Local population persistence as a pre-condition for large-scale dispersal of Idotea metallica(Crustacea, Isopoda) on drifting habitat patches

Abstract

Idotea metallica establishes self-sustaining populations exclusively on objects drifting at the sea surface. Large-scale transport of drift material with surface currents results in an efficient dispersal of the species. Two types of drifting objects are utilised (biotic and abiotic), providing quite different conditions of life. Ephemeral biotic substrata (mainly uprooted macroalgae) may be used for transport and food, however, resulting habitat destruction from feeding must be a major threat for local population persistence of I. metallica. Abiotic substrata or wood represent efficient vectors for long-distance dispersal due to their resistance to biodegradation, but do not provide food for this herbivorous species. In laboratory experiments, the spatially-limited conditions of drifting substrata were simulated in microcosms. Idotea metallica established persistent populations on both types of substrata. On abiotic substrata, however, where the animals were fed only on Artemia larvae, high variations in density and a reduced intrinsic rate of population growth increased the risk of population extinction. Idotea metallica avoids habitat destruction by limited feeding on macroalgae. In contrast, the coastally distributed congener Idotea baltica destroyed algal habitats by feeding about 10 times faster than I. metallica.