Overview
This lecture explores how predators regulate ecosystems, focusing on keystone species and the concept of trophic cascades, using classic ecological experiments and discoveries.
Ecosystems and Species Regulation
- Earth is home to diverse habitats, each with unique communities of plants and animals.
- The number of species and their populations in an ecosystem are determined by multiple factors.
Food Chain Theory and the Green World Hypothesis
- Early views suggested population sizes are regulated from the bottom up—producers limit herbivores, which in turn limit predators.
- This did not explain why herbivores don't consume all plant life.
- Fred Smith, Nelson Hairston, and Lawrence Slobodkin proposed that predators also limit herbivores, preventing overgrazing (Green World Hypothesis).
Robert Paine’s Starfish Experiment
- Paine removed starfish (Pisaster ochraceus) from a rocky coast to test the role of predators.
- Removing starfish led to a drastic decline in species diversity; mussels outcompeted other species and monopolized space.
- Paine introduced the concept of "keystone species"—species with a disproportionate impact on their ecosystem.
Keystone Species and Trophic Cascades
- Keystone species—like starfish—maintain the balance and diversity of ecosystems.
- Removing keystone species can cause cascade effects, simplifying communities and reducing biodiversity.
- Other species, when removed, had little effect, showing not all species have equal roles.
Sea Otters and Extended Trophic Cascades
- In Alaska, the absence of sea otters led to sea urchin overpopulation and the decline of kelp forests.
- Sea otters act as keystone species by controlling urchin populations, maintaining kelp forests.
- "Trophic cascade" describes how apex predators indirectly affect lower trophic levels, causing ecosystem-wide changes.
Killer Whales and Multi-level Cascades
- Decline in otter populations was linked to killer whales switching to otters after whale populations decreased due to hunting.
- This introduced a fourth trophic level, further illustrating complex cascade effects in food webs.
Broader Impacts and Conservation
- Keystone species and trophic cascades are found in many ecosystems, affecting global biodiversity.
- Ignoring top-down regulation risks ecosystem mismanagement.
- These discoveries have shifted ecological thinking from only bottom-up to including top-down controls.
Key Terms & Definitions
- Keystone species — A species whose impact on its ecosystem is disproportionately large relative to its abundance.
- Green World Hypothesis — The idea that predators control herbivore populations, preventing overconsumption of plants.
- Trophic cascade — A series of indirect effects through a food web initiated by changes in top predator populations.
- Apex predator — A predator at the top of a food chain with no natural predators.
Action Items / Next Steps
- Review definitions and examples of keystone species and trophic cascades.
- Consider implications of top-down versus bottom-up regulation in assigned readings.