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Hurricanes, Floods, Levees, and Nutria: Vegetation
Responses to Interacting Disturbance and Fertility Regimes
with Implications for Coastal Wetland Restoration
Tiffany B. McFalls
{
, Paul A. Keddy
{
, Daniel Campbell
{
, and Gary Shaffer
{
{
Department of Biological Sciences
Southeastern Louisiana University
Hammond, LA 70402, U.S.A.
Tiffany.McFalls@selu.edu
{
{
Biology Department
Laurentian University
935 Ramsey Lake Road, Sudbury
Ontario, P3E 2C6, Canada
ABSTRACT
MCFALLS, T.B.; KEDDY, P.A.; CAMPBELL, D., and SHAFFER, G., 2010. Hurricanes, floods, levees, and nutria:
vegetation responses to interacting disturbance and fertility regimes with implications for coastal wetland restoration.
Journal of Coastal Research, 26(5), 901–911. West Palm Beach (Florida), ISSN 0749-0208.
A primary cause of wetland loss in the Louisiana coastal zone has been the construction of flood control levees along the
Mississippi River. These levees restrict the inputs of freshwater, nutrients, and sediment that historically replenished
these wetlands. Wetland loss is compounded by other factors such as storms, introduced herbivores, and saltwater
intrusion. How do such simultaneous changes in fertility and disturbance regimes affect the vegetation of coastal
wetlands? Will proposed restoration strategies, such as freshwater diversions and protection from herbivores, increase
the productivity and accretion rates of coastal wetlands without sacrificing plant species diversity? During this 2-year
study, we applied five disturbance treatments (control, fire, herbivory, single vegetation removal, and double vegetation
removal) and four fertility treatments (control, sediment addition, fertilizer addition, and sediment +fertilizer addition),
using a split-plot factorial design with herbivory exclosures as main plots and species richness and total aboveground
biomass as dependent variables. We found that nutria, the principal vertebrate herbivore of the marsh, limited biomass
production. Other disturbances decreased biomass, but only to a limited extent in the absence of herbivores. The
sediment +fertilizer treatment, which simulated the additional nutrients and substrate material that a freshwater
diversion might deliver, significantly increased biomass production. Fertilizer significantly increased the biomass only in
the absence of herbivores. We had limited success in predicting species richness after 2 years. Only the most severe
disturbance decreased species richness, whereas fertilizer addition seemed to have a minor effect (p50.08). Sediment-
and nutrient-rich waters from freshwater diversions will likely mitigate negative impacts of nutria grazing on biomass
and have no effect on species richness. However, it should be noted that freshwater diversions will have the most impact
if nutria populations are reduced.
ADDITIONAL INDEX WORDS: Dynamic equilibrium model, fertilizer, fire, freshwater diversion, herbivory, Huston’s
model of diversity, Louisiana, oligohaline marsh, sediment.
INTRODUCTION
The Louisiana coastal zone experiences some of the highest
land degradation and loss rates in the world (Boesch et al., 1994;
Britsch and Dunbar, 1993; Gagliano, Meyer-Arendt, and
Wicker, 1981). Wetland loss is primarily driven by the
construction of flood control levees along the Mississippi River
during the past 2 centuries, which has severely restricted the
input of freshwater, nutrients, and sediment to itsdelta (Day et
al., 2000, 2007; Martin et al., 2000; Shaffer et al., 1992). Coastal
wetlands convert to open water if accretion does not keep pace
with relative sea level rise (eustatic sea level rise +subsidence).
With much of the Mississippi River watershedunder strict flood
control (Cowdrey, 1977), freshwater, nutrients, and sediment
are not available to rebuild the rapidly submerging wetlands of
the deltaic plain (Baumann, Day, and Miller, 1984; Martin et
al., 2000; Mossa, 1996). Wetland loss in the deltaic plain is
compounded and accelerated by multiple disturbances. Hurri-
canes and tropical storms periodically erode protective barrier
islands and directly affect wetlandsthrough scouring, sediment
and wrack deposition, and extensive salt burning of wetland
vegetation (Baldwin, Mckee, and Mendelssohn, 1996; Baldwin
and Mendelssohn, 1998; Boesch et al., 1994; Guntenspergen et
al., 1995). Saltwater intrusion from dredged canals for
navigation or oil exploration also leads to salt burning and loss
of wetland vegetation (Boesch et al., 1994; Turner, 1997). As
well, herbivory from the introduced rodent, nutria, (Myocastor
coypus [Molina]) severely reduces overall wetland biomass and
can lead to the conversion of wetland to open water (Carter,
Foote, and Johnson-Randall, 1999; Carter et al., 1999; Conner,
1989; Ford and Grace, 1998; Taylor and Grace, 1995). Primary
restoration strategies for wetlands in the delta now focus on (1)
diversions of Mississippi River water across levees to augment
inputs of freshwater, nutrients, and sedimentsto wetlands, and
(2) the control of nutria populations. Hence, restoration
strategies of wetlands in the delta of the Mississippi River will
change both fertility and disturbance regimes.
In this study, we examined the effects of multiple distur-
bances and fertility enhancements upon the plant diversity of
DOI: 10.2112/JCOASTRES-D-09-00037.1 received 8 April 2009;
accepted in revision 2 July 2009.
’Coastal Education & Research Foundation 2010
Journal of Coastal Research 26 5 901–911 West Palm Beach, Florida September 2010
an oligohaline marsh in the delta of the Mississippi River
(Platt, 1988; Saucier, 1963). The Huston (1979) dynamic
equilibrium model (DEM) of diversity was used as a conceptual
framework for the design, testing, and interpretation of our
experiments. We asked three questions: (1) How do simulta-
neous disturbance and fertility regimes affect the vegetation of
coastal wetlands that are already subject to a number of
perturbations and stressors, such as hurricanes, levees, and
introduced herbivores? (2) Will proposed restoration strategies,
such as herbivore protection and freshwater diversions, benefit
(i.e., increase biomass) the wetlands along the Gulf Coast? (3)
Can the productivity and accretion rates of rapidly submerging
coastal wetlands be increased without sacrificing plant species
diversity? Our primary focus was the consequences of possible
interactions of multiple disturbance and fertility treatments on
species richness and biomass.
The Huston (1979) DEM is particularly appropriate for
studying coastal wetlands in the Mississippi River delta,
because both historic anthropogenic changes have, and
proposed restoration strategies will, alter fertility and distur-
bance regimes. The DEM postulates that measurable relation-
ships exist between two fundamental factors of ecological
communities: disturbance and fertility (Grime, 1979; South-
wood, 1977). The DEM also offers probable mechanisms: the
rate at which biomass accumulation causes competitive
displacement, and the rate at which biomass loss allows
coexistence. As such, low disturbance rates require low
recovery rates (low fertility) to maintain high biological
diversity, but equally, high disturbance rates require high
recovery rates (high fertility) to maintain high biological
diversity. The Huston (1979) model includes both the interme-
diate disturbance hypothesis (Connell, 1978) and the unimodal
productivity–diversity hypothesis (Grime, 1973, 1979), produc-
ing a synthesis of two well-supported diversity models.
The DEM assumes that (1) the subject community is not at
equilibrium, as a result of periodic population reductions
(disturbances); (2) its component species have different
population growth rates; and (3) some environmental changes
affect all competing species in the same way (Huston, 1979).
Wetlands along the Gulf Coast, particularly oligohaline
marshes,arecertainlysubjecttomultipledisturbances
(storms, high salinity pulses, herbivory; e.g., Boesch et al.,
1994), which reduce populations of plant species. They are also
dominated by different functional types, including annuals and
perennial emergents and herbaceous vines (McFalls, 2004),
with fundamentally different population growth rates. As well,
environmental changes, such as increases in fertility through
nutrient input from Mississippi floodwaters or salt burning
from hurricane storm surges, affect all the component species
similarly at any one site (e.g., Boesch et al., 1994). As such,
many wetlands along the Gulf, and particularly, the oligoha-
line marshes, should be particularly suited to the DEM.
Rationale for Treatments
Disturbance Treatments
Disturbances, defined as events that destroy plant bio-
mass (Grime, 1977, 1979), strongly influence species diver-
sity and biomass patterns by creating heterogeneity in
ecological communities (Brewer, Levine, and Bertness,
1998; Connell, 1979; Shumway and Bertness, 1994; Watt,
1947). This heterogeneity is created from the differential
survival and recovery of species based on life history stra-
tegies, reduced competitive exclusion, and the changes
that occur in edaphic factors due to the disturbance (e.g., Al-
Mufti et al., 1977; Grime and Hunt, 1975; Grubb, 1977;
Skellam, 1951). Disturbance intensity, measured as the
proportion of biomass killed (Grime, 1979; Sousa, 1984),
dictates how far the system is perturbed. In this study, we
applied disturbance treatments of increasing intensity in the
following postulated order: control, fire, herbivory, single
vegetation removal treatment, and double vegetation removal
treatment.
Fire is a natural process in coastal wetlands, and prescribed
burning has been used historically as a management tool in
Louisiana marshes (Nyman and Chabreck, 1995). Fire is
generally used as a technique to increase diversity, but it
can also decrease biological diversity in wetlands if organic
matter in the soil is ignited, creating new depressions with
increased flooding (Lane, Day, and Day, 2006; Vogl, 1973;
White, 1994).
Herbivory can have significant effects on species composition
in wetlands, especially at small spatial scales (Bakker, 1985;
Bazely and Jeffries, 1986). Nutria are herbivores of particular
concern in Louisiana because they exert pressure on an already
stressed coastal system (Conner, 1989; Nyman, Chabreck, and
Kinler, 1993; Rejmanek, Gosselink, and Sasser, 1990; Taylor
and Grace, 1995). Nutria not only destroy large expanses of
vegetation, but they may subsequently prevent regeneration in
these areas (Carter, Foote, and Johnson-Randall, 1999; Shaffer
et al., 1992). Herbivory was expected to be a stronger
disturbance than fire based on the selective nature of
herbivores and the many published studies on the effects of
nutria grazing and grubbing (Ford and Grace, 1998; Llewellyn
and Shaffer, 1993; Myers, Shaffer, and Llewellyn, 1994;
Shaffer et al., 1992; Taylor et al., 1994). Nutria have a year-
long effect on vegetation, whereas fire is a one-time distur-
bance.
Vegetation removal treatments were included to simulate
extreme disturbances that can occur in deltaic wetlands, such
as erosion, wrack deposition, and salt burning, associated with
hurricane damage (Guntenspergen et al., 1995). A single
vegetation removal treatment was a pulse disturbance de-
signed to cause 100% mortality in adult plants, but to allow
regeneration from buried propagules. Although not a topic of
this article, the regeneration would allow an examination of the
role of the seed bank in vegetation recovery. Once again,
although not a topic of this article, the double vegetation
removal treatment was designed to examine the role of
propagule dispersal and colonization in vegetation recov-
ery. Plants were allowed to regenerate, but they were
periodically killed before producing seed. The vegetation
removal treatments using herbicide were expected to be the
two strongest disturbances because all aboveground and
belowground biomass was killed, whereas nutria herbivory
and fire treatments were expected to primarily remove
aboveground biomass only.
902 McFalls et al.
Journal of Coastal Research, Vol. 26, No. 5, 2010
Fertility Treatments
Increases in biomass production are brought about by
increases in available resources (Grime, 1979). Such increases
not only have profound effects on community interactions,
composition, and species richness (Grime, 1973, 1977, 1979)
but also can fundamentally alter ecosystem processes, such as
decomposition, nutrient cycling, and accretion (Craft and
Richardson, 1993). Fertility treatments were designed to
include factors that might affect both production and accretion
in Louisiana’s rapidly submerging coastal areas. The fertility
treatments serve, in part, to evaluate the potential for restoring
coastal wetlands by means of diversions of sediment and
nutrient-rich freshwater from the Mississippi River. Fertility
treatments of hypothesized increasing intensity were applied:
no fertility enhancement (control), sediment addition, fertilizer
addition, and sediment +fertilizer addition.
Sediment additions were designed to simulate the regular
sediment deposition that would occur during a normal year if
spring flooding occurred (Saucier, 1963). Sediment input
provides both mineral substrate and nutrients (Frey and
Basan, 1978; Johnston et al., 1984; Niering and Warren,
1980). The high productivity of riverine and deltaic wetlands is
often attributed to the regular deposition of nutrient-rich
mineral sediments in floodwaters (Day et al., 2000; Gorham
and Pearsall, 1956; Mitsch and Gosselink, 2000; Ranwell,
1964). However, sediment input in wetlands does not only
provide nutrients. It can also have negative effects by filtering
out species unable to cope with burial (Dittmar and Neely,
1999; Jurik, Wang, and Van Der Valk, 1994; Keddy, 2000;
Neely and Wiler, 1993; Van Der Valk, Swanson, and Nuss,
1983) and can, therefore, act as a disturbance under the DEM,
depending on the thickness of deposited sediment. Over time,
continued sediment additions will increase elevation and
reduce flooding, potentially increasing the pool of colonists
(Gough and Grace, 1998).
Fertilizer additions simulated the higher nutrient loadings
that would probably accompany spring flooding if water control
structures were not in place along the Mississippi River.
Increases in nutrients alone may also increase accretion in
wetlands through peat accumulation (Craft and Richardson,
1993). Additions of both sediment and fertilizer were designed
to more accurately simulate a spring flooding event, where
dissolved nutrients and suspended sediments are deposited on
the wetland surface. The actual extent of nutrient and
sediment inputs into wetlands from river diversions will
depend on the rate and timing of inputs, the landscape position
of wetlands, and the distance of wetlands from distributaries.
METHODS
Study Area
The research was conducted at Turtle Cove Experimental
Marsh (30u179N, 90u209W; 0.3 m elevation National Geodetic
Vertical Datum), located in the wetlands south of Southeastern
Louisiana University’s Turtle Cove Environmental Research
Station, 35 km northwest of New Orleans, Louisiana (Fig-
ure 1). This marsh is on the Manchac land bridge, a 10-km strip
of wetland that separates Lake Pontchartrain and Lake
Maurepas in the Lake Pontchartrain basin of southeast
Louisiana. At Hammond, Louisiana, 28 km to the north, mean
annual temperature is 19.3 uC (January, 9.9 uC; July, 27.6 uC),
and mean annual precipitation is 162.6 cm, based on 1971–
2000 average climate temperatures from the Southern Region-
al Climate Center (SRCC, 2004a, 2004b). The site receives
minor tidal influence (0.05–0.15 m), but wind-driven water
level fluctuations dominate (2002–2003 90% interval, 0.60 m;
maximum interval, 1.59 m; USACE, 2004). Short-lived peaks
in water levels are associated with tropical storms and
hurricanes. Mean salinity during 2002–2003 at the Louisiana
Universities Marine Consortium Lake Pontchartrain sta-
tion, 5 km to the east, was 1.66 ppt (range, 0.00–4.59 ppt;
LUMCON, 2004)—making it an oligohaline marsh (Cowardin
et al., 1979). The flora has been documented by Platt (1988),
and the vegetation in our study area was dominated by three
species: Schoenoplectus americanus (Pers.) Volk. ex Schinz &
Keller (39.0%), Polygonum punctatum Ell. (18.9%), and
Sagittaria lancifolia L. Nomenclature follows the Integrated
Taxonomic Information System (ITIS) used by the U.S.
Department of Agriculture (ITIS, 2005).
Experimental Design
This experiment was a randomized block design with a split-
plot factorial. Herbivore exclosures or areas open to mamma-
lian herbivory (40 360 m) were the main plot treatments.
Herbivory as the main plot is justified because nutria are
ubiquitous in coastal Louisiana, and as such, any research or
management decisions should include the effects of these
mammalian herbivores. Factorial combinations of fertility
treatments and disturbance treatments (besides herbivory)
were randomly allocated to 3 33-m subplots. A boardwalk,
335 m in length, provided access to the main plots. Access
inside of main plots was provided by 670 m of catwalk,
constructed to minimize damage to the organic soil.
Treatments
Herbivory Exclosures
In early 2002, three 40 360-m herbivore exclosures were
constructed and paired with three parallel areas of equal size
open to herbivory. Exclosures were designed to prevent nutria,
the principal vertebrate herbivores of the marsh, from entering
the plots, but the exclosures also excluded other less-common
herbivores, such as feral hogs (Sus scrofa L.), marsh rabbits
(Sylvilagus aquaticus L.), and muskrats (Ondatra zibethicus
L.). Exclosures consisted of approximately 1.5-m-tall wire
fences supported by pressure-treated wooden posts. They were
constructed from 1.83-m-tall, vinyl-coated, welded, 2-mm wire
fencing with 5 310-cm openings. The fencing was inserted at
least 45 cm into the substrate to prevent nutria from
burrowing into the exclosures. Where exclosures crossed
drainage areas, we reinforced them with additional fencing.
Additional fencing, at least 60 cm wide, was also placed on the
soil surface and attached to the fence to further discourage
burrowing. The few nutria that managed to enter the
exclosures were generally removed within a week.
Wetland Loss in Louisiana 903
Journal of Coastal Research, Vol. 26, No. 5, 2010
Fire
Prescribed burns were applied annually in late winter (April
23, 2002, and February 1, 2003) when water levels were low
and a large amount of natural fuel in the form of standing litter
was present. Fires were set using a propane torch designed for
vegetation burning (Model VT3-30C, Flame Engineering Inc.,
LaCrosse, Kansas).
Vegetation Removal
A standard backpack sprayer and the manufacturer recom-
mended levels of Rodeo aquatic-approved herbicide were
applied until complete mortality of vegetation was achieved.
For the single vegetation removal treatment, herbicide was
applied in May 2002. For the double vegetation removal
treatment, herbicide was first applied in May 2002 and then
reapplied in September 2002, May 2003, and July 2003.
Sediment Addition
Soil for the sediment treatment was obtained from bottom-
land sources in southeast Louisiana by local contractors. It was
hand-applied annually to a depth of 1 cm across the entire plot,
in late February to early March. This is similar to the sediment-
loading rates to wetlands from Mississippi River delta
diversions at Caernarvon and at West Pointe a
`la Hache,
Louisiana, which deliver 0.75–1.57 cm/y and 1.24–1.84 cm/y,
respectively (Lane, Day, and Day, 2006). These diversion
projects have pulses of discharge with most maxima in the
spring (Lane, Day, and Day, 2006), and as such, sediments are
also delivered as pulses in the spring, as in this study, but
extending over a several weeks, in contrast to this study.
Sediment in 2003 was analyzed by Louisiana State University’s
AgCenter Soil and Plant Test Laboratory for calcium (874 mg/
L), magnesium (110 mg/L), phosphorus (41.5 mg/L), potassium
(58.3 mg/L), sodium (57.3 mg/L), pH (4.84), and organic matter
(2.3%) (McFalls, 2004).
Fertilizer Addition
Slow release Osmocote 18–6–12 (N–P–K) was applied
annually at a rate of 215 g/m
2
, which provided 38.7 g N/m
2
/y,
12.9 g P/m
2
/y, and 25.8 g K/m
2
/y. It was applied once prescribed
burns were finished in early spring. These loading rates are
higher than the Mississippi River diversion at Caernarvon,
Louisiana, whichdeliver 8.9–23.4 g N/m
2
/y and 0.9–2.0 g P/m
2
/y
throughout a 260-km
2
marsh (Lane, Day, and Thibodeaux,
1999), but inputs at Caernarvon, Louisiana, are heterogenous
throughout the 260-km
2
marsh, and some areas received far
higher loadings (Mitsch et al., 2005). Our N loading rates are
similar to those modeled for the Maurepas diversion of 8.4–
87.7 g N/m
2
/y (Lane et al., 2003), which would feed the study
marsh once it became operational.
Fertilizer and Sediment Addition
The sediment +fertilizer treatment followed the same
protocols as the individual applications. Plots were fertilized,
and then, sediment was applied.
Figure 1. Study area. Location of Turtle Cove Environmental Marsh (TCEM) noted by arrow.
904 McFalls et al.
Journal of Coastal Research, Vol. 26, No. 5, 2010
Data Collection
In July 2003, all aboveground biomass was clipped from two
systematically chosen 0.25-m
2
areas just inside the 9-m
2
plot
perimeters. Samples were held in cold storage (5 uC) less than 3
weeks before they were sorted into live vs. dead material, dried to
a constant weight in a forced air oven at 80 uC for at least 48 hours,
and weighed on a digital laboratory balance to the nearest 0.01 g.
Species richness was assessed visually in the inner 4 m
2
(2 m 3
2 m) of each plot by collecting percentage of cover data by species
on April 28, June 25, July 29, August 27, and October 17, 2003.
Statistical Analysis
Separate analyses were conducted first on biomass, and
second, on richness, averaged over the five sampling times.
They were analyzed as a 2 3434 randomized block design
with split-plot factorial analyses of covariance (ANCOVA). A
randomized block design was used because of the large spatial
area of the experiment. For the split plot, the main plot
disturbance fixed effect was herbivory (two levels: no herbiv-
ory, by means of the exclosure; or herbivory, without
exclosure). In the subplots, fixed effects were factorial
combination of other disturbances (four levels: control, fire,
single vegetation removal, or double vegetation removal),
fertilizer addition (2 levels: none, fertilizer), and sediment
addition (two levels: none, sediment). We measured relative
elevation of individual plots, as measured in the center of the
plot using a laser surveying system, and proximity of the center
of the plot to the nearest flowing water as covariables because
these factors can influence flood level and duration.
Analyses were performed using general linear models in
SPSS Version 15.0. Homogeneity of variance was verified using
plots of residuals vs. predicted values, and normality was
verified by evaluating histograms of residuals. Biomass data
were square root–transformed to achieve homogeneity of
variance, and richness data were not transformed. No
interactions between blocks or covariables and the fixed effects
were included in the model.
RESULTS
Aboveground Biomass
Herbivory significantly reduced biomass (p50.010; Table 1;
Figure 2). On average, areas protected from nutria herbivory
had 1.4 times the biomass of areas open to herbivory.
Other disturbances also significantly affected biomass
(p,0.001), but there was a strong and significant interaction
with herbivory (p50.001; Table 1; Figure 3). When nutria
herbivory was combined with an additional disturbance, such
as fire, single vegetation removal, or double vegetation
removal, the effect of the other disturbances was amplified.
Without herbivores, fire had no effect relative to the control,
and single and double herbicide treatments were reduced in a
similar manner (open histograms, Figure 3). In the presence of
herbivores, there was a downward trend in biomass production
in our hypothesized order of disturbance intensity (control ,
fire ,herbivory ,single vegetation removal ,double
vegetation removal).
Sediment addition only slightly increased biomass (p5
0.072; Table 1), whereas fertilizer significantly increased
biomass by 1.3 times that of control plots (p,0.001). However,
there was a significant interaction between sediment and
fertilizer additions (p50.047; Table 1; Figure 4). The addition
of sediment alone did not increase biomass above control plots,
but the addition of sediment with fertilizer, which simulated
Mississippi River flooding events proximal to the outfall,
resulted in increased biomass compared with plots with only
fertilizer addition. The order of response in biomass production
Table 1. Split-plot analysis of variance table of aboveground biomass and species richness in 2003, in the second year of treatments of herbivory (Herbiv),
disturbance (Dist), fertilizer addition (Fert), and sediment addition (Sed). Covariables are proximity to closest flowing water (Prox) and relative elevation
(Relev) of plots. Analyses were made on square root–transformed biomass and untransformed species richness. Bolded pvalues are smaller than 0.10.
Biomass Richness
Source df MS FPMS FP
Block 2 15.8 3.78 0.206 4.98 2.16 0.314
Herbiv 1 331.9 78.83 0.010 1.01 0.44 0.574
Error A 2 4.2 2.31
Dist 3 232.5 33.20 0.000 25.16 12.73 0.000
Fert 1 99.0 14.14 0.000 6.27 3.17 0.080
Sed 1 23.5 3.35 0.072 0.03 0.01 0.909
Herbiv 3Dist 3 42.5 6.06 0.001 0.98 0.50 0.687
Herbiv 3Fert 1 86.6 12.36 0.001 0.00 0.00 0.974
Herbiv 3Sed 1 2.8 0.40 0.532 1.77 0.90 0.348
Dist 3Fert 3 9.3 1.33 0.275 0.34 0.17 0.914
Dist 3Sed 3 1.5 0.22 0.883 3.77 1.91 0.138
Fert 3Sed 1 28.8 4.11 0.047 0.04 0.02 0.889
Herbiv 3Dist 3Fert 3 8.3 1.18 0.325 1.84 0.93 0.432
Herbiv 3Dist 3Sed 3 35.0 5.00 0.004 1.38 0.70 0.555
Herbiv 3Fert 3Sed 1 0.4 0.06 0.815 1.20 0.61 0.440
Dist 3Fert 3Sed 3 0.2 0.03 0.993 2.01 1.02 0.392
Herbiv 3Dist 3Fert 3Sed 3 12.4 1.78 0.162 0.48 0.24 0.868
Prox 1 40.0 5.71 0.020 0.04 0.02 0.889
Relelev 1 11.6 1.65 0.204 3.58 1.81 0.183
Error B 58 7.0 1.98
Wetland Loss in Louisiana 905
Journal of Coastal Research, Vol. 26, No. 5, 2010
provides evidence that the hypothesized ranking of fertility
treatments was generally correct (control ,sediment only ,
fertilizer only ,sediment +fertilizer).
There was also a significant interaction between herbivory
and fertilizer addition (p50.001; Table 1; Figure 5). Where
herbivory was allowed, nutria significantly reduced biomass of
the fertilized plots—to the point that those plots were not
different from nonfertilized plots.
A complex, significant interaction occurred between herbiv-
ory, disturbance, and sediment addition (p50.004; Table 1;
Figure 6). In the absence of herbivores, sediment addition
increased biomass only in the fire treatments. In the presence
of herbivores, sediment addition increased the biomass of the
control treatments only. For some reason, it appears that
nutria determine whether added sediment will affect the
biomass of control or burned plots.
Species Richness
Herbivory did not cause any significant change in mean
species richness (p50.57; Table 3), although other distur-
bances did (p,0.001; Figure 7). Specifically, only the double
herbicide treatment reduced the mean species richness
compared with the control. Fertilizer addition decreased
species richness slightly, although it was not quite statistically
significant (p50.080; Figure 8). Sediment addition had no
effect (p50.909) on species richness. There were no
interactions.
DISCUSSION
Effects of Interactions of Fertility and Disturbance
on Biomass
We were successful in establishing a sequence of treatments
with increasing rates of biomass gain through the application
of sediment and fertilizer. As our predicted intensity of fertility
increased, biomass increased monotonically (control ,sedi-
ment addition ,fertilizer addition ,sediment and fertilizer
addition). We also successfully set up a sequence of treatments
with increasing rates of biomass loss through the application of
increasing disturbances. As our predicted disturbance intensi-
ty increased, biomass decreased monotonically (control .fire .
herbivory .single vegetation removal .double vegetation
removal).
The two-way and three-way interactions between herbivory,
other disturbance treatments, and fertility treatments did not
always follow the monotonic patterns shown by the main
treatment effects. Biomass decreased monotonically with
increasing disturbance when also exposed to herbivory, but
this decrease was less marked and not monotonic inside the
exclosures. Apparently, herbivory by nutria had an effect on
Figure 3. Effect of herbivory and other disturbance types on aboveground
biomass in July 2003 (mean 61 SE).
Figure 4. Effect of fertilizer and sediment additions on aboveground
biomass in July 2003 (mean 61 SE).
Figure 2. Overall effect of herbivory on aboveground biomass in July
2003 (mean 61 SE).
906 McFalls et al.
Journal of Coastal Research, Vol. 26, No. 5, 2010
biomass if another disturbance was also present. Similar
interactions of nutria herbivory and disturbances have been
observed in other studies in the Louisiana coastal marshes and
swamps (e.g., Brewer, Levine, and Bertness, 1998; Gough and
Grace, 1998). However, unlike some previous studies (Taylor et
al., 1994; Ford and Grace, 1998), we did detect a negative
interaction between herbivory and fire. Nutria apparently
selectively consumed biomass in burned plots, particularly if
the plot had a fertility enhancement.
As we increased fertility through fertilizer addition, biomass
increased only in the exclosures where herbivory was absent.
This suggests that nutriaconsume a great deal of the increased
vegetation that results from enhanced fertility, perhaps
because of an increased nutritive value of higher fertility plots
(White, 1993). Nutria are known to select specific species of
plants in their diet (Wilsey, Chabreck, and Linscombe, 1991),
although it is not known how this is related to their food
quality. The increase of marsh biomass with fertilization but
without herbivores has three important implications. First, it is
possible that increased fertility of coastal marshes might not, in
the long run, lead to more plant biomass but to more nutria
biomass. Second, it is a reminder that trophic effects may be
underestimated in coastal wetlands; in more saline habitats,
snails may replace nutria as agents that control biomass
(Silliman and Bertness, 2002). Third, it suggests that the
effects of alligators as predators on nutria might have
significant top-down effects by decreasing nutria and increas-
ing sensitivity of marshes to fertilization (Keddy et al., 2009).
Figure 5. Effect of herbivory and fertilizer addition on aboveground
biomass in July 2003 (mean 61 SE).
Figure 6. Effect of herbivory, other disturbances, and sediment addition
on aboveground biomass in July 2003 (mean 61 SE).
Figure 7. Effect of other disturbances on species richness averaged over
five sampling periods in April, June, July, August, and October 2003 (mean
61 SE).
Figure 8. Effect of fertilizer addition on species richness averaged over
five sampling periods in April, June, July, August, and October 2003 (mean
61 SE).
Wetland Loss in Louisiana 907
Journal of Coastal Research, Vol. 26, No. 5, 2010
Effects of Interactions of Fertility and Disturbance on
Species Richness
The DEM did not usefully predict the effects of the
treatments on species richness after 2 years of treatments.
Increased disturbance intensity did decrease species richness,
but only in the most severe disturbance treatment—double
vegetation removal by herbicide. Simultaneous disturbances
(herbivory with other disturbances) had no effect on species
richness.
Fertilizer addition had a slight but nonsignificant effect on
richness, and there was no interaction between fertility and
any disturbance, contrary to what is predicted by the DEM.
There was support for the individual components of the Huston
model, for the Connell (1978) intermediate disturbance
hypothesis, and for the Grime (1973, 1979) unimodal produc-
tivity–diversity hypothesis. Competitive displacement as a
result of fertilizer addition only slightly reduced species
richness in this Louisiana oligohaline marsh during our 2-year
study. This may indicate that these oligohaline marshes are
more like the systems described by the intermediate distur-
bance hypothesis portion of the DEM—systems with high
growth rates, such as intertidal zones and coral reefs (Connell,
1978; Sousa, 1984). Despite herbivory being shown to be an
intermediate disturbance in this study, at least as measured by
effects on biomass, it did not significantly affect species
richness during the course of this study. The Manchac area
also has intermediate to high disturbance rates, like systems
best described by the Grime (1973, 1979) model. Frequently,
high-disturbance rates in the experiment resulted in diversity
levels that might suggest a unimodal productivity–diversity
curve. Species richness only slightly decreased with fertilizer
addition, providing further evidence that the diversity patterns
within the community were controlled primarily by distur-
bance regimes during the course of this study. Species richness
patterns indicate that the Manchac area has high growth rates,
like systems that are best described by the intermediate
disturbance hypothesis. This information gives support to the
two components of the DEM, but when combined, they were not
able to accurately predict species richness responses after 2
years of treatments. However, Bakker (1985), who examined
herbivory in salt marshes, needed 3 years of data to detect a
diversity change, and Turkington et al. (2002) needed almost a
decade to see diversity changes. We expect that ongoing
monitoring of this experiment will lead to clearer trends.
Overall, the extensive marshes of the Manchac area have
relatively low diversity (Boshart,1997; Gough and Grace, 1998;
Thomson, 2000), on average just over 5 species per 4m
2
in our
experiment. The study area was dominated by just three
species (S.americanus, P. punctatum, and S. lancifolia), all of
which can produce dense canopies and become, dominants in
fertile areas (Boshart, 1997). Transplant experiments in these
marshes showed that competition from existing plants has a
negative effect on other species that might establish (Geho,
Campbell, and Keddy, 2007). The cover of existing plants, and
therefore, of these competition effects, might be reduced by
natural disturbances, from the small scale (herbivory) to the
large scale (hurricanes), but contrary to expectations, none of
our disturbance treatments increased plant diversity. The
Manchac area appears to already be at maximal diversity, as
shown by the highest richness in control plots. Why, then, is the
mean species richness so low? Gough, Grace, and Taylor (1994)
suggested that the abiotic stressors of increased salinity and
flooding kept the species pool at very low levels in the Manchac
area. Selective feeding by herbivores might compound this
effect by reducing establishment of species, such as southern
cattail (Typha domingensis Pers. (Geho, Campbell, and Keddy,
2007). Finally, it may be that dispersal of new species may
require decades to occur.
MANAGEMENT IMPLICATIONS
In summary, during 2 years, nutria decreased biomass, but
they had a neutral effect on species richness. The treatments
simulating a freshwater diversion apparently reduced the
negative effects of nutria on biomass. However, the small
increase in biomass when fertilizerwas applied in the presence
of herbivores indicates that nutria consumed a large proportion
of the extra biomass produced. The extra, potentially more
nutritious, food created by enhanced fertility, suggests that
increasing fertility throughout a large area may lead to larger
nutria populations (White, 1993). These results are consistent
with other evidence that predators, such as alligators, may
increase marsh biomass by reducing effects of herbivory
(Keddy et al., 2009). In our study, it appears that the effects
of herbivores did not completely remove the added production
because enhanced fertility increased biomass somewhat, even
in the presence of herbivores (that is, outsideof the exclosures).
Because the highest biomass was achieved when both fertilizer
and sediments were applied, floodwaters from the Mississippi
River may mitigate the negative effects of nutria grazing.
However, it should be noted that freshwater diversions will
have the most effect if nutria populations are reduced.
Based on the species richness data, there was no concomitant
decrease observed in diversity when productivity is increased.
This is positive information in terms of proposed freshwater
diversions, given that enhanced productivity generally yields
lower biological diversity (e.g., Auclair, Bouchard, and Pajacz-
kowski, 1976; Grime, 1979; Rosenzweig, 1971). It is surprising
that we found no decrease in species richness in response to our
fertility treatments. This suggests, however, that potential
eutrophication from freshwater diversions may not have
negative effects on plant diversity, at least in the short term.
More experimentation is needed to determine the long-term
effects of freshwater diversion nutrients on species richness.
Fire in areas of high nutria abundance should be avoided if
fertility is increased. The combination of fire and the additional
nutrients seems to promote heavy, localized herbivory, which
could lead to a positive feedback cycle of reduced accretion and
increased inundation. Other studies have also found that fire
tends to increase grazing pressure (Mcnaughton, 1984; Smith
and Kadlec, 1985; Smith, Kadlec, and Fonesbeck, 1984;
Svejcar, 1989; Woolfolk et al., 1975).
The most important general conclusion may lie, not in the
details of the interactions, but in the sheer number and
complexity of them. That is, no single factor—nutria grazing,
sediment, or fire—emerged as the dominant controlling factor
on either biomass or species richness. Although it is often
908 McFalls et al.
Journal of Coastal Research, Vol. 26, No. 5, 2010
tempting to try to manage wetlands as if single controls were
dominant, these data suggest otherwise. The sheer number of
interactions suggests that we need to view wetlands as arising
out of multiple, interacting factors, some of which we
understand, and some of which remain unknown. Hence,
multiple working hypotheses need to be entertained in
planning future research. For coastal wetlands as a whole,
interactions among the factors we manipulated, combined with
possible interactions from others that we did not manipulate
(e.g., salinity and alligator predation), need continuing atten-
tion both at the level of basic science and in habitat
management.
ACKNOWLEDGMENTS
We are indebted to the U.S. Environmental Protection
Agency for funding through the Wetland Protection and
Development Grant Program (R-82898001-01, R-82898001-
02, and R-82898001-03). Additional funding was provided by
The National Oceanic and Atmospheric Administration
through the Lake Pontchartrain Act. We offer our sincere
thanks to Dr. Nick Norton and to the entire Southeastern
Louisiana University Pontchartrain Basin Research Pro-
gram. This project could not have been completed without
much field assistance from M. Broussard Lombard, T.
Menzel, J. Smith, M. Clark, J. Zoller, M. Kaller, A. Roth, D.
Dardis and her teacher–research associate program, and
many others. Special thanks to R. Moreau and H. Reno at
Turtle Cove Environmental Marsh (TCEM) Research Station
for all of their support. Thanks also to the Louisiana
Department of Wildlife and Fisheries for permission to
construct TCEM and to conduct research on their land.
Thanks to M. Huston, M. White, R. Miller, M. Clark, M.
Kaller, and J. Willis for their comments on earlier drafts.
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