4.1Ecology and the Environment
The Organism in the Environment — Edexcel Linear - OL Revision Notes & Topical Questions
15 min read 17 parts Lesson 7 of 10 Unit quiz
Ecology Key Terms, Quadrats, Population Size and Biodiversity — EDEXCEL LINEAR - OL notes: Define population, community, habitat and ecosystem, use quadrats to estimate abundance and biodiversity, and explain why biodiversity matters.A population is a group of organisms of the sa…
- Worked example 1
- Worked example 2
- Worked example 3
- Worked example 4
- Percentage cover
- Species richness
Section 1 of 17Key terms in ecology
A population is a group of organisms of the same species living in the same place at the same time.
- so removing one can affect the whole community.
- such as light, mineral ions and water) interacting in an area at one time.
Section 2 of 17Investigating population size with quadrats
Ecology is the study of species distribution, abundance and their interactions with each other and the abiotic environment. Quadrats are square frames of wood or wire, such as 0.25 m2 or 1 m2, placed on the ground to record organisms inside. They measure:
Method (comparing two areas)
- lay two tape measures to mark a survey area, use a random number generator to give coordinates and place the quadrat there, count the species, repeat for at least 10 quadrats, then repeat in the second area and compare the averages.
- Abundance is influenced by biotic factors (competition, predation, food web interactions) and abiotic factors (light, minerals, water, pH, temperature, salinity).
Limitations
- organisms can be missed, especially if hidden under another species, so move leaves aside with a pencil or stick;
- identifying species may be difficult, so use a species key.
- Spell it "quadrat", not "quadrant".
Section 3 of 17Biodiversity and why it matters
Biodiversity is the range and variety of species on Earth or in an ecosystem. It considers species richness and also variation within species, their distribution and population size. High biodiversity means many different species that show variation and are evenly spread. Human activities such as waste production, deforestation and global warming are reducing biodiversity locally and globally.
Species depend on each other for food, for shelter (birds nesting in trees) and for maintaining the physical environment (tree roots stabilise soil so it is not washed away, giving a stable habitat for other plants).
- and diverse populations are more resilient to sudden environmental change or disease.
Section 4 of 17Measuring biodiversity and evaluating the method
Use the same quadrat method, but count the number of different species in each quadrat.
Section 5 of 17Worked examples
Distinguish between a community and an ecosystem. [2]2 marks
A community is all the populations of different species in an area. An ecosystem is the community together with the abiotic factors, interacting.
Explain why quadrats should be placed using random numbers. [2]2 marks
This avoids bias, so the sample is representative of the whole area.
A student counts 5, 8, 6, 9 and 7 daisies in 0.25 m2 quadrats. Calculate the mean number per quadrat and per m2. [3]3 marks
Mean = (5+8+6+9+7)/5 = 7 per quadrat. Per m2 = 7 / 0.25 = 28.
Explain why high biodiversity makes an ecosystem more stable. [3]3 marks
Organisms depend on many species rather than one for food and shelter, so loss of one species has a smaller effect, and the community can cope better with disease or change.
Section 6 of 17Measuring biodiversity step by step
Apparatus
- two tape measures, a quadrat, a random number generator and a species key.
- Method:
The area with the larger number of species is more biodiverse. However, this count does not show the distribution or population size of the species. Biodiversity is influenced by biotic factors (competition, predator-prey relationships, interactions in food chains and webs) and abiotic factors (light intensity, water availability, mineral availability, pH, temperature and salinity).
Section 7 of 17Limitations and CORMS evaluation
As with the population investigation, individuals can be missed if covered by another species (move leaves aside with a pencil or stick), and species can be hard to distinguish (use a species key).
Section 8 of 17How abiotic factors shape communities
Abiotic factors are non-living.
- and pollution reduces biodiversity;
- lichens are indicators of clean air because they are sensitive to sulfur dioxide.
Example investigation
- five glasshouses with 100 wheat plants each were kept at carbon dioxide levels from 0.03% to 0.20%, with other conditions the same.
- The dry biomass increased most between 0.03% and 0.10%.
- Carbon dioxide is a reactant in photosynthesis, so more produces more glucose for growth.
Remember to link each factor to a mechanism: a factor limits a population only because of what it does to survival, growth or reproduction, for example low oxygen reduces aerobic respiration in fish, and acidic soil reduces mineral uptake in many crops.
Section 9 of 17Evaluating the abiotic factor evidence
When evaluating the wheat data note the strengths and weaknesses.
- and glasshouse conditions differ from the field.
Section 10 of 17Worked examples
Describe how you would compare the biodiversity of two fields using quadrats. [5]5 marks
Mark a survey area in each field; use random coordinates to place the quadrat; count the number of different species in each quadrat; repeat for at least 10 quadrats in each field; compare the mean number of species.
State two variables to control in the quadrat investigation. [2]2 marks
Quadrat size and the day (conditions) on which the data are collected; also the way quadrats are placed.
Why does the biomass of wheat level off at high carbon dioxide levels? [2]2 marks
Another factor such as light, temperature or nutrients becomes limiting, so extra carbon dioxide cannot increase the rate of photosynthesis further.
A site has lots of one species but few others. Does it have high biodiversity? Explain. [2]2 marks
No. Biodiversity needs many different species; large numbers of one species do not give high species richness.
Section 11 of 17New pathogens
If a new pathogen enters an ecosystem, the populations living there may have no immunity or resistance, so their numbers fall sharply or the species may be wiped out locally. Ash dieback disease, caused by the fungus Hymenoscyphus fraxineus, has infected millions of ash trees in Europe since it was introduced from Asia, and many native ash populations are severely reduced.
When describing the effect of a pathogen on a food web, follow the chain: fewer trees means less food and shelter for herbivores and insects, so their populations also fall, and predators of those animals are affected in turn. This shows why the loss of a single species can alter a whole community.
Section 12 of 17Competition between species
Two species competing for the same resource cannot both thrive if one is better adapted: the better-adapted species outcompetes the other, and the less well-adapted species declines and may become extinct if its numbers fall too low to breed successfully.
Other biotic factorsfood availability (more food allows populations to grow, rainforests support many more species than deserts) and new predators (red foxes introduced to Australia in the 1800s for hunting have reduced native small mammals and birds and the food supply of native predators).
Competition is strongest between species with the same niche. A species with a wider diet or faster breeding is usually the better competitor. If resources are limited, the weaker competitor has less food, produces fewer young, and its population gradually declines. This is a good link to natural selection and to the idea of adaptation.
Section 13 of 17Worked example: red and grey squirrels
Scientists recorded squirrel populations in a woodland for 20 years; grey squirrels were introduced in year 6. A 5-mark answer should describe and explain.
- then decreased after the grey squirrels were introduced; the grey squirrel population increased after introduction;
- as the grey population rose the red population fell;
- by the end the red squirrel population was very small or gone.
- as the grey population rose the red population fell;
- such as unripe acorns); they may transmit disease;
- so red squirrels are outcompeted and decline.
Marking guidance
- answers about only one species, or without explanation, cannot get full marks.
- Give up to three marks for description and two for explanation (or the reverse).
- "Better adapted" must be linked to a specific advantage.
Section 14 of 17Exam technique for population graphs
You should be able to take information from charts, graphs and tables about how abiotic and biotic factors affect organisms in a community.
- then give a biological reason such as competition for food or spread of disease.
A model sentence
- "After year 6 the grey squirrel population rose from about X to Y while the red squirrel population fell from Z to near zero, because the grey squirrels competed for the same food and spread a virus." Replace X, Y and Z with values read from the graph.
- Always give units or years, and name both species in the comparison.
Section 15 of 17Worked examples
Explain why the introduction of a new pathogen can cause a native population to decline rapidly. [3]3 marks
The native organisms have not been exposed before so have no immunity or resistance. The pathogen spreads quickly and kills many individuals, so numbers fall sharply.
Give two reasons why grey squirrels have replaced red squirrels in the UK. [2]2 marks
They compete better for food and nest sites, and they carry a virus that red squirrels cannot resist.
Describe what a graph would show for red squirrels if grey squirrels outcompete them. [3]3 marks
Red numbers rise at first, then decline after grey squirrels arrive while the grey line rises, ending with very low red numbers.
Explain how an introduced predator can reduce the number of native birds. [2]2 marks
It preys on them and can eat more than balance allows, and it may reduce food for native predators.
Section 16 of 17Key terms
| Term | Meaning |
|---|---|
| Population | Group of the same species in one place at one time. |
| Community | All populations in an area. |
| Habitat | Place where an organism lives. |
| Ecosystem | Community plus its abiotic environment interacting. |
| Quadrat | Square frame used to sample organisms. |
| Percentage cover | Estimated percentage of a quadrat covered by a species. |
| Biodiversity | Range and variety of species in an area. |
| Species richness | Number of different species present. |
| Term | Meaning |
|---|---|
| Random sampling | Choosing positions by chance to avoid bias. |
| Representative | Typical of the whole area. |
| Species key | Tool used to identify species. |
| CORMS | Prompt: change, organism, repeat, measure, standardise. |
| Indicator species | Species whose presence shows environmental conditions. |
| Limiting factor | Factor in shortest supply that restricts a process. |
| Salinity | Salt concentration of water. |
| Term | Meaning |
|---|---|
| Pathogen | Microorganism that causes disease. |
| Outcompete | To be more successful in obtaining a resource, causing the other species to decline. |
| Introduced species | Species brought by humans into an area where it is not native. |
| Predator | Animal that kills and eats other animals. |
| Ash dieback | Disease of ash trees caused by a fungus. |
Section 17 of 17Exam technique
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