Kelp forests along the Pacific Coast are in decline. You are a scientist studying potential causes for this de-cline. Kelp (a form of brown algae) is eaten by two species of sea urchins

Question

The first urchin species is native to the Pacific, whereas the second species is an exotic species that was in-troduced from the Atlantic by human activity. The introduced urchins currently occur only near the seaports where they were first introduced, but they are expanding their geographic range. Sea otters prey on sea ur-chins. Sea otters were driven almost to extinction by hunting, but are now re-colonizing the Pacific coast. You conduct the following experiments and make the following observations:
• Kelp forests thrive in areas with sea otter populations except in areas where introduced sea ur-chins overlap with the sea otters.
• Removing sea urchins by divers results in kelp growth.
• When you census the sea urchin populations, you find that native sea urchins tend to outnumber exotic sea urchins except in areas where otters are present.
• When you enclose a single species of sea urchins with kelp in the field, you find that native sea urchins consume kelp at twice the rate than the exotic urchins.
Indicate the net impact on population numbers that the first species has on the second species in this food web by using the following symbols: 0 = no impact, + positive impact, – negative impact.

Exotic sea urchin (1)________ kelp
Native sea urchin (2)________ kelp
Sea otter (3)__________ exotic sea urchin
Sea otter (4) __________ native sea urchin
Native sea urchin (5) ____________ exotic sea urchin

What is the future of the kelp forests? Justify your statement.

What will be an ideal response?

Answer 1

ANS:
(1) –, (2) –, (3) +, (4) –, (5) –; The kelp forests will keep declining. Although the range expansion of otters controls native urchins, otters do not seem to like exotic urchins. Instead, the otters are releasing the exotic urchin from competition by the native urchin.