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Using Canals in Southern Florida to Measure Impacts of Urbanization on Herpetofaunal Community Composition
Oliver Ljustina and Shelby Barrett

Southeastern Naturalist, Volume 17, Issue 2 (2018): 202–210

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Southeastern Naturalist O. Ljustina and S. Barrett 2018 Vol. 17, No. 2 202 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 Southeastern Naturalist 203 O. Ljustina and S. Barrett 2018 Vol. 17, No. 2 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 Southeastern Naturalist O. Ljustina and S. Barrett 2018 Vol. 17, No. 2 204 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. Southeastern Naturalist 205 O. Ljustina and S. Barrett 2018 Vol. 17, No. 2 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 Southeastern Naturalist O. Ljustina and S. Barrett 2018 Vol. 17, No. 2 206 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 Southeastern Naturalist 207 O. Ljustina and S. Barrett 2018 Vol. 17, No. 2 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. Southeastern Naturalist O. Ljustina and S. Barrett 2018 Vol. 17, No. 2 208 (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). 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