Some Aleutian Islands are surrounded by urchin barrens while others are surrounded by kelp forests. Which of the following is the primary explanation for these differences? a. Kelp eat urchins, thus preventing their establishment b. Grazing by urchins prevents the formation of kelp forests. c. The kelp forest islands experience a substantially warmer climate than do the urchin barren islands. d. Kelp forest islands and urchin barren islands differ with respect to oceanic currents.