Of the many organisms found in marine ecosystems, sea otters play a un

Of the many organisms found in marine ecosystems, sea otters play a uniquely significant role in maintaining their environments. Sea otters (Enhydra lutris) are a keystone species, a species whose effect on their environment is disproportionately great relative to their size in numbers or weight. Keystone species keep the growth of populations of other species in check and are thereby a determining factor of the type and number of other species in a community; keystone species are so named because their role is analogous to that of a keystone in an arch: while keystones bear the least amount of pressure among the components of an arch, the arch collapses without them.

The sea otter's reputation as a keystone species is largely a consequence of its affinity for sea urchins. Sea otters are known to prey upon over 100 different species, but the bulk of their diet consists of sea urchins and other marine invertebrates. Sea urchins are such highly preferred prey that sea otters have been known to significantly deplete urchin populations. In marine ecosystems at equilibrium, however, sea otters are often the primary force that keeps populations of sea urchins in check. Without sea otters, sea urchins and other seafloor herbivores would graze on help holdfasts, causing kelp to drift away from their substrates and die. The aggregate result of many such events leads to the destruction of entire kelp beds, resulting in a loss of habitat and nutrients for other organisms in the ecosystem. The extended consequences include local extinction of other species and vast urchin barrens: subtidal areas where the unchecked population growth of sea urchins has caused a dearth of kelp and, therefore, other marine species.

This ecological cascade effect of secondary extinctions resulting from a decrease in otter populations and corresponding growth in urchin populations was observed throughout the 19th century along the Pacific coast of North America. Sea otters had been aggressively hunted for their rich pelts since before the 1600s, and this practice was not outlawed until an international treaty was signed in 1911 to prevent further exploitation. Prior to the enforcement of the treaty, hunters caused sea otter populations to decline; this in turn caused sea urchin populations to explode. Consequently, the urchin populations overexploited their primary food source, kelp, creating a preponderance of urchin barrens.

1. The author's tone towards sea otters could best be described as

(A) disdainful
(B) dispassionate
(C) concerned
(D) critical
(E) jovial



2. Which of the following would be the most likely title for this passage?

(A) The Sea Otter and the Sea Urchin
(B) Ecological Cascade Effects in Global Ecosystems
(C) Marine Holdfasts: The Key to Biodiversity
(D) A Study of Urchin Barrens
(E) The Sea Otter as a Keystone Species



3. The primary purpose of this passage is to

(A) advocate a strategy for keeping sea urchins in check.
(B) explain the absence of kelp in the North American Pacific coast.
(C) outline the dietary habits of sea otters.
(D) describe the significance of sea otters in their ecosystems.
(E) define the term "keystone species."



4. Based on the information in the passage, what is the most likely meaning of holdfast?

(A) organisms that dwell at the lowest level of a body of water
(B) a root-like structure that anchors an organism
(C) aquatic arthropods that live in marine environments
(D) rocks used by sea otters to dislodge prey and open shells
(E) the act of embracing feverishly



5. According to the passage, which of the following could be most reasonably inferred about kelp?

(A) Kelp forests provide a unique habitat for a variety of marine organisms.
(B) Kelp is an important food source for coastal Californians.
(C) Kelp forests occur worldwide throughout temperate and polar coastal oceans.
(D) Kelp is an important part of the sea otter's diet.
(E) A 1911 treaty reinvigorated kelp forests by requiring otter hunters to plant marine algae.