Vol.3, No.3, 234-241 (2013) Open Journal of Ecology
Persistence of herpetofauna in the urbanized rouge
river ecosystem
David A. Mifsud1,2*, John C. Thomas1
1Natural Sciences Department, University of Michigan-Dearborn, Dearborn, USA;
*Corresponding Author: jcthomas@umd.umich.edu
2Herpetological Resource and Management, Chelsea, USA
Received 11 January 2013; revised 9 March 2013; accepted 3 May 2013
Copyright © 2013 David A. Mifsud, John C. Thomas. This is an open access article distributed under the Creative Commons Attribu-
tion License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly
Over 100 years, urbanization has taken place
along the Rouge River watershed of southeast
Michigan, USA. To determine the impact(s) of ur-
banization on herpetofauna, species richness
and distribution in 122 wetlands along 13.0 km
of the urbanized Rouge River watershed were
monitored from early spring to late fall 2003.
Data were mapped using Geographic Informa-
tion Systems (GIS). Both amphibian and reptile
species richness were associated with wetland
size and hydroperiod. The invasive plants Allia-
ria petiolata and Rhamnus cathartica were coin-
cident with lower than average amphibian spe-
cies richness. In spite of the number of herpe-
tofauna bein g re lati v ely low, this stu d y identified
hydroperiod and wetland size as important fea-
tures that may contribute to amphibian and rep-
tile species sustainability in this highly disturb-
ed and fragmented urban landscape.
Keywords: Amphibian; Hydroperi od; Invasive
Plants; Reptiles; Wetland; Urban Herpet ology
Faced with increased urbanization and environmental
disturbance to their native ecosystems, the future of re-
sident amphibians and reptiles in SE Michigan (USA) is
uncertain. Herpetofauna are also susceptible to a severe
loss of genetic diversity because of life-history traits in-
cluding genetics, breeding strategies, habitat fragmen-
tation, and a low dispersal ability [1]. Habitat infringe-
ment from agriculture and urbanization contributes sig-
nificantly to amphibian and reptile decline worldwide [2-
6]. As human populations advance into undisturbed eco-
systems, anthropogenic pollutants challenge resident her-
petofauna [7-9]. Additional pressure on amphibians and re-
ptiles arise from the human transportation infrastructure
[10-12]. Some animals may adapt to a disrupted ecosys-
tem by altering their behavior, habitat, or range [13,14].
Hydroperiod greatly influences amphibian population
recruitment and composition [14,15]. The duration of
water in a site plays a key role in amphibian reproduction
and survival by influencing developmental rate, desic-
cation, and predation [16-19]. Wetland size and water du-
ration are also important considerations in assuring di-
versity [20,21]. Some observations suggest a more com-
plex relationship, where species diversity peaks at an in-
termediate hydroperiod, then drops in permanent wet-
lands [22]. Similarly, intermediate human exposure cor-
related with the highest species diversity [23].
This study was conducted in the Rouge River water-
shed in southeast Michigan, USA. During the 1980s, the
Rouge River was classified as one of the most polluted
rivers in the USA [24]. Historic and current challenges to
this wetland community arise from human population
growth: increased impermeable surfaces (e.g., roads, park-
ing lots); the widespread use of combined sewer over-
flows in the urban design, municipal, industrial, and non-
point discharges, contaminated sediments, habitat loss,
and degradation [25-29]. According to the Southeast Mi-
chigan Council of Governments (SEMCOG), nearly 100%
of the land is developed, and contains impervious surface
cover of at least 32% [26,27,30]. The latter is of concern
as 26% impervious surface cover (or higher) is consider-
ed “degraded” [31,32]. As low as 6% imperviousness can
negatively impact aquatic macro invertebrate species
abundance and diversity [33,34]. One major effect of im-
pervious surfaces is enhanced flooding, even from minor
rain events in the Rouge Rive watershed (<2.5 cm of rain)
[28]. Flooding promotes erosion, alters stream bed and
flow, increases pollutant load in receiving waters, de-
Copyright © 2013 SciRes. OPEN A CCESS
D. A. Mifsud, J. C. Thomas / Open Journal of Ecology 3 (2013) 234-241 235
creases ground-water recharge and water-table declines,
and generally impairs the resultant aquatic habitat [35].
Amongst the many possible stressors [1], increased
urbanization could alter Rouge River wetland hydrope-
riods and coincidentally associated herpetofauna. To exa-
mine urbanization and anuran and reptile population size
and species richness, characterization of 122 Rouge Ri-
ver wetlands was conducted. Wetland size, water quality,
plant species richness, and the presence of invasive plant
species were also examined.
2.1. Wetland Classification
Wetlands were identified and delineated using the Mi-
chigan Department of Environmental Quality [36]. Wet-
lands were considered “land characterized by the pre-
sence of water at a frequency and duration sufficient
to support, and that under normal circumstances does
support, wetland vegetation or aquatic life” [36]. Wet-
lands within the Rouge River survey were classified as
marsh, shrub-scrub, and forested, or complex (a mixture).
A marsh was considered a frequently or continually inun-
dated wetland, characterized by emergent herbaceous ve-
getation adapted to saturated soil conditions [37]. The
shrub-scrub was dominated by dense, woody, low stature
vegetation. Trees or shrubs dominated the forested wet-
lands, which lacked abundant herbaceous vegetation and
dried up seasonally [37]. Wetlands that were a combina-
tion of two or more wetland types (within 20 m) were
considered as complex.
2.2. Wetland Hydroperiod Assessment
Hydrology was recorded monthly at each site throu-
ghout the study period. Water depth was recorded up to 1
meter in marsh wetlands. For saturated wetlands, a soil
probe was used to measure the level of the subsurface
water within the top 30 cm of the soil profile. A lower
maximum water level was generally associated with
shorter periods of inundation. Watermarks, water-stained
leaves, drift lines, and buttressing roots were also used to
confirm wetlands from non-wetland locations.
2.3. Water Quality
Field measurements of conductivity, total dissolved
solids, pH, and temperature employed a Hanna 1700 me-
ter (Hanna Instruments, Woonsocket, RI, USA). Relative
turbidity was estimated using a rating 0 - 3 (none to high,
respectively). For nitrate and phosphate determination,
sterile bottles were used for water collections. A Hatch
Kit NI-14 (Hatch Company, Loveland, CO, USA) and
EPA Method 365.3 were used to measure (in triplicate)
nitrate and phosphate.
2.4. General Soil and Water Quality
The Wayne County Soil Survey map was used (Rouge
Program Office Data CD Volume 9, Wayne County
Rouge River National Wet Weather Demonstration Pro-
ject, Detroit, MI, USA) and the soil types were classified
as hydric soils (see [38]. Histosols form when drainage is
limited, interrupting the decomposition of biomass [37].
Using the wetland delineation protocol from the US
Army Corps of Engineers [39], histosols were determin-
ed visually. Reducing conditions were recorded as the
presence/absence of reduced iron (rust) in the soil strata.
Sulfidic odors were also recorded.
2.5. Herpetofauna Surveys
Historically, 14 amphibian species and 15 reptile spe-
cies frequented the Rouge River study region [40]. Ran-
dom surveys were conducted from March through Octo-
ber 2003, recording the number and species diversity of
herpetofauna. Calling surveys, visual observations, and
trapping were employed. All sites were surveyed bi-
monthly with a minimum of 16 visits per site throughout
the active season. Locations of all positively identified
animals were recorded using Global Positioning Systems,
(GPS), and a GIS database for analysis.
Amphibians that vocalize (anurans) to attract mates
were surveyed using a point-count system [41,42]. Sur-
veys were done bi-weekly at a fixed location and began
one-half hour after sunset and ended before midnight
throughout the active season. One of three call level
codes was used to categorize the intensity of calling acti-
vity for each species. Call level 1 was assigned if calls
did not overlap and calling individuals could be discre-
tely counted. Call level 2 was assigned if calls of indivi-
duals sometimes overlapped, but the numbers of indivi-
duals could still be discriminated. Call level 3 was as-
signed if the calling of many individuals overlapped or
the calling seemed nearly continuous. No calling was
rated as 0. Visual observations (binoculars, log flipping)
revealed animals in wetlands and forested areas that did
not call. Only positively identified animals were incur-
porated into the database. Aquatic survey sites were di-
vided into 2-meter blocks, each randomly surveyed bi-
weekly. Aquatic funnel traps were used when appropriate.
Un-baited funnel traps (3 - 4 L volume) were placed in
the water with some headspace at the top of the trap.
Traps were inspected after 24 h, and all trapped animals
recoded, and released. Netting was used on several 10 -
12 m portions of the wetland (separate from the trapping
sections). Generally, dip netting was concentrated in ar-
eas near submerged vegetation. Nets were swept in three
replications of two meters through the wetland. Am-
phibians and reptiles were recorded using 1 - 3 m GPS.
Copyright © 2013 SciRes. OPEN A CCESS
D. A. Mifsud, J. C. Thomas / Open Journal of Ecology 3 (2013) 234-241
Copyright © 2013 SciRes.
2.6. Statistical Methods
Diversity measurements were based on the presence of
species in each wetland. Using 14 species of amphibian
and 15 species of reptile that frequent the region [40], if
7 amphibian species were observed in a site, the amphi-
bian diversity value was considered 7 of 14 amphibian
species, or 0.5 value. Diversity was estimated using the
Shannon Diversity Index [43].
Water column maximum water levels throughout the
season were used for hydrological analysis. Linear reg-
ression modeling was used throughout [44]. The depen-
dent variable was generally species diversity whereas the
explanatory variable was varied with each test (the wet-
land type, maximum water level, invasive plant species,
pH, etc.). Mean values +/ SE are reported. Correlations
between the explanatory and dependant variables were
determined and the p-value and the R2 recorded. One-
way analysis of variance (ANOVA) was used to evaluate
the median differences between species diversity and dif-
ferent environmental variables. The presence or absence
of invasive plant species and any relationship to herpeto-
fauna and plants diversity was analyzed using the non-
parametric Mann-Whitney statistical test using JMP 5.01
software (SAS Institute Inc., Cary, NC).
2.7. Fieldwork Code Practice
During this study, efforts were made to reduce the risk
of transmitting potentially harmful organisms (i.e., bac-
teria, fungi, and invasive plant seed) and to minimize site
disturbance. Guidelines of the Declining Amphibian Po-
pulation Task Force [45] were used.
3.1. Rouge River Wetlands Location
The center of the study corresponded to 42 degrees 18'
42.82 “N 83 degrees 14’ 08.82” W (Figure 1). Rather
than classify land-use forms as residential, commercial,
and industrial land [42] or urban, suburban, and rural
[46], we chose to analyze potential habitats bordering the
river, surrounded by the diverse urban make-up of in-
dustrial and residential areas. Due to topography, the
borders of the river are relatively free of buildings, where
the river corridor creates a potential migratory path for
the animals studied.
One hundred and twenty two wetlands, 90.40 hectares,
were identified and delineated (Ta b l e 1 ). Most wetlands
were “small,” from 0.01 to 11.12 hectares2 and included
four wetland habitat types: forested, marsh, complex, and
scrub-shrub. In 122 wetlands, 62.3% were classified as
forested. Marsh communities were 23.8% of the sites.
The least common wetland was the scrub-shrub. Com-
plex wetlands comprised 12.3% of the total wetlands
considered during the study. From marsh and scrub shrub
wetlands, 57% and 43% had detectable histosols. Some
Forested (55%), Marsh (30%), and Scrub Shrub (15%)
demonstrated reducing conditions.
3.2. Herpetofauna Observed
The herpetofauna observed included eight amphibian
and eleven reptile species. The number of wetlands con-
taining each animal is shown in Figure 2. Only a few of
the total number of species were observed in most wet-
lands. The Eastern American Toad (Bufo americanus),
Western Chorus Frog (Pseudacris triseriata), Eastern Gar-
ter Snake (Thamnophis sirtalis), Midland Painted Turtle
(Chrysemys picta marginata), and Green Frog (Rana cla-
Figure 1. The study area along the Lower Rouge River, near
Dearborn, MI (USA). Wetlands are coded according to the
color legend shown above.
Table 1. Wetland size and herpetofauna observed in 122 wetlands along the Rouge River.
Type Hectare2 Total Wetlands Total Amphibans Amphibians/Hectare2 Amphibians/Wetland
Forested 10.4 76 28 2.7 0.4
Marsh 19.6 29 54 2.8 1.9
Complex 39.8 15 45 1.1 3.0
Scrub Shrub 2.5 2 1 0.4 0.5
Type Hectare2 Total Wetlands Total Reptiles Reptiles/Hectare2 Reptiles/Wetland
Forested 10.4 76 10 1.0 0.4
Marsh 19.6 29 22 1.1 0.8
Complex 39.8 15 30 0.8 2.0
Scrub Shrub 2.5 2 0 0 0
D. A. Mifsud, J. C. Thomas / Open Journal of Ecology 3 (2013) 234-241 237
Figure 2. (a) Distribution of amphibians (Figure 2(a)) in the
wetland habitats; (b) Distribution of reptiles (Figure 2(b)) with-
in the wetland habitats.
mitans melanota) were the most often observed. Examin-
ing the species found in each wetland habitat, more rep-
tiles and amphibians were found in marsh and complex
wetlands compared to the other habitats surveyed (Table 2).
3.3. Diversity, Wetland Area, Hydrology, and
Habitat Classes
Three surveyed wetlands contained the maximum spe-
cies richness seen (5 amphibian species per wetland).
Reptile species richness (overall) averaged 0.51 species
(SE of 0.12) per wetland with the maximum of 10 spe-
cies in one wetland. Shannon Diversity determinations
were highest in Complex and Mash wetlands (Table 2).
3.4. Wetland Area, Hydroperiod, and
Herpetofaunal Species Richness
Hydroperiod data (maximum water level) were exam-
ined with respect to species richness. ANOVA analysis
showed an association between amphibian and reptile
species richness and hydroperiod (Ta ble 3). Amphibian
and reptile species richness were also associated with
Table 2. Diversity determination using the Shannon index [43].
Symbol key, species richness (H), and evenness (Shannon’s Equ-
Habitat Amphibians +
Shannon Diversity
Index (H)
Equitability EH
Forested38 1.49 0.80
Marsh 76 2.17 0.94
Complex71 2.52 0.89
Scrub Shrub1 0 0
plant species richness (Ta b l e 3 ). Distance (153 m) from
the river system, water temperature, pH, conductivity, to-
tal dissolved solids, nitrate and phosphate did not show
significant effects on herpetofauna species richness (not
3.5. Plant Diversity and Wetland Area
The extent of wetland area was the most significant
determinant in contributing to plant species richness,
where the R2 is 0.90 (Ta ble 4). Some effects correlated
with phosphate, and total dissolved solids were also ob-
served (Table 4). Variables such as distance (153 m)
from the river, water temperature, pH, conductivity, tur-
bidity, and nitrogen levels did not significantly influen-
ced plant community composition. In plants, the mean
species richness per wetland (overall) was 16.02 species
(SE of 0.75). Plant species richness was greatest in fores-
ted wetlands (Figure 3).
3.6. Invasive Plant Species and Diversity
Initial regression analysis between amphibian and rep-
tile species richness versus invasive plant species reveal-
ed some potential interactions. Spatial Analyses were
done using ESRI ArcMap 8.3 with prevalence of garlic
mustard (Allia ria petiolata) compared to overall herpeto-
faunal species richness. For wetlands where garlic mus-
tard was abundant or dominant (n = 9), herpetofauna ne-
ver exceeded two species. In comparison, in wetlands
with little or no garlic mustard, as high as 8 species of
herpetofauna were observed (n = 82). Using the Mann-
Whitney test to compare the two sets of unpaired data,
amphibian species richness significantly declined with
the co-presence of Garlic Mustard (Alliaria petiolata) or
Buckthorn (Rhamnus cathartica) in wetlands (nearly 2
fold using a 2-tailed test, P value of 0.001).
Until the early 20th century and the emergence of the
auto industry, Southeast Michigan was largely forested or
used as farmland. Today urbanization has created many
paved roads that bisect the study site. Average daily traf-
fic along selected principal highways (Michigan Avenue,
Ford Road, and Telegraph Road) ranged from 39,000
Copyright © 2013 SciRes. OPEN A CCESS
D. A. Mifsud, J. C. Thomas / Open Journal of Ecology 3 (2013) 234-241
Table 3. ANOVA of amphibian and reptile species richness
compared to several environmental variables. *Indicates a sig-
nificant difference.
Amphibians Dftotal R2 F Ratio P-value
Wetland Area 121 0.17 4.93 0.0005*
Hydroperiod 121 0.44 6.46 <0.0001*
Plant Species Richness 121 0.20 5.81 <0.0001*
Reptiles Dftotal R2 F Ratio P-value
Wetland Area 121 0.44 14.96 <0.0001*
Hydroperiod 121 0.27 3.20 <0.0006*
Plant Species Richness 121 0.26 6.78 <0.0001*
Table 4. ANOVA of plant species richness compared to several
environmental factors. *Indicates a significant difference between
diversity and the variable considered.
Factors Dftotal R2 F Ratio P-value
Wetland Area 121 0.90 26.11 <0.0001*
Hydroperiod 121 0.23 2.43 0.0064
Dissolved Solids 121 0.62 2.23 0.008*
Phosphate 121 0.79 2.94 0.008*
12.5 11.5
MarshForested Scrub-shrubComplex
Plant Species/hectare
Figure 3. Density of plant species, density/hectare 2. Values
above bars indicate the actual value.
to 58,000 vehicles per day [47]. This single impediment
likely contributes to the paucity of herpetofauna in this
area [12]. A survey of the amphibian and reptile commu-
nity (including the Henry Ford mansion) in this frag-
mented ecosystem was done to determine animal po-
pulations and investigate species richness and distribu-
Hydroperiod and wetland size were both associated
with amphibian and reptile species richness (Table 3) [17,
18,21,22]. Altered hydroperiod and habitat fragmentation
resulting from intense agricultural land-use are known to
significantly lower amphibian diversity [48-50]. Under
fragmentation, “small” wetlands, such as those described
here, likely provide significant habitat and refuge oppor-
tunities for herpetofauna [22,28,49].
Plant species richness was also positively linked to
amphibian and reptile species richness. Wetland area was
the greatest determinant in plant species richness (Table
4). Invasive species garlic mustard and Buckthorn re-
stricted herpetofaunal species richness by a factor of 2.
This data is important, as the ecological plant community
greatly influences the nature of the wetland (Marsh,
Scrub Shrub, etc.) and amphibian and reptile populations.
For example, particular plant species are known to pro-
mote or inhibit herpetofauna diversity [23,51].
The observed herpetofaunal diversity (Table 2) provi-
ded a reflection of the historic species coverage in this
urban landscape [40,52]. Diversity of over 2.0 on the
Shannon Diversity Index was found in Marsh and Com-
plex habitats (Table 2). While encouraging, the total
number of individual species/site was quite small (Fig-
ure 2, Table 1), compared to a protected non-urban wet-
land containing more than 360,000 individuals counted
per year [31,53]. Given the low numbers of total indivi-
duals per species recorded, it is surprising that moderate
species richness was observed. Several reasons for this
could include; the reproductive prowess of the herpe-
tofauna studied here, the presence of Golf Course refugia
[28], upstream emigration, resilience, the adaptive nature
of these species, and perhaps even human re-introduction.
The presence of Marsh and Complex vegetation and the
general lack of fish (seen in only 3 marsh wetlands) in
most small wetlands also likely contributed to the re-
maining amphibian and reptile survival. Should road-
crossing impediments and other “stressors” [1] become
minimized, herpetofaunal abundance could increase in
this region. One caveat is that this single season survey
fails to account for innate dynamic population turnover
sometimes observed in these animals [54].
Assuming a large mortality of herpetofauna likely due
to the extensive road networks [10,12], this study found
that wetland hydroperiod and size were linked to herpe-
tofaunal species richness in relatively small Marsh and
Scrub Shrub wetlands along the Rouge River. Plant spe-
cies richness also had a positive effect on herpetofaunal
species richness. Thinking forward, designed corridors
and wetland connectivity could create accessible and lar-
ger habitat areas in this urban setting, supporting greater
amphibian and reptile populations and maintain species
diversity. Knowledge of wetland size and hydrology, to-
gether with a better understanding of the role of invasive
plant species, herpetofaunal migratory patterns, and the
use of buffer zones could greatly contribute to maintain-
ning amphibian and reptile abundance and diversity in
the urban Rouge River ecosystem [55]. This information
could also aid in environmental planning and manage-
ment to provide conservation-minded approaches to re-
duce the risks caused by human infringement to herpe-
Copyright © 2013 SciRes. OPEN A CCESS
D. A. Mifsud, J. C. Thomas / Open Journal of Ecology 3 (2013) 234-241 239
The authors wish to thank Rachel Mifsud, Mary Midsud, and Jim
Harding for their efforts, advice, and inspiration. Thanks to Sarajoy
Crew and Wade Johnson for statistics help, and to Robert Primeau and
Nakeeta Ward for their extensive field efforts. The authors acknowle-
dge Drs. Annette Sieg, David Susko, and Douglas A. Wilcox for their
helpful comments. This work received support from The Office of
Sponsored Research, University of Michigan-Dearborn and the Friends
of the Rouge.
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