Ecology Lecture Notes

Ecology: Organism Relationships and Environmental Interactions

  • Ecology studies the relationships between organisms, including competition and predation, as well as interactions with their environment.

Lecture Outline:

  • Niche Concept: Where an organism lives and its environmental requirements.
  • Competition and Predation: Their roles in shaping interactions.
  • Direct and Indirect Interactions: Expanding on concepts from the previous lecture.
  • Human Impacts and Trophic Cascades: How predators control food webs and the cascading effects.
  • Organism Roles in Food Webs: Whether some species have a disproportionately important role.
  • Conservation Importance: Why understanding these roles is critical for conservation.

Terminology:

  • Population: Individuals of the same species and their responses.
  • Community: Populations of different species interacting with each other.

Species Distribution:

  • Species distributions are influenced by basic requirements: eating, surviving, and reproducing.
  • Example: Humpback whale distribution is affected by resources, atmospheric variables, ocean temperature, biogeochemical nutrients, diet, population dynamics (feeding, breeding, resting), and human impacts.

Humpback Whale Distribution:

  • A study analyzing 148 studies over 40 years revealed patterns in humpback whale distribution.

  • Northern Hemisphere: Separate feeding and breeding populations.

    • Blue areas: Feeding regions (nutrient-rich, krill production).
    • Red areas: Breeding regions (warmer, tropical areas).

  • Antarctica: Separate foraging populations.

  • Spatial and temporal distributions vary; whales migrate to warmer areas to reproduce.

  • Temporal distribution: Humpbacks feed in Antarctica during summer and migrate to subtropics (e.g., Ningaloo) to give birth in June-August.

Ecological Niche:

  • The way an organism uses its environment.
  • Humpback whales: complex, with different places and times for different activities.
  • Simplified study: Plotting environmental dimensions to understand species requirements.

Rocky Shores:

  • Classic studies on temperate rocky shores due to their two-dimensional nature and strong zonation.
  • Example: Scottish rocky shore with barnacles, fucoid brown algae, and kelp at different shoreline levels.

Western Australian Coral Example:

  • Study on acropora corals in Cygnet Bay, Kimberley.
  • Subtidal zone (always submerged) vs. intertidal zone (between high and low tide).
  • Seasonal temperature variation shows significant heat stress on intertidal corals.
  • Corals are sensitive to heat stress with limited physiological tolerance, compounded by marine heat waves.

Zonation in Temperate Rocky Shores:

  • Wave action is predominant:
    • Higher up: specific organisms
    • Lower down: different organisms
  • Intertidal zone: Submerged twice daily.
  • Exposure levels determine species distribution.
  • Ballantyne's classification (1960s): Rocky shores categorized by wave exposure (1-8).
    • Example communities: Porphyra, Fucus, Thalamus, Elaria, kelps
  • Species distributions change based on wave exposure, showing niche variation by environmental parameters.
  • Rocky shores are ideal for manipulation experiments due to manageable scale.

Isle Of Man Rocky Shore Experiments:

  • Hawkins and Hartnell (1980s): Removed organisms (Ascophyllum seaweed) to study zonation.
  • Result: Fucus serratus (brown algae) extended its range upwards.
  • Conclusion: Zonation is not solely due to environmental tolerances but also competition.
  • Species interactions (ecology) drive distribution changes when organisms are removed or predators are introduced.
  • Competitive Exclusion Principle: Species with identical requirements cannot coexist.
    • Species can coexist if they use different components of a resource.
    • If resource use overlaps significantly, one species outcompetes the other.
  • Niches are more complex due to spatial and temporal variation.

Barnacle Competition in Scotland:

  • Barnacles (Semibalanus balanoides and Thalamus stellatus) coexist on rocky shores, suitable for testing the competitive exclusion principle.
  • Connell (1960s): Removed Semibalanus to study competition with Thalamus.
  • Experiment: Monitored barnacle survival in quadrats after selectively removing Semibalanus.
  • Results: Thalamus population expanded when Semibalanus was removed, indicating competitive restriction.
  • Competition for space restricts species range on rocky shores.
  • Interference competition: Competition for physical space.

Niche Concepts Revisited:

  • Niche is not only about environmental requirements but also shaped by interactions and environmental role.
  • Fundamental niche: Potential environment where an organism could exist.
  • Realized niche: Incorporates competition effects.
    • Thalamus barnacle demonstrates a realized niche shaped by competitive interactions.

Importance of Predation: Robert Paine's Experiments

  • Robert Paine (1933-2016) conducted predator removal experiments on the West Coast of the US.
  • Near Bay, Washington State: Intertidal communities with sessile and mobile organisms, including starfish and mussels.
  • Piaesta ocraseaus (purple starfish) and Mitelus californias (California mussel) were key species.
    • Mussel: Dominant competitor
    • Starfish: Generalist predator
  • Experiment: Starfish were removed from a 2m x 8m area, and the community was monitored.
  • After three months, species numbers shifted dramatically.
  • Barnacles dominated (60-80%), contrary to expectations based on competitive dominance of mussels.

Trophic Networks:

  • Removing a predator requires considering the entire trophic network.
  • Starfish food web: Starfish feed on mussels, barnacles, and gastropods, which in turn feed on benthic algae.
  • Starfish removal: Mussels become abundant, reducing barnacles and changing community structure.
  • Keystone species: Organisms with a disproportionately important role in structuring a food web.
  • Removal leads to dramatic community changes.

Trophic Cascades:

  • Changes in predator density lead to cascading effects.
  • Classic example: Killer whales, sea otters, sea urchins, and kelp forests.
  • Sea otters feed on sea urchins, which graze on kelp.
    • High sea urchin population: Low kelp density.
    • Low sea urchin population: High kelp density.
  • Sea otters control sea urchin populations; high sea otter populations result in fewer urchins and more kelp.
  • Killer whales now prey on sea otters due to prey switching, causing a trophic cascade.
  • High orca predation reduces sea otters, increasing sea urchins and decreasing kelp.

Orca Predation and Trophic Cascade Detail:

  • Science (1998) published a study showing the impact of killer whales.
  • Normal situation (1988): High otter abundance, high kelp density.
  • Predator switching (1997): Killer whales prey on otters, reducing otter populations.
  • Result: Increased sea urchins, high grazing, reduced kelp canopy.
  • However, the system is more complex than just direct trophic interactions.
  • Sea urchin predator (starfish) affects sea urchin condition and behavior.
  • Behavior change in sea urchins alters their distribution, affecting sea otter behavior.

Direct and Indirect Interactions:

  • Predators have direct effects on prey species and indirect effects through competition.
  • Starfish affect mussel abundance, leading to competitor release and increased community diversity.
  • Classic examples of trophic cascades come from marine communities.
  • Sharks cause behavioral shifts in mesopredators, affecting prey choice and algal communities.
  • Predator presence affects animal behavior, structuring food webs.
  • Non-lethal effects: Predation risk affects foraging behavior.

Summary of Trophic Cascades:

  • Top-down control: Predator removal affects lower trophic levels.
  • Bottom-up control: Changes at the bottom of the food web affect higher levels.
  • Trophic cascades occur in both terrestrial and aquatic ecosystems.

Importance of Particular Species in a Community:

  • Ecosystem function: Productivity, nutrient transfer.
  • Species richness: Hypothesis that more species contribute differently to ecosystem function and services.
  • Ecosystem services: Human use of ecosystem productivity.
  • Species diversity is not always equivalent to functional diversity.

Functional Diversity:

  • The role an organism performs (competitor, predator, nutrient recycler).
  • Example: Two organisms (A and B) with shared and unique traits.
    • Process 1 relies on shared traits. Both can perform process 1, with Organism A more abundant.
    • Process 2 relies on unique traits and can only be performed by Organism B. Making it vital for this function.
  • Organism B is critical for a hypothetical ecosystem process.
  • Removal of key organism would be detrimental to ecosystem function.

Functional Diversity Data:

  • Meta-data analyses of ecosystem functions (pollination, biological control, nutrient cycling).
  • Comparison of species richness vs. functional diversity in explaining ecosystem function.
  • Functional diversity demonstrates better explanatory power than species-based indices.

Lecture Summary:

  • Rocky shore ecology highlighted patterns of zonation and competition effects.
  • Trophic cascades, specifically top-down control, were discussed, with the sea otter-sea urchin-kelp system as a classic example.
  • Ecosystem functioning depends on functional diversity, not just species diversity.