The role of beaver in amphibian ecology and conservation in the Boreal Foothills (Alberta).

摘要

The Rocky Mountain foothills of west-central Alberta are a multi-use industrial landscape of deciduous boreal and coniferous cordilleran forests. Dam-building beaver active on small streams may create most of the breeding habitat for amphibians. My goal was to provide new ecological information that aids in preventing amphibian declines in this landscape and to evaluate techniques used for measuring breeding densities and juvenile recruitment. A major focus of this research is the use of beaver as a surrogate species for amphibian conservation. Results from call surveys and pitfall-trapping on 9 pairs of beaver-obstructed and unobstructed streams during 2001-2 showed that beaver may be a good indicator of the presence of wood frog, boreal chorus frog and western toad populations. The high percentage of ponds constructed by beaver in the study area (96%) suggests they are important in maintaining local amphibian populations. The distribution of beaver ponds, however, was not random. Stream order and proximity to cutblocks were important correlates of their occurrence in 15 watersheds examined with GIS. I also assessed whether succession of ponds mediated by beaver foraging in riparian zones affected amphibian populations. Results from call surveys on 57 beaver ponds in 2001-2 showed that wood frog densities were negatively related to % riparian canopy cover and positively related to pond age; the best a priori models included riparian canopy cover rather than pond age as a significant covariate. A mesocosm experiment in 2002-3 showed that higher growth and development rates of larval wood frogs in 5 old (>25 yrs) vs. 5 new ponds (10 yrs) coincided with reduced canopy cover plus higher indices of submergent vegetation, thermal degree days, and dissolved oxygen concentrations in old ponds. Unlike the common wood frog, identification of key breeding ponds for western toad (IUCN red-listed) was difficult due in part to its low density population and patchy distribution. However, results from pitfall trapping suggested that anthropogenic ponds (borrow pits) may be ecological sinks for the western toad. Management practices that avoid constructing ponds but protect beaver colonies and their food supply in riparian zones may ensure healthy populations of amphibians in the Boreal Foothills.