Skip to content
← Theoretical 2

Q30 - Parasite-Mediated Competition in Squirrels

Theoretical 2 Real exam question - full text reproduced under IBO's CC BY-NC-SA 4.0 license
Coevolution & Species Interactions

Parasite-mediated competition occurs in host species that live sympatrically (in the same place) and are differentially affected by parasite infections. Species that are badly affected by parasites may not be able to compete well with a sympatric species, except in situations where the parasite is absent.

Squirrels in Canada host the intestinal nematode Strongyloides robustus. However, nematode effects on the hosts are varied.

Researchers conducted a study to determine if parasite-mediated competition was occurring in squirrels. Four species of squirrel were trapped, and the presence of parasite eggs was determined. To assess the impact of the nematodes on the squirrels, the researchers measured Scaled Mass Index (SMI) as a measure of body condition; lower values indicate worse health.

The figure below shows the SMI for the four species. The error bars represent the standard error of the means. NOTE that the scales in the figures are different.

  • S: Southern flying squirrel (Glaucomys volans)
  • G: Grey squirrel (Sciurus carolinensis)
  • N: Northern flying squirrel (Glaucomys sabrinus)
  • R: Red squirrel (Tamiasciurus hudsonicus)

* Indicates a statistically significant difference.

The two nocturnal species of flying squirrels have different diets and foraging strategies from the diurnal red and grey squirrels.

SMI comparison for four squirrel species with and without parasite infection

True or False?

Q30.1. Northern flying squirrels and red squirrels are likely to be vulnerable to parasite-mediated competition.
Q30.2. Under conditions of high nematode infections, northern flying squirrels are likely to be outcompeted by grey squirrels.
Q30.3. Under conditions of high nematode infections, southern flying squirrels are likely to be outcompeted by northern flying squirrels.
Q30.4. If we were to increase the sample size of one of these species for greater credibility of the study results, the best choice would be red squirrels.
← Previous: Q29 Next: Q31 →

Question reproduced from IBO 2023, Theoretical Paper 2, licensed under CC BY-NC-SA 4.0 - attributed to the International Biology Olympiad. Open the full exam PDF