Ecology: Communities and Ecosystems

Communities

Defining Communities

• Individual: A single organism.
• Population: All individuals of the same species within an environment.
• Community: All populations combined in an environment.

Interspecific Interactions

Interspecific interactions are relationships between species, characterized by their effects on each participant:

• Positive (+)
• Negative (-)
• Neutral (0)

Examples of these interactions include:

• Competition
• Predation
• Herbivory
• Symbiosis, which further includes:
  • Parasitism
  • Mutualism
  • Commensalism

Competition

Competition can be visualized with the example of barnacles (Chthamalus and Balanus):

• When both species are present, their ranges are restricted.
• Chthamalus's range expands when Balanus is absent.

Competitive Exclusion Principle

• Two species cannot occupy the same niche (i.e., share the same resources/interactions).
• One species will either go extinct or evolve to use different resources.

Solutions to Competition: Resource Partitioning

• Spatial: Species utilize different physical areas.
  • Example: Anolis lizard species occupying different perches.
• Temporal: Species utilize resources at different times.
  • Example: Nocturnal spiny mouse species, where one becomes diurnal.

Resource Partitioning: Character Displacement

• Species evolve to occupy distinct niches, reducing competition.
• Example: Beak depth in Geospiza finches.
  • G. fuliginosa and G. fortis on Los Hermanos exhibiting different beak depths.
  • G. fortis on Daphne.
  • G. fuliginosa and G. fortis on Floreana and San Cristóbal, showing character displacement.

Symbiosis

Symbiosis involves species living close to each other, interacting in some way.

Mutualism (+/+)

Both organisms benefit.

• Examples: Lichens, nitrogen-fixing bacteria, mycorrhizae.

Commensalism (+/0)

One organism benefits, and the other is unaffected.

Parasitism (+/-)

One organism (parasite) benefits, and the other is harmed.

• A parasite causing host symptoms is a pathogen.

Predation and Herbivory (+/-)

One organism (predator or herbivore) eats another (prey or plant).

• Note the boom-bust cycle that each population follows. Invasive species disrupt this cycle.
• Plant-herbivore relationships are similar to animal predator-prey relationships.

Evolutionary Pressure

• Predation and herbivory put strong evolutionary pressure on both predator/herbivore and prey/plant.
• Prey want to avoid being eaten; predators want to eat more prey.

Prey/Plant Defense Mechanisms

• Physical defenses
• Chemical defenses
  • Some organisms evolve warning colors (aposematism).
• Cryptic coloration (camouflage) instead of warning colors.
• Mimicry: Copying another organism's