Graduate Thesis Or Dissertation
 

Temporal changes in the feeding habits of bullfrogs from three different habitat types in the Willamette Valley, Oregon

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  • Bullfrogs were introduced into Oregon in the early 1900s and have become widespread members of many wetland communities. Little is known of their ecology outside their native range, although they may play a role in the current declines observed in native herpetofauna. I analyzed the diets of 401 bullfrogs from one permanent and two ephemeral ponds at William L. Finley National Wildlife Refuge between May and September of 2001 and 2002 in order to better understand the breadth of bullfrog diets and feeding strategies exhibited at both the individual and population levels. A 1m3 sampling cube was used to obtain a density estimate of the available prey items in the pond environment. This data was compared to the stomach content data with a series of Spearman rank correlation analyses. Stomach contents were analyzed numerically and by mass so as to obtain an estimation of bullfrogs' impact on their community (numerical data) and the importance of prey items to bullfrogs (mass data). Arthropods dominated the numerical analysis. Although vertebrates were often numerically insignificant, when present they comprised the largest percentage of their respective samples. Amphibians were observed in the diets of bullfrogs from the ephemeral ponds, often comprising at least 40% of the pooled stomach contents for each sampling period. The most common amphibians in bullfrog diets from these ponds were larval anurans. Juvenile bullfrog diets (<76mm SVL) were restricted to terrestrial and aquatic arthropods, while adult bullfrogs exhibited a broader diet, consuming molluscs and various vertebrates in addition to arthropods. Feeding strategy analyses indicated that bullfrogs are generalist predators at the individual level. Some feeding specialization was observed at the individual level, but this was attributed to patchy distribution of resources in the pond environment. The results of the Spearman rank correlation analyses lent support to these findings.
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