Video: https://www.youtube.com/watch?v=hRGg5it5FMI
Some Animals Are More Equal than Others: Keystone Species and Trophic Cascades
Producers: Energy suppliers to whatever lives off of them
Herbivores: Animals that consume producers
Predators: Animals that feed on herbivores
The popular idea at the time was that the number of producers limits the number of herbivores, thus limiting the number of predators. However, it didn’t explain why herbivore populations didn’t simply grow to the point where they ate all the producers.
Fred Smith, Nelson Hairston, and Lawrence Slobodkin proposed that herbivores must be controlled by producers and predators.
Green World Hypothesis
The Green World Hypothesis proposed that the world is green because predators keep herbivores in check. Up until that time, no one thought predators had any role in regulating ecosystems.
Robert Paine put this idea to the test. He began by identifying all the organisms in an area and mapping out a food web. His observations showed that a large purple and orange starfish was at the top of the food chain. Paine asked “What happens if you remove the predator starfish from a singular outcrop?”
The ecosystem started to change rapidly. Although the top predator had been removed, the number of species in the rock decreased from 15 to eight and kept decreasing. Mussels monopolized all the available space, pushing other species out. Paine showed that a single species had a massive effect on the ecosystem. They are known as keystone species.
Keystone Species
Keystone species have a huge impact that extends beyond the species they primarily prey on. Not all species have the same impact on the ecosystem.
In another experiment, Robert Paine removed sea urchins from some pools and left others untouched. In the pools where he removed urchins, kelp started growing immediately. Urchins control the kelp, therefore it was a total violation of the Green World Hypothesis since nothing seemed to be controlling the urchin populations.
In another experiment, James Estes observed that the loss of otters reorganized an ecosystem to now be dominated by sea urchins that ate all the kelp. The experiment showed that sea otters controlled sea urchins, thus ensuring the population of kelp in trophic cascades. Sea otters were a keystone species and control the ecosystem from the top down.
In a follow-up experiment in response to declining otter populations, James Estes tested a hypothesis that stated orcas were eating otters because of decreasing whale populations. With orcas eating otters, sea urchin populations increased, thus decreasing kelp populations.
The removal of predators have profound effects on the variety of ecosystems as a whole.