Search ARMI Database

Beavers (Castor canadensis) are ecosystem engineers, altering hydrologic regimes, ecosystem processes, and modifying community structure. How beaver alter landscape patterns of wetlands or lentic habitat (e.g., ponds) and the distribution of obligate species remains an interesting unexamined question. A database of over 100 watersheds in southwestern Montana was used to compare the scale and pattern of lentic habitat and Columbia spotted frog (Rana luteiventris) detection between watersheds with and without signs of beaver presence.

Ambient ultraviolet-b (UV-B) radiation (280 – 320 nm) has increased at north-temperate latitudes in the last two decades. UV-B can be detrimental to amphibians, and amphibians have shown declines in some areas during this same period. We documented the distribution of amphibians and salmonids in 42 remote, subalpine and alpine ponds in Olympic National Park, Washington, USA. We inferred relative exposure of amphibian habitats to UV-B by estimating the transmission of 305 and 320-nm radiation in pond water. We found breeding Ambystoma gracile, A. macrodactylum and Rana cascadae at 33%, 31%, and 45% of the study sites, respectively. Most R. cascadae bred in fishless shallow ponds with relatively low transmission of UV-B. The relationship with UV-B exposure remained marginally significant even after the presence of fish was included in the model. At 50-cm water depth, there was a 55% reduction in incident 305-nm radiation at sites where breeding populations of R. cascadae were detected compared to other sites. We did not detect associations between UV-B transmission and A. gracile or A. macrodactylum. Our field surveys do not provide evidence for decline of R. cascadae in Olympic National Park as has been documented in Northern California, but are consistent with the hypothesis that the spatial distribution of R. cascadae breeding sites is influenced by exposure to UV-B. Substrate or pond depth could also relate to the distribution of R. cascadae in Olympic National Park.

Livestock grazing is a common land use across the western USA, but concerns have been raised regarding its potential to affect amphibian populations. We studied the short-term effects of full and partial livestock grazing exclosures on Rana luteiventris (Columbia Spotted Frog) populations using a controlled manipulative field experiment with pre- and post-treatment data (2002-2006). Despite a significant increase in vegetation height within grazing exclosures, we did not find treatment effects for egg mass counts, larval survival, or size at metamorphosis 2-3 years following grazing exclosure installation. Water samples taken in late summer showed concentrations of nitrite, nitrate, ammonia and orthophosphate that were low or near detection limits across all ponds and years. The results of this experiment do not support a hypothesis that limiting cattle access to breeding ponds will help conserve R. luteiventris populations in our study area. Further research is needed to evaluate regional variation and long-term effects of grazing exclosures on R. luteiventris populations.

Positive interactions among non-native species could greatly exacerbate the problem of invasions, but are poorly studied and our knowledge of their occurrence is mostly limited to plant-pollinator and dispersal interactions. We found that invasion of bullfrogs is facilitated by the presence of coevolved non-native fish, which increase tadpole survival by reducing predatory macroinvertebrate densities. Native dragonfly nymphs in Oregon, USA caused zero survival of bullfrog tadpoles in a replicated field experiment unless a non-native sunfish was present to reduce dragonfly density. This pattern was also evident in pond surveys where the best predictors of bullfrog abundance were the presence of non-native fish and bathymetry. This is the first experimental evidence of facilitation between two non-native vertebrates and supports the invasional meltdown hypothesis. Such positive interactions among non-native species have the potential to disrupt ecosystems by amplifying invasions, and our study shows they can occur via indirect mechanisms.

The American Bullfrog (Rana catesbeiana Shaw) is a widely introduced and invasive anuran that is frequently blamed for population declines of native species (Bury and Whelan 1984). Once established, Bullfrog populations are often either difficult or impossible to eradicate depending on habitat and landscape features (Schwalbe and Rosen 1988, Doubledee et al. 2003, Govindarajulu et al. 2005). Bullfrogs are representative of a large but neglected suite of invasive species that are characterized by: 1) a broad invasion that is well established in some areas; 2) a lack of obvious economic impacts compared to some other invasive species; and 3) a lack of reasonably feasible control methods. Despite demonstrated conservation concerns, invasive species like the Bullfrog do not tend to attract the resources necessary to attempt large scale management because of their lack of economic impact and the difficulty of control methods. This leaves biologists responsible for managing habitats invaded by such species with little hope and few options for promoting the persistence of sensitive native species. With these issues in mind, we consider the case of the Bullfrog, review management options, and suggest directions for future research with this and similar species.

A species account for the Tailed Frog (Ascaphus truei[/])