DP Biology HL D4.2 Stability and Change

DP Biology HL D4.2 Stability and Change - Full notes

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DP Biology HL | D4.2 Stability and Change

DP BIOLOGY · THEME D: CONTINUITY AND CHANGE

D4.2

Stability and Change

Full notes for HL · IB-style

Contents

1	Stability and sustainability of ecosystems
2	Requirements for a stable ecosystem
3	Tipping points and ecosystem collapse
4	Sustainable harvesting and agriculture
5	Eutrophication and biomagnification
6	Plastic pollution
7	Restoration, rewilding and ecological succession
8	Stability of ecosystems
9	Requirements for stability
10	Tipping points
11	Mesocosms
12	Keystone species and stability
13	Sustainable harvesting
14	Sustainable agriculture
15	Eutrophication
16	Biomagnification
17	Plastic pollution
18	Rewilding
19	Ecological succession
20	Mixed review: worked problems
21	Additional exam-style practice
22	Calculating biomagnification
23	Sustainability and human choices
24	Common misconceptions and links
25	Exam-style question with markscheme
26	Check your understanding
27	Summary and topic map

These are full teaching notes. Read each section in order, try the Think about it question before you read on, and do every Quick check before moving to the next section.

Before you start

Rate yourself on each statement now, and again after you've worked through the notes. Shade R (not yet), A (getting there) or G (confident).

I can…	R	A	G
explain what makes ecosystems stable and sustainable
explain the requirements for stability: energy supply, nutrient recycling, genetic diversity, climate
explain tipping points with the Amazon rainforest as an example
evaluate sustainable harvesting of natural resources and sustainable agriculture
explain eutrophication and biomagnification
explain the effects of plastic pollution
explain primary and secondary succession and rewilding

There is no additional HL content in this subtopic. SL and HL students study the same material.

Guiding questions

What features of ecosystems allow stability over long periods? How can human activities be made sustainable?

How this topic is examined

D4.2 is examined through case studies (fisheries, the Amazon, eutrophication of lakes), data on pollution and biomagnification, and evaluation of sustainable practices.

Strong answers link each human activity to a specific ecological process and give a named example.

Command term	What the examiner wants
State / Identify	a brief answer with no explanation needed
Outline	a brief account or summary
Describe	a detailed account, including trends or features, without reasons
Explain	a detailed account including causes, reasons or mechanisms
Distinguish / Compare and contrast	give the differences (and similarities), point by point
Calculate	a numerical answer with working and units
Deduce / Suggest	reach a conclusion from data; propose a reasonable explanation
Evaluate / Discuss	weigh strengths and limitations and reach a judgement

How to use these notes

Each section teaches one part of the syllabus in full. You'll see these boxes throughout; each has one job:

Box	What it's for
Think about it	a question to get you thinking before a section; the answer comes in the section
Key point	the one idea you must not forget
Common mistake	an error examiners see every year, and how to avoid it
In the real world	where the biology shows up outside the classroom
Try this	something quick you can do yourself to test the idea
Worked example	full solutions with IB-style marks (M1 method, A1 answer, R1 reasoning)
Quick check	short questions to test yourself at the end of each section

1 Stability and sustainability of ecosystems

Some ecosystems persist for millions of years.

Think about it

Some rainforests have existed for tens of millions of years, yet a few decades of clearing can destroy them. What kept them stable for so long?

A stable ecosystem persists over long periods with little change in its overall structure, and can recover from disturbances such as fire, storms or disease. Some ecosystems have been stable for very long times: parts of the Daintree rainforest in Australia are estimated to be over 100 million years old.

Sustainability is the use of resources at a rate that allows them to be replenished, so that they remain available indefinitely. Natural ecosystems are sustainable because energy is continually supplied by sunlight and nutrients are continually recycled (C4.2).

2 Requirements for a stable ecosystem

Energy, recycling, diversity and climate.

A continuous supply of energy, usually sunlight, captured by producers. Recycling of nutrients by decomposers, so that elements such as carbon, nitrogen and phosphorus are returned to the soil and atmosphere. Biodiversity and genetic diversity: ecosystems with many species have many interconnected food web links, so if one species declines, others can fill its role; genetic diversity allows populations to adapt. Climatic variables that remain within the tolerance of the community.

Keystone species and complex food webs also contribute: removing a keystone species can cause a cascade of changes (C4.1).

3 Tipping points and ecosystem collapse

Beyond a threshold, change becomes self-reinforcing.

A tipping point is a critical threshold beyond which a small additional change causes a large, often irreversible shift in an ecosystem to a different state. Positive feedback drives the change once the threshold is passed.

The Amazon rainforest recycles much of its own rainfall: trees take up water and release it by transpiration, which falls again as rain further inland. As deforestation increases, less water is recycled, the dry season lengthens, and more trees die, increasing fire risk. Scientists estimate that if around 20% to 25% of the forest is cleared, parts of the Amazon could pass a tipping point and change from rainforest to dry savanna. About 17% has already been lost.

Other examples include coral reefs shifting to algae-dominated states after repeated bleaching, and lakes switching from clear water to turbid, algae-dominated water after eutrophication.

Figure 1. Positive feedback towards a tipping point in the Amazon.

Quick check

1. Define tipping point.

2. Why could Amazon deforestation lead to savanna?

Answers: 1. threshold beyond which change becomes large and self-reinforcing 2. less transpiration reduces rainfall, more drought and fire

4 Sustainable harvesting and agriculture

Take no more than can be replaced.

Sustainable harvesting removes resources at a rate no greater than the rate of replacement. In fisheries, this requires knowing the population size and its growth rate, then setting a catch limit (the maximum sustainable yield) below that. Tools include quotas, minimum mesh sizes (allowing young fish to escape and breed), closed seasons, protected areas and monitoring. Overfishing, as in the North Atlantic cod collapse in 1992, shows the cost of unsustainable harvesting (A4.2).

Sustainable agriculture aims to produce food without degrading soil, water and biodiversity. Unsustainable practices include soil erosion from ploughing and overgrazing, excessive fertilizer use (causing eutrophication), pesticide overuse, monocultures (low biodiversity), clearing natural habitats, and high carbon footprints (machinery, fertilizer manufacture, transport, methane from livestock). More sustainable approaches include crop rotation, cover crops, reduced tillage, integrated pest management, agroforestry, reducing food waste, and eating less meat.

Practice	Problem	Sustainable alternative
Monoculture	low biodiversity, pest outbreaks	crop rotation, polyculture
Excess fertilizer	eutrophication	precise application, legumes
Ploughing bare soil	erosion	cover crops, reduced tillage
Pesticide overuse	harms non-target species, resistance	integrated pest management

Quick check

1. Define sustainable harvesting.

2. Give two features of sustainable agriculture.

Answers: 1. harvesting at a rate no greater than replacement 2. crop rotation, cover crops, reduced fertilizer, IPM, agroforestry

5 Eutrophication and biomagnification

Too many nutrients, and toxins that concentrate up food chains.

Eutrophication is the enrichment of water with nutrients, especially nitrates and phosphates from fertilizer run-off and sewage. The process: nutrients cause rapid growth of algae (an algal bloom); algae block light, so submerged plants die; algae and plants die and are decomposed by aerobic bacteria, whose respiration uses up dissolved oxygen, raising the biochemical oxygen demand (BOD); oxygen levels fall so low that fish and other animals die. This can create 'dead zones', such as in the Gulf of Mexico.

Biomagnification is the increase in concentration of a persistent, non-biodegradable toxin at each successive trophic level. Substances such as DDT (an insecticide), mercury and some industrial chemicals are absorbed but not broken down or excreted; they accumulate in fatty tissue. Each predator eats many prey, so the toxin becomes more concentrated up the food chain. Top predators receive the highest doses. DDT caused thin eggshells and population crashes in birds of prey such as peregrine falcons and ospreys until it was banned in many countries.

Figure 2. Biomagnification of DDT in an aquatic food chain.

WORKED EXAMPLE 1 Biomagnification factor

DDT concentration was 0.04 ppm in plankton and 25 ppm in fish-eating birds. Calculate how many times more concentrated it was in the birds.

Solution

25 ÷ 0.04 = 625 timesA1

Each consumer eats large quantities of prey; DDT is stored in fat and not excretedR1

Quick check

1. What causes eutrophication?

2. Why does dissolved oxygen fall?

3. Why do toxins biomagnify?

Answers: 1. nitrates and phosphates from fertilizer and sewage 2. aerobic decomposition of dead algae 3. not broken down or excreted; accumulate at each level

6 Plastic pollution

Plastics persist and fragment.

Plastics are durable and not readily biodegraded. Millions of tonnes enter the oceans each year. Macroplastics (large pieces) entangle animals and are swallowed by seabirds, turtles and whales, which may starve with stomachs full of plastic. Over time, sunlight and waves break plastic into microplastics (less than 5 mm), which are eaten by plankton, shellfish and fish, entering food chains. Plastics can also absorb toxic chemicals, which may then biomagnify.

Albatross chicks on Midway Atoll are fed plastic fragments by their parents, which mistake them for food. Reducing plastic use, improving recycling, and developing biodegradable alternatives are responses.

7 Restoration, rewilding and ecological succession

Ecosystems can recover, given time.

Ecological succession is the gradual change in the species composition of a community over time. Primary succession begins on bare ground with no soil, such as new volcanic rock or retreating glaciers: pioneer species such as lichens and mosses colonize first, break down rock and add organic matter, forming thin soil; grasses and small plants follow; then shrubs; and eventually a stable climax community (often forest). Secondary succession occurs where soil remains after a disturbance, such as an abandoned field or after a fire; it is faster.

As succession proceeds, soil depth, organic matter, biodiversity and biomass generally increase, and the community becomes more stable.

Rewilding restores ecosystems by reintroducing species (often keystone species and top predators) and reducing human management, allowing natural processes to recover. The reintroduction of wolves to Yellowstone in 1995 reduced elk overgrazing, allowing willows and aspens to recover along rivers, which benefited beavers and songbirds: a trophic cascade.

Figure 3. Primary succession.

Quick check

1. Distinguish primary and secondary succession.

2. What is a pioneer species?

3. Give an example of rewilding.

Answers: 1. primary: no soil; secondary: soil present after disturbance 2. first species to colonize bare ground 3. wolves reintroduced to Yellowstone

8 Stability of ecosystems

Resisting and recovering.

A stable ecosystem resists change (resistance) and recovers after disturbance (resilience). Stability depends on biodiversity, complexity of food webs, nutrient cycling and climate. Some ecosystems, like ancient forests, have remained stable for thousands of years.

WORKED EXAMPLE 2 Diversity and stability (standard)

Explain why diverse ecosystems tend to be more stable.

Solution

Complex food webs offer alternative food sourcesR1

Loss of one species has less overall effectR1

9 Requirements for stability

Energy, cycling, diversity.

Stable ecosystems need: a continuous energy supply (sunlight), efficient recycling of nutrients by decomposers, genetic diversity within populations, and climatic variables within tolerable limits.

Quick check

1. Why is nutrient recycling essential for stability?

Answers: 1. nutrients are finite and must be reused by producers

10 Tipping points

Sudden shifts.

A tipping point is a threshold beyond which an ecosystem changes rapidly to a new state that is hard to reverse. Example: deforestation of the Amazon may reduce rainfall enough to convert parts of the rainforest to savanna.

WORKED EXAMPLE 3 Amazon (exam level)

Explain how deforestation could cause the Amazon to reach a tipping point.

Solution

Trees recycle water by transpiration, generating rainfallR1

Loss of trees reduces rainfall; drier conditions favour savanna and firesR1

11 Mesocosms

Experimental ecosystems.

Mesocosms are small, enclosed experimental ecosystems (aquaria, sealed jars, outdoor enclosures) used to test the effects of variables such as light, nutrients or temperature on stability.

WORKED EXAMPLE 4 Mesocosm (standard)

Give one advantage and one limitation of mesocosm experiments.

Solution

Advantage: variables can be controlled and replicatedR1

Limitation: simplified, may not reflect real ecosystemsR1

Try this

Set up sealed jars containing pond water, plants and small invertebrates. Keep some in light and some in dark and observe over weeks.

12 Keystone species and stability

Disproportionate influence.

Removing a keystone species can destabilize an ecosystem: wolves in Yellowstone, sea otters in kelp forests, elephants in savannas (keeping woodland open).

Quick check

1. What happened to Yellowstone after wolves were removed?

Answers: 1. elk overgrazed willows and aspens; river banks eroded

13 Sustainable harvesting

Within limits.

Sustainable harvesting removes resources at a rate no greater than they are replaced. Fishing quotas, minimum sizes, seasonal closures, selective logging and certification help. Overexploitation causes population collapse.

WORKED EXAMPLE 5 Fisheries (exam level)

Outline three measures to make fishing sustainable.

Solution

Quotas limit catchR1

Minimum mesh sizes let young fish escapeR1

Closed seasons or no-take zones protect breedingR1

14 Sustainable agriculture

Feeding people without harm.

Agriculture can be unsustainable because of soil erosion, nutrient runoff, pesticide use, water depletion and carbon emissions. Sustainable practices include crop rotation, cover crops, reduced tillage, integrated pest management, and efficient irrigation.

WORKED EXAMPLE 6 Soil erosion (standard)

Explain how cover crops reduce soil erosion.

Solution

Roots bind the soilR1

Leaves protect soil from heavy rain and windR1

15 Eutrophication

Too many nutrients.

Fertilizer runoff adds nitrates and phosphates to water, causing algal blooms. Algae block light; plants below die; decomposers multiply and use up oxygen (high biochemical oxygen demand); fish and other animals die.

WORKED EXAMPLE 7 Sequence (exam level)

Outline the process of eutrophication.

Solution

Nutrient enrichment leads to algal bloomR1

Algae die; decomposition by aerobic bacteria uses oxygenR1

Low oxygen kills fish and invertebratesR1

Quick check

1. Suggest two ways to reduce eutrophication.

Answers: 1. buffer strips by rivers; precise fertilizer use; treating sewage

16 Biomagnification

Accumulating toxins.

Some pollutants (DDT, mercury, PCBs) are not broken down or excreted; they accumulate in tissues (bioaccumulation) and increase in concentration at each trophic level (biomagnification), harming top predators.

WORKED EXAMPLE 8 DDT (standard)

Explain why DDT harmed birds of prey more than smaller birds.

Solution

DDT concentration increases up food chainsR1

Top predators accumulate the highest levels; eggshells thinnedR1

In the real world

DDT caused eggshell thinning in bald eagles and peregrine falcons; after it was banned in the 1970s, their populations recovered.

17 Plastic pollution

Macroplastics and microplastics.

Macroplastics entangle and are swallowed by marine animals. Microplastics (fragments under 5 mm) are eaten by plankton and fish and enter food chains. Plastics persist for hundreds of years.

Quick check

1. Why are microplastics difficult to remove from ecosystems?

Answers: 1. very small, widespread and persistent

18 Rewilding

Restoring natural processes.

Rewilding restores ecosystems by reintroducing species (beavers, bison, wolves), removing barriers and reducing management, allowing natural processes to recover. Beavers create wetlands that increase biodiversity and reduce flooding downstream.

Quick check

1. Give one benefit of reintroducing beavers.

Answers: 1. dams create wetlands, increase biodiversity, slow floods

19 Ecological succession

Change over time.

Succession is the sequence of community changes over time. Primary succession begins on bare rock (pioneer species such as lichens), and secondary succession follows disturbance where soil remains. Communities change until a climax community forms.

WORKED EXAMPLE 9 Pioneers (standard)

Explain the role of pioneer species in primary succession.

Solution

Colonize bare rock; begin weathering and soil formationR1

Allow other species to establishR1

20 Mixed review: worked problems

Bringing the ideas together.

WORKED EXAMPLE 10 Mixed problem (exam level)

Evaluate the use of fertilizers in agriculture.

Solution

Increase crop yieldsR1

Runoff causes eutrophication; production uses energyR1

Precision application reduces harmR1

21 Additional exam-style practice

Paper 1 and Paper 2.

WORKED EXAMPLE 11 Paper 1 question (standard)

Which process directly causes oxygen depletion during eutrophication? A. algal photosynthesis B. decomposition by bacteria C. nitrogen fixation D. denitrification

Solution

BA1

WORKED EXAMPLE 12 Paper 2 question (exam level)

Explain biomagnification using an example.

Solution

Persistent pollutant (mercury/DDT) not excretedR1

Concentration increases at each trophic levelR1

22 Calculating biomagnification

Concentration factors.

Biomagnification factor = concentration in organism/concentration in its food (or the environment). Large factors indicate strong accumulation.

WORKED EXAMPLE 13 Factor (standard)

Water has 0.00005 ppm of a pesticide; fish-eating birds have 25 ppm. Calculate the concentration factor.

Solution

25/0.00005M1

= 500 000A1

23 Sustainability and human choices

Footprints.

Ecological footprints estimate the land area needed to supply a population's resources and absorb its waste. Reducing meat consumption, food waste and energy use lowers footprints.

Quick check

1. Why does eating less meat reduce an ecological footprint?

Answers: 1. less land needed: energy lost at an extra trophic level

24 Common misconceptions and links

Students often think…	Actually…
eutrophication kills fish directly with nutrients	fish die from lack of oxygen due to decomposition
biomagnification happens with all chemicals	only persistent, non-biodegradable substances
sustainability means no use of resources	it means use no faster than replacement

How D4.2 connects to the rest of the course

Topic	Connection
C4.2 Energy and matter	nutrient cycling and food chains
A4.2 Conservation	biodiversity loss
C4.1 Communities	keystone species
D4.3 Climate change	carbon and tipping points
B3.2 Transport	transpiration

25 Exam-style question with markscheme

Try this under exam conditions (about 15 minutes) before reading the markscheme.

Figure 4. DDT in a food chain.

(a) Calculate the increase in DDT from small fish to birds. [1]

(b) Explain biomagnification. [3]

(d) Outline two methods to make fishing sustainable. [2]

Markscheme

Part	Answer	Marks
(a)	25 ÷ 0.5 = 50 times	A1
(b)	DDT persistent, not broken down or excreted	R1
	stored in fat	R1
	concentrates at each trophic level as predators eat many prey	R1
(c)	nitrates/phosphates cause algal bloom	R1
	plants die from lack of light	R1
	decomposers use oxygen (BOD rises)	R1
	fish die from lack of oxygen	R1
(d)	quotas / mesh size / closed seasons / marine protected areas (any two)	A2

Examiner's comments. In (c), weaker answers skipped the role of decomposers.

26 Check your understanding

Section A: multiple choice

1. A tipping point is

A. a threshold beyond which change becomes self-reinforcing B. a stable state C. a type of pollution D. a keystone species

2. Eutrophication is caused mainly by

A. carbon dioxide B. plastics C. nitrates and phosphates D. pesticides

3. In eutrophication, oxygen falls because

A. algae photosynthesize B. decomposers respire C. fish breathe D. water warms

4. DDT harmed birds of prey by

A. blocking light B. causing eutrophication C. entanglement D. thinning eggshells

5. Pioneer species in primary succession include

A. oak trees B. lichens and mosses C. grasses only D. wolves

6. Sustainable harvesting means

A. taking no more than can be replaced B. no harvesting C. maximum harvesting D. only farming

Section B: structured

7. Explain the requirements for ecosystem stability. [4]

8. Evaluate methods of sustainable fishing. [4]

9. Explain the stages of primary succession. [4]

Answers

1. A 2. C 3. B 4. D 5. B 6. A

7. Energy supply, nutrient recycling, biodiversity and genetic diversity, stable climate.

8. Quotas, mesh sizes, closed seasons, protected areas; enforcement and data problems; cod collapse.

9. Pioneers on bare rock; soil formation; grasses; shrubs; climax community; increasing biodiversity.

27 Summary and topic map

Idea	Remember
Stability	energy, recycling, diversity, climate
Tipping points	Amazon to savanna; positive feedback
Sustainable harvesting	quotas, mesh size; cod collapse
Sustainable agriculture	rotation, cover crops, IPM
Eutrophication	nutrients, algal bloom, BOD, fish deaths
Biomagnification	persistent toxins concentrate up food chains
Plastics	macro- and microplastics
Succession	primary and secondary; rewilding

Before the exam, make sure you…

•link each impact to a process
•sequence eutrophication fully
•calculate biomagnification factors
•use named examples

Stability and Change on one page

Cover this page with a sheet of paper and try to rebuild it from memory. Whatever you can't recall is what to revise next.

Figure 5. Topic map for Stability and Change.

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