Ecosystems and Species Interactions

Ecosystems

  • Ecosystems arise from biotic and abiotic interactions.
    • Biotic factors: Living organisms (prey, competitors, predators, parasites, disease).
    • Abiotic factors: Non-living components (sunlight, temperature, water, soil, nutrients).

Interspecific Interactions

  • Symbiotic interactions:
    • Competition (-/-): Species compete for limited resources, leading to competitive exclusion.
    • Predation/Parasitism (-/+): One species benefits while the other is harmed.
    • Mutualism (+/+): Both species benefit (e.g., lichens: algae & fungus).
    • Commensalism (+/0): One species benefits, and the other is neither harmed nor helped (e.g., barnacles on a whale).

Predation and Evolution

  • Predation drives evolution.
    • Predator adaptations: Mechanisms to locate and subdue prey.
    • Prey adaptations: Mechanisms to elude and defend (spines, thorns, toxins, horns, speed, coloration).
  • Predation exerts strong selection pressure on both predator and prey.

Predator/Prey Population Cycles

  • Classic example: Snowshoe hare and Lynx population cycles.
  • Predator-prey interactions lead to population size fluctuations over time.

Niches

  • Niche: An organism’s ecological role (niche = job), distinguished from its habitat (habitat = address).
  • Fundamental niche: Potential niche an organism CAN occupy.
  • Realized niche: Actual niche an organism DOES occupy.
  • Competitive Exclusion:
    • No two similar species can occupy the same niche at the same time.
    • If Species 2 is removed, then Species 1 will occupy the whole tidal zone.
    • At lower depths, Species 2 out-competes Species 1, excluding it from its potential (fundamental) niche.
    • Example species: Chthamalus sp., Semibalanus sp.

Resource Partitioning

  • Reduces competition through the use of microhabitats.
  • "The ghost of competition past": Past competition has shaped present resource use.

Anti-Predator Adaptations

  • Hiding from predators: Avoiding detection through camouflage.
  • Warning predators: Advertising unsuitability as prey.
    • Aposematic coloration: Warning coloration (apo = away, sematic = sign/meaning).
    • Batesian mimicry: Harmless species mimics a harmful one.
    • Mullerian mimicry: Multiple harmful species resemble each other.

Aposematic Coloration

  • Common warning colors: Black, red, orange, and yellow signal "DON’T EAT ME!"
  • Aposematic species come to resemble each other.

Defense Mechanisms

  • Camouflage / Cryptic coloration helps organisms hide.

Community Structure

  • Characterizing a community:
    • Species diversity: Number of different species.
    • Composition: Dominant species (most abundant species or highest biomass).
    • Keystone species: Species with a disproportionately large effect on its environment relative to its abundance.
    • Changes over time: Succession.

Keystone Species

  • Influential ecological role: Exerting an important regulating effect on other species in the community.
  • Keystone species increases diversity in habitat.
    • Example: Pisaster ochraceous (sea star).
      • Removing sea stars decreases diversity, as mussels out-compete other species.
      • Presence of sea stars increases diversity on the Washington coast.
  • Beavers: Keystone species in Northeast & West.
    • Dams transform flowing streams into ponds, creating new habitat.

Species Diversity and Stability

  • Greater diversity = greater stability.
  • Greater biodiversity offers:
    • More food resources.
    • More habitats.
    • More resilience in the face of environmental change.

Impact of Reduced Biodiversity

  • Examples comparing communities: suburban lawn, agricultural “monoculture”, “old field”.
  • Historical examples:
    • Irish potato famine.
    • 1970 US corn crop failure.