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Resource Quantity and Quality in Cross-Ecosystem Food Web Subsidies


Prey moving from donor to recipient ecosystems (e.g. freshwater to terrestrial) are known as cross-ecosystem food web subsidies. These prey can affect the distribution and abundance of subsidized predators. Most investigators have focused on how the quantity of prey (body size and biomass) affect predators, often assuming that resource quantity is an accurate proxy for resource quality. In the present dissertation, I used meta-analysis, lab, and field experiments to show the importance of measuring resource quality and how shifting quality of prey subsidies (insects) can influence a subsidized predator (Tetragnatha sp. spiders). Specifically, I define resource quality as a function of body nutrient content (%N) and carbon:nitrogen (C:N) ratio. In chapter two using data from 30 studies and ~250 effect sizes, I performed meta-analyses and demonstrated that increases in metrics of resource quantity (body size and dry weight) are not synonymous with increases in metrics of resource quality. In fact, there was a negative correlation between body phosphorus content (%P) and resource quantity. In chapter three, I tested how terrestrial spiders (Tetragnatha nitens) responded to different levels of prey quality by rearing them on three different prey: midges (Chironomus dilutus), mosquitoes (Culex quinquefasciatus), and fruit flies (Drosophila melanogaster). Mosquitoes were the lowest quality resource and lowered the nutrient content and stoichiometry of spider predators compared to the other treatments. Spiders fed on mosquitoes also suffered delays in growth and development due to being fed on the lower quality resource. Lastly, in chapter four, I tested how terrestrial spiders (Tetragnatha sp.) were affected by an aquatic nutrient enrichment in experimental field mesocosms. Abundance, body size (tibia + patella length (TPL) of first walking leg) (mm), biomass (mg), or nutrient stoichiometry (C:N ratio) of the subsidized Tetragnatha sp. spider did not differ significantly between enriched and non-enriched pools. However, significantly higher nitrogen content (% body N) was in Tetragnatha sp. spiders from the Santa Ana River inflow in a constructed treatment wetland compared to a site farther along this aquatic nutrient gradient during July 2015. Nutrient enrichment of the aquatic ecosystem due to anthropogenic runoff during a summer monsoon event may explain this pattern.

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