Nonnative Phragmites australis (common reed) was introduced to North America about 150 years ago from Eurasia. Invasive haplotypes have replaced hundreds of thousands of hectares of native wetland plants. In these contexts, Phragmites displaces native flora, through both exploitative and interference competition, resulting in dense reed monocultures. No study has addressed the effects of Phragmites on inland aquatic food webs in North America. A few studies have addressed the effects of Phragmites on particular native fauna, with mixed results, mostly indicating negative or ambiguous effects. Few of these studies took an empirical approach in determining these effects. One basis for large scale Phragmites eradication programs is the purported negative impact of this species on native wildlife.
We evaluated the effects of Phragmites australis on native bullfrogs (Rana catesbeiana) by addressing two questions: 1) Does Phragmites affect larval amphibian survival, growth and development? and 2) If so, is there any evidence of local adaptation?
Results/Conclusions
Concurrent common garden and reciprocal transplant field experiments revealed a consistently strong positive influence of Phragmites on bullfrog larval performance. Tadpoles weighed 37% more and survived 34% better in the table experiment and were 31% heavier and survived 13% better in the field experiment (p from 0.027 to <0.0001). The experiments provided no evidence for local adaptation.
There are a variety of potential mechanisms that could underlie this positive influence of Phragmites. Given the concordance of results between experiments, it seems likely that part of the effect is mediated through food quality. Either Phragmites detritus, or the supported biofilm, provides superior food for amphibian larval growth as compared with deciduous leaf litter. Whatever its origins, the positive effect of Phragmites on bullfrogs merits further study particularly in the Western United States and elsewhere where both species are invasive and where bullfrogs are viewed as a threat to native fauna.
