Indirect effects of ultraviolet-B radiation on larval amphibians as mediated by food quality and trophic interactions Public Deposited

http://ir.library.oregonstate.edu/concern/graduate_thesis_or_dissertations/12579v75f

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  • Ultraviolet-B radiation (UVB) is an abiotic stressor in both terrestrial and aquatic systems. The stratospheric ozone layer, depleted due to anthropogenic activities and the cause of elevated UVB at earth's surface over the last four decades, is predicted to recover by 2065. However, UVB levels in aquatic systems may continue to increase as UVB penetration of the water column increases due to acidification, warming and changing precipitation patterns. Subsequent to the 1974 prediction and 1985 discovery of the "ozone hole", much work was done investigating the direct effects of UVB on organisms. Much less work has been done examining the role of UVB as a stressor in the context of communities and food webs. Amphibian population declines are one facet of the current crisis of loss in biodiversity, and some population declines may be linked to exposure to UVB radiation. When some amphibian embryos, larvae and juveniles are exposed to UVB, they can experience increased mortality, slowed growth and development, increased malformations, as well as increased susceptibility to other biotic and abiotic stressors including pathogens and competitors. My research moves beyond direct effects of UVB on amphibians and asks what indirect effects may be transmitted via trophic interactions when examined in the context of aquatic communities and food webs. I first focused on how a UVB-exposed natural diet affects early larval growth and development in two anuran species, Pseudacris regilla and Rana cascadae. Growth and development of R. cascadae did not differ between a UVB-exposed and UVB-shielded diet, but P. regilla grew less on the UVB-exposed diet. Previous work has shown that P. regilla is less vulnerable when exposed directly to UVB. My research emphasizes that the main effect of UVB on an organism may not be a direct effect. I next investigated whether P. regilla and Bufo boreas would distinguish between diets when offered both a UVB-exposed and UVB-shielded diet consisting of natural algal assemblage reared in the field. Bufo boreas demonstrated no selectivity, while P. regilla spent more time near the UVB-shielded algae. An ability to discern UVB-stressed food may allow larvae to avoid indirect UVB effects. Analyses in these two experiments indicate that there were no direct effects of UVB at the producer trophic level. Bioassays, however, did indicate a UVB-induced change in the producer trophic level with implications for growth of consumers. My findings imply that effects of UVB on adjacent trophic levels may not be predicted from analysis at one level. In the final study, I investigated the direct and indirect effects of ambient UVB on salamander larvae and a tritrophic freshwater community. Both UVB and predation were forces that structured the community, and there is potential for a direct effect of UVB on the salamanders to mitigate their role as predator. The field of UVB-research must continue to mature in its investigations of effects, both direct and indirect, in evermore real and complex contexts.
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