3.7: Ecological Interactions- Two-Species Model

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Two-species model allows to describe how two theoretical species might influence each other. For example, Species I may facilitate Species II: it means that if biomass (sum of weight) of Species I increases, biomass of Species II also increases (\(+\) interaction). There are also \(+\) and \(0\) interactions. Two species and three signs make six combinations:

	+	0	-
+	mutualism	commensalism¹	exploitation²
0	...	neutralism	amensalism
-	...	...	interference³

\(^1\) Includes phoresy (transportation), inquilinism (housing) and “sponging”.

\(^2\) Includes predation, parasitism and phytophagy.

\(^3\) Includes competition, allelopathy and aggression.

Mutualism: It sometimes called “symbiosis”. Two different species collaborate to make each other life better. One of the most striking example is lichenes which is algae-fungus mutualism.
Commensalism: Remember “Finding Nemo”? Clown fish lives inside actinia. This type of commensalism is called “housing”. Another example is suckerfish and shark, this is phoresy. Sponging happens when scavengers feed on what is left after the bigger carnivore meal.
Exploitation: This is the most severe interaction. Predation kills, but parasitism or phytophagy (the only difference is that second uses plants) do not.
Neutralism: Rare. Philosophically, everything is connected in nature, and if Species I and II live together, they usually interact, somehow.
Amensalism: This happens when suppressing organism is, for example, much bigger then the “partner”. Big trees often suppress all surrounding smaller plants.
Interference: Competition happens when Species I and II share same ecological niche, have similar requirements. Gause’s Principle says that sooner or later, one of them wins and another looses. Allelopathy is a mediated competition, typically through some chemicals like antibiotics. Most advanced (but least pleasant) is the direct aggression when individuals of one species physically eliminate the other one.