Answers to Hone Your Problem-Solving Skills Questions

1. The per capita interaction strength (IS) values between gulls and the species listed in the table are the following: ribbed limpet IS = ln (10/100)/10 = -0.23, gooseneck barnacle IS = ln (500/3,000)/10 = - 0.18, checkered limpet IS = ln (100/50)/10 = 0.07, mussel IS = ln (3,000/2,500)/10 = 0.02, microalgae IS = ln (500/100)/10 = 0.16.

2. Indirect effect 1: Removing gulls decreases the abundance of mussels because of increased competition with the gooseneck barnacle. Indirect effect 2: Removing gulls decreases the abundance of the checkered limpet because of increased competition with the ribbed limpet. Indirect effect 3: Removing gulls decreases the abundance of microalgae because of increased herbivory by the ribbed limpet. Indirect effect 4: Although the experiment cannot test for the effect on phytoplankton, it is likely that removing gulls decreases the abundance of phytoplankton because of increased herbivory by the gooseneck barnacle.

3. The effect of gulls would be even more positive. That's because gulls, by eating the ribbed limpet and gooseneck barnacle, reduce the competition for the mussel and checkered limpet (the interaction strength measurements for those species indicated that they indirectly benefited from their interaction with gulls). By reducing competition, excluding the mussel and checkered limpet allows those nonprey species to increase in abundance and prey more heavily on microalgae and phytoplankton.