Sea otter populations along the Pacific Coast of North America declined as a result of hunting. Sea urchins are a favoured food of sea otters. When the sea otter numbers declined, sea urchin populations increased. Sea urchin are herbivorous and feed on kelp. As a result, vast tracts of kelp forests were consumed by the urchins.Which one of the following statements is TRUE?

Question

BlackTom AI · Accepted Answer

Let’s parse the scenario first: sea otters keep sea urchin populations in check, and sea urchins eat kelp. When otters decline, urchins surge, kelp forests decline. Now evaluate each statement in light of this trophic cascade.

Option a: 'The sea otter and sea urchin have a symbiotic relationship that was disrupted by sea otter declines, which then resulted in the decline of the kelp.' This is misleading because the relationship is not symbiotic in the mutualistic sense; it is a predator–prey or consumer–prey interaction where otters predate urchins. The decline of otters reduces predation on urchins, not a disruption of a mutual benefit between otters and urchins. So this misstates the nature of their interaction and the causal chain.

Option b: 'The sea otter is a keystone species, the loss of which had a disproportionate impact on the entire kelp forest community.' This captures the core idea of keystone species: otters regulate a critical consumer (urchins) that, if released from predation, overgraze kelp and reshape the whole community. The outcome described—urchin overpopulation leading to kelp loss and forest decline—fits the keystone species concept well.

Option c: 'The sea otter and kelp have a symbiotic relationship that was disrupted by sea otter declines, which then resulted in the decline of the kelp.' This option wrongly asserts a direct symbiosis between otters and kelp. There is no mutualistic or symbiotic interaction between otters and kelp driving kelp decline; the key interaction is otters controlling urchin herbivory, not a direct otter–kelp symbiosis.

Option d: 'The sea urchin is a keystone species, the proliferation of which had a disproportionate impact on the entire kelp forest community.' Although urchins have a large ecological effect when abundant, they are not typically classified as a keystone species; their overabundance is a consequence of otter loss rather than a keystone role. The keystone label is misapplied here, since the primary driver of the community shift is otter loss, not urchin proliferation acting as a keystone.

In a broader ecological context, keystone species are those whose presence maintains community structure and diversity; when they disappear, many other species are affected. Here, the loss of sea otters removes top-down control on urchins, causing cascading effects on kelp forests, which aligns with the keystone concept described in option b.

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