Evaluating the effects of urbanisation on salamander abundances using a before-after control-impact design

Urbanisation represents a significant threat to semi-aquatic amphibian populations, especially stream-dwelling salamanders. Although studies of urbanisation effects on amphibians have been conducted, there is an urgent need to follow populations over longer time periods, account for imperfect detection and determine the response time to urbanisation. Consequently, we used a before-after control-impact (BACI) study design to estimate changes in abundances of larval and adult salamanders in streams affected by urbanisation. From 2005 to 2009, we used standard sampling techniques to obtain a count of salamanders in 13 first-order streams that underwent urbanisation of their catchments after the first year of sampling. Simultaneously, we counted salamanders in 17 streams that experienced no disturbance within stream catchments. Additionally, we measured environmental variables at each stream. We used Royle's binomial mixture model to estimate annual mean abundances and individual detection probabilities, and Bayesian inference was used to estimate population parameters for each stage and species. Although mean abundance estimates varied among years in control and urbanised streams, we found that urbanisation had a negative effect on larval and adult salamander abundances. Larval salamander abundances at sites 1year after urbanisation were significantly lower than abundances from control sites. Abundances of adult two-lined salamanders (Eurycea cirrigera) at urbanised sites were lower than abundances at control sites 2years post-urbanisation, and adult dusky salamander (Desmognathus fuscus) abundances at urbanised sites were lower than abundances at control sites 3years post-urbanisation. Maximum conductivity, sedimentation level and maximum stream channel width differed between urban and non-urban streams. Our results suggest that stream-dwelling salamanders exhibit little resistance to urbanisation. Our study also highlights the use of the BACI design to study how urbanisation affects populations in semi-aquatic habitats. We emphasise that inferences regarding urbanisation effects on population response may be compromised unless urban populations are compared to populations in control sites, especially for species in which populations fluctuate.