Some plants extract elements from the environment and store them in high concentrations in their aboveground biomass. This mechanism, termed "elemental hyperaccumulation," has been viewed as a system to deter competitors and other direct threats to the fitness of the plant. However, the selective processes that help maintain this trait are not fully understood. This dissertation utilizes a fern, Pteris vittata, which accumulates arsenic in its fronds, to address the questions surrounding the functional advantages associated with elemental hyperaccumulation and the implications this mechanism can have on the environment. Chapter 2 provides evidence that P. vittata are associated with higher concentrations of arsenic underneath the canopy in the field. We also demonstrate that these differences are substantial enough to have detrimental effects on growth in competitor plants ( Oxalis stricta). These results suggest that hyperaccumulated arsenic can provide P. vittata with an advantage for resource acquisition over their competitors. Chapter 3 measures the abundance of invertebrate taxonomic groups in association to environmental arsenic concentrations. We found that overall invertebrate abundance decreases with increasing soil and fern arsenic concentrations. Individual taxonomic groups vary in their relative abundance to environmental arsenic concentrations. With the exception of caterpillars, soil arsenic appears to be a better predictor of individual taxonomic abundance than fern arsenic concentrations. The abundance of invertebrate predators decline more than their potential prey relative to soil arsenic concentrations. These results suggest that arsenic hyperaccumulation may provide P. vittata with defense against herbivores. We conclude that defensive properties by arsenic hyperaccumulation comes from active avoidance of high arsenic concentrations within the plants, and in higher soil arsenic concentrations. The hypothesis that invertebrate deterrence is initially driven by soil arsenic concentrations, and can provide a functional advantage to the plant, constitutes an important and novel contribution to the literature. Chapter 4 addresses the relationship between environmental arsenic concentrations and the arsenic concentrations within the invertebrate community. Arsenic accumulates into individual invertebrates relative to soil arsenic concentrations and invertebrate arsenic concentrations correlate with a decline in abundance. Invertebrate predators also had higher arsenic concentrations relative to their prey. These results suggest that arsenic bioconcentrates into higher trophic levels. Individual taxonomic groups vary widely in their arsenic concentrations, but a specific species of Lepidoptera (Callopistria floridensis) contain arsenic concentrations three orders of magnitude higher than soil arsenic concentration from the same area. Chapter 5 focuses on the specific relationship between C. floridensis and P. vittata. Caterpillars grew slower on increasing concentrations of arsenic, but this effect was likely a result from consuming less material and not the negative effects of arsenic consumption directly. We conclude that the caterpillar disarms the negative effects of arsenic consumption by actively feeding on areas of the fern with low arsenic concentrations. This relationship represents a rare example of a specialist invertebrate herbivore capable of disarming the defensive properties associated with elemental hyperaccumulation.
|Advisor:||Shuster, Stephen M.|
|Commitee:||Hofstetter, Richard W., Ketterer, Michael E., Scott, Randall W., Service, Philip M.|
|School:||Northern Arizona University|
|School Location:||United States -- Arizona|
|Source:||DAI-B 76/10(E), Dissertation Abstracts International|
|Subjects:||Ecology, Environmental science|
|Keywords:||Arsenic, Callopistria floridensis, Hyperaccumulation, Plant-insect interactions, Pteris vittata|
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