Larval Anuran Stable Isotope Signatures and Stoichiometry
Across Multiple Geographically Isolated Wetlands in the
Southeastern United States
Carla L. Atkinson, Stephen W. Golladay, and Lora L. Smith
Southeastern Naturalist, Volume 16, Issue 1 (2017): 87–104
Full-text pdf (Accessible only to subscribers.To subscribe click here.)
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22001177 SOUTHEASTERN NATURALIST V1o6l(.1 1):68,7 N–1o0. 41
Larval Anuran Stable Isotope Signatures and Stoichiometry
Across Multiple Geographically Isolated Wetlands in the
Southeastern United States
Carla L. Atkinson1,*, Stephen W. Golladay2, and Lora L. Smith2
Abstract - Animals live in complex environments that vary spatially and temporally. This
heterogeneity strongly influences the availability and quality of food resources and has strong
impacts on growth and survival of consumers. Geographically isolated wetlands provide an
interesting system to study trophic relationships because they vary spatially and temporally
in hydrology and vegetation. Larval anurans play an important role in these wetland systems
because they are often the most abundant consumers. Yet, little is known about larval anuran
diet. Here we assessed the diet of 3 larval anurans (Acris gryllus [Southern Cricket Frog],
Hyla gratiosa [Barking Treefrog], and Lithobates sphenocephalus [Southern Leopard Frog]),
across 2 isolated wetland types (marsh and cypress savanna) using a stable isotope mixing
model, stable isotope analysis in R (SIAR). Furthermore, we assessed variation in basal resource
and anuran tissue stoichiometry (C:N). Our analyses suggested that larvae of these 3
species primarily function as herbivores and detritivores. All fed on a mix of algal resources,
detrital particulate organic matter, and litter originating from the wetland canopy. Barking
Treefrog had a lower C:N than the other two species, suggesting their dietary N requirements
may be greater. Understanding the trophic roles of these animals is essential in determining
their ecological significance and contributes to a more complete view of isolated wetlands in
the surrounding landscape.
Introduction
Understanding food-web structure and trophic relationships is central to ecology,
but can be difficult due to the complexity of natural communities. Organisms
live in spatially and temporally heterogeneous systems that can vastly differ in
resource abundance and productivity. Thus, food-resource availability and quality
interact to influence food and habitat selection by consumers and overall ecosystem
function (Marcarelli et al. 2011, Sitters et al. 2015). Furthermore, the availability
and quality of food resources can strongly influence the growth and survival of consumers.
Given this complexity, consumers often feed on a variety of resources that
may span trophic levels. Wetlands present a challenging environment to explore
these interactions because they vary in structure, hydrology (depth and permanence
of water), vegetation, and diversity and abundance of primary consumers even
when they are in close proximity to one another.
Larval anurans often play an important role in wetland food webs because they
can reach high densities and biomass, exhibit high per-capita consumption rates, and
1Department of Biological Sciences, University of Alabama, Tuscaloosa, AL 35487.
2J.W. Jones Ecological Research Center, Newton, GA 39870. *Corresponding author -
carlalatkinson@gmail.com.
Manuscript Editor: Brad Glorioso
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serve as important prey for other species (Gibbons et al. 2006, Pryor 2003). Phenotypic
plasticity of foraging behavior in response to resource variability can be one of
the principal causes of variation in growth and life-history parameters (Ghalambor et
al. 2007, Gotthard and Nylin 1995, Newman 1998, Nylin and Gotthard 1998). Both
the quantity and quality of food resources in wetlands can influence phenotypic expression,
determining the duration of larval stage and the timing of metamorphosis,
which are critical aspects of development (Hall and Wake 1999, Kupferberg 1997,
Stoler and Relyea 2013). Food quality influences growth and time of metamorphosis
of amphibians (e.g., Alvarez and Nicieza 2002, Pryor 2003, Skelly and Golon 2003),
and larvae often forage actively for high-quality resources, yet will change their
feeding and behavior in the presence of predators (Bestion et al. 2015, Lawler 1989,
Relyea and Werner 1999). Many amphibian larvae have been regarded as microphagous,
suspension-feeding herbivores, and detritivores (Altig et al. 2007, Duellman
and Trueb 1986, Whiles and Altig 2010) and viewed as feeding unselectively (Heyer
1976, Seale 1980), exhibiting little feeding-niche differentiation (but see Kupferberg
1997). While larval anurans are an abundant and diverse component of freshwater
wetlands, we still know relatively little about the trophic status and feeding ecology
of many species (Altig et al. 2007), especially of larval anurans in geographically
isolated wetlands, where diversity and biomass can be exceptionally high (Gibbons
et al. 2006, Leibowitz 2003), there are few grazing invertebrates (Battle and Golladay
2002), and amphibians likely provide significant transport of nutrient subsidies via
movements between wetlands and terrestrial habitats (Capps et al. 2015, Earl and
Semlitsch 2012). Previous studies (Opsahl et al. 2010, Verburg et al. 2007, Whiles et
al. 2006) have suggested a periphyton/larval anuran link, but this has not been examined
for individual species. Additionally, little has been done to examine the quality
of these resources.
Stable isotopes are commonly used in aquatic ecology to study food webs and
trophic structure (Finlay 2001, Post 2002, Vander Zanden and Rasmussen 1999).
The stable carbon isotope ratio (δ13C) of an animal’s tissues is a composite of the
food items assimilated by that individual (Fry 2006). Because 13C is a diet tracer
that is incorporated into body tissues with little fractionation, it has also been used
to examine inter- and intraspecific variations in diet (Hentschel 1998, Rossi et al.
2004). The nitrogen isotope ratio, as δ15N of tissues, also provides information
about an organism’s diet; because consumers are enriched in 15N by ~2–6‰ relative
to their food sources, δ15N provides useful information about trophic position (Post
2002, Vanderklift and Ponsard 2003).
Here we investigated the food resources larval anurans selected and assimilated
into their tissues in seasonally ponded, geographically isolated wetlands
in the southeastern US. Additionally, we examined anuran tissue stoichiometry
(C:N) across species and wetlands. We focused on 3 common species of anurans
across 2 wetland vegetation types (cypress savannas and marshes) in a relatively
undisturbed Pinus palustris Mill. (Longleaf Pine) forest in southwestern Georgia.
The goals of our study were to: (1) determine what food resources anurans were
primarily selecting and if these resources varied in quality, (2) discern if there was
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variation in diet across wetland types, and (3) assess whether there were differences
in tissue stoichiometry across species.
Field-site Description
Our study wetlands were located at Ichauway, an 11,500-ha privately owned
property of the Joseph W. Jones Ecological Research Center, in the Dougherty Plain
district of the Southeastern Coastal Plain physiographic province in southwestern
Georgia (Fig. 1). Ichauway has been managed primarily for forest and wildlife
since the 1920s and consists of multi-aged second-growth Longleaf Pine forest
with large areas of undisturbed native groundcover (Kirkman et al. 1999). Active
limestone dissolution and subsequent land subsidence in the region have resulted
in the development of numerous irregular-shaped geographically isolated wetlands
(Hicks et al. 1987). The wetlands are described as geographically isolated because
they are surrounded entirely by uplands and lack persistent and direct connections
to regional surface waters (Leibowitz 2003). The hydroperiod of wetlands in the
region is primarily controlled by precipitation, evapotranspiration, and shallow
groundwater (Kirkman et al. 1999); thus, the wetlands are intermittently inundated,
typically filling in late fall or winter and drying down in spring or summer. All wetlands
were inundated following heavy rains in March 2005 and remained flooded
throughout the study (May–August; Fig. 2). Wetlands began to dry down beginning
in August, and most were dry by September 2005. The climate in this region is humid
subtropical with an average annual precipitation of 132 cm and mean annual
low and high temperatures of 12.9 and 26.3 °C, respectively (University of Georgia
Weather Network 2016).
Prior classification of isolated wetlands based on characterization of vegetation,
soils, and hydrology, identified 3 vegetation types (i.e., marshes, cypress savannas,
and cypress gum swamps; Kirkman et al. 2000). Our study wetlands were limited
to marshes and cypress savannas due to greater anuran abundance in those wetland
types (Liner et al. 2008). Marshes are characterized by sandy soil, a treeless
open canopy, and diverse emergent vegetation dominated by a mixture of C3 and
C4 grasses and sedges (Opsahl et al. 2010). Cypress savannas are characterized
by sandy-clay to clay soil, an open canopy of Taxodium ascendens Brongn. (Pond
Cypress) and other C3 woody species, and an understory of mixed emergent vegetation
similar to marshes. The wetlands are surrounded by Longleaf Pine and Aristida
stricta Michx. (Wiregrass) forests that have been maintained with prescribed fire
(frequency of every 1–2 years) for the past 80 years.
Our focal anuran species were Acris gryllus (Leconte) (Southern Cricket Frog),
Hyla gratiosa Leconte (Barking Treefrog), and Lithobates sphenocephalus (Cope)
(Southern Leopard Frog). Larvae of the 3 species differ in body size and feeding
behavior. Larval Southern Cricket Frogs attain a total length of 42 mm and have
been described as benthic-feeding periphyton consumers (Altig and McDiarmid
2015, Altig et al. 1975). Southern Leopard Frog larvae range from 65 to 83 mm in
total length and are described as benthic omnivore/detritivores (Altig et al. 1975),
whereas larval Barking Treefrogs can attain 70 mm in total length and feed in the
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water column (Altig and McDiarmid 2015, Altig et al. 1975). All 3 species breed in
marshes and cypress savannas from winter through spring in the study area (Liner et
al. 2008, Smith et al. 2006) and were the most common species encountered during
the study.
Figure 1. The distribution of wetlands and the study wetlands in the landscape. The
wetlands were contained within Ichauway, a ~11,700-ha (~29,000-acre) area composed
primarily of Longleaf Pine and Wiregrass uplands. Land use surrounding Ichauway is
primarily center-pivot agriculture. Here we show the aerial view of the study area with the
study wetlands highlighted.
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Methods
Field sampling and sample processing
We selected 4 marshes and 3 cypress savannas (n = 7) for food-web analyses
of larval anurans, basal resources, and available invertebrates. The wetlands were
all in close proximity to one another (>10 km apart). While the cypress wetlands
were all on the western portion of the study area, we do not think this biased our
study. We sampled each wetland 3 times across the hydroperiod (Fig. 2) at ~6-
week intervals during 2005 (May 9–17, June 29–July 7, and August 16–23) for
basal resources and presumed primary and secondary consumers. Further, we
sampled these wetlands for water chemistry (dissolved organic carbon (DOC),
dissolved inorganic carbon (DIC), total dissolved carbon (TDC), ammonium
ion (NH4-N), nitrate (NO3-N), and soluble reactive phosphorus (SRP)) during
each collection date (see Supplemental Table S1, available online at http://www.
eaglehill.us/SENAonline/suppl-files/s16-1-S2311-Atkinson-s1, and, for BioOne
subscribers, at http://dx.doi.org/10.1656/S2311.s1). Basal resources included
periphyton (attached benthic algae and associated microbial communities), large
detrital material (CPOM: leaves and other plant detritus >1000 μm), and fine particulate
organic matter (FPOM; material of undetermined source <1000 μm and
>45 μm) that was suspended in the water column (see Supplemental Table S2,
available online at http://www.eaglehill.us/SENAonline/suppl-files/s16-1-S2311-
Atkinson-s1, and, for BioOne subscribers, at http://dx.doi.org/10.1656/S2311.s1).
We sampled periphyton with periphytometers, consisting of a plastic slide holder
Figure 2. The hydroperiod of the wetlands sampled during the study. The gauge height of
each wetland was recorded 8 times (except P51, which was recorded 5 times), biweekly
from 25 May 2005 to 30 August 2005 and converted to the percent total volume of inundation
of the wetland.
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fitted with 5 pre-ashed glass microscope slides, placed in each wetland ~3 weeks
prior to sampling. We collected POM by sieving water from open areas at least 10
m away from the shoreline. CPOM was primarily composed of senescent grasses
(Poaceae) and sedges (Cyperaceae) in the marshes and grasses, sedges, and Pond
Cypress leaves in the savannas. Three replicate samples were taken of each basal
resource at each time period. The stable isotope signatures and C:N of basal resources
did not significantly vary among the sampling periods (ANOVA with time
period as the factor and food resource as a blocking factor; δ13C: P = 0.08, δ15N:
P = 0.97, C:N: P > 0.06); thus, we averaged basal resources within resource category
across the 3 sampling periods. Our rationale for selection of potential basal
resources was based on the dominant sources of nutrition available in the wetlands
and feeding modes known for larval anurans (Altig and McDiarmid 1999).
A broader characterization of wetland primary producer isotopic composition has
been previously published (Opsahl et al. 2010).
Larval anurans were captured using 30-cm D-frame dip nets with 3-mm mesh
from all wetlands (sample sizes shown in Table 1). We attempted to collect larval
anurans each time, but all species were not present at each wetland on each date. Macroinvertebrates
were collected using dip nets with mesh sizes ranging from 500 μm to
5 mm and sorted in the field to compare trophic level with larval anurans. All samples
were immediately frozen at -20 °C, then dried at 50 °C and weighed for isotopic analysis.
We used whole specimens for macroinvertebrates and larval anurans, and only
grouped multiple individuals per wetland in cases where individuals provided insufficient
mass for analyses. Further details on field collection and sample processing
of detrital plant material, particulate organic matter, periphyton, larval anurans, and
macroinvertebrates have been previously described by Opsahl et al. (2010).
Table 1. Results showing the mean percent contribution (and 95% confidence intervals) to larval
anuran diet for each food resource and wetland from the SIAR mixing model for 3 species: Hyla
gratiosa (Barking Treefrog), Acris gryllus (Southern Cricket Frog), and Lithobates sphenocephalus
(Southern Leopard Frog).
Wetland type/ Mean % contribution to diet (95% CIs)
Wetland Species n Periphyton FPOM CPOM
Marsh
P15 H. gratiosa 5 43.9 (33.9–53.8) 42.6 (32.4–52.8) 13.5 (5.0–19.6)
P46 A. gryllus 4 38.2 (27.3–48.8) 27.0 (17.1–36.2) 34.8 (22.4–45.9)
H. gratiosa 9 30.7 (21.1–40.5) 43.1 (37.5–48.8) 26.2 (14.3–37.0)
P51 H. gratiosa 5 32.7 (18.4–44.6) 33.4 (19.2–45.4) 33.9 (19.8–46.1)
P53 H. gratiosa 10 40.3 (34.8–46.1) 33.8 (27.1–40.8) 25.9 (16.4–34.8)
L. sphenocephalus 3 46.5 (37.1–56.5) 24.1 (13.1–33.6) 29.4 (17.1–40.4)
Cypress Savanna
P27 L. sphenocephalus 3 38.9 (27.7–49.4) 29.7 (16.1–41.1) 31.4 (19.6–41.9)
P41 A. gryllus 3 60.3 (44.5–78.0) 20.9 (7.5–31.9) 18.7 (6.3–28.5)
H. gratiosa 3 75.0 (66.6–85.8) 13.7 (4.3–20.1) 11.3 (4.2–16.1)
L. sphenocephalus 4 63.7 (51.9–77.1) 20.8 (8.0–31.2) 15.4 (5.5–22.2)
P97 H. gratiosa 5 64.3 (48.6–79.3) 19.9 (6.9–29.1) 15.8 (5.6–23.0)
L. sphenocephalus 3 44.3 (37.2–50.8) 33.0 (23.8–42.9) 22.7 (10.2–33.5)
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Samples were analyzed for total C and N and δ13C and δ15N at the University of
Georgia’s Ecology Analytical Laboratory. The analytical precision for the stable
isotope measurements was ±0.1 for δ13C and ±0.2 for δ15N. To quantify the quality
of the basal resources, we calculated the molar C:N ratios of periphyton, FPOM,
CPOM, and larval anurans. Furthermore, we corrected for lipid content of the larval
anuran tissue when C:N exceeded 3.5, as recommended by Post et al. (2007). Following
this approach, we normalized δ13C values to account for lipid variation in
δ13C following the equation in Caut et al. (2013) derived for amphibian tadpoles:
δ13Cnormalized = -1.11 (δ13Cuntreated) + 0.37(C:N). Our statistical analysis was not limited
to composite samples of all species by wetland type as in Opsahl et al. (2010).
Our analysis was expanded to include 3 individual species per wetland: Barking
Treefrog, Southern Cricket Frog, and Southern Leopard Frog. Based on the δ15N
signatures of larval anurans in comparison to both basal resources and macroinvertebrates
in the wetlands (Fig. 3), we estimated the potential contribution of basal
food resources to larval anurans.
Statistical analysis and isotope mixing model
We used a MANOVA to test for differences in the isotope signatures (δ13C and
δ15N; dependent variable matrix) across the wetland types (marsh vs. cypress savanna)
and resource types (periphyton, FPOM, CPOM). To determine if there was
a statistically significant difference in the quality (C:N) of basal resources available
to larval anurans, we used a 2-way ANOVA with resource type and wetland type as
main effects. We also used a 2-way ANOVA (without an interaction) to determine
if C:N of larval anurans varied across species and wetland types (means per species
across wetland types). Significant ANOVAs were followed by Tukey’s HSD multiple
comparisons to determine if significant differences existed among resource
types and/or wetland types (α = 0.05).
We used a Bayesian isotope mixing model in the stable isotope analysis in R
(SIAR, v3.2.0) to determine the source contributions to larval anuran diet using
lipid-corrected data. SIAR projects probability distributions of sources to
consumer diets while accounting for uncertainty in source isotope signatures
and in trophic fractionation (Moore and Semmens 2008). Bayesian mixing
models allow the variation and uncertainties associated with isotopic estimates
and trophic enrichment to be propagated through the model, with outputs being
more reflective of the natural variability within a system. The SIAR model
is fit via Markov chain Monte Carlo methods producing simulations of source
dietary proportions using a Dirichlet prior distribution (Parnell et al. 2010). The
probability distributions are estimated using a sampling–resampling algorithm.
We used average fractionation for primary consumers ± the standard deviation
values (δ13C: 0.4 ± 1.2; δ15N: 3.4 ± 1.61) calculated from multiple stable isotope
studies (Post 2002, Vanderklift and Ponsard 2003). We used periphyton, CPOM,
and FPOM as potential basal resources in the mixing models because our δ15N
values, which are indicative of trophic level, have a similar trophic position as
primary conumer aquatic macroinvertebrates (Figs. 3, 4). Additionally, previous
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studies indicate that larval anurans are primarily herbivorous (Altig et al. 2007,
Whiles and Altig 2010). We averaged the basal resources through time because
δ13C, δ15N, and C:N did not vary with sampling date. We ran the SIAR mixing
model for 500,000 iterations, discarding the first 50,000 samples. The resulting
distributions of probability–density functions of feasible foraging solutions produced
by SIAR allowed direct identification of the most probable solution for the
sources supporting each species within a wetland (Parnell et al. 2010). We used
upper and lower 95% confidence intervals to describe the contribution for each
diet item (Phillips and Gregg 2003).
Results
Isotope signatures
Stable isotope signatures varied across basal resource type (Fapprox = 17.8, df = 4,
P < 0.0001) and between wetland types (Fapprox = 30.4, df = 2, P < 0.0001), and there
was an interaction between wetland type and resource type (Fapprox = 4.1, df = 4,
P = 0.003) (Figs. 3, 4). Larval anurans were enriched in δ15N by 0.52–5.6‰ relative
Figure 3. Graphs of the raw isotope signatures of the food resources and organisms sampled
in marsh wetlands. Shown are the average (mean ± standard error) isotope signatures of
the basal resources (periphyton, fine particulate organic matter, and coarse particulate matter),
larval anurans (not lipid corrected), crayfish, and predatory insects (e.g., Libellulidae,
Dytiscidae, Anisoptera).
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Figure 4. Graphs of
the raw isotope signatures
of the food resources
and organisms
sampled in cypress
savanna wetlands.
Shown are the average
(mean ± standard error)
isotope signatures
of the basal resources
(periphyton, fine particulate
organic matter,
and coarse particulate
matter), larval
anurans (not lipid corrected),
crayfish, and
predatory insects (e.g.,
Libellulidae, Dytiscidae,
Anisoptera).
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to periphyton, by 0.46–3.10‰ relative to FPOM, and by 0.22–2.29‰ relative to
CPOM across the wetlands (Figs. 3, 4). Furthermore, among macroinvertebrates,
crayfish (omnivores) and predatory insects, respectively, had a higher δ15N than
larval anurans.
C:N
We found a significant difference in C:N among basal resource types (F2, 176 =
18.6, P < 0.0001; Fig. 5), but resource C:N did not vary across wetland types (F1, 176
= 0.034, P = 0.85). Our Tukey’s HSD test showed that periphyton (P < 0.0001) and
FPOM (P < 0.0001) were different (lower C:N) than CPOM, but these 2 resources
did not differ in C:N from one another (P = 0.936). The C:N of larval anurans varied
across species (F2,57 = 12.14, P < 0.001), but not between wetland types (F1,57 =
0.207, P < 0.70). Barking Treefrog had a significantly lower C:N than both Southern
Cricket Frog and Southern Leopard Frog ( P < 0.004; Fig. 6).
Mixing models
Across wetland types, periphyton was generally the dominant resource assimilated
by larval anurans (Table 1). In the marshes, periphyton comprised 38.2% of
the diet in Southern Cricket Frog (site P46), 30.7–43.9% of the diet in Barking
Treefrog (all 4 sites), and 46.5% of the diet in Southern Leopard Frog (site P53). In
the cypress savannas, periphyton comprised 60.3% of the diet in Southern Cricket
Frog (site P41), 64.3–75.0% of the diet in Barking Treefrog (sites P41 and P97), and
38.9–63.7% of the diet in Southern Leopard Frog (all 3 sites). CPOM and FPOM
assimilation varied across wetlands and species, but generally represented an important
component in the diet of larval anurans (T able 1).
Figure 5. Stoichiometry of the larval anuran food resources. The C:N of each of the basal
food resources sampled. CPOM (n = 63) had a significantly higher C:N than both periphyton
(n = 59) and FPOM (n = 55), while periphyton and POM did not significantly differ.
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Discussion
Larval anurans likely are an important link in wetland food webs (Gibbons et al.
2006, Opsahl et al. 2010). Our results suggest that larvae of each of the 3 anuran
species obtained significant nutrition from algal-derived food (i.e., periphyton), but
FPOM and, to a lesser degree, vascular plant detritus (CPOM) also are potential
food resources. In comparison to other consumer taxa sampled during this study
(e.g., crayfish and predatory insects), larval anurans appear to be primary consumers
in these isolated wetland systems based on their relatively low δ15N. We were
not able to detect any other potential food resources. Our results suggest that larval
anurans likely feed selectively on periphyton and FPOM because they are higherquality
resources (lower C:N) and tend to have more microbes associated with them
than CPOM (Atkinson et al. 2009, Bonin et al. 2000, Kamauchi 2005). Previous
work has indicated that Barking Treefrog is primarily a pelagic feeder, whereas
Southern Cricket Frog and Southern Leopard Frog are both benthic feeders (Altig
and McDiarmid 2015, Altig et al. 1975). Our data suggest there is some difference
in diet among these 3 species, but all seem to feed on a mixture of the available
food resources.
Figure 6. The C:N of the 3 larval anuran species included in the study varied. Hyla gratiosa
(Barking Treefrog; n = 37) had a significantly lower C:N than A. gryllus (Southern Cricket
Frog; n = 7) and L. sphenocephalus (Southern Leopard Frog; n = 13).
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Through their foraging activities, larval anurans are contributing to organicmatter
processing by ingesting and recycling materials in the wetlands (Seale
1980). Studies on larval anurans in other systems have shown that there is little
functional redundancy between larval anurans and invertebrates in aquatic ecosystems
(Colon-Gaud et al. 2009, 2010; Whiles et al. 2013); thus, larval anurans are
the main primary consumers across multiple wetland systems. Within these geographically
isolated wetlands, there also seems to be little functional redundancy;
our results show that larval anurans are acting as grazing primary consumers (as
indicated by their low δ15N in comparison to crayfish and other invertebrates),
whereas most of the aquatic invertebrates are filter-feeders and predators (Battle
and Golladay 2001a, 2002; Opsahl et al. 2010). While other studies have shown
evidence of omnivory or ontogenetic diet shifts in anuran larvae (Schiesari et
al. 2009, Schriever and Williams 2013), our results suggest that larval anurans
in geographically isolated wetlands of the southeastern Coastal Plain are mainly
functioning as primary consumers. Seasonal wetlands are low-nutrient systems (see
Supplemental Table S1, available online at http://www.eaglehill.us/SENAonline/
suppl-files/s16-1-S2311-Atkinson-s1, and, for BioOne subscribers, at http://dx.doi.
org/10.1656/S2311.s1; Atkinson et al. 2011; Battle and Golladay 2001b, 2003), and
the recycling of nutrients bound in plant materials by larval anurans may be a critical
process in maintaining wetland productivity (Vanni 2002).
Reproductive output of amphibians in geographically isolated wetlands is
considerable, and following metamorphosis, amphibians often move considerable
distances (hundreds of meters) between breeding wetlands and upland habitats
(Gibbons 2003, Scott et al. 2013, Semlitsch and Bodie 2003, Smith and Green
2005); thus, pulsed migration of anurans may generate a substantial flow of nutrients
and energy (Earl and Semlitsch 2012, Seale 1980). For example, Gibbons et
al. (2006) showed that a single 10-ha isolated wetland produced >360,000 metamorphic
amphibians (>1400 kg of biomass) in 1 year, suggesting that energy and
nutrient subsidies by amphibians may be greater than previously realized. The
anuran species we studied emerge from the wetlands as terrestrial juveniles and
have the ability to move long distances (Semlitsch and Bodie 2003). Our results
indicate that the majority of their diet as aquatic larvae originates from wetlandderived
periphyton and FPOM. Following metamorphosis, anurans transport a
substantial amount of energy and nutrients (as biomass) out of the wetland into the
adjacent terrestrial habitat. The surrounding terrestrial Longleaf Pine landscape is
nutrient-poor in comparison to the abundant and often aggregated wetlands (Craft
and Chiang 2002, Martin et al. 2012), suggesting that migrations of anurans could
contribute significant subsidies to support landscape heterogeneity. Furthermore,
southeastern species tend to breed throughout the year and are less predictable than
northern populations (Pechmann et al. 1991), thus further enhancing the heterogeneity
of the movement of energy and nutrients.
Our study also showed that larval amphibians vary in their tissue C:N, which is
indicative of nutrient retention and not necessarily diet (Sterner and Elser 2002).
Barking Treefrog had a lower C:N than both Southern Cricket Frog and Southern
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2017 Vol. 16, No. 1
Leopard Frog, indicating it may have a higher dietary requirement for N. This finding
suggests that different species may have different nutrient requirements as has
been demonstrated in other studies (Burton and Likens 1975, Seale 1980). Surprisingly,
the 2 Hylidae species (Southern Cricket Frog and Barking Treefrog) in the
study were different in their C:N. Whether the differences we noted in C:N was
species-specific variation or due to differences in developmental stages of the larvae
we sampled (Capps et al. 2015) is unclear. Future research is needed to explore
the variability in tissue nutrient content among species to document the potential
role of the diverse suite of amphibians in wetlands and the surrounding landscape.
While we did not investigate ontogenetic changes in resource use and assimilation,
previous studies have shown that larval anurans can vary their diet across
developmental stages (Trakimas et al. 2011), but not in all cases (Schriever and Williams
2013). Capps et al. (2015) showed that larval Lithobates sylvaticus (LeConte)
(Wood Frog) undergo ontogenetic changes in tissue stoichiometry with developmental
stage such that they increase in phosphorus content as they grow. Thus, the
availability and quality of food and nutrients are likely important constraints on
development and metamorphosis of amphibians. This insight has important implications
regarding amphibian nutrient requirements and how those demands are met.
Further studies incorporating tissue stoichiometry and food selection are needed to
better understand production and population dynamics of amphibi ans.
Diet in larval anurans can also fluctuate in response to intraspecific density, interspecific
competition, food availability, predation, and water levels (Arribas et al.
2015, Bestion et al. 2015, Relyea and Werner 1999, Whiles et al. 2010). The ability
of anuran larvae to have a large trophic niche breadth demonstrates their capacity
for exploiting widely divergent or fluctuating conditions in isolated wetlands.
Shifts in the trophic niche of larval amphibians may strongly affect the structure
and function of the food web of isolated wetlands. Yet, our results should be taken
with some caution. Previous work has shown that gut clearance for stable isotopes
is not necessary with primary consumers (Jardine et al. 2005), but we did not allow
the gut contents to fully evacuate before freezing, thus potentially leading to
some bias. While this study revealed variability in diets of 3 larval anuran species
co-occurring in wetlands, it also highlights the need for additional research on the
trophic status of other larval anurans in these species-rich si tes.
Given the declines in both amphibians (Stuart et al. 2004) and the wetland habitats
they depend upon (Batzer and Baldwin 2012), it is essential to characterize their
habitat and feeding requirements in developing conservation strategies. Approximately
50% of the remaining geographically isolated wetlands in the southeastern
US have been dramatically altered (Lane et al. 2012), and these wetlands are not
protected under the US Clean Water Act, which has resulted in consequential reduction
in habitat (Leibowitz 2003). Declines in amphibian abundance and diversity
are occurring worldwide, yet little is known about the ecological consequences of
these losses (Stuart et al. 2004). Studies have shown that primary production, nutrient
cycling, and leaf-litter decomposition are affected by larval (Ranvestel et al.
2004, Rugenski et al. 2012) as well as juvenile and adult frogs (Beard et al. 2002).
Southeastern Naturalist
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2017 Vol. 16, No. 1
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Understanding the overall trophic status of larval anurans in isolated wetlands is
central to determining their ecological role and significance in the larger landscape.
Acknowledgments
Many people have supported this project, and it is with deep appreciation that we acknowledge
their contributions. S. Allums, T. Muenz, J. Fouke, M. Kaeser, B. Clayton, A.
Farmer, G. Miller, D. Steen, B. Cloninger, L. Cox, J. Brock, and J. Warren assisted in the
execution of this study. We appreciate the comments from K. Capps and B. van Ee on a
previous version of this manuscript. Research was conducted under Georgia Department of
Natural Resources Scientific Collecting Permit #29-WBH-09-151. Funding for this project
was provided by the Joseph W. Jones Ecological Research Center.
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