On a dry mass basis, normal plants contain 1-10 mg Ni/kg, accumulators 100-1,000 mg Ni/kg, and hyperaccumulators >1,000 mg Ni/kg. Metal hyperaccumulators are often found on serpentine soils: they may use metals for herbivore defense (defense hypothesis) or the metals may have other functions (elemental allelopathy, drought resistance, etc.). High metal levels can deter herbivores, increase plant resistance to herbivory, and increase plant tolerance of herbivory. Other associates of hyperaccumulators, such as pollinators, seed dispersers, and endophytic bacteria and fungi, may be affected by the high metal concentrations of hyperaccumulator tissues. Defensive effects have been shown against some herbivores and pathogens but Ni-based defense is not always effective. Herbivore feeding mode and evolution of metal tolerance can limit Ni-based defense. For example, field surveys have identified insects that feed on Ni hyperaccumulator tissues, and some contain elevated whole-body Ni levels. These herbivores mobilize metals into local food webs and the high Ni levels may defend these “high Ni insects” against predators. Tests of this hypothesis using Melanotrichus boydi (Heteroptera: Miridae) have reported mixed results, but provide a basis for future investigations.