Populations and ecosystems: WJEC A level Biology cram notes

Lesson 1 of 19

How do populations grow and change in size?

Cram notes for topic 3.5, size and , in WJEC A level Biology Unit 3.

A is all the organisms of one in one place at one time. Change in = (births + ) − (deaths + ).

The graph below shows the four phases of growth in a closed environment.

Line graph of number of individuals in thousands against time from 0 to 40 days. The S-shaped curve starts near 0, rises slowly to about 3 thousand at 5 days, rises steeply through 50 thousand at about 13 days, levels off at about 89 thousand between 22 and 30 days, then falls to about 58 thousand at 40 days. Labels read lag phase near the start, exponential phase beside the steep section, stationary phase above the plateau and death phase beside the fall. — The four phases of growth of a population in a closed environment. Original BioWelsh graph.

: few individuals, still adjusting, so growth is slow.
: plentiful resources, so is much greater than death rate.
: resources limit growth, so equals death rate.
: nutrients run out and toxic waste builds up, so death rate exceeds .

Lesson 2 of 19

What is carrying capacity and what limits population growth?

is the largest an environment can support for a long time. is all the and factors that limit growth.

Above the , competition, predation and disease increase, so the falls.
Below it, they decrease, so the rises: this makes numbers fluctuate around the .
Predator numbers rise after prey numbers rise, and fall after prey numbers fall.

Lesson 3 of 19

How does competition affect populations?

is between individuals of the same ; is between different .

increases with and regulates size.
In the better competitor can exclude the other, as when P. aurelia outcompetes P. caudatum for food.
Plants compete for light, water, space and mineral ions such as and .

Lesson 4 of 19

How do density-dependent and density-independent factors regulate populations?

A has a bigger effect at higher : competition, predation, parasites, disease and toxic waste. A affects the same proportion at any : flood, fire, drought or frost.

Lesson 5 of 19

How do ecologists sample organisms in a habitat?

avoids bias: lay two tapes at right angles, use as coordinates, place a , repeat many times and calculate a .

is number per m²; suits plants that are hard to count; is the percentage of containing a .
Use enough that the running levels off.
For mobile animals, use : = (n₁ × n₂) ÷ m.
This assumes marks do not affect survival, animals mix fully, and there are no births, deaths or migration between samples.

Lesson 6 of 19

How do transects show the distribution of organisms?

A is a line along an . Line record organisms touching the tape; belt use along it; interrupted belt place at regular intervals.

The below shows along a sand dune .

Kite diagram with distance from the strandline from 0 to 45 metres. Four rows are labelled with species names. Sand couch grass is widest at 10 metres and absent beyond 15 metres. Marram grass is present from 5 to 30 metres and widest at 15 to 20 metres. Red fescue is present from 20 to 45 metres and widest at 30 metres. Bird's-foot trefoil is present from 30 to 45 metres. A scale bar shows the width equal to 50% cover. — Kite diagram of percentage cover of four plant species along a sand dune transect. Original BioWelsh diagram.

The width of each kite shows at that distance.
Sand couch grass grows nearest the sea, marram grass on the yellow dunes, and red fescue and bird's-foot trefoil further inland.

Lesson 7 of 19

Specified practical: investigating the distribution of seaweeds on a rocky shore

Seaweeds form down a rocky shore because time exposed to air changes along the shore. Upper shore tolerate drying out; lower shore outcompete them lower down.

Specified practical

Investigating the abundance and distribution of seaweeds on a rocky shore

Unit 3, section 5, specified practical 1 (BioWelsh entry number)

Aim To find how seaweed changes down a rocky shore using an interrupted belt .

Investigating seaweed distribution on a rocky shore at a glance. Original BioWelsh diagram.

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Equipment

Tape measure
Tape measureMarks the .
Gridded quadrat
Gridded Estimates .
Ranging poles and spirit level
Ranging poles and spirit levelMeasure height.
Identification guide
Identification guideIdentifies .

Safety

Slippery rocks and incoming tideRiskFalling or being cut off.ControlGrip boots; check tide times.

Variables

Independent variableHeight on the shore

every 5 m down the .

Dependent variableAbundance of each seaweed

.

Controlled variablesQuadrat size, Interval between quadrats, Aspect and wave exposure

size

Same 0.5 m × 0.5 m .

Interval between

Every 5 m.

Aspect and wave exposure

Same stretch of shore.

Method, step by step

1)
On the day of the lowest tide, lay a tape from the top of the shore to the sea.
Why?
Follows the .
2)
Place a every 5 m and record of each seaweed.
3)
Measure the height of each with ranging poles and a spirit level.
Measuring the drop in height between two points on a shore with ranging poles. Original BioWelsh diagram.
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4)
Repeat the , calculate and draw a and shore profile.

Results and analysis

Example data

Distance along the at which each seaweed had its highest

	Highest cover / %	Distance / m	Height / m
Channelled wrack (Pelvetia canaliculata)	55	5	5.1
wrack (Fucus spiralis)	60	10	4.3
Bladder wrack (Fucus vesiculosus)	70	20	2.7
Serrated wrack (Fucus serratus)	65	30	1.2
Oarweed (Laminaria digitata)	80	40	0.1

Kite diagram with distance from the top of the shore from 0 to 40 metres. Channelled wrack is present from 0 to 10 metres and widest at 5 metres. Spiral wrack is present from 5 to 20 metres and widest at 10 metres. Bladder wrack is present from 15 to 30 metres and widest at 20 metres. Serrated wrack is present from 25 to 35 metres and widest at 30 metres, narrowing to nothing at 40 metres. Oarweed is present from 35 metres and widest at 40 metres. A scale bar shows the width equal to 50% cover. — Kite diagram of percentage cover of five seaweed species along a rocky shore transect. Original BioWelsh diagram.

Profile of the rocky shore along the transect. Original BioWelsh graph.

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What the results show

Each seaweed has a zone: channelled wrack is highest on the shore and oarweed lowest, with the three Fucus in between.

Upper shore tolerate drying out; lower shore grow faster and outcompete them lower down.

Worked calculations

Height at 10 m = 5.8 − (0.7 + 0.8) = 4.3 m

Evaluation

Cover estimated by eye.

Effect on resultsLess reproducible.

ImprovementSame person throughout.

Quadrats only every 5 m.

Effect on resultsBoundaries missed.

Improvement belt .

Going further

Choosing a method

: when conditions change along a gradient.
Random : when comparing two uniform areas.
: for mobile animals.

Model answer: describe the method

Describe how you would investigate the of seaweed down a rocky shore.

Original practice response · 6 indicative marks

Check tide times; survey on the lowest tide.
Lay a tape along the .
of size every 5 m.
Identify seaweeds and estimate .
Measure height with ranging poles.
Repeat , calculate , draw a .

What the answer needs to cover

along gradient
at regular intervals
Height measured
Repeats and
Tide safety

Exam practice

What is a transect?Show answer

A line laid along an to sample how the of organisms changes.

Line

Lesson 8 of 19

What are ecosystems, habitats and communities?

: the place where an organism lives.
: all the of different in one place at one time.
: a interacting with its environment; from a rock pool to an ocean.
The Sun supplies the energy for almost all through . Energy flows; nutrients cycle.

Lesson 9 of 19

What are gross and net primary production?

is the rate at which convert light energy into chemical energy. = − R, and is the energy available to the next .

Most light is not used: it is reflected, transmitted, the wrong , or is limited by other factors.
Harvest method: = increase in per m² ÷ time; dry to constant mass without burning.
Tropical rainforest has high (warm, wet, bright all year); deserts and tundra have low .

Lesson 10 of 19

How efficiently is energy transferred along food chains?

Only about 10% of energy passes from one to the next. = energy transferred ÷ energy in previous level × 100.

Energy is lost because not all of the organism is eaten, not all is digested, some is excreted, and most is released in as heat.
Carnivores are often more efficient than herbivores because their food is protein-rich and easier to digest.
So much energy is lost that food chains rarely have more than four or five levels.

Lesson 11 of 19

What do pyramids of numbers, biomass and energy show?

Pyramids of numbers and can be inverted, for example one oak tree supporting many caterpillars, or fast-reproducing phytoplankton. A is never inverted.

Lesson 12 of 19

How does succession change bare rock into woodland?

is the change in a over time. starts on bare rock with no soil; starts where soil is already present, so it is faster.

in lowland Britain

1) Lichens ()
2) Mosses
3) Grasses and ferns
4) Shrubs
5) Birch
6) Oak woodland (climax)

weather rock and, when they decompose, form and thin soil.
Each changes the conditions so new can grow and outcompete the old ones; the whole sequence is a .
The is stable. Soil depth, and number increase during .

Lesson 13 of 19

Why is the breakdown of dead organic matter important?

(bacteria and fungi) secrete onto dead matter, absorb the products and respire, releasing carbon dioxide and mineral ions such as ammonium. Without them, nutrients would stay locked in dead organisms.

Lesson 14 of 19

How is carbon cycled in an ecosystem?

removes carbon dioxide; by all organisms, including , returns it.
Carbon passes along food chains by feeding, and to dead matter by death and egestion.
Where decay is prevented, dead matter forms peat and ; combustion releases the carbon.
Oceans dissolve carbon dioxide; shells form chalk and limestone.

Lesson 15 of 19

How do human activities affect the carbon cycle and climate?

Burning and raise atmospheric carbon dioxide, from about 317 ppm in 1960 to about 425 ppm in 2024. This enhances the , causing and .

less , plus carbon dioxide from burning and decomposition.
ranges shift towards the poles and uphill; that cannot move or adapt may become .

Lesson 16 of 19

What is the role of bacteria in the nitrogen cycle?

Plants absorb to make amino acids, proteins, nucleic acids and chlorophyll. Bacteria carry out the steps of the nitrogen cycle.

: Rhizobium in and free-living Azotobacter convert N₂ to ammonium ions.
: release ammonium ions from proteins, and nucleic acids.
(aerobic): Nitrosomonas converts ammonium to nitrite; Nitrobacter converts nitrite to .
(anaerobic): Pseudomonas converts to nitrogen gas.
binds oxygen in , because does not work in oxygen.

The diagram below summarises the nitrogen cycle.

Flow diagram of six boxes: nitrogen gas in the air at the top; proteins and nucleic acids in plants on the left, and in animals below; ammonium ions, nitrite ions and nitrate ions in soil. A blue arrow from nitrogen gas to ammonium is labelled nitrogen fixation by Rhizobium in root nodules and by free-living Azotobacter. Feeding links plants to animals. Brown arrows from plants and animals to ammonium are labelled death, urea and faeces, and ammonification by decomposers. A grey arrow from ammonium to nitrite is labelled Nitrosomonas, nitrification. A purple arrow from nitrite to nitrate is labelled Nitrobacter, nitrification. A green arrow from nitrate to plants is labelled absorbed by roots. A red arrow from nitrate to nitrogen gas is labelled denitrification by Pseudomonas in anaerobic soil. — The nitrogen cycle and the bacteria involved. Original BioWelsh diagram.

Follow the arrows from nitrogen gas through ammonium, nitrite and to plant proteins.
returns nitrogen gas to the air.

Lesson 17 of 19

How do farming practices affect the nitrogen cycle?

Harvesting removes nitrogen from fields. Ploughing and drainage let air into the soil, which helps nitrifying bacteria and reduces . and in add nitrogen and increase yields.

Lesson 18 of 19

What causes eutrophication and algal blooms?

in six steps

1) leaches into water
2)
3) Light blocked; plants die
4) increase
5) respire and use up oxygen
6) Fish die

Draining wetlands destroys habitats for adapted to wet conditions, and the drained water carries into rivers.

Lesson 19 of 19

How do the ideas in population size and ecosystems fit together?