urban - Biology Department - Davidson College

lameubiquityMechanics

Feb 21, 2014 (3 years and 5 months ago)

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

Steven J. Price
1,2

, Robert A. Browne
1

and Michael E. Dorcas
2


1
Wake Forest University, Department of Biology, Winston
-
Salem NC 27109

2
Davidson 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

i

~ Poi(λ
i
)


Site
-
level abundance of salamanders was specified by

log(λ
i
) = 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 thei
r p
roperties. Comments by Dave Anderson, Melissa Pilgrim, Miles Silman, and Cliff Zeyl greatly improved the manuscript. J. And
rew

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

F
orest 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
Larva
2005
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
Larva
2005
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.