The ecological niches of parasites

The ecological niche as defined by Hutchinson (1957) is a multi-dimensional hypervolume determined by environmental (biotic and abiotic) variables within which a species can exist. Rohde (1979), discussing parasites and particularly ectoparasites of fishes, emphasised that the number of niche dimensions is almost infinite, but that a few dimensions are sufficient to characterise the niche volume of a parasite with a high degree of accuracy. These dimensions are host range (host specificity), microhabitats, macrohabitats of the host, geographical range, sex and age of the host, season, food and hyperparasites. The niches of marine parasites including many examples were discussed by Rohde (1993, 1994). Any animal or plant species has a niche that is restricted to varying degrees along all dimensions. Some parasites infect a wide range of host species (e.g. many trematodes), others are restricted to a single or a few host species (e.g. many monogeneans). Some parasites are always found in a narrowly defined microhabitat (e.g. certain didymozoid trematodes are always found on a small part of the mouth cavity of fish) whereas some larval trematodes, in contrast, infect the tissues in many body parts. Similarly, the degree of niche restriction for the other niche dimensions varies greatly. Niches are not static but may vary over time, depending on environmental conditions. For example, the size of the microhabitat of gill parasites of marine fishes may change with the oxygen content of the water. Nevertheless, niche preferences are, to a large degree, determined genetically. This is clearly shown, for example, by gill parasites. Even in the absence of potentially competing parasite species and of individuals of the same species, certain monogeneans always attach to the same parts of the gill filaments of a particular fish species.