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E.J. Tobin, J.M. Visser, J.K. Peterson, and P.L. Leberg
22001144 SOUTHEASTERN NATURALIST 1V3o(3l.) :1436,3 N–4o7. 43
Small-Mammal Occupancy in Freshwater Marshes of
Mandalay National Wildlife Refuge, Louisiana
Eric J. Tobin1,2,*, Jenneke M. Visser1,3, James K. Peterson3, and Paul L. Leberg2
Abstract - Small mammals are key consumers in the marsh food web and could serve as
indicators of a marsh’s potential to support higher-level predators. We studied how smallmammal
occupancy varied among plant communities in coastal Louisiana freshwater
marshes. We sampled small mammals at 36 sites on 4 different occasions during the late
spring in freshwater marshes of the Mandalay National Wildlife Refuge, LA. Mammalian
diversity was low; we captured only Oryzomys palustris (Marsh Rice Rats). Occupancy
modeling revealed a positive association between Marsh Rice Rat site occupancy and Sagittaria
lancifolia (Bulltongue Arrowhead) biomass. Our data suggest that subtle changes in
plant-species composition within a marsh may affect the distribution of the most common
small mammal in the ecosystem.
Introduction
Coastal Louisiana has undergone drastic habitat modification during the last
century, including major conversion of wetlands to open water (Barras et al. 2008,
Mitsch et al. 2009). In particular, freshwater marshes in Louisiana have experienced
major shifts in plant-species composition (Visser et al. 1999). Driving factors
behind these changes include water-level increase, salinity alterations, grazing
behavior by native and invasive species, lack of particulate deposition, and oil and
gas extraction activities (Gosselink et al. 1998, Penland et al. 2001, Visser et al.
2012). In Louisiana’s Terrebonne Basin, these complex and interacting environmental
changes have lead to a shift in freshwater-marsh vegetation types, such as
the replacement of Panicum hemitomon Schul. (Maidencane) with Eleocharis spp.
(spikerush) as the dominant plant (Visser et al. 1999).
Wildlife investigations and management activities in coastal wetlands are often
concerned with species responses across salinity zones due to a lack of fine-scale
habitat-use data (e.g., King and Michot 2002, Miller et al. 2009, Plattner et al. 2010,
Ysebaert et al. 2000). For wildlife management purposes, the Louisiana coastal
zone is divided into 5 habitats based on salinity: Taxodium sp. (cypress)–Nyssa sp.
(tupelo) swamp, and fresh, brackish, intermediate, and saline marshes (Palmisano
1972). Although cypress–tupelo swamps are classified as freshwater wetlands,
they are distinct from the other coastal-zone habitats because they are forests.
Vegetation-community structure varies widely within each of these marsh habitats;
Bulltongue, Maidencane, and Spikerush are all types of freshwater marshes (Visser
1Institute for Coastal Ecology and Engineering, University of Louisiana at Lafayette, Lafayette,
LA 70504. 2Department of Biology, University of Louisiana at Lafayette, Lafayette, LA
70504. 3School of Geosciences, University of Louisiana at Lafayette, Lafayette, LA 70504.
*Corresponding author - EricTobin09@gmail.com.
Manuscript Editor: Andrew Edelman
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et al. 1999, 2002) and each has a distinct species composition, structure, function,
soil, and hydrology (Sasser et al. 1996). Under current wildlife management
practices (e.g., Alligator mississippiensis Daudin [American Alligator] harvest
regulation), however, they are all placed in the same, homogenous classification of
freshwater marsh habitat.
Pioneering work has been done to define the differences in vegetation types
in Louisiana’s freshwater marshes, but little research has focused on the smallmammal
communities of these habitats. To date, only Abernethy et al. (1985) and
Martin et al. (1991) included freshwater marshes in assessments of small-mammal
diversity and abundance; however, these studies were not designed to examine
the influence of freshwater-marsh vegetation composition on small-mammal communities.
Small mammals play important roles in many ecosystems, such as seed
dispersal and insect control (Lowery 1974, Ryszkowski 1975). In freshwater-marsh
ecosystems, rodents serve as primary, secondary, and tertiary consumers and as
prey in both terrestrial and aquatic food webs (Eubanks et al. 2011, Gosselink et al.
1998, Lowery 1974, Negus et al. 1961). In the case of omnivorous rodents, their
variable trophic position likely allows for both top down and bottom up influence
on other species’ populations.
Our objective was to determine if variation in vegetation composition of freshwater
marshes in coastal Louisiana resulted in differences in small-mammal site
occupancy. Due to the low abundances of small mammals detected in Abernethy et al.
(1985) and Martin et al. (1991), and the disturbance caused to floating marsh habitats
by a large number of visits to the same site, we examined site occupancy instead of
measuring population densities. Occupancy modeling is a fairly new statistical tool
that estimates the probability that a species occurs at a site (MacKenzie et al. 2006),
and is well suited for application to the noncontiguous Louisiana freshwater marshes
that are interspersed with canals, rivers, lakes, and ridges. Occupancy modeling also
allows the inclusion of covariates such as vegetation, which is useful for comparing
marsh features associated with the occurrence of small mammals.
Methods
We selected Mandalay National Wildlife Refuge (MNWR) as the study area
because of its abundant freshwater marshes and relative ease of access. The refuge
is situated roughly 8 km west (29°31'12.44"N, 90°48'03.21"W) of Houma, LA
(USFWS2009). The climate is subtropical, with an annual average rainfall of 165
cm. The influx of freshwater from the Atchafalaya River results in classification
of MNWR as a freshwater system. Periodic flooding from the Atchafalaya River
deposits sediments and increases water level in MNWR swamps and marshes. Additional
freshwater comes via run-off from adjacent uplands and local streams. As
with the rest of Louisiana’s coastal marshes, climate change and hydrologic alterations
are affecting the refuge, leading to land loss and salt-water intrusion, with
catastrophic losses predicted for the near future.
Cypress swamps, freshwater marshes, upland ridges, Lake Hatch, and oil-field
canals are found on the refuge, and the Gulf Intracoastal Waterway crosses the
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parcel. Marshes are generally dominated by Sagittaria lancifolia L. (Bulltongue
Arrowhead) to the north of the waterway and Maidencane to the south. The refuge
is primarily managed for migratory and endangered birds. Faunal species diversity
of the refuge has not been described, but is likely similar to the species list for the
freshwater marshes and swamps of the Barataria-Terrebonne Estuary—another
freshwater wetland system in the region (Condrey et al. 1995).
We randomly selected 36 navigable sites for live trapping on MNWR using
ARCGIS (version 9.3.1), and we conducted an on-site determination of our ability
to traverse the marsh to set and retrieve traps. To allow access by boat, all of the
sites we selected for study were near canals. Due to the structural and ecological
fragility of floating freshwater marshes, we elected to place traps in a circular
layout (Fig. 1) because occupancy modeling relies only on detection and not abundance
measurements. This modification of a traditional linear trap-line is similar to
the point-count method commonly used when sampling birds for occupancy studies
(MacKenzie et al. 2006).
We employed standard (233 mm x 75 mm x 90 mm) live traps (model Folding
Trap-LFA, Sherman Trap Company, Tallahassee, FL) to non-lethally sample
small mammals, and selected quick oats as bait because they afforded the best
capture rate with the least attraction of Solenopsis invicta Buren (Red Imported
Fire Ants ) in a pilot study. We used Velcro to attach traps to 41-cm2 platforms.
Figure 1. Diagram of the general trap arrangement at small-mammal sampling sites in freshwater
marshes of the Mandalay National Wildlife Refuge, LA.
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The platforms consisted of plywood and foam layers that we glued together,
similar to the designs used by Desa et al. (2012), Hotaling et al. (2010) and Wolfe
(1985), but designed independently. We cleaned the traps weekly to prevent disease
and rancid odors and allowed them to air dry for at least 36 h before being
used again. At each site, we evenly spaced 10 traps along a 16-m-diameter circle
around a center point that was a minimum of 20 m into the marsh from the canal.
We marked the plot center with a PVC pipe and recorded the location using a GPS
unit. At each site, we installed traps in the afternoon and retrieved them early the
following morning to exploit the nocturnal nature of small mammals and prevent
the overheating of trapped animals.
We deployed a total of 360 traps (10 per site) in each of 4 sampling periods, for a
total of 1440 trap-nights during mid-May–mid-June 2011. All sites were trapped for
one night within each of the sampling periods, with a minimum of 4 and a maximum
of 5 days between samples. The number of sites sampled and sampling occasions
was a compromise between the need for statistical power and the resources (personnel,
traps, boat, and truck) available. Due to the fragile nature of floating marsh, we
limited the number of visits so that we measured small mammal and plant interaction
rather than researcher-induced habitat changes. Floating marsh root mats eventually
give way to open water if traversed too often. We placed sites a minimum of 200 m
apart unless there was a large (>8 m cross section) canal between them that we assumed
would be a barrier to animal movement. We considered movement across
large canals to be as unlikely as movement of >200 m because of high levels of boat
traffic and the presence of American Alligators in the canals. We made our site placements
so as to provide the maximum number of sites in the limited study area with
little likelihood of movement of small mammals between sites.
The number of sites sampled every day was determined by the average time
needed to check them in the morning before the animals became too stressed by
the heat. We sampled during the early summer, and daily high temperatures averaged
32 °C, daily low temperatures averaged 22 °C, and cumulative precipitation
was 0.58 cm (http://www.nws.noaa.gov/climate/). Water levels remained relatively
constant; however, we observed acute increases after periods of heavy precipitation
associated with thunderstorms. We emptied all traps placed on a given day between
sunrise (approximately 06:30) and 10:00 the following day. We strived to sample 10
sites per day, but sampled fewer sites on days with inclement weather. The timing
of site visits was structured to allow the sites to recover from researcher disturbance
between samples and to satisfy the assumptions of occupancy modeling: the area
does not change between sample periods, animals are not harmed by sampling, the
population remains closed, there is no movement between sites, sites do not become
colonized or extinct, each site has the same occupancy and capture probability, and
each site is homogeneous (MacKenzie et al. 2006).
We photographed captured animals and determined species, body metrics,
gender, and age characteristics. Before release, we applied an ear tag to identify
previously captured individuals to help us determine if movement occurred
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between sites. Recaptured animals were examined for any substantial change from
the previous sampling, such as bodily injury or change in reproductive status.
We harvested vegetation 14–18 June at five 0.25-m2 plots, located at every other
trap station at each site (a total of 180 stations), placed samples in black plastic
bags, and took them back to the laboratory, where they were refrigerated. We sorted
the collected aboveground biomass by species, stored samples in brown paper bags,
and dried them at 70 °C for a minimum of 48 hours. Species dry weight was determined
by weighing the bag with the sample to the nearest 0.1 g; the weight of the
bag was subtracted from this measurement to obtain each species’ dry weight. We
multiplied all weights by 4 to transform the sample weight to the dry weight per
m². We obtained total aboveground biomass for each plot by averaging the weights
of the species harvested from that plot (Evers et al. 1998).
The program PRESENCE (version 6.1) (available from http://www.mbr-pwrc.
usgs.gov/software/presence.html) was used to conduct occupancy modeling of
small mammals across sites (MacKenzie et al. 2006). In the occupancy model,
two parameters were estimated from covariates and occupancy data. The first parameter
estimated is psi (Ψ), the probability of a site being occupied. The second
parameter estimated was probability of detection at a site (p). We used a single-species,
single-season model that held probability of detection constant across sites and
sampling sessions. Assuming a constant detection probability was reasonable because
trap placement, time of deployment, and baiting were standardized, while weather
conditions varied randomly across sites and the sampling season. The equation for
determining Ψ for a single-species, single-season model with covariates is: logit(Ψi)
= β0 + β1xi1 + β2xi2 + ... + βuxiu, where β0 represents intercept, βu represents covariates
associated with site i, and xi are the values for the covariates at site i. PRESENCE
evaluated null, full, and covariate models using the biomass of each of the 5 most common
plant taxa as the covariates, both singularly and with additive effects, for a total
of 31 models. We did not include climatic covariates because they tended to either be
relatively invariant, such as temperature, or too spatially heterogeneous to monitor at
each sample site, such as rainfall. Model averaging was used to estimate parameter and
unconditional standard errors (Burnham and Anderson 2002).
We used using Program R (Version 2.12.1) (R Foundation for Statistical Computing)
to perform spatial autocorrelation on biomass of each of the 5 most common taxa
to determine the spatial independence of sites. Spatial autocorrelation uses distance
and site characteristics to determine if sites in a given system fall out into spatial
clusters (Lichstein et al. 2002). We also evaluated whether capture of individual
mammals was due to spatial proximity of sites, using a Mantel test (XLSTAT 2010,
New York, NY). We compared a matrix of similarity of sites based on capture status
to a matrix of geographic distance between sites; statistical significance of the matrix
association was calculated by running 10,000 permutations.
Results
The only small-mammal species we captured was Oryzomys palustris Harlan
(Marsh Rice Rat). Marsh Rice Rats were captured at 21 of the 36 sampled sites, for
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a total of 34 individuals captured during the study. We recaptured only 3 individuals:
2 at the same site of their initial capture and one that had traversed a floating
marsh and one small canal to move 1.5 km between sites over a 5-day period. The
individual that had moved was a juvenile male, and it is likely that this movement
is indicative of the lack of male philopatry rather than overall species behavior.
The captured Marsh Rice Rats included 7 females, 25 males, and 2 individuals that
escaped before being sexed. We classified 3 juveniles, 6 sub-adults going through
molts, and 25 adults.
We collected 33 plant species across the refuge. We identified 5 dominant plant
taxa that made up the majority (89%) of the live biomass and occurred at the majority
of the sites (Fig. 2): Alternanthera philoxeroides Grisebach (Alligatorweed),
Bulltongue Arrowhead, spikerush, Leersia oryzoides L. Swartz (Rice Cutgrass), and
Typha spp. (cattail). Alligatorweed was the most common plant and was found at every
site. The most dominant species were cattail and Bulltongue Arrowhead, at 42%
and 28% of total biomass, respectively. Alligatorweed, spikerush and Rice Cutgrass
made up 10%, 4%, and 4% of total biomass, respectively. We found no significant
correlations (Pearson’s r ≤ 0.1018) between the biomass of the 5 most common plant
taxa, so we used the biomass of each of these 5 taxa in subsequent analyses.
The results of the spatial autocorrelation analysis showed that all sites were
spatially independent (Fig. 3); strong positive autocorrelation only occurred within
Figure 2. Dry live biomass of the 5 most common plant species at each site sampled in the
Mandalay National Wildlife Refuge. The 36 sites are listed left to right by decreasing number
of Marsh Rice Rats captured, then by decreasing Bulltongue Arrowhead biomass, and
last by increasing cattail biomass. Bars represent the average from 5 plots.
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2014 Vol. 13, No. 3
the first 200 m for the biomass of most plant species among sites. The results of the
Mantel test indicated no association between the distance among sites and Marsh
Rice Rat captures (r = -0.01, P = 0.787).
We ran occupancy models with null, full, singular, and additive effects of the
5 most abundant plant taxa. After removing models in which SE could not be estimated
for 1 or more parameters, the highest ranked model, based on the lowest
Akaike information criterion corrected for small sample sizes (AICc), retained
Bulltongue Arrowhead as the only covariate (Table 1). A goodness-of-fit test of the
saturated model (χ² = 27.9, P ≥ 0.02) suggested that the data were overdispersed.
However, the variance-inflation factor was relatively small (ĉ = 1.93) compared to
an expectation of 1. Burnham and Anderson (2002) suggested that there is no need
to adjust AICc values for overdispersed data if ĉ is between 1 and 4, so we present
unadjusted estimates of AICc. Results of model averaging returned a βS. lancifolia of
15.0 (SE calculated from unconditional variances = 8.0), which indicated a positive
association between Marsh Rice Rat occupancy and Bulltongue Arrowhead
biomass. The lowest level of Bulltongue Arrowhead biomass observed in this study
offered a 100% probability of occupancy. There was little support for other covariates
containing information on Marsh Rice Rat occupancy because model-averaged
parameter estimates for all other vegetation covariates were near zero with large
Figure 3. Spatial autocorrelation of the 5 most common plant species (Alligatorweed, Rice
Cutgrass, spikerush, Bulltongue Arrowhead, and cattail) on sampled sites in Mandalay National
Wildlife Refuge, LA (n = 180). Spatial lag (in meters) is plotted against covariance
for each site.
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Table 1. Results of occupancy modeling for the presence of Marsh Rice Rats in freshwater marshes of coastal Louisiana. Only the 5 most common vegetation
species were used in the analysis. Included on the table are the 15 most informative models ordered by AICc. AIC is corrected for small sample sizes
(AICc), Model likelihood is AIC weight of the given model relative to the top model, and -2LL is 2 times the negative log-likelihood.
Model covariates AICc ΔAICc AIC weight Model likelihood # of parameters -2LL
Sagittaria lancifolia 149.69 0.00 0.2837 1.0000 2 145.60
S. lancifolia + Leersia oryzoides 151.04 1.35 0.1511 0.5326 3 144.86
S. lancifolia + Eleocharis spp. 151.62 1.93 0.1131 0.3985 3 145.44
S. lancifolia + Alternanthera philoxeroides 152.83 3.14 0.0617 0.2176 3 146.65
S. lancifolia + L. oryzoides + Eleocharis spp. 153.09 3.40 0.0578 0.2039 4 144.78
Eleocharis spp. 153.31 3.62 0.0464 0.1637 2 149.22
Eleocharis spp. + A. philoxeroides 154.09 4.40 0.0329 0.1159 3 147.91
S. lancifolia + L. oryzoides + Eleocharis spp. + Typha spp. 154.57 4.88 0.0298 0.1049 5 144.11
S. lancifolia + Eleocharis spp. + A. philoxeroides 154.58 4.89 0.0275 0.0968 4 146.27
S. lancifolia + L. oryzoides + Eleocharis spp. + A. philoxeroides 154.97 5.28 0.0244 0.0859 5 144.51
A. philoxeroides 154.63 4.94 0.0240 0.0846 2 150.54
Total biomass 154.84 5.15 0.0216 0.0762 2 150.75
L. oryzoides + Eleocharis spp. 155.07 5.38 0.0201 0.0710 3 148.89
Eleocharis spp. + Typha spp. 155.39 5.70 0.0172 0.0605 3 149.21
Null model 155.48 5.79 0.0157 0.0553 1 151.39
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standard errors. A Mantel test indicated no significant association between the biomass
of Bulltongue Arrowhead and spatial proximity of sites (r = 0.03, P = 0.456).
The estimated overall Ψ for all sites using the null model was 0.8032 (SE = 0.1557)
with probability of detection at 0.2767 (SE = 0.0639). Therefore, we estimate that
about 80% of sampled sites on MNWR were occupied by Marsh Rice Rats, although
the species was captured at only 21 (58%) of the sampled sites.
Discussion
Our results suggest that variation in plant composition in freshwater marshes affects
Marsh Rice Rat occupancy over a small spatial scale. Marsh Rice Rat ecology
and population dynamics in salt marshes and terrestrial edge habitats (barrier islands,
levees, etc.) have been extensively described (Eubanks et al. 2011, Kruchek
2004, Negus et al. 1961, Smith 1980, Smith and Vrieze 1979, Wolfe 1985), but little
is known about this marsh-obligate species in coastal freshwater-marsh habitat.
Different wetland plant communities reflect differences in the environment, such
as soil structure and composition, flooding regimes, and water chemistry (Sasser et
al. 1996). These changes in the environment can directly or indirectly affect Marsh
Rice Rat occupancy through characteristics of the plants. The relationship between
Bulltongue Arrowhead and Marsh Rice Rat occupancy found in our study may be
related to several characteristics of plant-community structure and Marsh Rice Rat
ecology. Marsh Rice Rats are omnivorous (Gosselink et al. 1998), and their diet
includes crabs, insects, green vegetation, seeds and fruits of marsh plants, snails,
fishes, and carrion (Davis and Schmidly 1997, Lowery 1974, Wolfe 1985). Populations
fluctuate seasonally and are largely driven by resource availability (Bloch
and Rose 2005, Lowery 1974). Marsh Rice Rats’ resource-based home ranges are
extremely variable in size, ephemeral, and overlap with conspecifics (Negus et al.
1961). Bulltongue Arrowhead is perennial and grows in thick clumps, with broad,
rigid leaves that allow Marsh Rice Rats to move under cover but above the water
of the marsh, helping them to avoid aerial predators, including herons and raptors.
Another feature of Bulltongue Arrowhead is that it forms floating mats (Sasser et
al. 1996) that rise and fall with the water levels, providing reliable living space and
sites for Marsh Rice Rats to rest and rear young. Marsh Rice Rats utilize all parts of
plants as food (Davis and Schmidly 1997, Lowery 1974, Wolfe 1985) and, because
Bulltongue Arrowhead has relatively high biomass, this species could potentially
be an important food source. Marsh Rice Rats have shown an affinity for broadleaved
plant cover in other marsh environments and the species would likely be
extirpated if marshes shift to woody vegetation, despite the cover shrubs provide,
as the species has not been found to inhabit sites dominated by shrubs (Eubanks et
al. 2011, Hofmann et al. 1990, Negus et al. 1961).
Bulltongue Arrowhead has been shown to have a higher salt tolerance than related
species and is a good candidate for use in restoration of freshwater marsh that
has suffered saltwater intrusion and conversion to open water (Martin and Shaffer
2005). A full understanding of how Marsh Rice Rats and Bulltongue Arrowhead
interact may be important to the success of recovery efforts and the longevity of
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Louisiana’s coastal freshwater marshes. More research is needed on behavior, diet,
plant use, and life history of small mammals in these systems, as well as the influence
of plant–animal interactions.
Acknowledgments
We would like to thank the staff of the Mandalay National Wildlife Refuge and the
University of Louisiana at Lafayette Institutional Animal Care and Use Committee
(IACUC No. 2010-8717-007) for their help in making this project possible. Dr. Scott
Duke-Sylvester’s help with the data analysis for this project is greatly appreciated. We
extend our gratitude to Andrew Edelman and the two anonymous reviewers for their patience
and comments, which have greatly improved this manuscript. We would also like
to thank Edward Delaney IV for his assistance in editing figures, and Jennifer Rasch and
Clara Weidman for their technical support. Support for E. Tobin was provided by the Louisiana
Environmental Health Association.
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