This excellent post at The Fisheries Blog uses a vist to the Monterey Bay Aquarium's kelp forest to explain the concept of trophic downgrading. The post references the seminal Science report on the topic co-authored by James A. Estes and John Terborgh. Their book, Trophic Cascades, is the first presentation of the science in book form and provides background and details on this subject.
From The Fisheries Blog...While visiting the Monterey Bay Aquarium for the first time, I was reminded of an article in Science discussing the consequences of globally removing large apex predators from our ecosystems. Below I share some information from my visit and from this article. I walked up to the renowned three-story display of the kelp forest in the Monterey Bay Aquarium and stared. Despite the crowds and clatter of people around me, this beautiful floating forest with a diverse array of fish darting through the swaying stalks and leaves was mesmerizing. At the front of the crowd, an employee of the aquarium stood describing how kelp forests provide nursery grounds and habitat to a large number of fish species. Being a native to the east coast, I had never before had the opportunity to see a kelp forest. As I stood watching the display, I could hear my undergraduate ecology professor in my head describing how kelp forests held one of the most common examples of a key stone species, the sea otter, and top-down control of an ecosystem. I also remembered that kelp are the fastest growing plant in the world (20in/day); nearly double the growth rate of bamboo. Despite this amazing growth rate, kelp can be decimated into “sea urchin barrens” (former kelp forests where sea urchins cover the substrate) in the absence of sea otters. Sea otters eat invertebrates and small fish, but most importantly to kelp, sea urchins. Sea urchins are herbivorous and can clear-cut a kelp forest quickly and with relative ease by scraping the base of the kelp off the sea floor. Without the pressure of sea otters to control the population of sea urchins, kelp forests and the habitat they provide to fishes can be destroyed. Read more