D4.2Theme D — Continuity and change
Stability and change — IB Biology HL Revision Notes & Topical Questions
15 min read 12 parts Lesson 13 of 18 Unit quiz
Curriculum Equation Key قوانين الرقم الهيدروجيني والاتزان الأيوني - pH & Acid-Base Equilibrium FormulasFast Exam Solution
pH = -log[H⁺] | pOH = -log[OH⁻] | pH + pOH = 14 | [H⁺][OH⁻] = 10⁻¹⁴
Word Equation & Conditions: الرقم الهيدروجيني = -لوغاريتم تركيز أيون الهيدروجين | pH + pOH = 14
Location & Specifications: الاتزان الأيوني، كيمياء 3 ثانوي، التحصيلي، و AP Chemistry.
Lesson summary
D4.2 Stability and change — IB BIOLOGY HL notes: Explain ecosystem stability, keystone species, sustainability, pollution, ecosystem restoration and ecological succession.Stable ecosystems have efficient nutrient cycling so they are self-supporting, high biodiversity, resistance …
- Amazon and Congo rainforests
- Supply of energy
- Recycling of nutrients
- Genetic diversity
- Climatic variables within tolerance levels
- Experiment
Part 1 of 10Features of stable ecosystems
Features of stable ecosystems
Stable ecosystems have efficient nutrient cycling so they are self-supporting, high biodiversity, resistance to change (consumer populations that stay steady so resources are not overused) and high photosynthesis.
Amazon and Congo rainforests
- Amazon and Congo rainforests some have stayed in their state for tens of millions of years.
- They are highly diverse, with millions of invertebrate species, tens of thousands of plant species, thousands of bird and fish species and hundreds of mammal species in the Amazon.
- High light and moisture give high photosynthesis.
- Detritivores such as termites, slugs and worms, and decomposers such as fungi recycle organic matter for the trees.
- Water is cycled by transpiration, condensation and rain.
Healthy ecosystems are stable but not static, since natural selection always acts.
Part 2 of 10Requirements for stability and investigating stability: mesocosms
Supply of energy
- Supply of energy a reliable source, usually sunlight converted by many photosynthetic organisms, and chemical energy passes up food chains.
Recycling of nutrients
- Recycling of nutrients it is essential so that supplies do not run out.
- Decomposers such as bacteria and fungi break down proteins and nucleic acids in dead matter and waste.
- Carbon is released as carbon dioxide and minerals such as nitrates and phosphates enter the soil for producers.
- Decomposers need oxygen, moisture and suitable temperature, and an ecosystem that is too hot or dry cycles nutrients less and is less productive.
- Removing nutrients, as when timber is taken from fallen trees or crops are harvested, interrupts cycling and reduces productivity.
Genetic diversity
Genetic diversity the number of different alleles in a population.
- It lets natural selection act on favourable alleles, so a population can adapt to environmental change.
- A limited number of alleles reduces the chance that one is favourable.
Climatic variables within tolerance levels
- Climatic variables within tolerance levels genetic variation helps only up to a point.
- Temperature and rainfall changes beyond the tolerance of a species force it to migrate or face extinction.
- Human-caused climate change is moving climatic variables beyond tolerance in some ecosystems.
Mesocosm
Mesocosm an experimental container simulating a natural ecosystem, used to study responses to factors such as nutrients and light.
- All factors except the variable studied can be controlled.
- Examples are land tanks for sewage effects on ponds, underwater enclosures for temperature or dissolved carbon dioxide on oceans,
- and greenhouse-like buildings with trees for carbon passage through a rainforest.
Realism
- Realism enclosed, highly controlled mesocosms may be unrealistic,
- and larger ones that share more real features,
- such as mixing of water layers, improve realism.
Part 3 of 10Building a mesocosm
Design points
- A transparent container lets sunlight reach the producers.
- Autotrophs convert light energy to chemical energy.
- Small primary consumers such as zooplankton or invertebrates can be included if the mesocosm is large enough.
- Secondary consumers should not be included, because the food chain cannot support them and it would be unethical to let primary consumers be eaten.
- Sealed systems are more controlled than open ones, and prevent organisms and substances entering or leaving.
Experiment
- Experiment alter one factor,
- such as light or temperature,
- and set up an identical control mesocosm at the same time to show that changes result from the altered factor.
Terrestrial mesocosm
1
Put drainage material such as gravel in the bottom of a clear container.
2
Add a layer of charcoal to help prevent mould.
3
Add sphagnum moss or filter paper as a separating layer.
4
Add soil or compost for organic material and micro-organisms.
5
Plant slow-growing producers such as mosses and ferns.
6
Water the medium before sealing with a lid, avoiding too much water.
7
Place in a light location at a stable temperature.
- Aquatic mesocosm
1
Use organic substrate from the bottom of a lake or pond as the base layer for nutrients and microorganisms.
2
Add pond or lake water, not tap water.
3
Add healthy aquatic plants.
4
Add only primary consumers such as water fleas or snails, and no more than the system can support.
5
Place in a light location at a stable temperature.
- Nature of science
- IB guidelines say animals should be used only when there is no alternative, with no cruelty and with measures to remove distress.
- Many mesocosms need no animals.
- If animals are needed, use a limited number of herbivores with enough food, safe temperatures and a short investigation,
- and return them to their natural environment afterwards.
Part 4 of 10Deforestation of the Amazon and keystone species
Deforestation of the Amazon and keystone species
Effect on self-sufficiency
- Effect on self-sufficiency transpiration puts water vapour above the rainforest, cooling the air and affecting air movement and rainfall.
- Fewer trees change local temperature and rainfall, which affect photosynthesis and nutrient cycling.
Tipping point
- Tipping point a point beyond which the Amazon is no longer stable,
- because so many trees are removed that temperature and rainfall change greatly and may pass the tolerance limits of species.
- The area of loss needed is uncertain, so how close the ecosystem is to losing stability is unknown.
Percentage loss of rainforest
(change in area ÷ original area) × 100, with the change being initial minus final cover.
- Keystone species = species with a disproportionate effect on the structure and function of their ecosystem, whose removal can cause the loss of other species or collapse.
- Protecting them maintains stability for other species.
| Species | Role |
|---|---|
| Sea otter (Pacific Northwest kelp forests) | Controls sea urchins, which would otherwise overgraze kelp and leave no food or habitat for other species |
| Beaver | Builds homes in running water, slowing flow and creating wetland habitat |
| Elephant (African grasslands) | Eats shrubs and small trees, keeping the grassland for grazers such as zebra and so for predators such as lions |
Part 5 of 10Sustainable harvesting
Sustainable harvesting
Sustainable harvest
Sustainable harvest one where the resource is replaced as fast as it is harvested, so it does not run out, with use regulated and monitored.
Black cherry (terrestrial plant)
- It is a slow-growing hardwood in North America used for furniture, so clear felling would leave no timber for many years.
- Selective felling chooses individual trees and leaves canopy gaps that encourage forest floor growth.
- Enough trees are left to flower and seed so saplings germinate.
- Regular monitoring checks that new growth keeps pace with logging.
Alaska pollock (marine fish)
- Alaska pollock (marine fish) the largest sustainably certified fishery, certified by the Marine Stewardship Council.
- Pollock grow fast and can breed from 3–4 years.
- Nets barely touch the seabed, so they do not damage the habitat.
- Bycatch is under 1%, and it cannot be sold, so fishermen avoid it.
- Research vessels and trained onboard observers monitor the fishery.
- Areas with high salmon bycatch are closed.
Part 6 of 10Sustainability of agriculture and eutrophication
- Agriculture supplies food and materials, and sustainability depends on many factors.
| Factor | Issue and responses |
|---|---|
| Soil erosion | Clearing trees and shrubs removes roots that bind soil, so topsoil washes or blows away; crops give only partial cover and farmers plant cover crops between seasons |
| Leaching and nutrient run-off | Rain washes soluble fertilisers into water; apply small amounts when no rain is forecast, or use organic fertilisers |
| Fertiliser supply | Chemical fertilisers are expensive, in high demand and energy-intensive to make, so cost follows global energy prices; organic fertilisers help |
| Pollution | Pesticides, herbicides and fungicides protect yield but can kill non-target species; biological control (ladybirds eating aphids) and genetically modified resistant crops reduce use |
| Carbon footprint | Fossil fuels for transport, machinery and fertiliser release carbon dioxide; renewable energy and efficiency reduce it |
- Eutrophication = excess nutrients such as nitrates and phosphates entering lakes, rivers, estuaries and coastal waters,
- causing excess growth of plants and algae.
1
Mineral ions from fertiliser leach from farmland into waterways.
2
Algae grow rapidly in an algal bloom that blocks sunlight, so submerged plants cannot photosynthesise and die.
3
Algae die as competition for nutrients grows.
4
Decomposing bacteria feed on the dead matter and multiply.
5
Their aerobic respiration uses up dissolved oxygen, raising the biochemical oxygen demand (BOD).
6
Fish and insects may be unable to survive, and dead zones can form in oceans and fresh water.
Part 7 of 10Biomagnification and plastic pollution
Biomagnification
Biomagnification the increase in concentration of persistent, non-biodegradable pollutants with each trophic level up a food chain.
- Bioaccumulation is the build-up of pollutants within an organism or one trophic level.
- Concentration rises because biomass falls at higher levels, so top consumers eat many contaminated organisms.
DDT
- DDT a persistent insecticide widely used in the mid-20th century.
1
DDT sprayed on crops leached into waterways and entered freshwater and marine ecosystems.
2
It entered food chains via plankton and accumulated in fish.
3
Birds of prey eating fish accumulated higher concentrations.
4
Eggshells thinned and broke under the incubating parent, reducing young and bird populations.
- It is now banned worldwide except where it is needed to control malaria mosquitoes.
- Mercury
- released by coal-fired power plants and gold mining.
- Microorganisms convert it to toxic methyl mercury, which accumulates in fish.
- Large predatory fish such as tuna and swordfish contain high levels, a risk to humans who eat them.
Plastics are non-biodegradable and harm land and water habitats.
- Macroplastics are items over 5 mm such as bags, bottles, packaging and fishing nets.
- Microplastics are pieces under 5 mm, formed when macroplastics are broken by wave action or UV rays.
Marine harm
- Animals eat plasticturtles mistake bags for jellyfish, and albatrosses feed plastic to chicks that may starve with full stomachs.
- Animals get caught in plastic such as fishing line, causing injury and death.
- Breakdown releases toxins that can biomagnify, and tiny particles enter the food chain.
Nature of science
- Nature of science clear communication of findings, such as wildlife documentaries, changes public behaviour.
- People petition companies to cut plastic packaging, shop at low-plastic stores, recycle better and choose items such as bamboo toothbrushes and paper straws.
Part 8 of 10Restoring ecosystems and causes and types of succession
Restoring ecosystems and causes and types of succession
Rewilding
Rewilding ecosystem-level restoration of natural processes to improve stability.
Strategies
- Species reintroductionsapex predators control herbivores and let vegetation recover, raising plant and overall biodiversity,
- and keystone species can change ecosystem structure.
- Habitat connectivitywildlife corridors such as hedgerows through farmland link habitat pockets so organisms reach more resources and populations grow.
- Limiting human influencestopping logging, fishing or agriculture, and using management such as controlled grazing or burning.
Hinewai Reserve, New Zealand
- Hinewai Reserve, New Zealand once farmland, it is now privately owned for restoration.
- Non-native species were removed at first.
- It is then managed with minimal intervention so native communities return by succession, and people may walk there.
Restoring ecosystems and causes and types of succession
Ecological succession
Ecological succession progressive change in the species of a community over time, which shows ecosystems are dynamic.
Triggers
- Abioticvolcanic activity creating new land, or fire destroying part of an ecosystem and leaving unfilled niches.
- Bioticthe death and decomposition of an organism altering soil and allowing a new series of species.
Primary succession
Primary succession colonisation of newly formed land (cooling volcanic magma, new rocky islands) or newly exposed land (landslides, retreating glaciers),
- including sand dunes and bare rock.
Secondary succession
Secondary succession succession on previously occupied land, such as after wildfire or deforestation,
- which starts later because soil is already present.
Part 9 of 10Stages of primary succession
1
Wind-blown seeds and spores land on bare rock.
2
Pioneer species, often mosses and lichens, colonise first.
- They germinate easily and withstand harsh conditions with little nutrients or water.
3
Dead organisms decompose to form soil.
4
Seeds of small plants and grasses grow in the shallow, nutrient-poor soil, and their roots hold it in place.
5
As these plants die, the soil deepens and becomes nutrient-rich.
6
Larger plants, shrubs and small trees grow, needing more water stored in deeper soil.
7
Eventually the soil supports large trees, which become dominant.
8
The climax community forms, with all the plant and animal species, depending on location,
- such as rainforest in the tropics or deciduous woodland in temperate regions.
- A climax community is stable but not always the most biodiverse stage.
1
Change in conditions
- Change in conditions each stage changes the biotic and abiotic conditions.
- Lichens break up the rock surface, and with dead organic matter this forms basic soil.
- Grass roots stabilise soil so it holds more moisture and nutrients.
- New species often make conditions less suitable for earlier ones, such as lichens that cannot grow on soil.
- Pioneer species are well adapted to harsh conditions but poor competitors, so later species out-compete them for light.
- Trees shade out shrubs and smaller plants.
2
General principles
- General principles as succession progresses:
- Larger plants can be supported.
- Primary production rises.
- Species diversity rises.
- Food webs become more complex.
- Nutrient cycling increases.
Part 10 of 10Cyclic succession and human influence
1Cyclic succession
Cyclic succession a changing cycle of communities instead of a single unchanging climax, over long periods such as an oak's lifetime.
2Grazed woodland (wood pasture)
- Grazed woodland (wood pasture) a mosaic of grassland, woodland and scrub.
1
Grazers such as cattle graze open grassland and eat tree seedlings, preventing tree growth.
2
Thorny scrub such as brambles arises in places.
3
Tree seedlings survive within the thorns, which deter grazers.
4
Trees grow up through the scrub, which dies off as it loses light under the canopy.
5
At the end of its life the tree dies and falls, leaving an open area for grass, restarting the cycle.
- Human influence
- succession tends towards a climax community set by the biome, such as temperate forest, but human activity often interrupts it.
- Grazing by sheep and cattle stops tree seedlings establishing, so forest does not develop.
- Draining wetlands for farmland, development or peat fuel prevents peat bog formation.
- Interruption can maintain habitats, since without grazing, heathlands, meadows and chalk grasslands would be lost.
Key terms
| Term | Meaning |
|---|---|
| Keystone species | a species with a disproportionate effect on its ecosystem. |
| Mesocosm | a small experimental model of an ecosystem. |
| Eutrophication | excessive nutrient enrichment of water causing algal blooms and oxygen loss. |
| Biomagnification | increasing concentration of a persistent pollutant at higher trophic levels. |
| Ecological succession | progressive change in the species of a community over time. |
| Pioneer species | the first species to colonise bare land. |
| Climax community | the stable final community of a succession. |
| Rewilding | restoring ecosystems to a self-sustaining state. |
| Tipping point | a point beyond which an ecosystem moves to a new state. |
Avoid these mistakes
1
Stable ecosystems do not change at all — natural selection still acts on their species.
2
Biomagnification and bioaccumulation are the same — bioaccumulation is build-up in an organism, and biomagnification is the rise up trophic levels.
3
The climax community is the most diverse stage — it is stable, not always the most diverse.
4
Pioneer species dominate the climax community — they are poor competitors and are out-competed.
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