The decline of the Oregon Spotted Frog (Rana pretiosa), a Pacific Northwest endemic now federally listed as threatened, has been attributed to several aspects of ecosystem alteration, primarily habitat degradation and loss. The introduced American Bullfrog (Rana (Aquarana) catesbeiana ) has been widely implicated in those declines, but the basis of that contention has been difficult to characterize. The bullfrog occurring at every site of recent Oregon Spotted Frog extirpation has focused concern about its impact.
Here, I present a suite of interconnected studies that examine the behavioral ecology of both species to better understand the potential for bullfrog-mediated Oregon Spotted Frog extirpation. I quantified Oregon Spotted Frog anti-predator behavior from the only known population successfully co-occurring with bullfrogs (Conboy Lake) and a population devoid of bullfrog impact (Big Marsh), and compared these behaviors to the predatory traits of the bullfrog. The initial study revealed that captive-reared individuals from the Oregon Spotted Frog population that has successfully co-occurred with bullfrogs respond faster to a predatory stimulus (measured as latency to response) than Oregon Spotted Frogs from a population not to exposed to bullfrogs. Subsequent field investigations of the approach distance allowed by a predator stimulus before taking evasive action (termed the flight initiation distance: FID) conducted with the Oregon Spotted Frog population co-occurring with bullfrogs first demonstrated that FID of recently metamorphosed bullfrogs is consistently greater than that of recently metamorphosed Oregon Spotted Frogs. Further, examination of FID across all post-metamorphic age classes of Oregon Spotted Frogs revealed that older frogs do not allow as close approach as recently metamorphosed Oregon Spotted Frogs. This age class shift in FID did not occur in the Oregon Spotted Frog population not exposed to bullfrogs. In the latter population, FID did not differ among age classes.
Since the bullfrog might be driving this age-based change in anti-predator behavior, I explored the variation in strike distance of bullfrogs from the site of co-occurrence in both the field and laboratory to determine the extent of overlap with Oregon Spotted Frog FID. I found that the bullfrog strike distance significantly overlaps the FID of all ages of Oregon Spotted Frogs from the bullfrog-free site but only that of youngest (recently metamorphosed) frogs at the site of co-occurrence. Older Oregon Spotted Frogs from the site of co-occurrence generally escaped at distances greater than the strike distance of bullfrogs.
I also collected > 880 bullfrogs from the site of co-occurrence and analyzed the stomach contents to assess their dietary trends. I found that bullfrogs consume Oregon Spotted Frogs at the site, but do not eat the larger (older) frogs. Moreover, the body size ratio between Oregon Spotted Frogs as prey and bullfrogs as predators suggests that nearly all of the adult size distribution of bullfrogs at Conboy would be incapable of preying on adult Oregon Spotted Frogs.
Collectively, these studies strongly suggest that bullfrogs have altered the escape behavior of Oregon Spotted Frogs at Conboy Lake and that most adult Oregon Spotted Frogs at Conboy may have a size-based release from predation by bullfrogs. Implicit in this finding is that bullfrogs may pose a real threat via predation to other Oregon Spotted Frog populations with which they might come into contact where the distribution of bullfrog body sizes differ substantially from that at Conboy Lake.
|Advisor:||Duffield, Deborah, Shepherdson, David|
|Commitee:||Hayes, Marc, Lutterschmidt, Deborah, Pan, Yangdong, Zelick, Randy|
|School:||Portland State University|
|School Location:||United States -- Oregon|
|Source:||DAI-B 78/09(E), Dissertation Abstracts International|
|Keywords:||American bullfrog, Aquarana catesbeiana, Escape theory, Flight initiation distance, Introduced species, Optimal discriminant analysis, Oregon spotted frog, Rana catesbeiana, Rana pretiosa|
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