Hyperaccumulated Ni has been shown to defend plants against a wide array of herbivores. Since Ni is not significantly reabsorbed during leaf senescence, much of the metal-based defense remains intact when leaves abscise. Thus, once metal-enriched leaves are incorporated into vegetation litter, they may also negatively affect detritivores Detritivores are a “valve” through which carbon and nutrients flow from leaf litter into the soil, therefore metal-enriched litter could have significant impacts on nutrient cycling in communities with high densities of hyperaccumulators. We conducted feeding studies to examine this possibility using a Ni hyperaccumulator, Senecio coronatus, and two bioassay detritivores, an isopod (Porcellio sp.) and a millipede (Orthoporus texicolens). Senecio coronatus is unique in that both hyperaccumulating and non-accumulating populations exist on South African serpentine soils. This characteristic provides a natural source of high- and low-Ni leaf litter from the same species. We also conducted a series of feeding trials using the same arthropod species and Ni-amended artificial diets.
Results/Conclusions
For both detritivores, neither growth nor mortality was affected by the presence of Ni in Senecio litter. Porcellio individuals were also not affected by increasing Ni levels in artificial diets. Orthoporus individuals, however, exhibited significant mass loss above a threshold Ni level of 240 ppm. These data suggest that detritivore responses to Ni-enriched litter will vary among species, and may not be as detrimental as herbivore interactions with Ni. However, we have recently published an in situ litter decomposition study that was conducted concurrently with these feeding assays. In this study, Ni-enriched litter decomposed more slowly than low-Ni litter indicating a possible Ni effect on detritivorous organisms. Large detritivores, such as the species used in our assays, were excluded from the decomposing litter by mesh bags, but small organisms (e.g., springtails, mites) were still able to access the litter. Thus, the severity of detrimental Ni effects on detritivores may be correlated with mass of individual detritivores. Future studies will need to examine the microfauna associated with decomposition as well as fungal and bacterial decomposers.
