SummaryExtraordinarily high leaf metal concentrations in metal hyperaccumulator plants may serve as an elemental defence against herbivores. However, mixed results have been reported and studies using comparative approaches are missing.We investigated the deterrent and toxic potential of metals employing the hyperaccumulator Arabidopsis halleri. Effects of zinc (Zn) and cadmium (Cd) on the preferences of three Brassicaceae specialists were tested in paired-choice experiments using differently treated plant material, including transgenic plants. In performance tests, we determined the toxicity and joint effects of both metals incorporated in an artificial diet on the survival of a generalist.Feeding by all specialists was significantly reduced by metal concentrations from above 1000 lg Zn g À1 DW and 18 lg Cd g À1 DW. By contrast, metals did not affect oviposition.Generalist survival decreased with increasing concentrations of individual metals, whereby the combination of Zn and Cd had an additive toxic effect even at the lowest applied concentrations of 100 lg Zn g À1 and 2 lg Cd g À1 . Metal hyperaccumulation protects plants from herbivory resulting from deterrence and toxicity against a wide range of herbivores. The combination of metals exacerbates toxicity through joint effects and enhances elemental defence. Thus, metal hyperaccumulation is ecologically beneficial for plants.

Young individuals of the bottom-biting (i.e. sediment-ingesting) common carp Cyprinus carpio and gudgeon Gobio gobio consumed significant amounts of nematodes in laboratory experiments, whereas the selective-feeding roach Rutilus rutilus did not. In mesocosm enclosure experiments in the field, C. carpio strongly decreased the nematode abundance within 4 days, whereas the bottom-biting bream Abramis brama did not affect the abundance until after 14 days. In controlled experiments with a known number of prey, C. carpio but not A. brama significantly reduced the number of nematodes, and G. gobio reduced the nematode abundance dependent on the size of the fish, with smaller fish causing a greater reduction. Cyprinus carpio consumed the nematodes and did not just mechanically kill them in the sediment, as shown by dissection of the fish intestine. Morphometric analysis of the branchial baskets indicated that the mesh width of C. carpio, but not of A. brama, is suitable for consuming meiobenthos. The results indicate that the meiobenthos is a food resource for certain bottom-feeding freshwater fishes.

Several plant species are able to not only tolerate but also hyperaccumulate heavy metals in their aboveground tissues. Thus, in addition to secondary metabolites acting as organic defences, metal hyperaccumulators possess an elemental defence that can act as protection against antagonists. Whereas several laboratory studies have determined potential relationships or trade-offs between organic and inorganic defences, little is known about whether these traits are interconnected in the field and which factors determine the compositions of organic defences and elements of leaf tissues most. To target these questions, we collected young leaves of Arabidopsis halleri, a Brassicaceae capable of hyperaccumulating Cd and Zn, as well as soil samples in the field from 16 populations. We detected wide variation in the composition of glucosinolates-the characteristic secondary metabolites of this plant family-among plants, with two distinct chemotypes occurring. Distance-based redundancy analyses revealed that variation in glucosinolate composition was determined mainly by population affiliation and to a lesser degree by geographic distance. Likewise, elemental composition of the leaves was mainly influenced by the location at which samples were collected. Therefore, the particular abiotic and biotic conditions and potential genetic relatedness at a particular locality affect the plant tissue chemistry. A slight indication of a trade-off between glucosinolate-based organic and inorganic defences was found, but only in the less abundant chemotype. A large variation in defence composition and potential joint effects of different defences may be highly adaptive ways of protecting against a wide arsenal of biotic antagonists.

Several Brassicaceae species are able to hyperaccumulate extraordinarily high levels of heavy metals in their aboveground tissues. This rare and complex trait named metal hyperaccumulation may offer effective protection against antagonists. Several laboratory studies provide evidence that metal hyperaccumulation acts as an elemental defense. Here, we investigated whether this elemental defense can be confirmed in the field and whether plant growth is affected when plants can hyperaccumulate metals from the soil. Plants of Arabidopsis halleri (L.) O'Kane & Al-Shehbaz, which can hyperaccumulate cadmium (Cd) and zinc (Zn), were cultivated in pots in unamended (control) or metal-amended soil (test) for 2 months. The pots containing the plants were then transferred to a common garden. Leafdamaging insect visitors were monitored over a period of 3 months. Plant size and shoot mass were measured directly before and at the end of the field exposure. Furthermore, before and at the end of the field exposure we determined leaf concentrations of Cd and Zn, as well as glucosinolates, which are characteristic secondary metabolites of the Brassicaceae that are known to act potentially as organic defense compounds. Growth of the test plants on metal-amended soil led to a significant increase in leaf Cd and Zn concentrations when compared to control plants on unamended soil. After two initial months of growth under standardized greenhouse conditions, plants grown on different soils did not differ in size or biomass. In contrast, by the end of the 3-month field exposure, test plants were larger than control plants. During field exposure, control plants were visited by more herbivores, suggesting that plants growing on metal-amended soil were well defended against herbivores. Total glucosinolate concentrations were significantly higher in test compared to control plants only after the initial growth period under standardized conditions, but not by the end of the field exposure, pointing to joint effects of defenses particularly in young plants. Our results demonstrate that metal hyperaccumulation affords an effective elemental defense and enables increased growth under field conditions.

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