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

Abstract

1. 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. 2. 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. 3. 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. 4. 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 1 year 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 2 years post-urbanisation, and adult dusky salamander (Desmognathus fuscus) abundances at urbanised sites were lower than abundances at control sites 3 years post-urbanisation. Maximum conductivity, sedimentation level and maximum stream channel width differed between urban and non-urban streams. 5. 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.

Modeling structured population dynamics using data from unmarked individuals

“…However, our results are consistent with Price et al (2012), who found reduced survival and abundance in populations of D. fuscus at urbanized and urbanizing sites, similar to our study area, relative to undisturbed stream sites. Additionally, our study occurred during years with belowaverage precipitation, which resulted in reduced baseflows in these headwater stream habitats, possibly leading to lower survival and recruitment of juveniles.…”

“…2C). As expected, survivorship was higher for adults than juveniles (Grant et al 2010, Price et al 2012. Recruitment accounted for the majority of individuals gained at locations (median annual per capita recruitment ¼ 0.87), while immigration rates were very low, with a median estimate of 0.05 immigrating individuals ( juveniles and adults) per sampling location annually (Fig.…”

“…Observers recorded all individuals found, as well their snout-to-vent lengths. We assumed that the population abundance could change between years, but that local populations were closed between the June and July sampling events, a reasonable assumption because reproduction does not occur during this time frame and studies of other northern dusky populations in the Shenandoah mountains (E. H. C. Grant, unpublished data) and the western Piedmont of North Carolina (Price et al 2012) indicate that survivorship is high and movement rates are low over this time frame.…”

Environmental and Anthropogenic Factors Influencing Salamanders in Riparian Forests: A Review

“…Researchers estimated that the conversion of riparian forest to urban land near Davidson, North Carolina, resulted in the decline of southern two-lined salamander populations by up to 44% [144]. Investigations concerning the effects of urbanization on both adult and larval stages of salamanders found that urbanization was associated with decreases in survival and colonization probabilities [145,146]. Another study asserted that high water velocities in urban streams, due to increased amounts of impervious surfaces and loss of riparian forests in watersheds, resulted in decreased larval salamander retention [42].…”

“…Although many studies have found that spreading urbanization can reduce or decimate salamander populations in riparian forests [42,99,[143][144][145][146], there is a need for more research on the mechanisms that drive those responses. It is important to know how urbanization impacts stream habitat so that we can mitigate changes in stream morphology and subsequent shifts in salamander populations.…”