Stability & Change

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Apuntes de la lección

Features of Stable Ecosystems

Stable ecosystems have efficient nutrient cycling, allowing the system to be self-supporting.
They have high biodiversity, which contributes to their stability.
They show resistance to change, meaning consumer population sizes do not change significantly, so resources are not overused.
They have high levels of photosynthesis, which supports the food chain.
Examples include tropical rainforests such as the Amazon and Congo, which have remained stable for tens of millions of years.
In these rainforests, organic matter is cycled by detritivores (e.g. termites, slugs, worms) and decomposers (e.g. fungi), and water is cycled through transpiration and rainfall.
Even stable ecosystems are not entirely static; natural selection and evolutionary change always act on species.

Requirements for Ecosystem Stability

A supply of energy is essential; for most ecosystems this is sunlight, converted to chemical energy by photosynthesis.
Many photosynthetic organisms (plants or algae) must be present to capture energy and pass it up food chains.
Recycling of nutrients is essential; without it, nutrient supply will run out.
Decomposers such as bacteria and fungi break down dead organisms, releasing carbon dioxide and minerals like nitrates and phosphates.
Conditions must be suitable for decomposers: enough oxygen and moisture, and suitable temperatures.
Genetic diversity (the number of different alleles in a population) allows natural selection to act on favourable alleles, enabling adaptation to environmental change.
Climatic variables (e.g. temperature, rainfall) must remain within tolerance levels; extreme changes may force species to migrate or face extinction.
Human activities are causing climate change so rapidly that climatic variables are changing beyond tolerance levels in some ecosystems.

Investigating Ecosystem Stability: Mesocosms

A mesocosm is an experimental container in which a naturally occurring ecosystem is simulated.
Mesocosms allow control of all factors except the variable being studied, making them useful for investigating ecosystem responses.
They can be set up in many ways: water tanks on land, underwater enclosures, or large greenhouse-like buildings to replicate a rainforest.
Mesocosm experiments may be considered unrealistic due to their enclosed nature and high level of control.
Realism can be improved by designing large mesocosms that share more features of a real ecosystem (e.g. mixing of water layers).
When building a mesocosm, the container should be transparent to allow sunlight to reach producers.
Autotrophs must be included so light energy can be converted to chemical energy.
Small primary consumers may be included if the mesocosm is large enough, but secondary consumers should not be included because there is not enough energy to sustain them and it could be unethical.
Sealed systems are more controlled than open systems, and a control mesocosm must be set up to demonstrate that any change is due to the altered factor.

Building Terrestrial and Aquatic Mesocosms

A terrestrial mesocosm uses a clear container with layers: drainage material (gravel), charcoal to prevent mould, sphagnum moss or filter paper for separation, and soil or compost for organic material and microorganisms.
Slow-growing producers such as mosses and ferns are planted, and the container is sealed with a lid after watering.
Once stabilised, plants release enough water vapour during respiration to maintain moisture levels.
An aquatic mesocosm uses organic substrate from a lake or pond, lake or pond water (to provide microscopic organisms and avoid tap water chemicals), and healthy aquatic plants.
Small aquatic organisms such as water fleas or water snails can be added, but only primary consumers and not more than the mesocosm can support.
Both types should be placed in a light location with stable temperature.
Ethical guidelines for animals in mesocosms: investigations should only involve animals if no alternatives exist, must not be cruel, and should remove potential causes of distress.
If animals are required, only a limited number of herbivorous animals should be included, with enough food, controlled temperature, short duration, and return to natural environment at the end.

Ecosystem Stability: Deforestation in the Amazon

Deforestation affects the self-sufficiency of the Amazon ecosystem by influencing temperature and rainfall.
Transpiration from trees releases water vapour, cooling the air and affecting air movement and rainfall.
Changes in tree numbers can therefore influence local temperatures and rainfall, which in turn affect photosynthesis and nutrient cycling.
Scientists are concerned that the Amazon could reach a tipping point beyond which it is no longer stable.
At this tipping point, so many trees are removed that temperature and rainfall patterns change significantly, and climatic factors may change beyond tolerance levels for some species.
There is uncertainty about how much rainforest loss would trigger this tipping point, so we do not know how close the Amazon is to losing stability.
Percentage loss of rainforest can be calculated as: (change in rainforest area / original rainforest area) × 100.

Keystone Species

Keystone species have a disproportionate effect on the structure and function of their ecosystem.
Their removal can cause significant changes, including loss of other species and possible ecosystem collapse.
Protecting keystone species helps maintain ecosystem stability, benefiting other species.
Sea otters in kelp forests control sea urchin populations; without them, sea urchins overgraze vegetation, leaving no food or habitats for other species.
Beavers build homes in running water, slowing flow and creating wetland habitats that support many other species.
Elephants in African grasslands consume shrubs and small trees, maintaining grassland and food for grazers like zebras, which in turn supports large predators like lions.

A keystone species removed

Sustainability in Ecosystems and Agriculture

A sustainably harvested resource is one that is replaced as rapidly as it is harvested, so it does not run out.
Sustainable use requires careful regulation and monitoring to prevent over-harvesting.
Black cherry is a slow-growing hardwood; sustainable harvesting involves selective felling, leaving enough individuals to produce seeds, and regular monitoring.
The Alaska pollock fishery is the largest sustainably certified fishery, certified by the Marine Stewardship Council (MSC).
Pollock are fast-growing and can reproduce from age 3–4 years; nets have minimal contact with the sea bed, bycatch is less than 1%, and monitoring is carried out by research vessels and trained individuals.
Soil erosion occurs when land is cleared, removing roots that hold soil together; topsoil is easily washed or blown away, reducing soil available for crops.
Leaching and nutrient run-off from synthetic fertilisers can wash into waterways; this can be reduced by applying fertilisers in small volumes, avoiding rain, and using organic fertilisers.
Chemical fertilisers are expensive, supply may not meet demand, and production is energy-intensive; organic fertilisers can reduce these difficulties.
Agrochemicals (pesticides, herbicides, fungicides) can enter the environment and kill non-target species; biological pest control and genetic modification can reduce their use.
Agriculture's carbon footprint comes from fossil fuel use for transport, machinery, and fertiliser production; transitioning to renewable energy and energy-efficient practices can help.

Eutrophication

Eutrophication occurs when water bodies receive artificially large inputs of nutrients such as nitrates and phosphates, often from fertiliser leaching.
This causes excess growth of plants and phytoplankton, known as an algal bloom.
Algal blooms block sunlight, causing aquatic plants below the surface to die as they cannot photosynthesise.
Algae also die when competition for nutrients becomes too intense.
Decomposing bacteria feed on dead organic matter and increase in number, respiring aerobically and using up dissolved oxygen.
This creates an increased biochemical oxygen demand (BOD), rapidly decreasing dissolved oxygen so fish and insects may not survive.
Dead zones in oceans and freshwater occur when there is not enough oxygen to support aquatic life.

An algal bloom

Effects of Pollution: Biomagnification

Biomagnification is the increase in concentration of persistent or non-biodegradable pollutants with ascending trophic level through a food chain.
It differs from bioaccumulation, which is the build-up of pollutants within an organism or within a single trophic level.
Pollutants become more concentrated at higher trophic levels because total biomass decreases, so top predators consume many smaller organisms and receive a larger dose.
DDT was a widely used insecticide that leached into waterways, entered food chains via plankton, and accumulated in fish and then birds of prey.
DDT caused thinning of eggshells and reduced reproductive success, leading to reduced bird populations; it is now banned worldwide except for essential mosquito control.
Mercury released from coal-fired power plants and gold mining can be converted by microorganisms into highly toxic methyl mercury, which accumulates in fish and can harm humans who eat large predatory fish like tuna or swordfish.
Plastics are non-biodegradable; macroplastics (>5 mm) and microplastics (<5 mm) cause harm.
In marine habitats, animals may eat plastic (e.g. turtles mistaking bags for jellyfish, albatrosses feeding plastic to chicks), become caught in it, or ingest toxins that biomagnify.
Clear communication of scientific findings can change public behaviour, e.g. reducing plastic packaging, recycling, and choosing non-plastic items.

Restoring Ecosystems

Human activities such as deforestation and overharvesting can destabilise ecosystems.
Ecosystem restoration (rewilding) aims to improve stability by restoring natural ecosystem processes.
Strategies include species reintroductions, especially apex predators to control herbivores and allow vegetation recovery, and keystone species to alter ecosystem structure.
Improving habitat connectivity through wildlife corridors (e.g. hedgerows) allows organisms to roam larger areas, access more resources, and increase population size.
Limiting human influence may involve preventing logging, fishing, or agriculture, and using ecological management techniques like controlled grazing or burning.
Hinewai Reserve in New Zealand was once farmland and is now privately owned for restoration; initial human intervention removed non-native species, but it is now managed with minimal intervention to allow native communities to restore by succession.
The public can enjoy walking in the Reserve, but human activities are limited.

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Preguntas de práctica

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1.What is required for an ecosystem to be stable over an extended time period? I. An external supply of soil nutrients II. Genetic diversity III. An energy supply
Easy
- AI and II only
- BI and III only
- CII and III only
- DI, II and III
2.Which of the following statements about keystone species is correct?
Easy
- AKeystone species are always top predators.
- BKeystone species do not affect the diversity of their surrounding ecosystems.
- CThe influence of a keystone species on its ecosystem is equivalent to that of any other species.
- DRemoving a keystone species risks ecosystem collapse.
3.Which of the following considerations should be taken into account when building a mesocosm to test the impact of a particular environmental variable?
Medium
- AAny type of container can be used; no secondary consumers; inclusion of autotrophs; producers should be slow-growing.
- BTransparent container; food chains of up to three organisms can be included; inclusion of autotrophs; producers should be slow-growing.
- CTransparent container; no secondary consumers; inclusion of autotrophs; a control mesocosm should be built.
- DTransparent container; no secondary consumers; inclusion of autotrophs; soil should be sterilised to prevent contamination.
4.The stability of an ecosystem can be investigated using a mesocosm. What ethical guidelines should be followed when using animals in a mesocosm? I. Animals should be used in preference to alternative organisms. II. Enough food should be available for any animals present. III. Animals must be returned to their natural environment at the end of an investigation.
Medium
- AI and II only
- BII and III only
- CI and III only
- DI, II and III
5.A region of rainforest with an area of 4,100,000 km² was reduced to an area of 3,855,000 km² over a ten-year period. Which statement about deforestation in the Amazon rainforest is correct?
Medium
- AOnly a small area of rainforest is needed to sustain the cycle of transpiration and rainfall that maintains the ecosystem.
- BThe percentage decrease in area was 6%.
- CScientists know the area of rainforest that needs to be maintained in order to avoid passing a tipping point in rainforest stability.
- DThe Amazon rainforest will be able to recover even if deforestation continues until the stability tipping point is reached.
6.Which of the following are features of a stable ecosystem? (Select all that apply.)
Medium
- AEfficient nutrient cycling
- BHigh biodiversity
- CRapid and constant change in species composition
- DHigh levels of photosynthesis
- EDependence on external nutrient supply
7.Keystone species have a disproportionate effect on the structure and function of their ecosystem.
Easy
True or false?
8.Match each term with its correct description.
Medium
- Biomagnification
- Bioaccumulation
- Eutrophication
- Increase in concentration of persistent pollutants with ascending trophic level
- Build-up of pollutants within an organism or single trophic level
- Excess nutrient input causing algal blooms and oxygen depletion

Más preguntas

9.A simple aquatic mesocosm is shown with a green plant labelled A. Why is A an important component of the mesocosm?
Easy
- AA provides carbohydrates and oxygen to support the mesocosm.
- BA provides protection for the primary consumers from predators.
- CA converts heat energy to chemical energy.
- DA helps to maintain a stable temperature.
10.Place the stages of eutrophication in the correct order.
Medium
- Leaching of fertilisers into waterways
- Rapid growth of algae (algal bloom)
- Death of aquatic plants and algae due to reduced light and competition
- Increase in decomposing bacteria and oxygen depletion
11.Which of the following should be included when building a laboratory mesocosm? (Select all that apply.)
Medium
- AA transparent container
- BAutotrophs
- CA control mesocosm
- DSecondary consumers
- EFast-growing producers
12.A sustainable resource is one that is replaced as rapidly as it is harvested.
Easy
True or false?
13.Which of the following are examples of keystone species? (Select all that apply.)
Medium
- ASea otters
- BBeavers
- CElephants
- DTermites
- EMosses
14.In a sealed mesocosm, organisms and substances cannot enter or leave.
Easy
True or false?
15.A mesocosm was set up to study the effect of increasing global temperatures on the biomass of aquatic autotrophs and heterotrophs. As the temperature increased from 21 °C to 27 °C, the biomass of autotrophs decreased while the biomass of heterotrophs increased. Which statement best explains this result?
Medium
- AHeterotrophs gained energy directly from the increased heat, so their biomass rose.
- BHigher temperatures reduced photosynthesis in autotrophs, so less energy was available, but heterotrophs consumed the dying autotrophs.
- CAutotrophs migrated out of the mesocosm as it warmed, leaving more space for heterotrophs.
- DHeterotrophs photosynthesised more efficiently at higher temperatures, outcompeting the autotrophs.
16.A small island formed about 200 years ago. Ecologists want to estimate the present-day mean percentage cover of grass on the island, which has a surface area of about 100 square metres. Which method is most appropriate?
Medium
- ACount every grass plant on the island and divide by the total area.
- BPlace a single 1 m² quadrat in the centre of the island and record the grass cover.
- CUse randomly placed quadrats across the island and calculate the mean percentage cover.
- DMeasure the height of grass in several areas and average the values.
17.If the biologists only wanted to study the abundance of broad-leaved trees on the island, they could have used frequency rather than percentage cover. Why is this possible?
Medium
- AFrequency is easier to record for large, slow-moving organisms.
- BBroad-leaved trees are too large to fit inside a quadrat.
- CFrequency counts only the number of individuals, which is more accurate for trees.
- DPercentage cover is only used for animals, not plants.
18.Nutrient-rich sewage from coastal settlements causes algal blooms, which reduce light intensity on the ocean floor. What is the most likely general effect on the marine community?
Medium
- AIncreased photosynthesis by bottom-dwelling plants, raising oxygen levels.
- BDeath of submerged aquatic plants, followed by increased decomposer activity and reduced dissolved oxygen.
- CIncreased diversity of fish species near the ocean floor.
- DReduced growth of algae due to competition for nutrients.
19.Since 2013, starfish numbers in some large marine ecosystems have fallen by up to 80%. Starfish are detritivores and also prey on mussels and limpets. Some scientists suggest this decline may have a positive impact. Which statement best evaluates this claim?
Hard
- AIt is correct because fewer starfish means more food for other predators.
- BIt is incorrect because starfish are a keystone species and their loss will reduce biodiversity.
- CIt could be positive for mussel and limpet populations, but negative overall because starfish help recycle nutrients and control prey populations.
- DIt is correct because starfish are secondary consumers and their removal increases energy available to primary consumers.
20.Scientists want to simulate a naturally occurring ecosystem to investigate how global warming might affect its sustainability. Which approach best simulates the ecosystem while allowing control of the variable?
Hard
- AObserve a natural ecosystem without changing anything and record temperature over time.
- BBuild a large mesocosm that includes producers, primary consumers and nutrient cycling, and compare it with an identical control mesocosm kept at the original temperature.
- CPlace a single plant in a sealed jar and measure its growth at different temperatures.
- DCollect water samples from a natural lake and measure their temperature in the laboratory.
21.A scientist collects data on the number of two species in a mesocosm before and after changing the light level. Which statistical test is most appropriate to determine whether the change in species distribution is significant?
Medium
- AChi-squared test
- Bt-test
- CSpearman's rank correlation
- DSimpson's index
22.A region of rainforest with an area of 4,100,000 km² was reduced to an area of 3,855,000 km² over a ten-year period. What was the percentage decrease in rainforest area?
Medium
- A6.0%
- B5.9%
- C6.4%
- D94.0%
23.Which statement about the Amazon rainforest is correct?
Medium
- AOnly a small area of rainforest is needed to maintain the transpiration and rainfall cycle.
- BScientists know exactly how much rainforest must remain to avoid a tipping point.
- CThe rainforest will recover even if deforestation continues past the tipping point.
- DDeforestation can change local temperature and rainfall, affecting ecosystem stability.
24.A team used 20 mesocosms (400-litre tanks on land) to test the effect of invasive species A and B on native frog, toad and newt species. The mesocosms contained the native species alone, with A, with B, and with both A and B. Why was it important to include mesocosms with no invasive species?
Medium
- ATo provide a control showing the natural change in native species without invasive species.
- BTo increase the total number of organisms in the experiment.
- CTo allow the invasive species to establish before the native species were added.
- DTo test the effect of a different environmental variable.
25.In a sustainably managed fishery, the population of fish is monitored and catch limits are set. Which condition must be met for the fishery to be sustainable?
Medium
- AThe fish are replaced as rapidly as they are harvested.
- BOnly large, old fish are caught.
- CFishing is stopped completely every other year.
- DThe fish are farmed rather than caught in the wild.
26.A lake receives run-off containing nitrate fertiliser from nearby farmland. Which sequence of events is most likely to follow?
Hard
- AAlgal bloom → increased light penetration → increased photosynthesis by submerged plants → increased oxygen.
- BAlgal bloom → reduced light penetration → death of submerged plants → increased decomposer activity → reduced dissolved oxygen.
- CReduced nitrate levels → reduced algal growth → increased dissolved oxygen → increased fish survival.
- DIncreased nitrate levels → increased decomposition → increased dissolved oxygen → algal bloom.
27.The concentration of DDT in the tissues of organisms in a food chain was measured. The concentration was highest in birds of prey. Which process explains this?
Medium
- ABioaccumulation
- BBiomagnification
- CEutrophication
- DDecomposition
28.Mercury released from coal-fired power plants is converted by microorganisms into methyl mercury. Which statement best explains why large predatory fish such as tuna can contain dangerous levels of methyl mercury?
Hard
- AMethyl mercury is excreted rapidly, so it does not accumulate.
- BMethyl mercury is persistent and becomes more concentrated at higher trophic levels.
- CLarge fish absorb methyl mercury directly from the water through their gills.
- DMethyl mercury is broken down by large fish, so it does not harm them.
29.Which of the following is a consequence of soil erosion caused by clearing land for agriculture?
Medium
- AIncreased nutrient content in the topsoil.
- BLoss of nutrient-rich topsoil, reducing crop growth.
- CIncreased soil stability due to removal of tree roots.
- DDecreased leaching of fertilisers into waterways.
30.A farmer wants to reduce the leaching of fertiliser into a nearby river. Which practice would be most effective?
Medium
- AApplying fertiliser just before heavy rain is forecast.
- BUsing large volumes of synthetic fertiliser in one application.
- CApplying fertiliser in small volumes at times when rain is not forecast.
- DPloughing the field immediately after applying fertiliser.
31.Which statement best describes the difference between bioaccumulation and biomagnification?
Hard
- ABioaccumulation occurs across trophic levels, while biomagnification occurs within one organism.
- BBioaccumulation is the build-up of pollutants within an organism or trophic level, while biomagnification is the increase in concentration with ascending trophic level.
- CBioaccumulation only occurs in aquatic ecosystems, while biomagnification only occurs in terrestrial ecosystems.
- DBioaccumulation refers to nutrients, while biomagnification refers to toxins.
32.A restoration project reintroduces a keystone predator into a degraded ecosystem. Which outcome is most likely?
Hard
- AHerbivore populations increase, reducing plant diversity.
- BHerbivore populations are controlled, allowing vegetation to recover and increasing overall biodiversity.
- CThe predator becomes the only species in the ecosystem.
- DNutrient cycling stops because the predator consumes all the herbivores.
33.Which of the following is an example of improving habitat connectivity in a restoration project?
Hard
- ABuilding a fence around a nature reserve.
- BPlanting hedgerows to connect small patches of woodland.
- CRemoving all predators from an area.
- DIntroducing a non-native species to increase biodiversity.
34.Which of the following are requirements for an ecosystem to be stable over an extended time period? (Select all that apply.)
Medium
- AA reliable supply of energy
- BRecycling of nutrients
- CGenetic diversity within populations
- DClimatic variables remaining within tolerance levels
- EAn external supply of soil nutrients that never runs out
35.Which of the following statements about biomagnification are correct? (Select all that apply.)
Hard
- AIt involves persistent or non-biodegradable pollutants.
- BConcentration increases at higher trophic levels.
- CIt is the same as bioaccumulation.
- DIt can lead to harmful effects in top predators.
- EIt only occurs in terrestrial ecosystems.
36.Bioaccumulation is the increase in concentration of a pollutant with ascending trophic level.
Medium
True or false?
37.Keystone species always occupy the highest trophic level in a food chain.
Medium
True or false?
38.Eutrophication can lead to the formation of dead zones where dissolved oxygen is too low to support aquatic life.
Medium
True or false?
39.Match each agricultural problem with its cause.
Medium
- Soil erosion
- Leaching
- Carbon footprint
- Pesticide pollution
- Rainfall washing soluble fertiliser into waterways
- Removal of roots that hold soil together
- Combustion of fossil fuels for machinery and fertiliser production
- Killing of non-target species by agrochemicals
40.Place the stages of secondary succession after a disturbance in the correct order.
Hard
- Disturbance removes existing community
- Pioneer species colonise the area
- Species diversity increases
- Competition increases and some species are outcompeted
- Climax community establishes
41.Complete the sentence about mesocosms.
Easy
A mesocosm is an experimental ____ in which a naturally occurring ecosystem is simulated.
42.A region of rainforest with an area of 4,100,000 km² was reduced to an area of 3,855,000 km² over a ten-year period. Calculate the percentage decrease in rainforest area.
Medium
43.Name the process by which the concentration of a persistent pollutant increases at successively higher trophic levels in a food chain.
Medium
44.What is the name of the experimental container in which a naturally occurring ecosystem is simulated?
Medium
45.What term describes the increase in biochemical oxygen demand caused by decomposing bacteria in a eutrophic lake?
Medium
46.A region of rainforest with an area of 4,100,000 km² was reduced to an area of 3,855,000 km² over a ten-year period. Calculate the percentage decrease in rainforest area. Give your answer to one decimal place.
Medium
47.A lake has a dissolved oxygen concentration of 8.0 mg/L before a sewage spill. After the spill, the concentration falls to 2.4 mg/L. Calculate the percentage decrease in dissolved oxygen concentration.
Hard
48.In a mesocosm experiment, the biomass of autotrophs at 21 °C was 120 g. At 27 °C it was 84 g. Calculate the percentage decrease in autotroph biomass.
Medium
49.Complete the sentence about eutrophication.
Medium
Excess nutrients cause an algal ____ that blocks sunlight, leading to the death of submerged plants and a decrease in dissolved ____.
50.Complete the sentence about biomagnification.
Medium
Biomagnification is the increase in concentration of persistent pollutants with ascending ____ level, and it can lead to harmful effects in top ____.
51.Identify three key features of a stable ecosystem.
Hard
52.Explain why succession in the Hinewai Reserve is likely to occur more quickly than in examples of primary succession.
Hard
53.Evaluate the benefit of mesocosm experiments in understanding interactions between organisms in their natural environment.
Hard
54.Describe how projects such as the Hinewai Reserve help to restore biodiversity.
Hard
55.Suggest what benefit was provided by Ulex europaeus (gorse) in the initial stages of the Hinewai Reserve restoration process.
Hard
56.A region of rainforest with an area of 4,100,000 km² was reduced to an area of 3,855,000 km² over a ten-year period. Calculate the percentage decrease in rainforest area.
Medium
- A7 %
- B5 %
- C6 %
- D8 %
57.A region of rainforest with an area of 4,100,000 km² was reduced to an area of 3,855,000 km² over a ten-year period. Calculate the percentage decrease in rainforest area. Give your answer to one decimal place.
Medium
- A8 %
- B6 %
- C5 %
- D7 %
58.A lake has a dissolved oxygen concentration of 8.0 mg/L before a sewage spill. After the spill, the concentration falls to 2.4 mg/L. Calculate the percentage decrease in dissolved oxygen concentration.
Hard
- A63 %
- B70 %
- C77 %
- D84 %
59.In a mesocosm experiment, the biomass of autotrophs at 21 °C was 120 g. At 27 °C it was 84 g. Calculate the percentage decrease in autotroph biomass.
Medium
- A33 %
- B30 %
- C27 %
- D36 %

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