Does selenium hyperaccumulation confer protection from herbivory? A field study

Hyperaccumulation is the ability of a plant to accumulate metals or metalloids several magnitudes higher than non-accumulating plants in the same soil.  Plants hyperaccumulating selenium (Se) can contain up to 1% Se of dry weight.  The ecological significance of this phenomenon has been researched in laboratory and greenhouse settings, with most studies currently supporting an herbivory/pathogen defense hypothesis, also called the elemental defense hypothesis. In this study, the effectiveness of Se as an herbivore deterrent was investigated through field studies, allowing us to determine whether previous greenhouse and laboratory studies are relevant in nature. The effect of Se accumulation on herbivory deterrence was observed by examining arthropod loads of two hyperaccumulating species (Astragalus bisulcatus and Stanleya pinnata) as well related non-accumulating species (Camelina microcarpa, Astragalus americana, Descurainia pinnata, Medicago sativa, and Helianthus pumilus) over two growing seasons (2004, 2005) at Pine Ridge Natural Area in Fort Collins, CO. It was found that Se hyperaccumulating plants had significantly lower arthropod loads when compared to non-accumulating controls.  Arhropod species richness was also found to be lower on Se hyperaccumulating plants.  On average, arthropods collected on Se hyperaccumulating plants contained significantly higher Se concentrations than those found on non-accumulating species.  In addition, arthropod species were identified with high Se content (>100 mg kg -¹ DW), indicating a high degree of Se tolerance, and perhaps feeding specialization. In conclusion, these field studies lend support to the elemental defense hypothesis, and also identify potential arthropod specialists that may have overcome this plant defense.