Quantifying variation and chemical correlates of bladderwrack quality - herbivore population makes a difference

Abstract

1. Herbivory is an ecologically and evolutionarily strong interaction in marine littoral communities. However, among-host variation in quality for herbivores has not been documented comprehensively in these systems although it is a pre-requisite for selection through herbivory, and assuming an additive genetic component, for evolutionary responses. Host quality is composed of a multitude of plant traits and can thus be measured all-inclusively only in terms of herbivores' performance. In geographically structured interactions, herbivore populations may show differentiation of the traits related to host use ability, with consequent among-population variation in performance and its correlates with host traits. 2. We reared the herbivorous isopod Idotea baltica from three local populations on 30 distinct clones of its major host species, the brown alga Fucus vesiculosus. We further screened a wide array of metabolites, water content and toughness of the host clones and correlated those with the herbivore performance. 3. Food consumption of herbivores differed among host clones in all herbivore populations indicating that herbivory can generate phenotypic selection for algal traits through selective feeding. 4. The dependency of herbivore growth rate on host clone varied among the herbivore populations; among-clone variation was the highest in the herbivore population having the highest average growth rate. 5. Accordingly, the covariation patterns of algal traits with herbivore performance differed among herbivore populations. Proteinic amino acids enhanced and phlorotannins impaired growth rate but differently so depending on the population. The herbivore population having the highest average growth rate was also most sensitive to phlorotannins. Toughness of the thallus reduced growth rate in the population having on average the smallest body size and lowest growth rate. 6. That the variation in host quality explained a large proportion of variance in herbivore performance suggests that plant chemistry and structural characteristics may act as selective agents for herbivore traits and play a fundamental role in ecological differentiation of herbivore populations. © 2011 The Authors. Functional Ecology © 2011 British Ecological Society.