁻¹ for aquatic ones (since in water you're often measuring a volume rather than a flat area).

How energy moves through a food chain

Once a producer has stored chemical energy as biomass, that energy doesn't just sit there forever — it gets passed along the food chain every time one organism eats another. Each feeding step is called a trophic level.

The arrows represent the transfer of energy by feeding. When a primary consumer eats a plant, it digests the tissue and uses the released chemical energy either to fuel its own respiration, or to build new biomass of its own — muscle, organs, and so on. That new biomass is then available to whatever eats the primary consumer next, and so the chain continues, right up until decomposers finally break down whatever's left when an organism dies.

Why so much energy is lost along the way

Here's the crucial fact for this whole topic: only around 10% of the energy at one trophic level makes it into the biomass of the next level. The other roughly 90% is lost to the environment. It doesn't vanish — it's just not usable by the next consumer. It's lost in several ways:

Calculating efficiency of energy transfer

Calculating efficiency of biomass transfer

Since biomass is essentially a physical measure of how much chemical energy an organism has stored, you can also calculate the efficiency of biomass transfer between trophic levels — it tells you the same underlying story, just using mass instead of energy units.

Scientists use dry biomass for this (not "wet" biomass) because the amount of water an organism holds can vary a lot and would distort the comparison. Dry biomass is found by drying a sample in a low-heat oven and weighing it repeatedly until the mass stops changing.

Ecology has a nested vocabulary — like Russian dolls, each term contains and builds on the one before it. Getting these four terms straight (and being able to define them precisely, not just vaguely) is worth serious exam marks.

Habitat

A habitat is simply the place where an organism lives. Habitats vary enormously in scale — a desert is a habitat, but so is a single tree, or even a rotting log (these tiny habitats are called microhabitats).

Species differ in how fussy they are about habitat. Specialists can only survive in one very specific type of habitat. Generalists can cope in a wide range of habitats — which also makes them more likely to become invasive species if introduced somewhere new, since they can establish themselves almost anywhere and disrupt the normal balance of species interactions.

Population

A population is all of the individuals of one species living in a habitat. Two measurements matter here:

• Abundance — the size of the population (how many individuals).
• Distribution — exactly where within the habitat those individuals are located.

Community

Species never live in isolation — they interact with other species around them. A community is multiple populations living and interacting in the same area. A garden pond community, for example, includes fish, frogs, newts, snails, dragonflies, pondweed, and water lilies — all interacting.

Ecosystem

An ecosystem is a community and its interactions with the non-living parts of its habitat — things like sunlight, temperature, soil, and water. Within an ecosystem, energy flows in one direction and nutrients get recycled. Ecosystems vary hugely in size (a garden pond vs. the open ocean) and complexity (a desert vs. a rainforest), and no ecosystem is ever fully self-contained — migratory animals move between them constantly.

Why does a species thrive in one place and struggle in another? The answer is a combination of biotic factors (living influences) and abiotic factors (non-living, physical/chemical influences).

Biotic factors — the influence of other living things

These arise from the activity of other organisms: predation, food availability, competition (both intraspecific — same species competing for resources — and interspecific — different species competing), cooperation, parasitism, and disease.

Abiotic factors — the influence of the non-living environment

These include light intensity and wavelength, temperature, water turbidity, humidity, soil/water pH, salinity, soil composition, and oxygen/CO₂ concentration.

If a habitat is the "address" where a species lives, a niche is its "job" — the full role that species plays within that habitat. This includes what it eats, which other species rely on it, what time of day it's active, and exactly where in the habitat it lives and feeds.

This doesn't mean similar-looking species can't coexist — it just means there must always be some subtle difference in their niche. Classic example: several species of warbler can all live in the same spruce tree, but each one feeds at a different height and location within the tree, avoiding direct competition.

The niche a species occupies directly determines its abundance (number of individuals) and distribution (where it lives). If two species end up competing for a similar niche, both of their populations will typically be smaller than they would be without that competition.

You can't realistically count every single organism in a large or complex habitat — so ecologists use sampling to estimate the abundance and distribution of species instead.

Random vs. Systematic Sampling

Frame Quadrats

A square frame placed in the habitat to define a sample area — used to study sessile (non-moving) organisms like plants. Sizes vary: a 1 m² quadrat suits small plants or limpets on a rocky shore, while a 400 m² quadrat (usually marked out with string, not an actual frame!) suits large organisms like trees.

Data that can be recorded using a frame quadrat:

• Presence/absence of a species
• Species frequency — how many individuals are present
• Species abundance — using the ACFOR scale (Abundant, Common, Frequent, Occasional, Rare, or None)
• Percentage cover — the % of the quadrat covered by a species (quadrats are often subdivided into smaller squares to make this easier to estimate)

Point Quadrats

A vertical frame with holes along the top, through which pins are lowered to the ground. Useful in habitats with dense plant cover, where a frame quadrat's boundaries would be hard to see clearly. Whatever species a pin touches is recorded as present.

Transects

A transect is a line laid across a habitat (often with a tape measure) to see how species abundance and distribution change along a gradient — for example, moving away from a hedgerow, or moving up a rocky shore away from the sea.

Measuring Abiotic Factors

Representing Results: Kite Diagrams

A kite diagram shows both abundance and distribution at once. Along a central horizontal line (representing the transect), each species gets its own "kite" — the width of the kite shows its abundance at that point (it extends equally on both sides of the line), while its position along the x-axis shows the distance along the transect.

Ecosystems aren't static — they're dynamic, constantly changing over time. This process of change is called succession, and both the biotic (living) and abiotic (non-living) conditions shift as it progresses.

Primary Succession

Primary succession happens when newly formed or newly exposed land is gradually colonised by an increasing number of species. This land has never supported life before — think cooled volcanic rock, a new rocky island, bare rock exposed by a landslide, or ground revealed as a glacier retreats. Crucially, there is no soil at all when it begins.

Step by step:

1. Pioneer species (usually mosses and lichens) are the first to colonise bare rock. They're specially adapted to germinate easily and survive harsh conditions — low nutrients, little water.
2. As pioneer species die and decompose, their organic matter forms a thin layer of soil.
3. Seeds of small plants and grasses land in this soil and begin to grow. Their root networks help hold the soil in place, preventing erosion.
4. As these plants die and decompose, the soil deepens and becomes more nutrient-rich, eventually able to support shrubs and small trees (which need more water — now available in deeper soil).
5. Eventually the soil is deep enough, nutrient-rich enough, and can hold enough water to support large trees, which become the dominant species.
6. The final, stable community is called the climax community. Its exact type depends on location — a rainforest in the tropics, deciduous woodland in temperate regions.

Succession changes its own conditions

This is the part students often miss: at each stage, the newly arrived species actively change the local environment — often making it more suitable for the next wave of species, but less suitable for themselves.

• Lichens break apart rock and add organic matter, creating soil — but lichens themselves can't grow on soil, so once soil forms, lichens disappear, out-competed by the very species that soil now allows to grow.
• As trees grow tall and block out light, shrubs and smaller plants beneath them are out-competed and die off.

This is why pioneer species are rarely found in a mature climax community — they're brilliant at surviving harsh conditions but poor competitors once conditions improve.

Secondary Succession

Secondary succession is very similar to primary succession, but it takes place on land that has previously been occupied — for example, after a wildfire or deforestation. The key difference: soil is already present, so the process starts partway through, skipping the "bare rock → pioneer species → soil formation" stages.

Humans can prevent succession

Human activity often interrupts succession, stopping a climax community from ever developing. Regular mowing prevents woody plants establishing on a lawn; grazing livestock prevent new plants from establishing on pasture. A stable community that develops because of human intervention (and wouldn't otherwise exist) is called a plagioclimax — heathland is a classic example.

• I can define GPP and NPP, and explain the relationship between them
• I can calculate NPP, GPP, or R given the other two values
• I can explain the four main causes of energy loss between trophic levels
• I can calculate the efficiency of energy transfer between trophic levels
• I can calculate the efficiency of biomass transfer, and explain why dry mass is used
• I can define and distinguish habitat, population, community, and ecosystem
• I can give examples of biotic factors affecting populations
• I can give examples of abiotic factors affecting populations
• I can explain what a niche is and why no two species can share one
• I understand the difference between random and systematic sampling, and when to use each
• I can describe how to use frame and point quadrats, and what data they collect
• I can describe the four types of transect and when each is used
• I can interpret a kite diagram, identifying both abundance and distribution
• I can describe the stages of primary succession from bare rock to climax community
• I can explain how species change conditions to favour some species and disfavour others during succession
• I can distinguish primary succession from secondary succession
• I can explain what a plagioclimax is and give an example