Lecture Notes: Keystone Species and Trophic Cascades

Introduction

• Exploration of various ecosystems: jungle, desert, forest, plains, mountains, and seashores.
• Each habitat contains communities of plants and animals, with species present in different numbers.
• Key question: What determines how many species live in a place and population sizes?

The Birth of an Ecological Experiment

• Robert Payne's Experiment (1963)
  • Location: Rocky Pacific Shore
  • Action: Removed a purple starfish, pisaster ocius, from the ecosystem.
  • Goal: Understand the role of predators in the ecosystem.

The Green World Hypothesis

• Origin: Classroom at the University of Michigan

  • Question posed by Professor Fred Smith: Why is the tree green?
  • Initial thought: Chlorophyll makes a tree green.
  • Bigger picture: How food chains regulate populations.

• Traditional View

  • Producers limit herbivores; herbivores limit predators.

• New Idea by Smith, Hairston, and Slobodkin

  • Herbivores are controlled not just from bottom-up but also top-down by predators.
  • This hypothesis suggests predators keep herbivores in check, maintaining the green world.

Predator's Impact: Payne's Experiments

• Payne's Observations

  • Identified food chain on Pacific coast.
  • Noted starfish as top predators.

• Starfish Removal Experiment

  • Method: Consistently removed starfish from a specific area.
  • Result: Ecosystem simplified, species diversity decreased drastically.
  • Conclusion: Predators like starfish regulate community structure significantly.

Keystone Species Concept

• Keystone Species

  • Definition: Species that have a large impact on its ecosystem.
  • Removal of such species can lead to ecosystem collapse.

• Trophic Cascades

  • Concept: Effects of one species on others through a food web.
  • Example: Sea otters controlling sea urchin populations, influencing kelp growth.

Case Study: Sea Otters and Kelp Forests

• James Estes' Study

  • Location: Uan Island, Alaska
  • Finding: Otters control urchin populations; loss of otters leads to loss of kelp forests.

• Human Impact

  • Wailing led to reduced whale populations, affecting orca diets.
  • Result: Orcas started preying on otters, leading to increased urchins and decreased kelp.

Broader Implications

• Global Ecological Changes

  • Removal of key predators like wolves, sharks, and lions affects ecosystems worldwide.
  • Insight has changed ecological and conservation perspectives.

• Conservation and Ecology

  • New understanding of top-down controls provides tools for conservation.
  • Ignoring top-down effects invites ecological mistakes.

Conclusion

• Predators play a crucial role in regulating ecosystems through top-down control.
• Continued research is necessary to fully understand these dynamics and prevent ecological collapse.