Geographic covariation of chemical quality of the host alga<Emphasis Type="Italic"> Fucus vesiculosus</Emphasis> with fitness of the herbivorous isopod<Emphasis Type="Italic"> Idotea baltica</Emphasis>

Environmental and/or genetic among-site variation in plant quality may influence growth and fecundity of specialized herbivores inhabiting a particular site. Such variation is important as it generates spatial variation in selection for traits related to plant–herbivore interaction. Littoral macroalgae are known to respond plastically to environmental variation by modifying their chemistry or morphology. We studied geographic variation in phlorotannin, nitrogen, protein, and sugar (fucose, mannitol, and melibiose) concentrations of the brown alga Fucus vesiculosus at 12 sites separated by 0.5 to 40 km in the naturally fragmented Archipelago Sea in the northern Baltic Sea. By this regional variation in algal chemistry we attempted to explain among-population variation in size and fecundity of the crustacean herbivore Idotea baltica. We observed high spatial variation in all the measured chemical characteristics of F. vesiculosus, as well as in female size and the number of eggs produced by the herbivores. Spatial variation in nitrogen or protein contents of the alga did not explain the variation of herbivore traits. However, egg size positively covaried with spatial variation in the concentration of mannitol, the major storage carbohydrate of the alga. Such a positive relationship may arise if I. baltica can utilize the nutritive value of a mannitol-rich diet thereby being better able to provision the developing eggs with energy-rich metabolites. Unexpectedly, the concentration of phlorotannins, secondary metabolites having a putative role in defense against herbivory, positively covaried with the size of the herbivore. Among-population variation in host plant chemistry and covariation of that with herbivore growth and reproduction imply that herbivores respond to the local quality of their host plants, and that geographical structuring of populations has to be taken into account in studies of plant–herbivore interactions.Communicated by M. Kühl, Helsingør