Modeling the Effects of Urbanization on Stream Salamander Abundances Using a Before-After Control-Impact Design

Steven J. Price1,2 , Robert A. Browne1 and Michael E. Dorcas2

1Wake Forest University, Department of Biology, Winston-Salem NC 27109 2Davidson College, Department of Biology, Davidson NC 28035

Objective• To estimate larval and adult abundances before, during and after urbanization

and compare abundances to those of populations in streams that are not urbanized

Introduction

Summary and Conclusions

Methods

Acknowledgements

Results

Map of study sites. Circles represent stream locations that were urbanized after the first year of sampling. Triangles are control sites.

• Urbanization of stream catchments affects nearby stream ecosystems.

•Urban streams are characterized by:

• Altered hydrologic flow patterns.

• Increased sedimentation, which modifies channel morphology.

• Decreased water quality from runoff.

• 30 first-order streams in Charlotte-metropolitan area, NC were sampled annually from 2005-2009

•13 streams urbanized after first year of sampling (i.e., 2005); 17 control (undisturbed) streams

•Salamander counts gathered through dipnetting and trapping

• Sites surveyed two times each year from March to early May.

•Amphibians, especially salamanders, represent the dominant vertebrate biomass in many streams.

•Salamanders may be particularly sensitive to urbanization due to a bi-phasic life cycle and complex habitat requirements.

• Responses of urbanization may differ among species and stages (i.e., larvae vs. adults)

•Abundances (λ) at the local-level were modeled with the Bayesian binomial mixture model developed by Royle (2004) such that: Ni|λi ~ Poi(λi)

•Site-level abundance of salamanders was specified by log(λi) = β0 + β1* urban, where urban was a vector of 1 or 0 dependent on if a site was urbanized (1) or control (0).

• Detectability of salamanders was specified by cij|Ni ~ Bin(Ni,pij)

•Site-level detection was modeled by logit(pij) = α0 + α1* cover + α2* detritus + α3* rain

•Our models used uninformative priors. Posterior summaries were based on 300,000 Markov chain Monte Carlo iterations with a 30,000 sample burn-in and a thinning rate of 5.

Collecting salamanders at an urban stream.

Urban streams often have increases in sedimentation and modified channels

• We counted a total of 6558 dusky and two-lined salamanders between 2005 and 2009 [3889 two-lined salamanders (298 adults and 3591 larva) and 2669 dusky salamanders (974 adults and 1695 larva)].

•Detection probabilities for salamanders varied among years, with covariates cover, detritus, and rain having positive, negative, no effects dependent on stage and species.

Abundance Estimates

We thank students in the Davidson College Herpetology Laboratory, particularly W. Anderson, K. Cecala, G. Connette, E. Eskew, E. P. Hill, C. McCoy and D. Millican, who helped collect data for this study. W. R. Costenbader, K. Coffey, S. Davies, R. Harper, L. Hobbs and D. Testerman provided assistance locating study sites. F. Bragg, J. Bragg, B. Eakes, K. Killian, D. Seriff, M. Strawn, T. Waters, and A. White allowed us to sample salamanders on their properties. Comments by Dave Anderson, Melissa Pilgrim, Miles Silman, and Cliff Zeyl greatly improved the manuscript. J. Andrew Royle also provided advice on statistical analysis. This material is

based upon work supported by the Department of Energy under Award Number DE-FC-09-075R22506. Funding was provided by the Department of Biology at Davidson College, the Davidson Research Initiative funded by the Duke Endowment, the Department of Biology at Wake Forest University, National Science Foundation grant (DEB-0347326) to M.E.D., and Duke Power.

•All salamander species and stages decreased in abundance in urbanized streams and abundances differed from control sites after urbanization.

•Species that inhabited terrestrial environments (i.e., two-lined salamander) declined more rapidly than primarily aquatic species (i.e., dusky salamander).

•Larval salamanders declined more rapidly than adults likely from increases in sedimentation and changes in water flow patterns.

•Previous investigations have indicated that amphibian populations may not respond to urbanization for decades; our findings suggest that response time for stream salamanders is rapid.

•By using a BACI design we were able to separate variability in salamander counts among populations due to natural fluctuations from variability in salamander counts among populations due to urbanization.

Pre-development

Post-development

The northern dusky salamander (Desmognathus fuscus) is common in Piedmont streams.

2005

2006

2007

2008

2009

-4 -3 -2 -1 0 1 2

A. Adult dusky salamander B. Larval dusky salamander

C. Adult two-lined salamander D. Larval two-lined salamander

2005

2006

2007

2008

2009

-4 -3 -2 -1 0 1 2

2005

2006

2007

2008

2009

-4 -3 -2 -1 0 1 2

2009

2008

2007

2006

2005

-4 -3 -2 -1 0 1 2

Parameter estimates for urbanization effects Parameter estimates for urbanization effects

A. Adult Dusky Salamander B. Larval Dusky Salamander

C. Adult Two-Lined Salamander D. Larval Two-Lined Salamander

Table 1. Abundance estimates with 95% credible intervals (CI; in parentheses) of dusky salamanders in streams that did not undergo urbanization of catchments and in stream catchments that were urbanized after 2005.

Year Adult Larva Adult Larva2005 12.22 (6.80, 28.99) 13.86 (8.31, 29.76) 16.65 (6.71, 54.46) 27.02 (12.40, 76.13)2006 40.61 (12.17, 154.47) 16.89 (11.45, 27.91) 40.65 (8.70, 216.01) 8.39 (4.12, 19.09)2007 46.57(15.94, 165.83) 56.76 (40.65, 93.69) 39.32 (10.18, 184.58) 30.50 (17.87, 61.31)2008 23.22(10.71, 74.81) 5.77 (4.06, 8.81) 13.07 (4.15, 60.44) 5.70 (2.77, 12.56)2009 24.41 (12.29, 78.57) 26.81 (19.41, 38.17) 9.92 (3.52, 44.89) 12.56 (7.07, 23.91)

Non-urban Catchments Urbanized Catchments

Year Adult Larva Adult Larva2005 8.97 (3.92, 29.43) 37.04 (32.65, 42.18) 15.11(3.89, 83.92) 32.91 (24.98, 43.39)2006 16.53 (5.39, 93.22) 32.55 (27.79, 38.59) 9.73 (1.60, 103.75) 21.61 (15.15, 31.13)2007 56.43(40.77, 87.44) 221.85 (124.08, 397.82) 30.22 (17.91, 57.05) 41.14 (18.65, 90.46)2008 2.73 (1.59, 5.85) 49.30 (38.32, 65.04) 0.99 (0.25, 4.54) 17.37 (10.15, 30.17)2009 1.62 (0.81, 3.36) 77.56 (54.12, 128.90) 0.40 (0.05, 2.89) 12.21 (6.35, 26.76)

Non-urban Catchments Urbanized Catchments

Table 2. Abundance estimates with 95% CI (in parentheses) of two-lined salamanders in streams that did not undergo urbanization of catchments and in stream catchments that were urbanized after 2005.

Effects of Urbanization

Figure 1. Estimates of β (effect of urbanization) on abundances of A) adult dusky salamanders, B) larval dusky salamanders, C) adult two-lined salamanders, and D) southern two-lined salamanders detected in 30 streams in the Charlotte-metropolitan area, NC, USA. Error bars indicate 95% CI.

High levels of sedimentation in urban streams likely leads to decreases in salamanders

Two-lined salamanders inhabit forests during non-breeding

season. Forests are reduced in urbanized catchments.