Snails in the genus Nucella consume barnacles and are eaten by crabs. When crabs

Question

Snails in the genus Nucella consume barnacles and are eaten by crabs. When crabs are present, barnacle densities are higher than when crabs are absent. The data below come from a study of this system. The goal of the study was to detemine how much of the indirect effect of crabs on barnacles is due to snail density (density-mediated) and how much is due to snail behavior (trait-mediated). In this experiment, snails were kept in tanks in the lab, along with tiles covered with barnacles. One tile in each tank faced up (toward the surface of the water) and was considered a "risky" habitat. One tile faced down near the bottom of the tank and was considered a "refuge'' habitat. Half the tanks were exposed to crab scent - water from a tank holding crabs was added. IMPORTANT: The risky habitat started out with more barnacles because risky habitats in nature have more barnacles. For each of your answers, refer to the specific data that support your conclusions. What is the behavioral response of snails to crabs? How does the behavioral response on its own affect per capita barnacle consumption? How does it affect final barnacle density? Manual removal of snails was used to mimic the effect of crabs eating snails and reducing their density. How does reduced snail density affect per capita barnacle consumption? How does it affect final barnacle density? Which effect of crabs on snails (changed behavior or reduced density) has the biggest effect on barnacles (or are they about the same)? Figure 1 (below): proportion of snails in the refuge habitat when exposed to crab scent or not Figure 2 (right): Mean (± SE) (a) per capita consumption of barnacles by snails and (b) final barnacle abundance in risky and refuge habitats after exposure to the presence and absence of crab scent and different levels of snail removal (0%, 8% and 16%) for 36

Explanation / Answer

a. The proportion of snail population is more in the presence of crab compared to the absence of crab. There is an initial increase in the proportion, in the presence of crab, which then remained constant. In the absence of crab, the proportion of snail decreased in the first few days which later remained more or less constant.

b. The percapita barnacle consumption varied in risky and refuge habitats. In the risky habitat, per capita consumption of barnacles is more in the absence of crab. However, it showed a different pattern of barnacle consumption in refuge habitat. The percapita barnacle consumption in refuge habitat shows a slight increase in the presence of crab. It doesn’t show much variation in situations of the presence or absence of crab.

It resulted in an increase in barnacle density in risky habitat, in the presence of crab. Even in the absence of crab, the density of barnacles increased linearly. However, in refuge habitat, there is a slight increase in density of barnacles in the absence of crab. In the presence of crab, in refuge habitat, there is a decrease in barnacle density after 8 days, after the removal of snail population.

c. Reduced snail population seems did not show any effect on barnacle consumption, both in presence and absence of crab, and both in risky and refuge habitats. In all the said situations, the graph shows an increase in consumption with time, which is linear. However, in risky habitat, the consumption decreased slightly after 8 days, where as in refuge, it shows a slight increase in consumption after 8 days, after removal of snail.

The density of barnacles increased in the absence of crab, but did not vary much in its presence, in risky habitat. In refuge habitat, when there is no crab, the density increased even after removal of snail, at a constant rate; whereas in the presence of crab, after 8 days, there is a decrease in density of barnacle.

d. The above results show that the behavior of snails has its effect on barnacles. It is obvious from the experiment that the consumption of barnacles vary with risky and refuge habitats. In risky habitat, there is no much increase in barnacle population in the presence of crab, but it increased in its absence; whereas, in refuge habitat, the barnacle density increased only in the absence of crab and it decreased in the presence of crab.

Related Questions

Navigate

• Browse (All)
• Browse S
• Subjects

---

---

Integrity-first tutoring: explanations and feedback only — we do not complete graded work. Learn more.