MUTUALISM: Both species benefit.
Example: in pollination mutualisms, the pollinator gets food (pollen, nectar), and the plant has its pollen transferred to other flowers for cross-fertilization (reproduction).
COMMENSALISM: One species benefit, the other is unaffected.
Example: cow dung provides food and shelter to dung beetles. The beetles have no effect on the cows.
COMPETITION: Both species are harmed by the interaction.
Example: if two species eat the same food, and there isn’t enough for both, both may have access to less food than they would if alone. They both suffer a shortage of food.
PREDATION AND PARASITISM: One species benefit, the other is harmed.
Example: predation—one fish kills and eats parasitism: tick gains benefit by sucking blood; the host is harmed by losing blood.
AMENSALISM: One species is harmed, the other is unaffected.
Example: A large tree shades a small plant, retarding the growth of the small plant. The small plant has no effect on the large tree.
NEUTRALISM: There is no net benefit or harm to either species. Perhaps in some interspecific interactions, the costs and benefits experienced by each partner are exactly the same so that they sum to zero.
Example: Neutralism is also sometimes described as the relationship between two species inhabiting the same space and using the same resources, but that has no effect on each other. In this case, one could argue that they aren’t interacting at all.