IB Biology HL C4.1 Populations and Communities Questions

Practise IB Biology HL C4.1 by selecting sampling methods, interpreting growth and food-web data, and reasoning about interactions between species.

Exam points

Define a population and community, then distinguish a same-species population from interacting populations of different species.
Design quadrat or transect sampling for sessile organisms, using representative placement and known area to estimate abundance or density.
Estimate a mobile-animal population with capture-mark-release-recapture, applying N = n₁ × n₂ / n₃ and stating its assumptions.
Relate carrying capacity to limiting resources and explain population fluctuations as environmental capacity is reached.
Explain density-dependent negative feedback using competition, predation, disease, or resource limits in population data.
Interpret exponential and sigmoid growth curves, label their phases, and explain changes using limiting factors and population rates.
Relate colony living, division of labour, or foraging specialization to cooperation and survival or food intake.
Classify interspecific relationships from species effects, including direct or indirect interactions shown in ecological data.
Explain mutualism through exchanged resources or protection and identify benefits to both species in named examples.
Distinguish endemic from invasive species and evaluate how an introduced species affects native populations and biodiversity.
Test species association with chi-squared: state hypotheses, calculate expected quadrat counts and degrees of freedom, then interpret the critical value.
Interpret predator-prey graphs to explain foraging, density-dependent population changes, and time-lagged cycles.
Distinguish top-down from bottom-up control and use mesocosm or food-web data to trace trophic cascades.
Explain how named plants release allelochemicals that inhibit neighbouring-plant germination or growth.

Covered syllabus points

C4.1.1—Populations C4.1.2—Population size estimation C4.1.3—Random quadrat sampling C4.1.4—Capture-mark-release-recapture C4.1.5—Carrying capacity C4.1.6—Negative feedback control C4.1.7—Population growth curves C4.1.8—Modelling sigmoid growth C4.1.9—Competition vs. cooperation C4.1.10—Community C4.1.11—Interspecific relationships C4.1.12—Mutualism as interspecific relationship C4.1.13—Endemic vs. invasive species C4.1.14—Tests for interspecific competition C4.1.15—Chi-squared test C4.1.16—Predator-prey relationships C4.1.17—Top-down vs. bottom-up control C4.1.18—Allelopathy and antibiotics

[Maximum number: 3]

The photograph shows Japanese knotweed, Reynoutria japonica, which is native to Japan, China and Korea. It has been introduced into other regions of the world and is now an invasive alien species in parts of Europe and North America. Dense groups of stems often develop with a height of up to 3 metres, which prevent other plants from growing.

(a)

State the biological term for a group of organisms of the same species living in an area.

[ 1 ]

(b)

Suggest two ways in which a plant such as Reynoutria japonica could prevent the growth of other plants.

[ 2 ]

[Maximum number: 2]

The Atlantic cod is considered in many countries to be endangered due to overfishing. Describe two methods that could be used to estimate the cod population.

[Maximum number: 4]

White sage (Salvia apiana) is a native Californian shrub. Slender oat (Avena barbata) is a grass, originally from the Mediterranean, which was introduced to California. The map shows the distribution of the two species in relation to one another in an area near Santa Ynez, California.

(a)

Using the scale bar, determine the area of quadrat A1, giving the units.

[ 1 ]

$9.0 \mathrm{~m}^{2} / 9 \mathrm{~m}^{2}$

Units required for the mark.

(b)

Outline how chi-squared can be used to test for an association between the distributions of the two species.

[ 3 ]

a. in each quadrat determine the presence/absence «of plants» of each species
b. null hypothesis is that the presence of one is random in relation to the presence of the other plant
OR
alternate hypothesis is that the presence of one is associated with the presence or absence of the other
c. $x^{2}=\sum \frac{(O-E)^{2}}{E}$
d. accept alternative hypothesis/reject null hypothesis if the difference between observed and expected is statistically significant / p<0.05 / calculated $X^{2}$ higher than tabulated $X^{2}$ / critical value
OR
it supports the association between the two species if the difference between observed and expected is statistically significant / p<0.05 / calculated $X^{2}$ higher than tabulated $X^{2} /$ critical value

All question bank results loaded

a	competition for/taking/blocking/limiting access to light/sun;
b	competition for/taking/limiting supplies of water/nutrients (from the soil);
c	allelopathy/release of toxins;

Covered syllabus points

Question 1

Question (a)

Question (b)

Question 2

Question 3

Question (a)

Question (b)