Species Interactions

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•  Interactions between species in a community can be classified according to their effect

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

  • The interactions between species in a community can be classified according to their effect on the organisms involved


Herbivory is the act of eating only plant matter (e.g. primary consumers are considered herbivores)

  • Herbivores may employ different feeding strategies (e.g. mucivores feed on plant sap, granivores feed on seeds, etc.)

Herbivory can be either harmful or beneficial to the plant species as a whole:

  • Certain types of beetle may feed voraciously on the leaves / foliage of crop plants (folivores), causing crop failure
  • Fruit-eating animals (frugivores) spread the seeds from a fruit in their faeces, promoting overall seed dispersal  

Examples of Herbivores



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 Relationship (Arctic Fox vs Snowshoe Hare)



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 transported to plankton-rich waters by whales)
  • Parasitism – One species benefits to the detriment of the other species (ticks or fleas feed on the blood of their canine host)

Types of Symbiotic Relationships


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•  Local examples to illustrate the range of ways in which species can interact within a community


Mutualism describes an ongoing interaction between two species whereby both species benefit from the interaction

  • Honey bees gather food (nectar) from flowers and distributes pollen between plants (mediating plant life cycle)
  • Plover birds pick food morsels from between the jaws of crocodiles, cleaning the crocodiles teeth in the process
  • Zooxanthellae (algae) photosynthesise within the protective environment of the polyp’s endodermis (feeding the coral)



Commensalism describes an ongoing interaction between two species whereby one benefits and the other is unaffected

  • Remora attach to the underside of larger predatory fish (e.g. sharks) and feed off the uneaten food scraps
  • Monarch butterflies can safely store poisonous chemicals produced by milkweeds, discouraging predators from eating it
  • Decorator crabs remove small fragments of tissue from sea sponges and uses them as a source of camouflage



Parasitism describes an ongoing interaction between two species whereby one species benefits at the other's expense

  • Ticks infest the skin and fur of host animals (such as humans), feeding off the host and potentially causing disease
  • Leeches attach to the skin and drinks the blood of the host animal until fully engorged
  • Tongue-eating louses eat the tongue of a fish (it may then function as a replacement tongue – stealing ingested food)


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•  The symbiotic relationship between Zooxanthellae and reef-building coral reef species

Reef-building coral will form a symbiotic relationship with the photosynthetic unicellular algae – Zooxanthellae

  • Coral are colonial organisms made up of individual polyps that are connected by a layer of living tissue
  • The algae lives within the cells of the coral’s endodermis (innermost lining of the animal)

The coral provides the algae with a protective environment and source of inorganic compounds:

  • Coral polyps secrete calcium carbonate to build a stony skeleton which encases the polyps (and zooxanthellae)
  • Coral polyps also recycle the waste products of the algae and supplies the zooxanthellae with carbon dioxide

The zooxanthellae, in turn, provides the coral polyps with a necessary source of nutrition:

  • The algae supplies the coral with oxygen, glucose and other organic molecules (produced via photosynthesis)
  • The algae also helps the coral to remove necessary waste products

Mutualistic Relationship Between Algae and Coral

coral anatomy

Coral Bleaching

It is the zooxanthellae within the polyp endodermis which gives the coral its vibrant pigmentation

  • When there is a large scale loss of zooxanthellae from the coral (due to environmental stress), bleaching occurs
  • When bleaching occurs, coral begins to starve and will die unless the zooxanthellae are restored

Conditions which can cause coral bleaching include:

  • Changes in light availability (e.g. sedimentation may increase the opacity of the oceanic waters)
  • Temperature increases (water temperatures in excess of 30ºC can irrevocably stress the zooxanthellae) 
  • Ocean acidification (the build up of carbon dioxide concentrations in the ocean can lower pH and stress the algae)

Healthy versus Bleached Coral

coral bleaching