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An abstract of the thesis of Catherine Mary Callison for the Master of Science in Biology presented July 18, 2001. Title: Factors Influencing Oviposition Among Pond-Breeding Amphibians: Exotic Vegetation, Oviposition Braces, and Cover. I examined oviposition in four pond-breeding amphibians (northwestern salamander [Ambystoma gracile], long-toed salamander [Ambystoma macrodactylum], Pacific treefrog [Hyla regilla], northern red-legged frog [Rana aurora aurora]), at Burlington Bottoms, a lowland riverine site in northwestern Oregon, to determine whether differential use of native versus exotic plant substrates occurs. I found differential use in all four species, but use was inconsistent with the hypothesis that selection for native plants (or selection against exotic plants) was occurring. If selection was occurring, the pattern implied that reed canarygrass (Phalaris arundinacea), a widespread exotic in this lowland system, was sometimes favored. However, inconsistent use of reed canarygrass led me to examine the alternative that substrate strength (measured as density, diameter, and mass), rather than plant status, might be the basis of selection. Species used for oviposition differed in strength, but eggs were not consistently laid on braces based on my strength measures. Failure to find support for this hypothesis led to examination of a third hypothesis for two species (red-legged frog and northwestern salamander), that cover characteristics of the oviposition brace or nearby vegetation might influence egg mass location. Analysis of structural complexity of species used as a brace (as percent cover within 15 em of the egg mass) did reveal a pattern consistent with complexity, but that was species-specific. Red-legged frogs selected braces with significantly more nearspace cover; northwestern salamanders selected braces with sparser cover. Comparison of marginal shrub and tree cover to the number of red-legged frog egg masses revealed that ponds with < 50% shrub/tree cover had few « 5) masses. Ponds with ~ 50% shrub/tree cover had many (~ 10) masses. More complex vegetation may provide greater protection for egg masses or ovipositing red-legged frogs but further investigation is clearly warranted. Elucidating northwestern salamander cover requirements will require examining more occupied ponds. In particular, more refined examination, using experimental manipulation, is needed to verify the cover relationships revealed in this study. Meanwhile, managers should treat cover as important in red-legged frog oviposition life history, paying special attention to minimizing loss of marginal shrub and tree cover. FACTORS INFLUENCING OVIPOSITION AMONG POND-BREEDING AMPHIBIANS: EXOTIC VEGETATION, OVIPOSITION BRACES, AND COVER by CATHERINE MARY CALLISON A thesis submitted in partial fulfillment of the requirements for the degree of MASTER OF SCIENCE in BIOLOGY Portland State University 2001 Dedication In memory of my father, Richard Worth Coldren (1927-2000), and my brother, Peter Worth Coldren (1960-2000): Two of myfirst teachers and supporters who gave me the courage to try new things and helped me believe in myself.