Species Interactions

Species Interactions

Types of Species Interactions

  • Ecologists classify interactions by effect: positive (+), negative (-), or neutral (o).
  • Consider trophic (feeding) and symbiotic relationships.
  • Species interactions are difficult to classify because of the different ways in which interactions affect particular species.
  • Interactions have both ecological and evolutionary consequences.

Four Broad Categories of Species Interactions

  • Predation: Trophic interaction where a predator kills/consumes prey.
    • Includes carnivory, herbivory, and parasitism.
  • Competition: Nontrophic interaction where species overlap in resource use, negatively affecting growth, reproduction, and/or survival.
  • Positive Interactions (Facilitation): At least one species benefits, none are harmed.
    • Mutualism: Both species benefit.
    • Commensalism: One benefits, the other is unaffected.
    • Amensalism: One is harmed, the other is unaffected.
  • Interaction categories are part of a continuum.
    • Difficult to classify, affect species differently.

Predation

  • Carnivory: Capture and avoidance mechanisms.
    • Success depends on balancing energy costs/benefits.
    • "Active pursuit" predators: fast, strong jaws.
    • "Stealthy ambush" predators: strike from hiding.
    • Venom and morphological adaptations.
  • Prey defenses:
    • Morphological: tough skin, shells, spines.
    • Avoiding detection: hiding, crypsis (camouflage).
    • Chemical: repel predators.
    • Warning signals: visual or acoustical defenses, bright colors.

Mimicry Systems

  • Batesian mimicry: harmless species mimics a dangerous one.
  • Mullerian mimicry: multiple dangerous species converge on a common warning signal.

Herbivory

  • Widespread but specialized, many insects feed on few plant species.
  • Vertebrate herbivores are often generalists.
  • Herbivores consume parts of plants.
  • Plant defenses include chemical and physical (thorns, spines).

Parasitism

  • Effects vary from weak to lethal.
  • Parasites specialize on few hosts; hosts have many parasites.
  • Parasites can have their own parasites.
  • Microparasites: small, live inside hosts, cause disease.
  • Macroparasites: larger, not always closely associated with hosts.
    • Ectoparasites: live outside hosts.
    • Endoparasites: live inside hosts.

Predator-Prey Dynamics

  • Populations linked and change in response to each other.
  • Predator-prey interactions influence population dynamics.
  • Example: Canada Lynx and snowshoe hare populations cycle.
  • Predators can alter communities.
  • Example: Sea stars preferentially feed on mussels, creating bare space for other sessile invertebrates to settle.
  • Keystone species: large community effects despite abundance.

Competition

  • Species compete for resources (food, water, light, space).
  • Resource set determines a species' niche.
    • Fundamental niche: Physiological capabilities.
    • Realized niche: Interactions with other species.
  • Limiting resource: Resource strongly influencing population size.
  • Competitive exclusion: One species eliminates another.
  • Competitive coexistence: Species coexist despite shared resources.
    • Interference competition: Direct interference with access to limiting resources.
    • Exploitation competition: Relies on the relative efficiency with which each species uses the resource.
  • Resource partitioning: Using resources differently to reduce competition.
    • Can lead to character displacement (morphological/behavioral divergence).
  • Environment/disturbances alter competition outcomes.

Positive Interactions

  • Mutualisms, commensalisms, symbiotic relationships.
  • Obligate (necessary) or facultative (optional).
  • More common in stressful environments (deserts, marshes, alpine).
  • Alpine plant example: neighbors provide thermal protection at high elevations.