Species Interactions

In nature no species exists in total isolation – all organisms interact with both the abiotic environment and other organisms

  • If two species interact directly within a shared environment, they share a positive association (they co-exist)
  • If interactions within an environment are mutually detrimental to both species, they share a negative association (do not co-exist)

Positive Associations

1.  Predator-Prey Relationships

  • Predation is a biological interaction whereby one organism (predator) hunts and feeds on another organism (prey)
  • Because the predator relies on the prey as a food source, their population levels are inextricably intertwined
    • If the prey population drops (e.g. due to over-feeding), predator numbers will dwindle as intra-specific competition increases
    • If the prey population rises, predator numbers will increase as a result of the over-abundance of a food source

Predator-Prey Relationships (Arctic Fox vs Snowshoe Hare)


2.  Symbiotic Relationships

  • Symbiosis describes the close and persistent (long-term) interaction between two species
  • Symbiotic relationships can be obligate (required for survival) or facultative (advantageous without being strictly necessary)
  • Symbiotic relationships can be beneficial to either one or both organisms in the partnership:
    • Mutualism – Both species benefit from the interaction (anemone protects clownfish, clownfish provides fecal matter for food)
    • Commensalism – One species benefits, the other is unaffected (barnacles are transported to plankton-rich waters by whales)
    • Parasitism – One species benefits to the detriment of the other species (ticks and fleas feed on the blood of their canine host)  

Types of Symbiotic Relationships


Negative Associations

1.  Competition

  • Competition describes the interaction between two organisms whereby the fitness of one is lowered by the presence of the other
  • Competition can be intraspecific (between members of same species) or interspecific (between members of different species)
  • Limited supplies of resources (e.g. food, water, territory) usually triggers one of two types of responses:
    • Competitive exclusion – One species uses the resources more efficiently, driving the other species to local extinction
    • Resource partitioning – Both species alter their use of the environment to divide the resources between them

Competitive Exclusion vs Resource Partitioning

niche differentiation