Item IE029006: Changes in a population of organisms in a food web (worms) can affect the population of its predator (robins).

The diagram below shows the feeding relationships between populations of organisms in an area. The arrows point from the organisms being eaten to the organisms that eat them.

Using only the the relationships between the organisms shown in the diagram, if most of the worms are killed, which of the following statements describes what will happen to the number of robins and why?

  1. The number of robins will increase because there are fewer worms to eat them.
  2. The number of robins will decrease because there are not enough worms for them to eat.
  3. The number of robins will stay the same because the worms are killed, not the robins.
  4. The number of robins will stay the same because a change in the population of worms will not affect any other population of organisms.
Distribution of responses
Answer Choice
Overall
Grades
Gender
Primary Language
 
n = 1477
6–8
n = 859
9–12
n = 616
Male
n = 694
Female
n = 766
English
n = 1337
Other
n = 104
A. The number of robins will increase because there are fewer worms to eat them. 6% 5% 7% 5% 6% 6% 9%
B. The number of robins will decrease because there are not enough worms for them to eat. 82% 84% 80% 81% 83% 82% 79%
C. The number of robins will stay the same because the worms are killed, not the robins. 7% 7% 7% 8% 6% 7% 9%
D. The number of robins will stay the same because a change in the population of worms will not affect any other population of organisms. 5% 5% 6% 6% 5% 5% 4%