Dr. Ray L. Winstead
Professor of Biology, Indiana University of Pennsylvania

Interactions Between Two Species

1     2     Type of Interaction     General Nature of Interaction and Example

0     0     Neutralism              Neither population affects the other.
                                        Squirrel (0), Butterfly (0)

+     +     Mutualism*              Interaction favorable to both and obligatory.
                                        Lichens: algae (+, gains water and protection),
                                        fungus (+, receives food produced by algae)

+     +     Protocooperation        Interaction favorable to both but not obligatory.
                                        Marine crab (+, gains camouflage),
                                        Sponge (+, transported to food)

+     0     Commensalism*           The commensal (1) benefits while the host (2) is not
                                        affected.  Spanish Moss (+, support), Tree (0)

+     -     Predation               The predator (1), usually larger, kills and eats the
                                        prey (2); Snake (+, gains food), mouse (-, eaten)

+     -     Parasitism*             The parasite (1), usually smaller, attacks the host (2)
                                        Tapeworm (+, gains food), Human (-, loses energy)

-     0     Amensalism              Population 1 inhibited, 2 not affected
                                        Small bush (-,deprived of sun's energy), large tree (0)

-     -     Competition             Indirect inhibition of each species by the other when
           (Resource Use type)       common resource is in short supply.
                                       Arctic Fox (-) and Arctic Owl (-) both competing for
                                       same food source (Lemmings)

-     -     Competition             Direct inhibition of each species by the other.
        (Direct Interference type)      Mutual predation: Bream (-,eaten by Bass)
                                        Bass (-, when small eaten by Bream)

(* types of symbiosis - two species living together closely)
0  not affected by the association / interaction
+  benefits from the association
-  loses from the association

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