BIBO assert that a dcterms:Agent is an equivalent class to foaf:Agent. This means that all the individuals belonging to the foaf:Agent class also belongs to the dcterms:Agent class. This way, dcterms:contributor can be used on foaf:Person, foaf:Organization, foaf:Agent and foaf:Group. Even if this link is not done in neither the FOAF nor the DCTERMS ontologies this is a wide spread fact that is asserted by BIBO.