Explain why commensalism is difficult to demonstrate and think about why other interactions (called amensalism (0/-) and spite (-/-) relationships) are exceedingly rare or nonexistent (think about this in an evolutionary context.)
Commensalism is a long-term, close association between two species in which one benefits and the other is unaffected. For instance, many of the bacteria that inhabit our bodies seem to have a commensal relationship with us. They benefit by getting shelter and nutrients and have no obvious helpful or harmful effect on us.
Commensalism is difficult to demonstrate because many apparent commensalisms actually turn out to be slightly mutualistic or slightly parasitic (harmful to one party, see below) when we look at them more closely. For instance, biologists are finding more and more evidence that our normal microbial inhabitants play a key role in health.
A relationship between two organisms in which one organism is harmed or inhibited and the other is unaffected. Examples of amensalism include the shading out of one plant by a taller and wider one and the inhibition of one plant by the secretions of another (known as allelopathy).
Spite provides an explanation for how natural selection can favor harmful behaviors that are costly to both the actor and the recipient; spite is typically considered a form of altruism that benefits a secondary recipient.