Black-legged ticks carry Lyme disease, a common illness in the United States. Black-legged ticks live in forests and meadows. The ticks feed on small rodents like chipmunks as they grow. The chipmunks feed on acorns from oak trees.

Question

Researchers investigated factors that affect tick populations in their habitats. They recorded the number of black-legged ticks, acorns, and chipmunks in an area over several years. The table below shows their results.

Year Acorns Per
Square Meter Chipmunks Per
5 Acres Ticks Per
100 Square Meters
1994 18 12 10
1995 4 56 3
1996 10 8 14
1997 4 26 4
1998 15 9 14
Data adapted from Ostfeld et al. 2006. PLoS Biology. Licensed under CC BY 4.0.

Which of the following claims best explains the data?
A.
When the chipmunk population decreases, the ticks begin to eat acorns, which increases the tick population and decreases the number of acorns.
B.
When the tick population increases, the ticks cause the chipmunk population to decrease, and this causes the oak trees to stop producing acorns.
C.
A shortage of food for chipmunks in a certain year decreases the chipmunk population in the next year and the tick population in the year after that.
D.
When a lot of acorns are on the ground in a certain year, it is likely that there will be large populations of chipmunks and ticks in that year.

Answer 1

To analyze the data and claims, we need to look at the relationship between acorns, chipmunks, and ticks over the years provided. The key aspects to consider include how the food supply impacts the populations of chipmunks and ticks.

Let's evaluate the options:

A. This claim suggests that ticks eat acorns when chipmunk populations decrease. However, ticks are ectoparasites that feed on the blood of animals, not on acorns. This claim is not supported by the ecological relationships presented.

B. This claim implies that the ticks directly cause a decrease in the chipmunk population and that this, in turn, affects acorn production. However, while there may be a correlation between the populations, there's no evidence here to suggest that ticks impact chipmunk populations in such a cascading way, and the connection to oak trees and acorn production is not substantiated in the data.

C. This claim posits a shortage of food (in this case, acorns) leading to fewer chipmunks in the subsequent year, which would then correlate with a lower tick population in the year after that. This idea aligns with the observed data of declines in chipmunk numbers relating to lower acorn counts, which can indirectly affect tick populations.

D. This claim suggests that high acorn availability correlates with higher populations of chipmunks and ticks. This, too, is somewhat consistent with the data, as years with more acorns tend to have more chipmunks, which may support a higher tick population.

After considering all claims, C is the most comprehensive as it provides a direct causal link: a decrease in food sources (acorns) influences the chipmunk population, which in turn affects the tick population. Thus:

C. A shortage of food for chipmunks in a certain year decreases the chipmunk population in the next year and the tick population in the year after that.