Two important marine food webs off the west coast of North America are the phytoplankton $\rightarrow$ zooplankton $\rightarrow$ fish web and the kelp $\rightarrow$ sea urchin $\rightarrow$ sea otter web.How do these two food webs compare in terms of the primary control of energy flow?
A) Both have primarily bottom-up control; they are controlled by phytoplankton and kelp, respectively.
B) Both have primarily top-down control; they are controlled by fish and sea otters, respectively.
C) The first has primarily bottom-up control by phytoplankton; the second has primarily top-down control by sea otters.
D) The first has primarily top-down control by fish; the second has primarily bottom-up control by kelp.
E) Both food webs vary so much in resource availability that it is impossible to determine primary control.

Question

Two important marine food webs off the west coast of North America are the phytoplankton $\rightarrow$  zooplankton $\rightarrow$  fish web and the kelp $\rightarrow$  sea urchin $\rightarrow$  sea otter web.How do these two food webs compare in terms of the primary control of energy flow?
A) Both have primarily bottom-up control; they are controlled by phytoplankton and kelp, respectively.
B) Both have primarily top-down control; they are controlled by fish and sea otters, respectively.
C) The first has primarily bottom-up control by phytoplankton; the second has primarily top-down control by sea otters.
D) The first has primarily top-down control by fish; the second has primarily bottom-up control by kelp.
E) Both food webs vary so much in resource availability that it is impossible to determine primary control.

Quizplus · Accepted Answer

The phytoplankton-zooplankton-fish food web is primarily controlled from the bottom-up by phytoplankton, while the kelp-sea urchin-sea otter food web is primarily controlled from the top-down by sea otters. This is because sea otters are predators of sea urchins, which in turn feed on kelp. Therefore, the presence of sea otters has a top-down effect on the abundance of sea urchins, which in turn affects the abundance of kelp.

Two Important Marine Food Webs Off the West Coast of North