Communities and Ecosystems

Focus

• Recent focus on biotic interactions in biological communities.
• Impact of these interactions on population sizes and ecosystem-level effects.
• Case study to illustrate concepts in a real-world context.
• Introduction of key terms related to communities and ecosystems.

Case Study: Coastal Marine Ecosystem

• Key Species:
  1. Giant Kelp
     • Major primary producer.
     • Type of large brown algae, grows up to 200 feet.
     • Forms kelp forests, providing shelter and food substrate for many species.
     • Important for humans: food source, used in cosmetics, paint, and nut milk alternatives.
     • Sequesters carbon like terrestrial forests.
     • Physical structure attenuates wave energy, preventing coastal erosion.
  2. Sea Urchins
     • Omnivores eating at different trophic levels, including giant kelp.
     • Habitat: rocky ocean bottom, coral reefs.
     • Threats: pollution, ocean acidification, dredging, overharvesting.
     • Overabundance leads to overgrazing of kelp.
  3. Sea Otters
     • Top predator, part of the weasel family.
     • Eat benthic invertebrates, mainly sea urchins.
     • Considered a keystone species due to their significant ecosystem impact.

Keystone Species

• Plays a key role in the ecosystem, impact disproportionate to abundance.
• Often higher trophic level species.

Trophic Interactions and Cascades

• Direct Effects:
  • Otters eat urchins, regulating their population.
  • Urchins eat kelp.
• Indirect Effects:
  • Otters indirectly benefit kelp by controlling urchin populations.
• Trophic Cascade:
  • Decrease in otters → Increase in urchins → Decrease in kelp (Reciprocal population effects).
  • Can occur from either a decrease or increase in predator populations.
• Ecosystem Impact:
  • Kelp forests can turn into urchin barrens due to trophic cascades, leading to ecosystem transformation.

Historical and Current Data

• Sea Otters:
  • Threatened in the U.S., endangered globally due to hunting for fur.
  • Populations plummeted due to the fur trade, rebounded after hunting moratorium.
  • Current threats: pollution, disease, oil spills.
• Trophic Cascade Evidence:
  • Historical data shows fluctuation in otter, urchin, and kelp populations correlating with hunting moratorium and otter population changes.

Complex Ecosystem Interactions

• Orcas and Sea Otters:
  • Industrial whaling reduced great whale populations, orcas shifted to sea lions, then sea otters as prey.
  • Increased predation on sea otters due to lack of traditional prey.
• Bald Eagles:
  • Shifted diet from sea otter pups to seabirds (e.g., ptarmigans), increased fecundity.

Summary

• Trophic cascades observed in nature; systems are complex with multiple interacting factors.
• Important to consider broader community interactions to understand ecosystem dynamics.