Ticks feed on the blood of mice and other mammals. In 2012, ecologists studied the interaction between ticks and the white-footed mice in an oak forest. They found that when tick density is high, fitness of white-footed mice declined; they would characterize the interaction between ticks and mice as . In 2013, tick density was low and the researchers found no effect on mouse fitness; they would characterize the species interaction between ticks and mice as .

parasitism, commensalism

Explanation:

Parasitism is a form of interaction between two organisms in which the parasite lives on the host, obtain all its nutrients from the host and end up stifling the growth of the host. In summary, parasites reduce the fitness of their hosts by gradually killing them.

Commensalism is a form of interaction between two organisms in which the commensal benefits and the host is unaffected. The fitness of the host neither decrease nor increase.

Hence, when ticks (which feeds on the blood) reduced the fitness of white-footed mice in 2012, it is a parasitic relationship. When the blood feeding had no effects in 2013, it is a form of commensalism.