I measured amphibian occurrence in wetlands restored under programs implemented by the U.S. Department of Agriculture's Natural Resources Conservation Service during 2010–2011 in two regions of sites in the Mid-Atlantic region of the United States. One cluster was in Delaware and Maryland on the Delmarva Peninsula and the other was in southeastern Virginia and northeastern North Carolina. I compared larval species richness and relative abundance among 17 restored wetlands, 12 natural wetlands, and 8 wetlands (ditches) in active agricultural fields. Based on larval occurrence, I documented 56 and 60% of the expected number of anurans and salamander species, respectively, known to use ponds and ephemeral wetlands in this region. Of the total number of species encountered, 71% used restored wetlands, 76% used natural wetlands, and 35% used the ditches in agricultural fields. Mean number of species did not differ significantly among the three habitat types due to wide confidence intervals. Total species richness in restored wetlands was not correlated with age (years since restoration), percentage of canopy cover, or percentage of emergent aquatic vegetation. Wetlands restored through Natural Resources Conservation Service and private landowner partnerships, such as those included in this study, support amphibian populations that help mitigate natural habitat loss in these two regions.
Amphibians are important components of Mid-Atlantic wetlands and their surrounding uplands because they are key sources of energy exchange between these habitat types (Davic and Welsh 2004; Gibbons et al. 2006; Hocking and Babbitt 2014). Larval amphibians are herbivores, predators, and competitors that regulate the diversity and abundance of species at all trophic levels, including that of primary producers (Dickman 1968; Seale 1980). Keystone species, such as the eastern newt Notophthalmus viridescens, regulate species diversity and abundance at low trophic levels (Fauth and Resetarits 1991; Fauth 1999). Adult frogs and salamanders are predators of many species and prey to many others. Their roles in aquatic and upland habitats have been diminished because of the dramatic loss of wetlands in the United States since initial colonization by Europeans (Tiner 1984; Dahl 1990; Noss et al. 1995). Indeed, negative associations often exist between agriculture and amphibian use of breeding wetlands (Faulkner et al. 2011). Other stressors, such as landscape alteration, herbicides and pesticides from agricultural areas, urbanization, and disease also contribute to the widely recognized worldwide decline in amphibians (Blaustein and Olson 1991; Houlahan et al. 2000; Wake and Vredenburg, 2008).
The U.S. Department of Agriculture's Natural Resources Conservation Service (NRCS; http://www.nrcs.usda.gov/wps/portal/nrcs/site/national/home/) and Farm Service Agency (http://www.fsa.usda.gov/) administer the Wetlands Reserve Program (http://www.nrcs.usda.gov/wps/portal/nrcs/main/national/programs/easements/wetlands/) and the Conservation Reserve Program (http://www.fsa.usda.gov/programs-and-services/conservation-programs/conservation-reserve-program/). These programs provide support for permanent or long-term restoration of wetland habitats in the Mid-Atlantic region and elsewhere in the United States. The U.S. Department of Agriculture's Conservation Effects Assessment Project (https://www.nass.usda.gov/Surveys/Conservation_Effects_Assessment_Project/) is a multiagency effort to assess the environmental effects of conservation practices and programs, including wetland restoration. Studies of amphibian responses to wetland restoration have been completed in the Prairie Pothole Region (Balas et al. 2012; Mushet et al. 2012) and the Mississippi Alluvial Valley (Waddle et al. 2013; Walls et al. 2013), but appear to be lacking for other parts of the country.
Freshwater wetlands in the Mid-Atlantic region support a diverse and species-rich amphibian fauna. Twenty-five species of anurans and five species of salamanders that use ephemeral wetlands and ponds for breeding occur in this region (Beane et al. 2010; Powell et al., 2016). Agricultural lands are considered suitable habitat for six anurans in this region when aquatic habitats are present (Mitchell et al. 2006). To my knowledge, no previous study has addressed the effects of U.S. Department of Agriculture wetland restoration efforts on amphibian populations in the Mid-Atlantic region. My objectives were to determine patterns of amphibian species richness in three wetland categories (restored, natural, and historical [unrestored wetlands on active agricultural fields]) and if restored wetlands support native amphibian communities under U.S. Department of Agriculture conservation programs.
Materials and Methods
A total of 48 wetland and historic wetland sites in Delmarva (northern region) and the southeastern Virginia and northeastern North Carolina region (southern region) were selected initially for study by NRCS personnel. Nine of the 18 restored wetlands were constructed under the Conservation Reserve Program, five under the Wetlands Reserve Program, and two under the NRCS Conservation Technical Assistance program. Two study sites selected by NRCS personnel were of unknown origin. I have not included site-specific information here because U.S. Department of Agriculture contractors cannot reveal anything that might allow someone to identify private landowner locations (Natural Resources Conservation Service 2009).
Restored wetlands (Figure 1) sampled for amphibians (17) were human excavated or impounded temporary depressions with no surface outlet (Brooks et al. 2013). One restored wetland remained dry throughout the study and I could not sample it. Terraces and deep depressions occurred in none of the sampled wetlands, although two had small depressions in the substrate that held water beyond July and dried later. Each wetland had gradual slopes that resulted in maximum water depths of ∼1.5 m. There were no tree canopies on these wetlands. Fifteen wetlands supported aquatic vegetation. Eleven of the restored wetlands in this study occur on the Delmarva Peninsula and six occur in southeastern Virginia and northeastern North Carolina.
I defined natural wetlands (Figure 1) as those with no human manipulation to the basin or water levels. The 12 natural wetlands varied from recognized Delmarva bays (natural, usually circular, wetlands) with partial canopies to shallow forested wetlands with full canopies primarily consisting of red maple Acer rubrum, and sweetgum Liquidambar styraciflua. Most were single depressions not connected to other wetlands, although a site in Maryland was one of four wetlands that are connected during periods of high water. Two of those selected by NRCS remained dry during the entire study. All six natural sites in Delmarva and three in Virginia and North Carolina that could be sampled were shallow surface depressions with gentle slopes. One site in Virginia was a wooded, shallow forested wetland that lacked water throughout the study. One of the wetlands in North Carolina considered natural was a deep drainage ditch and one was a shallow, highly ephemeral forested area adjacent to an abandoned logging road.
Historical sites were former (destroyed) wetlands currently on lands with active agriculture, mostly corn Zea mays and soybean Glycine max production. These sites were selected for study by NRCS based on aerial photos and agency personnel on the ground. The 16 historical wetlands included 8 without standing water during the entire study and 8 with one or more drainage ditches that served to reduce the water table to enable crop production (Figure 1). These ditches varied from about 0.75 to >2 m deep and held water for variable lengths of time. There was no tree canopy cover at any of these sites. Five of the historical sites sampled were in Delmarva and three were in southeastern Virginia and northeastern North Carolina.
I visited all wetlands twice during 2010 and 2011: spring (April–May) and summer (June–July). Because private landowner sensitivities did not allow night access, sampling was limited to daylight hours. Thus, I determined species richness by identifying aquatic larvae in samples obtained with an aquatic dip net (D-ring aquatic dip net with a fine mesh [∼1-mm-diameter] bag, Ward's Biology Supply Company, Rochester, NY). To standardize dipnet samples, I extended the dip net perpendicular to and 1 m from the wetland edge, drove the flat side of the bag frame down to the substrate, and quickly pulled larvae and debris back to shore. I identified and counted all larvae in each dipnet sweep, then released them in the wetland. Sweep stations were approximately 5–10 m apart to avoid affecting results in adjacent samples (Magurran 2004). I sampled wetlands with multiples of five sweeps (up to 20) adjusted to wetland size. Twenty sweeps covered all or most of the perimeters of the largest ponds. I used the highest relative abundance value of larvae in each wetland sampled in 2010–2011 for comparison of this metric among the three habitat types.
Wetland age data (time since restoration) were provided by NRCS. I visually estimated percentage of canopy cover over each wetland in roughly 10% increments during full leaf-out while standing in the middle of the depression, but did not include the surrounding forest that may have shaded some parts of the wetland for short periods of time. Restored and historical wetlands lacked forest canopies. Because the wetlands were small and the margins could be clearly delineated, I visually estimated percentage of the entire surface area covered by emergent aquatic vegetation in each wetland during spring sampling sessions. I evaluated mean larval species richness and relative abundance of larvae for the three wetland habitat types collectively in the Mid-Atlantic region. I then compared data in northern and southern clusters separately because of the effect of species distribution patterns (see below). I used SYSTAT 11 for statistical tests following Zar (2009) and accepted α = 0.05 as the level of significance. I used linear regression to compare relationships of species richness with environmental variables. I used ANOVA to compare species richness samples among the three habitat types. I used nonparametric statistics (Kruskal–Wallis one-way ANOVA on ranks, H) when assumptions of normality were not met.
I encountered 21 species of amphibians (17 anurans, 4 salamanders) in the Mid-Atlantic wetlands sampled (Table 1). I found 13 anurans and 2 salamanders in the northern region and 12 anurans and 3 salamanders in the southern region. I detected six frog species (northern cricket frog Acris crepitans, green treefrog Hyla cinerea, carpenter frog Lithobates virgatipes, pickerel frog Lithobates palustris, wood frog Lithobates sylvaticus, New Jersey chorus frog Pseudacris kalmi, and one salamander species (spotted salamander Ambystoma maculatum) only in the northern region and one frog (southern toad Anaxyrus terrestris) and one salamander (Mabee's salamander Ambystoma mabeei) only in the southern region (Table 1). I detected three species of salamanders in natural wetlands, one in restored wetlands, and none in any of the historical sites. Four species, all anurans, occupied all three habitat types and I found one species (southern leopard frog Lithobates sphenocephalus) in all habitat types in both regions (Table 1). The low number of sites with water in the southern regions prevented meaningful statistical comparisons of species richness between the two regions.
Mean amphibian species richness based on occurrence of larvae in restored and natural sites was 50–60% higher than the mean for historical sites, but were not significantly different (H = 1.089, P < 0.05; Figure 2). Mean number of larval species was not significantly different among the three habitat types in the northern region (F = 1.089, P = 0.359) or the southern region (F = 0.899, P = 0.445; Figure 3). Relationships of larval species richness with restored wetland age (months; P = 0.354), percentage of canopy cover (P = 0.317), and abundance of emergent aquatic vegetation (P = 0.194) were also not significantly different. Mean larval relative abundance in restored wetlands was higher than in natural and historical wetlands in both 2010 and 2011 (Figure 4), although there were no significant differences (F = 0.511–3.162, P = 0.070–0.624) among the samples due to large confidence intervals.
A total of 30 species of amphibians (25 anurans, 5 salamanders) that use ephemeral wetlands and ponds for reproduction occur in the Mid-Atlantic Coastal Plain (Dodd 2013; Powell et al. 2016), of which I detected 17 and 4, respectively, in my study sites. Number of anuran species (13) in the northern region of the study was higher than the number of anurans (11) in the southern region. One (marbled salamander Ambystoma opacum) of the four species of salamanders occurred in both areas. Species-specific distribution patterns of five anurans and one salamander that occur only in the northern region and three anurans and one salamander that occur only in the southern region contributed to the difference between regions (Petranka 1998; Powell et al. 2016).
Results of field studies involving animal capture depend on the technique used (Mitchell et al. 1993). Using a dip net to sample larvae captures a subset of the species that occur in ephemeral wetlands and ponds. Adding recordings of anuran male vocalizations, a method that could not be used in this study, may have increased detection of other species. It would not ensure that all of the salamander species would be encountered, however. Larval development periods of fall- and winter-breeding salamanders that lay eggs in ephemeral wetlands and ponds extend into May or later (Petranka 1998; Mitchell and Gibbons 2010). I thus assumed sampling with dip nets in the two seasons detected larvae of all the salamanders in the wetlands studied here.
Larval occurrence provided evidence of species that actually reproduced in these wetlands. Studies of anuran occurrence based on detection of calling males assume that all of the species recorded use the wetland for breeding (Weir and Mossman 2005). However, this assumption is not supported by several researchers who found discrepancies between number of species relying solely on calling males and presence of larvae (Shaffer et al. 1994; Mazanti 2000). Larval occurrence may be a more accurate indicator for occupancy studies than sole use of auditory detection techniques.
Variation in number of species detected in this study among wetlands is consistent with results of numerous amphibian studies, especially anurans (e.g., Church 2008; Walls et al. 2013). Variation in numbers and relatively small sample sizes contributed to the lack of significant differences in species richness among the wetlands I sampled. Amphibian species richness was significantly related to pond age in a study of amphibian diversity on the Eastern Shore of Maryland at a different region of sites (Merovich and Howard 2000), but not in the regions in my study. Variation in larval relative abundance at historical sites between years was demonstrated by extreme numerical differences primarily in anuran occurrence.
Several factors affected numbers of species detected in this study. Use of a single capture technique may have limited the number of species detected that actually occur in the Mid-Atlantic region (Heyer et al. 1994). Several of the restored and natural sites in the southern region were not comparable to wetlands in the northern region or other southern region wetlands. The three “natural” sites in North Carolina included a manmade drainage ditch dug in the 1800s or earlier, a depression formed by an old roadbed that stopped surface water flow, and an undefined shallow area in part formed by small spoil mounds adjacent to a dirt road that appeared to rarely hold water. One natural site in southeastern Virginia dominated by loblolly pine Pinus taeda was a similar shallow area that appeared to seldom hold water. Two of the three restored sites in North Carolina were wide, deep ditches in peat deposits characteristic of this area (Ingram 1987).
The low pH of water on peat lands may have contributed to the lack of amphibian larvae at these sites. A pH below 6.3 limits anuran sperm mobility (Freda and Taylor 1992), and pH at natural and restored sites in this area are well below this value (Walbridge and Richardson 1991; Ducey et al. 2011). Restoration site selection for this study was conducted by state NRCS staff and intended to be representative of restorations within the Coastal Plain physiographic province of their state. Landowner permission also constrained initial site selection (J. Miller, personal observation). Involving the appropriate taxon biologist in the site selection process in future Conservation Effects Assessment Project projects that include amphibians, or perhaps any animal group, would ensure more appropriate site comparability. Choice of target species and knowledge of their habitat requirements would help direct the excavation to create the most appropriate basin topography. It would be prudent to seek the advice of a knowledgeable herpetologist to provide recommendations on wetland design that incorporate breeding habitat requirements of the area's species. A geomorphologist or soil scientist could assist with delineation of subsurface water tables and water retention abilities such as clay content of soils.
Most restoration projects could benefit from development of management plans with clearly stated goals, including input from the landowner (Bailey et al. 2006; Mitchell et al. 2006). Once restoration has been completed, guidelines in the management plans could provide steps to prevent establishment of predatory fish and exotic vegetation. Postrestoration assessment instructions in the management plan would determine if target species have colonized the wetland and whether there is evidence of reproduction (Balcombe et al. 2005; Vasconcelos and Calhoun 2006; Brown et al. 2012). Removal of invasive and native woody vegetation by mechanical means, herbicide treatments, and fire in restored wetlands during periods of drought would help to maintain the desired hydroperiod. Long-term monitoring and habitat management would ensure that restored wetlands remain ecologically functional and used by amphibians in the region. Several publications provide guidelines that would assist landowner's implementation of restoration actions (Semlitsch 2000; Bailey et al. 2006; Mitchell et al. 2006; Ausden 2007).
Former wetlands on agricultural lands restored through NRCS conservation programs in the Mid-Atlantic region provide habitats for diverse assemblages of amphibians (Mazanti 2000; Merovich and Howard 2000; this study). My results showed that restored wetlands support levels of amphibian diversity similar to the reference natural wetlands included in this study. They also demonstrate the effectiveness of conservation efforts implemented under NRCS wetland programs, and support the findings of Balas et al. (2012), Waddle et al. (2013), and Walls et al. (2013), who studied amphibians in other regions. Wetlands restored through NRCS and private landowner partnerships, such as those included in this study, support amphibian populations that help mitigate natural habitat loss in these two regions.
Please note: The Journal of Fish and Wildlife Management is not responsible for the content or functionality of any supplemental materials. Queries should be directed to the corresponding author for the article.
Data S1. Archived data for analyses of amphibian species richness in restored, natural, and historical wetlands collected during 2010–2011 in Delmarva (peninsula containing Delaware and eastern Maryland and Virginia; northern region) and southeastern Virginia and northeastern North Carolina (southern region) for the U.S. Department of Agriculture's Natural Resources Conservation Service. Study sites (column A) are numbered sequentially to avoid possible identification of private landowner properties. Abbreviations and column identifications: (B) n = northern region, s = southern region; (C) H = historical wetlands, N = natural wetlands, R = restored wetlands; (D) wetland age; (E) percentage of canopy; (F) percentage of aquatic vegetation; (G) larval species richness; (H) relative abundance in 2010; (I) relative abundance in 2011.
Found at DOI: http://dx.doi.org/10.3996/092015-JFWM-085.S1 (11 KB XLSX).
Reference S1. Bailey MA, Holmes JN, Buhlmann KA, Mitchell JC. 2006. Habitat management guidelines for amphibians and reptiles of the southeastern United States. Montgomery, Alabama: Partners in Amphibian and Reptile Conservation, Technical Publication HMG-2.
Found at DOI: http://dx.doi.org/10.3996/092015-JFWM-085.S2; also available at http://www.separc.org/products/habitat-management-guidelines-for-herpetofauna (7186 KB PDF).
Reference S2. Dahl TE. 1990. Wetlands: losses in the United States 1780's to 1980's. Report to Congress, Department of the Interior, U.S. Fish and Wildlife Service, Washington, D.C.
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Reference S3. Mazanti L. 2000. Use of constructed wetlands in agricultural environments as breeding habitat for frogs. Wetland Science Institute–Wetland Restoration Information Series No. 4.
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Reference S4. Mitchell JC, Breisch AR, Buhlmann KA. 2006. Habitat management guidelines for amphibians and reptiles of the northeastern United States. Partners in Amphibian and Reptile Conservation, Technical Publication HMG-3:1-108.
Found at DOI: http://dx.doi.org/10.3996/092015-JFWM-085.S5; also available at http://northeastparc.org/habitat-management-guidelines/ (7445 KB PDF).
Reference S5. Noss RF, LaRowe ET III, Scott JM. 1995. Endangered ecosystems of the United States: a preliminary assessment of loss and degradation. U.S. Department of the Interior, National Biological Service, Biological Report 28:1–58.
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Reference S6. Tiner RW. 1984. Wetlands of the United States: current status and recent trends. , Washington, D.C.: U.S. Fish and Wildlife Service.
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Diane Eckles directed the mid-Atlantic Conservation Effects Assessment Project wetlands project initially and was instrumental in ensuring that at least one animal group was included in the multi-disciplined study. I am grateful to Tom Desterling for showing me around Delmarva and helping with aquatic sampling. I appreciate the editorial comments from Megan Lang, David Mushet, Charles Wewa, and Susan Walls on earlier versions of the manuscript. I thank the Associate Editor, and two reviewers for the Journal of Fish and Wildlife Management for helpful comments and suggestions that improved this manuscript. I am grateful to all the private landowners, The Nature Conservancy, the Natural Heritage Program in Delaware, and the U.S. Fish and Wildlife Service for allowing me to study amphibians on their lands. I followed “Guidelines for use of live amphibians and reptiles in field research” published in 1987 by the American Society of Ichthyologists and Herpetologists, The Herpetologists' League, and Society for the Study of Amphibians and Reptiles. Permits were provided by Delaware Department of Natural Resources, Maryland Department of Natural Resources, North Carolina Wildlife Resources Commission, and the Virginia Department of Game and Inland Fisheries.
This project was supported financially by funds from the US Department of Agriculture through NRCS. I declare no conflict of interest with this manuscript.
Any use of trade, product, website, or firm names in this publication is for descriptive purposes only and does not imply endorsement by the U.S. Government.
Citation: Mitchell JC. 2016. Restored wetlands in Mid-Atlantic agricultural landscapes enhance species richness of amphibian assemblages. Journal of Fish and Wildlife Management 7(2):490–498; e1944-687X. doi: 10.3996/092015-JFWM-085
The findings and conclusions in this article are those of the author(s) and do not necessarily represent the views of the U.S. Fish and Wildlife Service.
Present address: Florida Museum of Natural History, University of Florida, Gainesville, Florida 32611