Human Impact on the Environment, Grade 11: The 25-Mark Alien Species Task and Five Corrections

There is a formally assessed practical in this topic worth 25 marks, and the questions inside it add to 24.

We counted them: 2 plus 1 plus 6 plus 6 plus 4 plus 3 plus 2. That is 24, against a printed total of 25. Somebody marking a perfect script has to decide for themselves where the twenty fifth mark lives.

The same task then tells learners to sample a control site containing "natural alien vegetation", when the table they are filling in is headed indigenous vegetation and the entire comparison depends on it. This page gives the task with a working mark split, the content of all five units, and five places where the printed account is wrong or incomplete.

Where this fits in the curriculum

Subject Life Sciences
Grade 11
Term 4
Topic Topic 11, Human impact on the environment
Units Unit 1 the atmosphere and climate change. Unit 2 water. Unit 3 food security. Unit 4 loss of biodiversity. Unit 5 solid waste disposal
Formal assessment Practical task, investigate the impact of alien species on biodiversity, 25 marks
Marks Our worksheet is 70

Finding 1: the assessed task does not add up

The task is printed with a total of 25. Its seven mark allocations add to 24.

Question What is asked Marks
1 Name the alien invasive plants on your site 2
2.2 Write down your hypothesis 1
3 Sample the invaded site and complete the first column 6
4 Sample the comparison site and complete the second column 6
5 Pool the class results and work out the average 4
6 Do your results support your hypothesis 3
7 Why take an average across several sites 2
Printed total 25
Actual total 24

Question 2.1 is the one with no mark on it. It asks whether the learner thinks the alien species increase, decrease or have no effect on biodiversity, and it is the question the whole hypothesis is built on. Giving it the missing mark makes the task add to 25 and rewards the thinking the task is actually about.

Decide before the class starts, and tell them. A task that does not add up gets marked differently by every teacher in the school, and the learner who loses out is the one who did it properly.

Finding 2: the control site is described as the wrong kind of site

Question 4 says to find land that "seems to be undisturbed and contains natural alien vegetation".

The table that question fills in is headed "Site with indigenous vegetation". The whole investigation compares an invaded patch against an uninvaded one. Sampling a second patch of alien vegetation would compare alien against alien and could not test the hypothesis at all.

What the task means
Site one: land covered by the alien invasive plant
Site two: undisturbed land with indigenous vegetation, as similar as possible in everything else
The comparison is the point. If both sites are invaded, there is no result

The task does allow for a school with no undisturbed ground nearby to leave that part out. If you do, say so in the report, because a conclusion drawn from one site only is exactly what question 7 is asking about.

How to run the task

It costs nothing. Twenty metres of string marked every five metres, four sticks, and a notebook.

  1. Find a patch covered by one alien invasive plant. Black wattle, pine, bugweed, blackjack and Port Jackson willow are the examples given, and in most South African schoolyards at least one of them is within walking distance
  2. Mark out 5 m by 5 m with the string and the four sticks. That is a 25 square metre sample area, and the twenty metres of string is exactly its perimeter
  3. Count how many different species are present, not how many individuals. Three kinds of ant, two of beetle, one butterfly and three bugs is nine species, not nine hundred ants
  4. Repeat on an indigenous patch
  5. Pool the class results and average them

The distinction in step 3 is the one that decides the marks. Species richness counts kinds. Abundance counts individuals. A learner who counts every ant has measured the wrong thing and will get a number that cannot be compared with anything.

Unit 1: the atmosphere and climate change

Idea What it means
The greenhouse effect Greenhouse gases trap heat energy radiating from the Earth's surface, which keeps the surface warm. This is natural and life depends on it
The enhanced greenhouse effect Human activity adds more greenhouse gases, more heat is trapped, and the Earth warms
The three main greenhouse gases Carbon dioxide, methane and nitrous oxide
Where they come from Burning fossil fuels in homes, vehicles and industry, deforestation and burning vegetation, landfill and sewage sites, and agriculture
Climate change The consequence: droughts and desertification, floods, melting ice and rising seas, shifting vegetation and loss of biodiversity
Carbon footprint The carbon released, directly and indirectly, by a person, an industry, a power station or a mine

The distinction that earns a mark: natural against enhanced. A learner who writes that the greenhouse effect is a problem has lost it. The greenhouse effect is why the planet is habitable. The enhanced greenhouse effect is the problem.

Finding 3: ozone depletion and global warming are two different problems

You will read that "the ozone layer protects us from the ultraviolet rays of the Sun; CFCs and other greenhouse gases have broken down the ozone layer".

That sentence merges the two biggest environmental stories of the last fifty years into one, and they are not the same story.

Ozone depletion Global warming
What gets through Ultraviolet radiation Heat is trapped on the way out
Where The stratosphere, high up The lower atmosphere
Caused by CFCs and halons Carbon dioxide, methane, nitrous oxide
Effect on people Skin cancer, cataracts Heat, drought, flood, sea level

CFCs really are greenhouse gases as well, and very powerful ones, which is where the confusion starts. But the sentence as written says that other greenhouse gases broke down the ozone layer, and the three main ones it named three bullets earlier do not. Carbon dioxide does not destroy ozone. Methane does not destroy ozone.

If an exam question asks what causes ozone depletion, the answer is CFCs, not greenhouse gases. If it asks what causes global warming, the answer is the greenhouse gases, and CFCs are a small part of that list. Mixing the two is the single most common error in this topic.

Unit 2: water

Theme What the topic says
Wetlands Store water and release it slowly, so rivers run all year. Destroy them and you get floods in the wet season and drought in the dry one
Availability Boreholes draw too much groundwater. Poor ploughing and stripping vegetation make rain run off instead of soaking in, which also causes soil erosion
Exotic plantations Absorb very large amounts of water and lower the water table
Sewage Water carrying waste, urine and faeces is purified at works and returned to rivers or reused
Fertiliser runoff and thermal pollution Cause algal blooms and eutrophication
Toxic substances From mining and industry, damaging health and the environment

Finding 4: eutrophication is described without the step that kills the fish

The summary lists algal blooms and eutrophication as results of fertiliser runoff, and never once mentions oxygen.

Oxygen is the whole mechanism. Without it the topic cannot explain why a river full of plant growth ends up full of dead fish, and that is precisely what the exam asks.

Step What happens
1 Fertiliser runoff adds nitrates and phosphates to the water
2 Algae and plants such as water hyacinth grow rapidly: the algal bloom
3 The bloom blocks light, so the plants beneath it die
4 Decomposing bacteria multiply on the dead material and use up the dissolved oxygen
5 Fish and other aerobic organisms suffocate. This is the mark

Learn it as a sequence, not as a list. A question on eutrophication nearly always asks for the chain, and the chain is worthless without step 4.

Unit 3: food security

Pressure How it reduces food security
Exponential human population growth More people, the same land
Alien plants Invade agricultural land and take it out of production
Monocultures Produce a great deal of food and can be wiped out by one pest or one bad season
Loss of wild varieties Shrinks the gene pool we would need to breed a replacement crop
Climate change Droughts and floods
Wastage Food is lost at every step from field to plate

The monoculture argument is the one worth teaching properly, because it is the same argument as the gene pool argument in genetics. A field of genetically identical plants has no variation for anything to survive on, which is exactly why a single disease can take all of it.

Unit 4: loss of biodiversity

Threat Examples given
Habitat destruction Agriculture including monocultures and overstocking, alien plantations, golf estates, and urbanisation
Alien plant invasion Controlled mechanically, chemically and by biological control
Poaching Rhino, elephant and others
Over-exploitation Harvesting of medicinal plants

The sixth extinction is the framing the topic uses, and it is worth saying what the first five were: five mass extinctions in the fossil record, all caused by physical events. The sixth is the one with a cause that can read about itself.

Unit 5: solid waste disposal

Topic What it covers
Landfill Where most solid waste goes, and how dumpsites are managed
Methane from dumpsites Can be captured and burnt for heating and lighting. Methane is itself a greenhouse gas, so capturing it does two jobs at once
The three Rs Reduce, reuse, recycle, in that order of priority
Nuclear waste Must be isolated for a very long time

Finding 5: the summary drops the three Rs

The unit teaches reduce, reuse and recycle. The content summary for that unit has three bullets, and none of them mentions any of the three.

What survives into the summary is landfill, methane and nuclear waste. A learner revising from the summary alone would never know the three Rs were in the course, and they are the one part of this entire topic that a seventeen year old can act on without anyone's permission.

The order matters and is examinable. Reduce first, because waste that is never made needs no handling. Reuse second. Recycle third, because recycling still costs energy and transport. Most learners put recycling first.

What this topic costs

The assessed task costs nothing. String, four sticks and a notebook. There is no apparatus to buy and we are not going to invent any.

Optional Price In stock Why
Pollution wall chart R85,00 2 Covers the ground of Units 2 and 5 in one picture
Water pollution wall chart R85,00 2 Pairs with the eutrophication sequence above
Air pollution wall chart R85,00 1 Unit 1
Environmentally friendly energy wall chart R85,00 1 The carbon footprint conversation

Stock on all four is thin, between one and two each, and we would rather say so than take an order we cannot fill. More at educational wall charts.

What you should be able to do by the end

  • Separate the greenhouse effect from the enhanced greenhouse effect, and say why only one of them is a problem
  • Name the three main greenhouse gases and where they come from
  • Keep ozone depletion and global warming apart, including which gases cause which
  • Give the eutrophication sequence in order, with the oxygen step in it
  • Explain why a monoculture is fragile in terms of the gene pool
  • List the threats to biodiversity and what is being done about alien plants
  • Give the three Rs in priority order and say why reduce comes first
  • Design a valid sampling comparison with an invaded site and an indigenous control

Free worksheet and marking memo

Both free, no sign up, straight to the PDF.

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