IB DP Biology- B4.2 Ecological niches -FA 2025- IB Style Questions For HL Paper 2

Question 

Brandt’s voles (Lasiopodomys brandtii ) are small herbivorous rodents that live in the steppe grassland of Inner Mongolia. These mammals live in large family groups and feed on the roots, stems and leaves of a wide variety of plants that grow near their burrows. Their feeding activities have been found to have a large impact on the grassland vegetation. Researchers investigating the impact of these voles on the vegetation marked out plots of land and recorded how the biomass of two types of grasses, rhizome grasses and bunch grasses, changed in the presence and absence of voles from the years 2013 to 2017.

(a) Identify the year in which the greatest biomass of bunch grasses was recorded in plots with voles present.

(b) Compare and contrast the mean biomass of the two types of grass in 2013 in the presence and absence of voles.

(c) The voles derive nutrition from only one type of the grasses. Deduce with a reason which type.

In a second study carried out from 2019 to 2020, researchers looked at how the presence of voles changed the plant volume of one type of bunch grass, Achnatherum splendens, in measured plots of land. This was done by measuring the change in plant volume of A. splendens in the plots.

(d) Calculate the mean difference in plant volume in the plots in the presence and absence of voles over the period of study.

(e) Outline how these results appear to contradict the results of the previous study.

(f) Suggest one advantage of measuring plant volume rather than plant biomass.

Shrikes (Lanius spp.) are predatory, carnivorous birds that feed on small animals, including voles. Shrikes were seen visiting the marked plots in the second study on plant volume of  A. splendens, but not in the first study on biomass of two types of grass. The graph shows the number of visits shrikes made to 15 different marked plots of land according to percent coverage by the bunch grass A. splendens.

(g) Calculate the percentage of plots that had two visits by shrikes.

(h) State the relationship between number of shrike visits to the plots and percentage of plot covered by A. splendens.

The researchers prepared plots, some with and some without voles. Shrikes were present in all plots. At the end of one month, the mean changes in root biomass and plant volume of A. splendens in the plots were measured.

(i) Suggest a reason for the change in the plant volume of A. splendens in the plots over the period of this study.

In a final experiment, the researchers prepared plots containing varying levels of cover of A. splendens, as well as the plants the voles normally feed on. Shrikes were not excluded from the plots. The amount of time the voles spent feeding and the mortality rate of the voles was recorded over a 3-week period.

(j) Describe, with a reason, the condition that favours survival of the voles.

(k) The researchers concluded that the voles can modify their ecosystem in order to survive. Discuss this conclusion with reference to all the data in the question.

▶️Answer/Explanation

(a) 2015;

(b) Comparing biomass without voles present:
a. bunch grass biomass and rhizome biomass are similar/not significantly different without voles;
Comparing biomass with voles present:
b. bunch grass biomass is higher than rhizome biomass with voles;
Comparing the changes in biomass due to presence of voles:
c. decrease with rhizome grasses but only a slight decrease/less decrease no (significant) change/ with bunch grasses;

(c)  rhizome grass as lower biomass/less when voles are present;

(d) 1.1 (m3) (accept answers from 1 or 1.0 to 1.2);

(e) A. splendens/bunch grass reduced by/with voles in this study but bunch grass not reduced/increased /only rhizome grass reduced by/with voles in the previous study/OWTTE;

(f) a. plants do not have to be dried/killed;
b. habitat/ecosystem/environment not changed/damaged/harmed (as much);
c. volume shows how much area/height/space is occupied/how large the plant is overall/OWTTE;
d. allows repeated measurement of the same plants;
e. allows change/increase in volume / growth to be measured/monitored;

(g) 20;

(h)  more visits with more cover (by A. splendens)/positive correlation;

(i) a. increase (without voles) due to photosynthesis/growth / due to roots not being eaten;
b. decrease (with voles) due to voles feeding on roots/shoots/leaves/grass/A. splendens;

(j) a. low area/cover as this has lower mortality;
b. low area/cover due to less shrikes/predation;
c. low area/cover as this has higher feeding frequency;

(k) a. Award one mark if
the answer states that the conclusion is supported by the
research/data/graphs/studies
OR
the answer states that voles can modify their ecosystem, with an example of how
the research/data/graphs/studies shows this;
b. voles reduce the volume/area of A. splendens (2nd/4th data);
c. voles reduce root biomass/kill roots of A. splendens (4th data);
d. voles don’t eat bunch grass (1st data) yet reduced the bunchgrass A. splendens (2nd data);
e. lower mortality with lower area covered by A. splendens (5th data);
f. fewer shrikes/shrike visits with less A. splendens (3rd data);
g. less predation/mortality due to shrikes with less A. splendens (5th data);
h. higher vole feeding frequency with lower area covered by A. splendens (5th data);