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Date	November 2020	Marks available	2	Reference code	20N.3.HL.TZ0.18
Level	Higher level	Paper	Paper 3	Time zone	TZ0 / no time zone
Command term	List	Question number	18	Adapted from	N/A

Question

The elimination of wolves (Canis lupus) from Yellowstone National Park in 1926 and their reintroduction in 1995 provided the opportunity to examine ecological interactions with elk (Cervus elaphus). The National Park Service made efforts between the 1920s and 1968 to reduce the size of elk herds by culling (selective killing) because of concerns about overgrazing.

The graphs provide the trends for the numbers of wolves and elk in Yellowstone National Park from 1900 to 2004.

[Source: Ripple, W.J. and Beschta, R.L., 2004. Wolves and the Ecology of Fear: Can Predation Risk Structure Ecosystems?
BioScience, 54(8), pp.755–766 by permission of Oxford University Press on behalf of the American Institute of Biological
Sciences. Translated and reprinted by permission of Oxford University Press on behalf of the American Institute of Biological
Sciences. Please visit: https://academic.oup.com/bioscience/article/54/8/755/238242.]

Outline the changes in elk population between the years 1930 and 2004.

[3]

a.

List two biotic factors, other than wolf predation or culling, that could affect the elk population.

1.

2.

[2]

b.

Markscheme

a. between 1930 and 1968 the numbers were reduced;

b. after 1968/in the 1970s the numbers increased;

c. in 1990s1995 the numbers decreased again;

d. lowest in 1968 and highest in 1990;

a.

a. relative rates of natality;

b. disease/lack of food;

c. competition for the same resources;

d. immigration/emigration/migration;

b.

Examiners report

The majority of candidates had no difficulty outlining the changes in elk population.

a.

Many candidates could list two biotic factors, others did not pay attention to "biotic" and included abiotic factors.

b.

Syllabus sections

Option C: Ecology and conservation » Option C: Ecology and conservation (Core topics) » C.1 Species and communities

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