IB Biology SL C4.1 Populations and Communities Questions

Practise IB Biology SL C4.1 by estimating populations, reading community and growth data, and testing species interactions with quadrat and statistical evidence.

Exam points

Define a population and community, distinguish same-species from multi-species groupings, and interpret richness, abundance, or distribution data.
Use random quadrats or transects to estimate sessile populations, scaling representative counts by quadrat and habitat area.
Estimate a mobile-animal population with capture-mark-release-recapture, applying N = n₁ × n₂ / n₃ and its assumptions.
Relate carrying capacity to limiting resources and explain how density-dependent pressures regulate population growth.
Interpret exponential or sigmoid growth data, label growth phases, and explain slowing or plateaus through limiting factors.
Use behavioural or resource-distribution evidence to explain cooperation and competition within a species.
Classify interspecific relationships from their effects on each species, including competition, predation, parasitism, and mutualism.
Explain mutualism through exchanged resources or protection and identify benefits to both species in examples.
Distinguish endemic from invasive species and evaluate effects of invasive populations on native species and biodiversity.
Test interspecific competition or species association using ecological observations and chi-squared expected counts, degrees of freedom, and critical values.
Interpret predator-prey data to explain changing populations, foraging, and time-lagged effects.
Trace top-down control through a food web and predict trophic-cascade effects when a predator is removed.
Explain how a named plant's allelochemicals inhibit the germination or growth of neighbouring plants.

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: 1]

What is a population?

Organisms of the same genus living in an ecosystem

Organisms living together and interacting in the same habitat

Organisms of a species living together in the same area

Organisms that can breed together

[Maximum number: 1]

In an area of forest measuring 100 m by 100 m , samples were taken to estimate the number of silver maple (Acer saccharinum) trees in the forest. The number of trees counted in each of five areas of $400 \mathrm{~m}^{2}$ was recorded.

Square sampling grid with five recorded tree counts placed in separate cells: 3, 5, 4, 5 and 8.

Approximately how many silver maple trees are in the $10000 \mathrm{~m}^{2}$ area of forest?

125

625

[Maximum number: 2]

A school football field has plantain plants (Plantago media) growing on it as well as grass. Each plantain plant has a short stem with a circle of wide leaves. Outline how the size of the plantain population could be estimated using a quadrat method.

[Maximum number: 3]

The image shows a castor bean tick (Ixodes ricinus) which is commonly found in woodlands or forest areas. This tick can carry bacteria that cause Lyme disease in humans.

Close-up photograph of a tick, showing an oval patterned body, projecting mouthparts and four pairs of jointed legs.

Explain how the capture-mark-release-recapture method could be used to estimate the size of a tick population.

[Maximum number: 1]

In 1789 Gilbert White, a naturalist, observed eight breeding pairs of swifts (Apus apus) in the English village of Selborne. On average, each pair of swifts produces two offspring per year. This would allow the population to rise to 1030 swifts over 200 years. A bird survey carried out in 1983 revealed only 12 breeding pairs in this village.

What could have prevented the numbers rising to 1030 ?

I. The number of nesting sites remained the same.
II. The food supply of the swifts remained constant.
III. Predatory birds in the area were exterminated.

I only

I and II only

II and III only

I, II and III

[Maximum number: 1]

What can limit a population from growing?

An increase in natality

A disease affecting predators

A decrease in mortality

A disease affecting the population

[Maximum number: 4]

Reproduction can cause populations to increase rapidly. Draw a labelled graph showing a sigmoid population growth curve.

[Maximum number: 10]

A group of nine semi-wild Asian elephants (Elephas maximus) from an elephant camp in Myanmar were trained to pull two ends of a single rope to get access to food in food trays located on a table behind a barrier. When both elephants pulled the rope, the table moved towards them making the food trays accessible. The task required two elephants to work together in cooperation for mutual benefit to obtain food.

Two elephants behind a barrier face food trays on a table. A rope runs around the apparatus towards both elephants, with two distances marked 3 m.

The graph shows the rate of cooperation between pairs of elephants in pulling the rope when there were two food trays, one at each end of the table (as shown in the diagram), or when there was one food tray placed in the centre of the table. The cooperation rate was the number of successful cooperative attempts divided by the number of all attempts in one session. The experiment was repeated for many sessions on successive days.

(a)

Cooperation rate plotted by session number: with two trays it generally rises towards one; with one tray it falls to zero in later sessions.

Using the graph, describe how the elephants cooperated when two food trays were available.

[ 2 ]

(b)

Outline the reason that the one-tray experiment was carried out for fewer sessions than the two-trays experiment.

[ 1 ]

(c)

Suggest a reason for the difference in the response of the elephants between the two-tray and one-tray experiments.

[ 1 ]

(d)

Compare and contrast the frequency of monopoly and fighting when there is a change from two trays to one tray of food.

[ 2 ]

(e)

Suggest a reason for the observed changes in competition.

[ 1 ]

(f)

The probability of a fight occurring between two elephants depends on differences in their dominance rank in the group. Rank difference was calculated using the rank of the elephant starting the fight minus the rank of the target elephant. The researchers modelled a graph of the probability of elephants fighting back when another elephant attempted to start a fight if presented with two trays or one tray of food.

Probability-versus-rank-difference curves for two trays and one tray. Both fall as rank difference increases, but the one-tray curve falls later and more abruptly.

Estimate the probability of a fight occurring when an elephant of rank 5 initiates a fight with an elephant of rank 2 if they are presented with two trays of food.

[ 1 ]

(g)

Compare and contrast the chances of a fight occurring with two trays and one tray of food.

[ 2 ]

[Maximum number: 6]

The relative abundance of different grass species in the 17 plant communities of the Serengeti ecosystem in Tanzania is presented in the graph below. The communities are listed along a transect that runs from the dry south-eastern boundary of the park (community 1), north and west across the plains and woodlands to Lake Victoria (community 17).

(a)

State the grass species that is most abundant in plant community 1.

[ 1 ]

(b)

Both communities 1 and 17 have a low overall abundance of grasses. Suggest a reason for this in

[ 2 ]

(i)

Both communities 1 and 17 have a low overall abundance of grasses. Suggest a reason for this in community 1.

[ 1 ]

(ii)

Both communities 1 and 17 have a low overall abundance of grasses. Suggest a reason for this in community 17.

[ 1 ]

(c)

Evaluate the conclusion that there are trends in the distribution of plants along the transect of Serengeti grass communities.

[ 3 ]

[Maximum number: 2]

The images show a southern cassowary (Casuarius casuarius johnsonii) and the fruit of Ryparosa kurrangii. The fruit grow suspended from the main trunk of the tree on long stems.

The southern cassowary is a large, flightless bird found in New Guinea and Queensland, Australia, which feeds on the fruits of rainforest trees. Many of these tree species contain large seeds that are stripped of their flesh as they pass along the cassowary gut, and may be transported long distances before being ejected in fecal material.

The graph shows the results of an investigation in which seeds of the rainforest tree species Ryparosa kurrangii were exposed to one of three conditions before being transferred to soil to assess the effects of consumption by the cassowary on seed germination:
- seeds left intact inside the fruit (control)
- some flesh roughly removed
- seeds consumed by cassowaries and passed through their gut.

Populations of the southern cassowary are experiencing serious decline as a result of collision with road vehicles which kills many adults, wild dogs which kill chicks and juveniles, and wild pigs which destroy cassowary habitat. Deduce two effects of this decline on R. kurrangii.
1.
2.

[Maximum number: 3]

Reef-building corals are an association between two organisms: coral polyps and Zooxanthellae.

Coral-polyp section labelled Tentacle, Mouth and Pharynx above a shaded calcium-carbonate skeleton labelled CaCO₃ skeleton.

(a)

State the relationship between Zooxanthellae and coral reef species.

[ 1 ]

(b)

Describe the exchange of materials between the coral's polyps and Zooxanthellae.

[ 2 ]

[Maximum number: 2]

12. A survey was made of the intertidal zone at Butter Lump Bay, Great Cumbrae, Scotland. The three species of barnacle found were Elminius modestus (EM), Chthamalus montagui (CM) and Semibalanus balanoides (SB). The kite diagram shows the vertical distribution of these three species from the low tide mark at 0 m to 2.25 m above low tide.

E. modestus is an invasive barnacle while the others are native species. Analyse the data to show how it supports this statement.

[Maximum number: 1]

Caenorhabditis elegans and Caenorhabditis briggsae are two species of nematode that co-exist in rotting plant material. A study was conducted in France to determine the relative population of each species in samples of apples.

Two stacked-bar charts show the proportions of sampled apples containing different population sizes of C. elegans and C. briggsae on dates from July 2009 to January 2011. Shading and patterns distinguish the nematode-population categories.

Suggest, with a reason, the type of interaction that may exist between the two nematodes.

[Maximum number: 3]

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.

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

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

in each quadrat determine the presence/absence «of plants» of each type
null hypothesis is that the presence of one is random/independent 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
$x^{2}=\sum \frac{(O-E)^{2}}{E}$
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

[Maximum number: 2]

The sea snail Nucella ostrina and the sea star Pisaster ochraceus are predators of the mussel Mytilus trossulus. The mussels live on rocks at the edge of the sea and feed on phytoplankton and zooplankton. The zooplankton feed on the phytoplankton.

Nucella ostrina

Pisaster ochraceus

Mytilus trossulus

Groups of 50 mussels were transplanted to an experimental area and protected from predation until the start of the experiment. Researchers then investigated the effect of the predators on the population of the mussels over a period of 60 days.

Compare and contrast the effects of the predators on the population of the mussels.

[Maximum number: 1]

Humans have persecuted predators such as coyotes (Canis latrans), causing severe reduction in the number of large carnivores throughout the world. The diagram represents part of a food web in Texas, USA.

What is a possible outcome for the lagomorph population in this ecosystem if coyote numbers were controlled by killing them to reduce losses of domestic livestock?

Decrease because there are fewer predators

Increase because there are fewer foxes to eat them

Decrease because there are more rodents for foxes to eat

Increase because, although there are more foxes, there are also more rodents

[Maximum number: 1]

The paradise tree (Ailanthus altissima) produces a chemical called ailanthone which inhibits the growth of other plants. What is this process?

Allelopathy

Endemism

Mutualism

Herbivory

All question bank results loaded

a	competition is low with two trays as no/little opportunity to grab food from both trays OR when there is only one tray the elephants are more desperate to obtain the food for themselves;
b	fighting occurs less often than monopoly as fighting is potentially harmful;
c	monopoly shows wider range (than fighting) as more likely to attempt this type of competitive behaviour;
d	one elephant could be larger than the other/one could be female while the other is male;

a	reduced rates of seed germination;
b	reduced dispersal;
c	decreased population size/possible extinction of R.kurrangii;

Covered syllabus points

Question 1

Question 2

Question 3

Question 4

Question 5

Question 6

Question 7

Question 8

Question (a)

Question (b)

Question (c)

Question (d)

Question (e)

Question (f)

Question (g)

Question 9

Question (a)

Question (b)

Question (i)

Question (ii)

Question (c)

Question 10

Question 11

Question (a)

Question (b)

Question 12

Question 13

Question 14

Question 15

Question 16

Question 17