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The Elemental Defence Hypothesis, proposed by Boyd and Martens in 1992, suggests that high foliar metal concentrations deter herbivore feeding and this protection from herbivory may be one factor explaining the adaptive value of metal hyperaccumulation. However, lower foliar metal concentrations than those occurring in hyperaccumulators, may also confer this advantage, but the benefits of metal uptake by this group of plants (known as accumulators) has been relatively less-studied. Despite this potential advantage, metal accumulation is a relatively rare phenomenon, suggesting it may have costs as well as benefits. A field survey of metallicolous populations of the hyperaccumulator Thlaspi caerulescens J. & C. Presl. (recently renamed as Noccaea caerulescens (J. & C. Presl.) F. K. Meyer) (Brassicaceae) and the accumulator Rumex acetosa L. (Polygonaceae), found on zinc (Zn) contaminated mining sites, revealed between-population differences in chewing herbivore damage, in the efficiency with which they uptake soil Zn into their shoots (measured as the concentration factor), and the foliar Zn concentration of T. caerulescens. However, foliar Zn concentration was not correlated with damage within a species. In a series of pot experiments using two populations of R. acetosa, the foliar Zn concentration was manipulated through the addition of Zn to the soil and through differences in Zn uptake rate between populations. This thesis investigated how these manipulations influenced herbivory by generalist Helix aspersa Müller (Helicidae), and how plant competitive ability (in terms of biomass) was determined by a combination of population identity, soil Zn concentration and presence of herbivores. When two R. acetosa populations were grown under 1500 and 45,000 mg/kg soil Zn concentrations, population differences were found in shoot biomass and competitive ability. The outcomes of intra- compared with inter-population competition depended on soil Zn concentration. When herbivores were present, shoot damage was low, usually < 15% of foliage removed. Snail preference was dependent upon the interaction between population identity and soil Zn concentration, partially supporting the Elemental Defence Hypothesis.