Stability and change

One that **stays roughly the same over time** and **returns to balance after a disturbance**.

It belongs to the **whole ecosystem working together**, not to any single organism — it emerges only at the level of the whole system.

The ability to **recover and return to its normal state** after a disturbance.

**1)** a continuous supply of energy, **2)** recycling of nutrients, **3)** genetic diversity within populations, **4)** steady climatic/abiotic variables.

Energy is **lost as heat** at each trophic level and **cannot be recycled**, so it must keep coming in (as sunlight).

The chemical elements (C, N, P) are **finite**, so **decomposers return them** to the soil/water for producers to reuse.

Variation means **some individuals survive** a new disease or stress, so a whole population is **not wiped out** by one change.

Species have **overlapping roles**, so if one is lost **another can fill the role** — the web is not broken and the ecosystem returns to balance.

**Energy flows through** (lost as heat); **matter/nutrients cycle round** (recycled).

Any event that knocks an ecosystem out of its steady state — e.g. **fertiliser run-off, wildfire, or pollution**.

A threshold beyond which a disturbance causes a **self-reinforcing change** that flips the ecosystem into a **new stable state** it cannot easily recover from.

A **deflection** is recoverable (the system bounces back); past a **tipping point** the change feeds itself and the system settles in a new state.

The over-enrichment of water with **nitrate and phosphate** (often from fertiliser run-off), causing an **algal bloom** and then a fall in dissolved oxygen.

**Algal bloom** → light blocked, plants die → **decomposers respire** and use up the oxygen (high **BOD**) → **fish die**.

That **decomposers are using up a lot of dissolved oxygen** breaking down dead organic matter, leaving little for fish and other aerobic organisms.

It removes the **vegetation and roots** that bind the soil; the **bare soil** is then **washed away by rain and blown away by wind**.

Because **decomposers multiply and respire aerobically** as they break down the dead algae and plants (high BOD) — not because the algae 'use it up'.

Using it so it lasts indefinitely — taking **no more than is naturally replaced**, so the stock is not depleted.

When **more is removed than is replaced** each year — the stock declines and can **collapse**.

A **small, enclosed experimental ecosystem** (e.g. a sealed tank or fenced plot) used to study how an ecosystem behaves.

Strength: **controlled, cheap, repeatable and safe/ethical**. Limitation: **small and simplified**, so it may not match a real ecosystem.

The **increase in a persistent pollutant's concentration at each higher trophic level** of a food chain.

It is **persistent** (not broken down or excreted), and each predator **eats many prey**, so it keeps all of their pollutant and the concentration multiplies up the chain.

The **top predator** — it has the **highest** concentration of the pollutant.

**Bioaccumulation** = build-up within **one organism** over its life. **Biomagnification** = increase **up the food chain**, level by level.

The ability of **one genotype** to produce **different phenotypes** in response to **different environments**, within an individual's lifetime.

**No.** The DNA stays the same — only the **environment** changes how the genes are expressed.

**No** — it happens within one individual's lifetime and is **not passed on** to offspring.

Natural selection changes **alleles in a population over generations** (heritable). Plasticity changes the **phenotype of one individual** because of its **environment** (not heritable).

A moth species reared at different **temperatures** develops different **wing colours** despite the **same genotype** (or: an Arctic hare's white winter / brown summer coat).

**Same genotype → different phenotype, caused by the environment.**

**Almost no free oxygen** and a **high level of carbon dioxide** (a 'reducing' atmosphere).

**Oxygen rose** (to ~21%) and **carbon dioxide fell** (to a low level).

**Photosynthesis** — it releases oxygen as a waste product.

**Cyanobacteria**, then later **algae and plants**.

The time, billions of years ago, when **oxygen from photosynthesis built up** in the atmosphere for the first time.

**Photosynthesis fixed CO₂** into organic carbon, which was then **buried as fossil fuels** or **locked in limestone**.

In **fossil fuels** (coal, oil, gas) and in **limestone** (carbonate from marine shells).

It enabled efficient **aerobic respiration** and formed the **ozone layer**, making **complex life** and life on land possible.