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Ecology

Population ecology

Population ecology is how a group of one species changes in number, and what stops it growing forever.

CAPS

Grade 11 · Life Sciences · Ecology

Life Sciences is two papers. Use the strand to know which one.

  • Where it is examined

    Grade 11 works through energy transformations, gas exchange, excretion, population ecology and human impact. The same two-paper pattern is used.

  • How to write it

    Name the structure, then say what it does. A labelled diagram only scores if the labels are the biological names. “It helps the body” is not a function.

  • How a marker reads it

    A common mix-up is to think a population can grow exponentially forever in a real field, or to call all the species in the field one population.

A 2-mark “explain” wants two facts, or one fact and its reason. Extra sentences that contradict the first one can cancel a mark.

sun

Diagram

Worked theory

Worked theory. Quelea birds on a grain field. One season the grain is rich, so the flock grows quickly. The field cannot feed an endless flock. When grain runs out, deaths and departures rise and the flock stops growing. That levelling is carrying capacity, not a decision by the birds to be tidy. If a predator increases after the flock peaks, the prey curve falls next. A drought is a limiting factor even if no predator is involved. Counting birds in sample plots and scaling up is an estimate, not a photograph of every bird.

More worked examples

  • A 20 hectare camp

    A camp of 20 hectares of decent veld carries 15 cattle through a normal year. The herd grows to 40 because calves were not sold. By August the grass is gone, the cattle lose weight, and three die. The carrying capacity was 15, not 40. Rain the next year does not immediately restore it, because the roots were grazed out. A predator was never involved. Food was the limit.

  • Ducks on a park pond

    A few ducks on a new pond breed, and for a while the number climbs fast. Then nesting sites and food run out. The number levels off. That level is the carrying capacity of this pond, not of ducks in general. Add bread from visitors and the level can rise for a season, then fall when the bread stops. A predator is not required. The pond’s own limits are enough.

In ordinary words

Population size is how many. Density is how many in a given area. Growth often starts slow, becomes fast, then levels off at the carrying capacity, the number the place can support. Limiting factors include food, water, space, disease and predators. Predator and prey numbers rise and fall out of step: more prey, then more predators, then fewer prey, then fewer predators.

In South Africa

Quelea on grain, or goats on an overstocked village veld, is carrying capacity you can see.

Part 1 / 4

What it is

Population ecology is how a group of one species changes in number, and what stops it growing forever.

More worked examples

Board walkthroughs on this topic. Open one, copy the working, then try the next.

Example 1 / 20

40 mice are caught, marked and released. A week later 50 are caught, 10 of which are marked. Estimate the population.

How we got there

Read this like a teacher at the board. We go slowly, name every number, and say why we take each step.

Explain in another language · 11

1. What is the question asking?

Read the question twice. Circle the unknown. Underline every number and unit. The paper is asking: 40 mice are caught, marked and released. A week later 50 are caught, 10 of which are marked. Estimate the population.

Before any working — hunt first

Given

Numbers and units already in the sentence: 40 mice · 50 are · 10 of. Copy each one into a column. A name without a number is not given.

Missing

After you copy the given, the letter that still has no number is missing. The paper is pointing at: which are marked. Circle that word — that is the missing variable.

Find

We are required to find: which are marked. Write that as a symbol on the left, with a question mark, before you touch a formula.

Calculate

Write 40 × 5010=200 with empty letters. Cross out every letter that already has a number. Calculate the letter that is still standing — that gives which are marked.

In other words — as if you have never seen this before

If you have never seen this before: Lincoln index N = M × CR. We will name a method in words, write the formula with no numbers, then substitute one line at a time.

2. What do we already know?

M=40, C=50, R=10.

3. Which method — and why this one?

Lincoln index N=M×CR.

4. Step-by-step working

  1. 01 / 05

    Write the formula

    N=(M × C)R.

    Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.

  2. 02 / 05

    Calculate

    40×5010=200.

    40×5010=200

    Why this step: We only do one job per line so a marker can follow the marks — and so you can find a slip without starting over.

  3. 03 / 05

    Step G — Sense-check the size

    Ask: is this the kind of number a Grade 10–12 paper would want? A speed of 3 000 ms for a learner walking, or a probability bigger than 1, means a unit or algebra slip. Estimate first: round the numbers and do the sum in your head.

    Why this step: Markers write ‘unrealistic’ in the margin when the method is right but the size is wild. A 10-second head check often finds a missing 10² or a unit that never changed.

  4. 04 / 05

    Step F — Box the answer with its unit

    About 200 mice.

    Why this step: A number without a unit is half a mark on a Physics paper, and a marker can miss a Maths answer that is buried in working.

  5. 05 / 05

    Step H — What the marker is looking for

    On an NSC memo you usually earn: (1) a method mark for naming the right formula or idea in words, (2) an accuracy mark for substituting the given numbers with units, (3) an accuracy mark for the boxed answer with its unit. Write those three lines even if you are unsure of the arithmetic — blank pages score zero.

    Why this step: First-timers chase the final number and leave the method blank. The memo still pays out for the idea and the substitution even when the last line is wrong.

5. The answer

About 200 mice.

Copy onto your answer sheet — in this order

Formula: 40×5010=200

Answer: About 200 mice.

6. Check it like a marker

Assume no births, deaths or migration between samples.

7. A trap first-timers fall into

First-timers often grab the first formula they remember, mix units (cm with m, minutes with seconds), or drop a minus sign. Write the formula in words first: “this equals that, because…”

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