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2018 SOUTHEASTERN NATURALIST 17(2):202–210
Using Canals in Southern Florida to Measure Impacts of
Urbanization on Herpetofaunal Community Composition
Oliver Ljustina1,* and Shelby Barrett1
Abstract - Urban ecosystems provide habitat for a variety of amphibian and reptile species,
but in most places, these communities are understudied. Gradients of urbanization have
been used to examine how herpetofaunal communities respond to anthropogenic disturbance.
We used visual-encounter surveys along human-made canals that track a gradient of
urbanization as a system to examine changes in aquatic and semiaquatic herpetofauna. We
found substantial changes in herpetofaunal community composition along the urbanization
gradient, primarily driven by the association of exotic invasive amphibians with canals
adjacent to urban areas relative to canals adjacent to natural areas.
Introduction
Urbanization drastically alters ecosystems and is a contributor to global biodiversity
loss (Alberti 2005, Marzluff and Ewing 2001, McKinney 2006). However,
urbanized areas are being increasingly appreciated as functioning ecosystems that
present novel challenges to persisting organisms (Donihue and Lambert 2015). Gradients
of anthropogenic disturbance have been used to demonstrate how organisms
respond to urbanization, and are often defined by levels of anthropogenic surfacedisturbance
(McKinney 2008). Flood-control canals might provide a convenient
means of sampling an urbanization gradient for changes in community composition
of aquatic and semiaquatic species. Southern Florida supports a diverse native herpetofauna,
as well as several dozen non-native species (Krysko et al. 2011, Meshaka
2011, Meshaka and Layne 2015, Meshaka et al. 2000), creating an ideal system for
herpetological research. The degree to which exotic reptile and amphibian species
contribute to herpetofaunal community structure in relation to urbanization might
be determined by observing herpetofaunal community composition along a gradient
of urbanization delineated by canals.
Occurrence of a range of taxa along gradients of urbanization have been used in
various studies to demonstrate the impact of urbanization on ecosystems, the results
of which have shown that urbanization is generally correlated with decreasing species
diversity and increasing exotic species richness (Clergeau et al. 1998, Germaine
and Wakeling 2001, Kowarik 2008, McKinney 2008). Canals in southern Florida
might represent a system in which to examine such compositional shifts in plant
and animal communities along a readily accessible and easily sampled gradient of
urbanization. Changes in fish community composition have been demonstrated in
southern Florida canals relative to more natural hydrologic features (Gandy and
1Department of Biological Sciences, Southeastern Louisiana University, Hammond, LA
70402. *Corresponding author - oliver.ljustina@selu.edu.
Manuscript Editor: Brad Glorioso
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Rehage 2017, Trexler et al. 2000). However, we are not aware of studies using
canals in southern Florida to examine compositional changes in semi-aquatic and
terrestrial animal communities along urbanization gradients.
Flood-control and irrigation canals have caused drastic alterations to hydrology
and nutrient regimes in southern Florida ecosystems, with major impacts on
plant and animal communities, as well as human populations (Childers et al. 2003;
Sklar et al. 2001, 2005). Flood-control canals in southern Florida extend from state
and federally managed lands and agricultural areas, through residential and urban
areas, along a gradient of increasing urbanization.
Previous studies have described shifts in herpetofaunal composition and diversity
in southern Florida (Cassani et al. 2015, Forys and Allen 1999, Meshaka
et al. 2008, Smith 2006), often describing increased contributions from exotic
herpetofauna over time. These studies suggest habitat modification is at least
partially responsible for perceived community shifts, but do not offer fine-scale
examination of herpetofauna in urbanized areas. In this study, we employed visual-
encounter surveys to determine if an urbanization gradient along canals could
be used as a system to quantify compositional changes in semi-aquatic herpetofaunal
communities.
Field-site Description
We selected six 1-km–long transects in Miami–Dade County, with the intention
of achieving a roughly uniform spatial distribution extending from relatively undisturbed
areas to residential neighborhoods (Fig. 1). Transects C-4 West and C-4
Central were adjacent to a Cladium jamacense (Crantz) Kük (Sawgrass) marsh on
both the northern and southern banks. An approximately 2.5-m–tall earthen levee
separated the canal transects from the marsh on the northern bank and a 2-lane
highway (Tamiami Trail) on the southern bank. The C-4 East transect was adjacent
to a tract of land dominated by the exotic invasive Melaleuca quinquenervia (Cav.)
S.T. Blake (Broad-Leaved Paper Bark) along the northern bank, and a strip mall
and residential housing along the southern bank. The Snapper Creek West transect
was adjacent to a 6-lane highway (Florida Turnpike) on the western bank and residential
housing on the eastern bank. The Snapper Creek East transect was adjacent
to residential housing on the northern bank, and Dadeland Mall and residential
housing along the southern bank. The C-100A transect was surrounded by residential
housing along both banks. Transect structure was generally typified by low-cut
grassy banks on the shore, and a shallow limestone ledge beginning at the water
line, which dropped precipitously in depth ~1–2 m from the water’s edge. Though
generally consistent, we will discuss some notable variations in canal structure
elsewhere in this paper.
Methods
The first author conducted visual-encounter surveys (VES) along the transects
(Guyer and Donnelly 2012) at night between the hours of 20:00 and 01:00. He
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walked along the water’s edge at a steady pace while scanning the bank and the water
within 1 m of the water-line with a Husky® 1000-lumen flashlight and visually
identified and counted all reptiles and amphibians observed. Surveys took place
2–3 times per week between 27 April 2016 and 10 July 2016, though never during
rain. The total number of surveys was variable between transects (min–max = 6–10,
median = 9; Table 1).
We quantified urbanization in ArcMap 10.2.2 (ESRI 2014) using the National
Land Cover Database (NLCD) 2011 (Homer et al. 2015). The NLCD 2011 was
acquired from the Multi-resolution Land Characteristics Consortium (MRLC)
online data portal. The NLCD classification scheme divides developed cover into
4 classes: developed, open space; developed, low intensity; developed, medium
intensity; and developed, high intensity. We included these 4 classes in the analysis
of percent urbanization for the area surrounding each transect. We created a buffer
of 289 m for each transect; Semlitsch and Bodie (2003) recommended this distance
as a natural buffer for maintaining biodiversity in riparian and wetland ecosystems.
The NLCD was a subset of the area of the transect buffers. We calculated the area of
developed land within the buffers, then divided by the total area of the transect buffer,
and multiplied by a hundred to calculate percent urban cover for each transect.
We used percent impervious cover (roadways, buildings, etc.) within the buffer to
quantify urbanization (McKinney 2002).
Figure 1. A simplified schematic, drawn to scale, of the canal transects surveyed in this
study. The gray bars represent the 1-km–long transects surveyed (transect indicated in
parentheses). Canal names are shown in bold. The line down the middle of the schematic
approximately denotes the Miami Urban Development Boundary. The star on the inset map
indicates the location of this study in Miami-Dade County, FL.
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Table 1. Percent impervious cover (roads, roof-tops, etc.) in surrounding matrix, and number of surveys performed at each transect are at the top of the
table. Species counts are expressed as total number of individuals encountered over all surveys by transect.
C-4 Snapper Creek
Transect West Central East West East C-100A
Surveys conducted 9 6 9 8 9 10
Percent impervious cover 9.32% 18.85% 45.09% 89.74% 69.72% 84.86%
Anura
Eleutherodactylus planirostris (Cope) (Greenhouse Frog) 0 0 41 171 113 314
Hyla cinerea (Schneider) (American Green Tree Frog) 0 1 0 0 0 0
Lithobates grylio (Stejneger) (Pig Frog) 1 1 0 0 0 0
Lithobates sphenocephalus (Cope) (Southern Leopard Frog) 34 22 13 0 0 0
Osteopilus septentrionalis (Dumèril and Bibron) (Cuban Tree Frog) 0 0 0 1 0 0
Rhinella marina (L.) (Cane Toad) 0 0 12 12 60 372
Serpentes
Nerodia fasciata pictiventris (Cope) (Florida Banded Water Snake) 9 3 0 0 0 0
Nerodia floridana (Goff) (Green Water Snake) 0 0 1 0 3 9
Nerodia taxispilota (Holbrook) (Brown Water Snake) 0 6 1 3 7 65
Chelonia
Apalone ferox (Schneider) (Florida Softshell Turtle) 1 3 9 0 0 0
Pseudemys sp. (a cooter) 0 1 2 2 0 0
Sternotherus odoratus (Latreille, in Sonnini and Latreille) (Common Musk Turtle) 0 0 2 0 0 0
Trachemys scripta elegans (Wied-Neuwied) (Red-eared Slider) 0 0 0 3 0 7
Crocodylia
Alligator mississippiensis (Daudin) (American Alligator) 4 1 0 0 0 0
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We employed PRIMER-6 (Clarke and Gorley 2006) to determine if the
herpetofauna community structure differed from random by applying non-metric
multidimensional scaling (NMS) with Bray–Curtis similarity, 50 iterations, and
10,000 permutations. We ran the ANOSIM test to determine the level of significance
associated with perceived community structure. We conducted the BEST procedure
with the BVSTEP option and a Spearman correlation and 99 permutations to determine
which species most influenced community structure, where P represents
species correlation with community structure. We employed the SIMPER function
using Bray–Curtis similarity to determine which transects were most dissimilar in
terms of present herpetofauna.
Results
We observed a total of 14 species of reptiles and amphibians along the 6 canal
transects (Table 1). Percent impervious cover was highest at the Snapper Creek
West transect, and lowest at the C-4 West Transect (Table 1). The ANOSIM function
suggested a significant difference in herpetofaunal community structure
across the different transects (ANOSIM Global R = 0.686, P < 0.001). Eleutherodactylus
planirostris (Greenhouse Frog), Rhinella marina (Cane Toad),
Lithobates sphenocephalus (Southern Leopard Frog), Nerodia taxispilota (Brown
Watersnake), and Apalone ferox (Florida Softshell) were primarily responsible
for observed community structure (P = 0.995) (Fig. 2). Transects C-4 West and
C-4 Central were the least dissimilar transects (average dissimilarity = 58.70),
followed by Snapper Creek West and Snapper Creek East (average dissimilarity
= 65.51), and Snapper Creek East and C-100A (average dissimilarity = 70.05).
Snapper Creek East and C-4 West, as well as C-100A and C-4 West were the most
dissimilar pairs of transects (average dissimilarity = 100.0), followed by Snapper
Creek West and C-4 West (average dissimilarity = 99.79).
Discussion
Visual-encounter surveys along the canal transects were easy to perform, and
yielded sufficient encounters to describe general patterns of herpetofaunal community
composition as it relates to urbanization. However, detection rates for
most species were generally low in this study, suggesting that alternate sampling
methodologies should be considered, depending on which species and hypotheses
are being addressed. Surveys along the more natural transects failed to detect many
native species that could reasonably be expected to inhabit canals, such as Amphiuma
means Garden (Two-toed Amphiuma), Acris gryllus (LeConte) (Southern
Cricket Frog), and Kinosternon baurii (Garman) (Striped Mud Turtle) (Meshaka et
al. 2000). Our VES were effective in detecting the anuran community along canals
in this study; therefore, this method could be used to address future hypotheses
concerning anurans and their relationship with urban areas.
The herpetofaunal community residing along canal banks in southern Florida
demonstrates significant structure primarily driven by the stark change in
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amphibian species composition; exotic invasive species, namely the Greenhouse
Frog and Cane Toad, dominate transects with high impervious-surface cover, and
the native Southern Leopard Frog dominates transects with low impervious-surface
cover. These amphibian species only overlap in the C-4 East transect, where impervious-
surface cover was intermediate relative to the other transects (Table 1).
In addition to surrounding matrix composition, variations in canal structure,
not quantified here, likely contributed to community structure. For example, even
though structurally similar in that grass was cut by maintenance crews regularly
Figure 2. (A) Non-metric multidimensional scaling plot, showing the rank similarity of transect
surveys in terms of species composition and abundance; stress = 0.09; (B–F) Bubble
plots showing relative abundance and distribution of various species across transects and
sampling events, with bubble size correlating positively with relative abundance (B) Greenhouse
Frog, (C) Cane Toad, (D) Southern Leopard Frog, (E) Brown Watersnake, (F) Florida
Softshell.
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(though not as frequently at C-4 West and C-4 Central), it is worth noting that bank
vegetation was composed of native Muhlenbergia capillaris (Lam.) Trin. (Hairawn
Muhly) at C-4 West and C-4 Central, whereas residential lawn grass was the dominant
vegetation along the banks of the other transects. Similarly, there were notable
variations in aquatic vegetation density and species. Variations among transects in
canal depth and width as well as the extent of the littoral zone might also have influenced
the resident herpetofaunal community. Other abiotic factors not measured
here likely impacted detection at different sampling events: air temperature; water
temperature, turbidity, and salinity; and general weather conditions such as cloud
cover and wind speed all potentially contributed to detection of particular species
during a given survey. Future studies would benefit from careful examination of
vegetation, canal characteristics, and other abiotic variables as potential factors
affecting herpetofaunal community variation along urbanization gradients in southern
Florida.
It is also possible, and likely, that added daytime sampling would contribute
further to observed structure. We incidentally observed Basiliscus vittatus Wiegmann
(Brown Basilisk), Iguana iguana (L.) (Green Iguana), and Norops sagrei
Dumèril and Bibron (Brown Anole) along most urban transects, but never C-4
West and C-4 Central. Although commonly observed during the day, we did not
observe any of these species during night surveys. The ubiquity and hyperabundance
of the Brown Anole in urbanized areas (O. Ljustina, pers. observ.) makes
them worthy of consideration in any study of urban herpetofaunal communities
in southern Florida.
The absence of Nerodia fasciata pictiventris (Florida Watersnake) from all but
the 2 transects with the least amount of impervious-surface cover is noteworthy
because this species is described as a habitat generalist (Gibbons and Dorcas 2004;
but see Todd et al. 2016, 2017). Whether the Florida Watersnake is truly absent
from areas that are more disturbed or whether they simply occur at lower densities
relative to areas that are more natural requires intensive, long-term sampling. Nerodia
floridana (Florida Green Watersnake) was absent from surveys of 3 transects
(Table 1); however, it is worth noting that we encountered this species along the
levee while traveling to and from C-4 West and C-4 Central, suggesting that more
extensive sampling is warranted. The apparent absence of the Greenhouse Frog and
Cane Toad from habitats that were more natural warrants further investigation, particularly
given the dispersal of Cane Toads through natural areas in other portions
of its introduced range (Doody et al. 2009).
Our findings demonstrate that herpetofaunal community composition responds
to urbanization similarly to other taxonomic groups examined in different geographic
areas, with increasing abundances of invasive species and reduced native
species diversity associated with increasing levels of urbanization (McKinney
2008). The absence of some species in our study that would seem likely to persist
in urbanized areas poses interesting questions regarding the biotic filtering mechanisms
associated with urban ecosystems.
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Acknowledgments
We thank Jennifer Rehage and Maureen Donnelly for providing helpful insight on methodologies.
We are grateful to Brian Crother and Gary Shaffer for reviewing the manuscript.
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