Herbivory and microhabitat preferences ofIdoteaspp. (Isopoda) in the northern Baltic Sea

Feeding rates and microhabitat preferences of Idotea baltica, I. chelipes and I. granulose were investigated experimentally to elucidate their role in the local declines of Fucus vesiculosus in northern Baltic littoral communities. The mean ingestion rates (mg Fucus AFDW per 10 mg Idotea AFDW per day) varied from 1.1 to 1.6 in males and from 2.6 to 3.5 in females, the average daily consumption per individual being 2.1 mg Fucus AFDW in I. Baltica females, 1.6 mg in I. granulose and 0.7 mg in I. chelipes females. The feeding rates were significantly affected by the quality of fucoid tissues, the palatability of which increases with the algal age. The old eroding Fucus thalli also are most attractive microhabitats for I. baltica and I. chelipes compared to apical segments and thalli with epiphyte load. These results suggest that normally young Fucus plants escape harmful grazing in the presence of more attractive adult algae with a capacity for renewal and compensatory growth. If the demographic structure of Fucus populations breaks down due to unsuccessful reproduction, the effects of concentrating Idotea populations may become deleterious and result in the destruction of the F. vesiculosus vegetation.