Many amphibian populations are experiencing decline, and it has been argued that invasive plant may contribute to this decline. Invasive plants are not only novel organisms, with chemical arsenals to which many amphibians are naïve, but often change the structure of the entire habitat by rapidly establishing a monoculture or dominance of a single species, thus reducing habitat complexity. Invasive plants have a potential to affect amphibians and other partially to fully aquatic organisms, more so than the threat invasive plants pose to non-aquatic organisms. Invasive plants may have detrimental effects on native amphibians through many pathways. This study compares growth and survival of enclosed Rana clamitans and R. catesbeiana tadpoles in non-invaded and invaded wetlands. Invaded wetlands had well-established populations of Typha angustifolia or Phragmites australis. Five wetland complexes were used in this study. Each complex includes three independent ponds of comparable sizes- one each of predominately T. angustifolia or P. australis, and one predominantly native plants. In the spring, R. clamitans eggs were caught and places in 1L enclosures. Emerging tadpoles, in groups of ten, were grown and monitored weekly in each enclosure. This procedure was repeated in late June with clutches of R. clamitans and R. catesbeiana.
Results/Conclusions
Our study revealed no significant differences in tadpole size after 10 weeks across the three frog groups in the native non-invaded wetlands. Tadpoles in early cohorts of R. clamitans had a higher growth rate in T. angustifolia invaded wetlands (P<0.001). Similarly, early cohorts of R. clamitans experienced higher initial growth rates in P. australis invaded wetlands. Late cohorts of R. clamitans and R. catesbeiana showed similar responses throughout the growth period. A comparison of the chemical characteristics of the ponds revealed higher dissolved oxygen and nitrate content in T. angustifolia invaded ponds than in the other two pond types (F=13341.854 and F=94586.790, P < 0.001 and d.f.=2/21578 in both cases There were differences in water temperature, where non-invaded ponds were warmer than either of the invaded ponds (P<0.001, d.f.=2/21578, F=83.235). Finally, ammonium content was higher in P. australis-invaded ponds (P<0.001, d.f.=2/21578, F=8151.641). These results indicate that the specific identity of the invading species may be less important than the presence of an invasion in the case of these plants. Despite the preservation of total wetland area via no-net-loss policies, if the preserved wetlands are of low quality and dominated by invasive plants, the net effect for amphibians may be negligible.
