Population Ecology — Drills · Grade 12 Science

Chapter: Ecology, Populations & Biodiversity

What these drills covers

By the end of Population Ecology you should be able to:

6 questions, multiple choice, numeric and written answer, each with a worked explanation.

Vocabulary used: population, exponential growth, logistic growth, carrying capacity (K), limiting factor, survivorship curve.

Problem 1

The maximum population size an environment can sustain over the long term is its
  1. survivorship curve
  2. carrying capacity
  3. exponential rate
  4. biotic potential
Answer and explanation

carrying capacity

Carrying capacity (K) is the sustainable maximum set by the environment's limiting factors.

Problem 2

A population of N = 100 has r = 0.4 per year in a habitat with K = 500. Using dN/dt = rN((K - N)/K), how many individuals are instantaneous growth rate in individuals/year?
Answer and explanation

32 individuals/yr

(500 - 100)/500 = 0.8; 0.4 × 100 × 0.8 = 32. This is an instantaneous rate in the continuous model, not an exact finite-year increase.

Problem 3

A population of 2,000 records 90 births and 40 deaths in a year (ignore migration). What is the annual growth rate, as a percent?
Answer and explanation

2.5 %

(90 - 40)/2,000 = 50/2,000 = 0.025 = 2.5%.

Problem 4

A species that produces enormous numbers of offspring, most of which die young (a Type III survivorship curve), is typified by
  1. humans
  2. elephants
  3. oysters and many fish
  4. large mammals in general
Answer and explanation

oysters and many fish

Type III species produce many offspring with little parental care, so most die early — like oysters and many fish.

Problem 5

Give one example of a density-dependent limiting factor and explain why it counts as density-dependent.
Answer and explanation

Disease is density-dependent: as a population grows more crowded, individuals contact one another more often, so disease spreads faster and kills a larger fraction of the population. Its effect strengthens as density rises. (Competition for food and predation are other examples.)

Problem 6

A population grows at 5% per year. Using the rule of 70, estimate its doubling time in years.
Answer and explanation

14 years

Doubling time ≈ 70 ÷ 5 = 14 years.

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