Direct and indirect effects

Parasites can have a multitude of direct and indirect effects on their hosts and on surrounding communities. Direct effects occur, for example, when parasites cause mortality of their hosts, thus reducing host population size. Parasites can also cause sub-lethal effects such as reducing host fecundity or manipulating the behaviour of their hosts. For example, the rhizocephalan parasite Sacculina carcini (a barnacle relative) makes male crabs behave like females and move into deeper waters.

Parasites can also indirectly affect other organisms than their hosts. This can happen when parasites reduce host population sizes and thus release other organisms from competition or predation (density-mediated indirect effects). Or it happens when parasites change the behaviour or other traits of their hosts which in turn has effects on other organisms than the hosts (trait-mediated indirect effects).

As parasitism is a trophic interaction between a parasite and its host, parasites can also affect the structure and dynamics of food webs by adding and mediating trophic interactions. We study the various effects of parasites using experimental approaches and food web analyses with the aim to understand the ecological role of parasites in marine ecosystems.