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\begin{document}

\title{Grazer host density mediates the ability of parasites to protect
foundational plants from overgrazing}



\author[1]{Joseph Morton}%
\author[2]{Samantha Huff}%
\author[3]{Emory Wellman}%
\author[2]{Brian Silliman}%
\affil[1]{Affiliation not available}%
\affil[2]{Duke University}%
\affil[3]{University of Florida}%


\vspace{-1em}



  
  \date{May 24, 2024}


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\selectlanguage{english}
\begin{abstract}
Like many top consumers, parasites can regulate feeding of their prey
via trait-mediated means. If parasites modify the feeding behavior of
ecologically important grazers, they may have cascading effects on the
structure and functioning of whole plant communities. The extent to
which parasites can influence plant communities in this way is largely
dependent on the strength of their behavioral alteration, their
prevalence in host grazers, and the density of those hosts. Recent
experiments and comparative surveys in southeastern USA salt marshes
revealed that common larval trematode parasites suppress the per capita
grazing impacts of the marsh periwinkle (Littoraria irrorata),
generating a trophic cascade that protects foundational marsh plants
from drought-associated overgrazing. Here, we conducted a field
manipulation wherein we modified grazer host density while holding
infection prevalence constant at an ecologically relevant level (20\%)
to determine whether the indirect, facilitative effects of parasites on
marsh plants varied with the density of grazers. We found that parasites
had significant positive impacts on marsh net primary productivity at
moderate densities of snails ([?]50 snails/ 0.5 m2), but that the positive
effects of parasites were negligible at lower densities. Our results
confirm the findings of previous studies that parasites can protect
marsh plants from overgrazing at sufficiently high prevalence, but show
that their ability to do so depends on host density.%
\end{abstract}%



\sloppy


\emph{Abstract}

Like many top consumers, parasites can regulate feeding of their prey
via trait-mediated means. If parasites modify the feeding behavior of
ecologically important grazers, they may have cascading effects on the
structure and functioning of whole plant communities. The extent to
which parasites can influence plant communities in this way is largely
dependent on the strength of their behavioral alteration, their
prevalence in host grazers, and the density of those hosts. Recent
experiments and comparative surveys in southeastern USA salt marshes
revealed that common larval trematode parasites suppress the per capita
grazing impacts of the marsh periwinkle (\emph{Littoraria irrorata} ),
generating a trophic cascade that protects foundational marsh plants
from drought-associated overgrazing. Here, we conducted a field
manipulation wherein we modified grazer host density while holding
infection prevalence constant at an ecologically relevant level (20\%)
to determine whether the indirect, facilitative effects of parasites on
marsh plants varied with the density of grazers. We found that parasites
had significant positive impacts on marsh net primary productivity at
moderate densities of snails ([?]50 snails/ 0.5 m\textsuperscript{2}), but
that the positive effects of parasites were negligible at lower
densities. Our results confirm the findings of previous studies that
parasites can protect marsh plants from overgrazing at sufficiently high
prevalence, but show that their ability to do so depends on host
density.

\par\null

\emph{}

\emph{Introduction}

A sizable body of research has been devoted to the role of top predators
in organizing communities and increasing ecosystem stability through
trait-mediated or density-mediated control of herbivorous prey
populations (Beschta \& Ripple 2009, Estes et al. 2011, Rosenblatt et
al. 2013, Ripple et al. 2014). Though small, parasites can also regulate
grazer populations and trigger trophic cascades, with powerful
ramifications for the structure and functioning of natural communities
(Wood et al. 2007, Buck \& Ripple 2017, Morton \& Silliman 2020). In
fact, because parasites can initiate trait-mediated indirect effects via
both consumptive and nonconsumptive pathways, they may be even more
likely than predators to trigger trait-mediated trophic cascades (Buck
\& Ripple 2017, Buck 2019). Unlike predators, however, parasites only
attack one host per life stage, meaning that the potential for parasites
to exert trait-mediated influence on ecosystems is largely a function of
the number of infected hosts (Buck 2019). The extent to which parasites
alter the structure and functioning of ecological communities through
cascading trait-mediated indirect effects necessarily depends on the
myriad processes that underlie disease transmission, the specifics of
the host traits being modified, and the ecological context in which
those traits are expressed. While there are many well-publicized
examples of parasites inducing dramatic changes in the behavior or
appearance of their host species, most changes to host traits take the
form of subtle alterations to behaviors such as feeding and locomotion.
Any detectable cascading effects as a result of these subtle changes may
only emerge when conditions favor high infection prevalence/intensity.
However, if the ecological influence of host behavior scales with host
density, the effects of parasites at a given prevalence may not be
detectable at high host densities when the aggregate ecological impacts
of hosts drown out the aggregate effects of parasites. Likewise, it may
be difficult to discern the influence of parasites at low host densities
when the ecological impacts of host behavior are negligible.

In southeastern USA salt marshes, the keystone gastropod\emph{Littoraria
irrorata} (hereafter referred to as snails or grazers) can form
high-density consumer fronts (\textasciitilde{}100--1000
snails/m\textsuperscript{2}) in response to sublethal drought stress
that weakens plant defenses (Silliman et al. 2005). High densities of
grazing snails can yield cascading vegetation loss even after the
initiating drought stress has passed, converting large swaths of highly
productive marsh into relatively unproductive mudflat habitats (Silliman
et al. 2005, 2013). In this system, parasitism by larval digenean
trematodes can be prevalent among snails within these consumer fronts
(\textgreater{}30\%), likely because the mudflat areas generated by
overgrazing are attractive to birds, which are definitive hosts of these
trematodes (Morton 2018, Morton \& Silliman 2020, Sharp \& Angelini
2020). Previous experiments and observational studies revealed that one
common trematode, \emph{Parorchis acanthus} , can increase plant
ecosystem resistance to die-off from drought-associated overgrazing by
reducing per capita grazing rates of snails (Morton 2018, Morton \&
Silliman 2020).

Both infection prevalence (0 -- \textgreater{}40\%) and density
(\textasciitilde{}50--2000 snails/m\textsuperscript{2}) of adult snails
vary considerably within marsh die-off areas throughout the southeastern
USA (Silliman \& Zieman 2001, Angelini et al. 2015, Morton \& Silliman
2020). Thus, the ability of trematode parasites to confer ecosystem
resistance to snail overgrazing is likely highly context dependent.
Previous field manipulations of \emph{P. acanthus} prevalence found that
infection prevalence was roughly proportionate to reductions in the
top-down impacts of grazers on marsh plants (Morton \& Silliman 2020).
Consistent with this, multi-site surveys found that the magnitude of
grazer- induced damage (radular wounds) to plants along die-off borders
decreased with increased prevalence of infection. Damage also increased
with snail density (Morton \& Silliman 2020). These results suggest that
the ability of parasites to ameliorate stress and slow the rate of
die-off expansion is likely dependent on both infection prevalence and
grazer density, but the latter has yet to be experimentally tested.
Therefore, a more robust understanding of how this parasite operates as
an agent of ecosystem resistance in this system necessitates determining
at what levels of host density parasites are likely to generate
cascading trait-mediated impacts.

Here, we experimentally determine at which host densities parasites
effectively ameliorate snail impacts on marsh vegetation via
trait-mediated indirect effects. We did so by manipulating the density
of grazer hosts in the field while holding infection prevalence constant
at an intermediate value. We predicted that at high grazer densities,
any ameliorating effects of parasitism on marsh plants would be
overwhelmed by the magnitude of snail grazing. At low grazer densities,
we predicted that parasitism would not lead to meaningful reductions in
snail grazing that would translate to positive effects on plant growth
and reproduction. We anticipated that parasites would be most effective
at increasing ecosystem resistance to overgrazing at moderate snail
densities, where grazers' top-down impacts on marsh vegetation are just
beginning to emerge.

\emph{Methods}

To determine how trematode infection influences marsh productivity at
different levels of grazer host density, we conducted a field
manipulation where we modified grazer host density while keeping
infection prevalence constant. In June 2016 we established 0.5
m\textsuperscript{2} caged plots in a structurally homogenous swath of
smooth cordgrass (\emph{Spartina} \emph{alterniflora} ) marsh within the
Hoop Pole Creek Clean Water Reserve in Atlantic Beach, North Carolina,
USA (34\selectlanguage{ngerman}°42'25.12'' N, 76°45'1.14'' W). The site was characterized by a
relatively uniform elevation, very low snail densities (\textless{}1
adult snail per m\textsuperscript{2}) and minimal visible signs of snail
grazing.

Snails used in the experiment were collected from a marsh die-off area
where snails were abundant and infection prevalence was known to be high
(Morton \& Silliman 2020). Collected adult snails (shell length
\textgreater{} 15mm) were transported back to the lab where their
infection status was determined using a previously described cercariae
shedding method that produces no false-positives (Morton 2018, Morton \&
Silliman 2020). We marked the shell of each infected snail with a red
dot using a non-toxic, water resistant paint pen while uninfected snails
were marked with a blue dot (Henry \& Jarne 2007). Snails were kept in
separate aquaria and provisioned with damp cordgrass wrack for
\textasciitilde{}3 weeks until they were deployed in the field.

Roofless cages (0.7 × 0.7 × 1 m) were constructed from untreated wooden
posts and galvanized hardware mesh. A strip of copper tape was applied
to the inner base of each cage, just above the sediment, to discourage
snail escapes. Caged plots were spaced at least 1-m apart to assure
independence of replicates---a design confirmed from past studies
(Silliman \& Zieman 2001, Silliman \& Bertness 2002, Morton \& Silliman
2020). Cages were buried 10-cm into the substrate to prevent snail and
mud crab migration in and out of cages, and to inhibit belowground
connections between plants inside and outside of the cages. Each plot
was assigned to one of 8 snail density treatments (20, 40, 50, 60, 70,
80, 90, and 100 snails/per 0.5 m\textsuperscript{2}) and one of two
parasite addition treatments (0 and 20\% infection prevalence). This
resulted in 16 total treatments (n = 4 replicates per treatment). The
snail densities used spanned the full range of adult snail densities
observed within marsh die-off areas at this site. The 20\% infection
prevalence used in the experiment reflected the average naturally
occurring summertime infection prevalence value for snails within local
die-off areas (Morton \& Silliman 2020). Uncaged plots marked at the
corners with colored PVC flags (n = 4) and partial cages with one open
side (n = 4) served as cage controls.

Before the beginning of the experiment, we removed any mud crabs and
snails from cages. We took measurements of several marsh characteristics
in all plots at the beginning of the experiment. We counted all \emph{S.
alterniflora} stems and measured the heights of ten randomly selected
stems in each plot. Random selection of stems was accomplished by
tossing a plastic dowel into plots and measuring the first 10 stems
touching the dowel. We constructed a height-to-biomass regression by
collecting 30 cordgrass stems of varying sizes from the marsh directly
adjacent to our experimental site. The stems were washed, their length
measured, and were dried at 70\selectlanguage{english}@C until they reached a constant weight.
We used the resulting height-to-biomass regression to estimate standing
cordgrass biomass in each plot. We also counted all juvenile
(\textless{} 0.5 cm in diameter) and adult fiddler crab burrows at both
the beginning and end of the experiment, because these organisms are
known to influence cordgrass growth by oxygenating the sediment through
burrowing (Bertness 1985, Daleo et al. 2007, Angelini \& Silliman 2012,
Gittman \& Keller 2013, Raposa et al. 2018).

Infected and uninfected snails within each plot were counted twice
weekly and replaced as necessary to maintain the assigned snail density
and infection prevalence treatments for the 3-month duration of the
experiment. During monitoring, any predatory mud crabs found within
plots were removed and their burrows plugged with marsh sediment to
discourage successful re-occupation.

We took final metrics of marsh vegetation characteristics in September
2016, 12 weeks after the beginning of the experiment. All snails were
removed and dissected to confirm infection status. Because snail grazing
had dramatically reduced stem densities in many plots, we were able to
measure all stems within each plot to generate final plot biomass
estimates by calculating and summing individual biomass estimates.
Belowground biomass cores (15-cm diameter x 25-cm height) were also
taken from the center of plots. Cores were washed and root material
separated from rhizomes. Roots and rhizomes were dried to a constant
weight at 70@\selectlanguage{english}C and weighed.

The possible influence of caging artifacts on marsh invertebrates and
cordgrass growth was assessed using paired t-tests. Specifically, we
compared the number of total fiddler crab burrows, final stem density,
initial aboveground biomass, final aboveground biomass, change in
aboveground biomass, and number of flowering stems between cage controls
and open controls.

Because fiddler crabs can positively influence cordgrass growth
(Bertness 1985, Gittman \& Keller 2013), we examined the relationship
between burrow density and cordgrass stem density using a linear model.
We validated model fit by examining model residuals, their distribution
relative to fitted values, and normal Q-Q plots (\emph{car} package,
Chambers and Hastie 1992).

In accordance with our hypotheses, models including the interaction
between parasite infection (uninfected versus infected) and snail
density (20, 40, 50, 60, 70, 80, 90, or 100 snails per 0.5
m\textsuperscript{2}) were fit for each response metric (belowground
biomass, aboveground biomass, shoot density, and number of flowering
stems). Additionally, all models initially included fiddler crab burrow
density as a covariate. However, this covariate was ultimately not
significant for any model, and was therefore dropped and each model
re-run to include only the interaction term. We analyzed the interactive
effects of infection status and snail density on belowground biomass
using a two-way analysis of variance (ANOVA), with data log-transformed
prior to analysis to meet model assumptions. Change in aboveground
biomass was analyzed with a linear model, with initial aboveground
biomass values for each plot subtracted from corresponding final values.
Model assumptions were verified through assessments of homogeneity of
variance (Levene's test, \emph{P} \textgreater{} 0.05) and examination
of fitted residuals and normal Q-Q plots. The number of cordgrass
inflorescences was modeled with a negative binomial generalized linear
model (\emph{glmmTMB} , Brooks et al. 2017). Model appropriateness and
fit were confirmed through examination of simulated scaled residuals
(\emph{DHARMa} , Hartig 2022).

For each response metric, the significance of the interaction model was
examined in an Analysis of Deviance Table using Wald chi-square tests
(\emph{car} package, Chambers \& Hastie 1992). For all models, Tukey's
post-hoc comparisons were used to assess pairwise differences for any
significant treatment or interactive effects in the models
(\emph{emmeans} package, Lenth et al. 2021). All analyses were performed
in the R statistical computing environment (v. 4.1.3; R Core Team 2018).

\emph{Results}

Initial estimated plant biomass was not significantly different among
treatments (\emph{P} = 0.057, one-way ANOVA). Additionally, there were
no significant differences in the densities of fiddler crab burrows,
plot elevation, and porewater salinity among treatments
(\emph{P}\textgreater{} 0.6, one-way ANOVA, for all response variables).
The final mean infection prevalence for each treatment did not differ
from initial assigned conditions. No false negatives were observed when
snails were dissected following the end of the experiment. In treatments
where they had been added, the mean weekly deviation in snail density
never exceeded 10\% for any plot. Exogenous snails very rarely found
their way into experimental plots and were removed during weekly snail
counts (\textless{}1 snail per plot per week). Those that were
discovered within plots were typically below the size threshold (shell
height \textless{}10 mm) associated with active grazing (Silliman \&
Bertness 2002). On average, less than one mud crab per week was removed
from caged snail exclusion controls.

Cages used in this study did not have any detectable effects on
cordgrass growth or the abundance of resident fiddler crabs
(\emph{P}\textgreater{} 0.07 for all t-tests, Table S1). Resident
fiddler crab abundance tracked positively with cordgrass stem density,
though the relationship was weak (F\textsubscript{1,62} = 4.108,
\emph{P} = 0.047, Table S2, Fig. S1). We found no effects of snail
density (F\textsubscript{1,7} = 1.367, \emph{P} = 0.241) nor parasitism
on cordgrass belowground biomass (F\textsubscript{1,7} = 0.045, \emph{P}
= 0.834, Fig. S2).

Infection status and densities of snails significantly interacted with
one another to affect aboveground biomass changes (\emph{P} = 0.037,
Table S3). Aboveground growth was generally higher in plots with
infected snails compared to those with uninfected snails (infected: 13 g
\selectlanguage{ngerman}± 1.67 {[}estimated marginal mean ± standard error{]}, uninfected: 0.269
± 1.67, \emph{P} \textless{} 0.0001), and plots with parasitized snails
were able to sustain aboveground production at higher snail densities
than those with uninfected snails (Fig. 1). On average, plots with
uninfected snails began to experience decreases in aboveground biomass
at 64 snails/0.5 m\textsuperscript{2}, while net losses in aboveground
biomass in plots with infected grazers began at densities of 84
snails/0.5 m\textsuperscript{2} (Fig. 1). We also found that,
unsurprisingly, plots with greater densities of snail grazers lost more
aboveground biomass (or demonstrated a gain that was much smaller than
in plots with lower snail densities) (Fig. 1). The point at which snail
density and infection status interacted to alter aboveground biomass was
50 snails/0.5 m\textsuperscript{2}. When marsh plots were stocked with
fewer than 50 snails, similar levels of aboveground growth occurred,
regardless of parasite infection. However, trends in aboveground biomass
production diverged at snail densities of 50 (infected: 22.4 g {[}lower
and upper 95\% CIs, 18.61-26.2{]}, uninfected: 12.1 g {[}8.33-15.9{]}),
with plots containing uninfected snails maintaining higher amounts of
aboveground biomass at every density greater than or equal to 50 snails.

Although the number of inflorescences appeared to differ according to
the presence of parasitized snails (Table S4), post-hoc tests revealed
no significant increases flowering in plots with infected snails
(\emph{P} = 1, Table S5). Plots with 20 snails had more flowering
cordgrass stems than did plots with 50 (\emph{P} = 0.035), 60 (\emph{P}=
0.02), or 70 (\emph{P} = 0.008, Table S5) snails. No other clear
differences in quantity of inflorescences existed between levels of
snail density (Fig. S2, Table S5). Because plots with high snail
densities (\selectlanguage{english}[?]80 snails/0.5 m\textsuperscript{2}) were highly denuded by
the conclusion of the experiment, the number of inflorescences could not
be determined for the highest snail density treatments (Table S5).

\emph{Discussion}

Our field manipulation of grazer host density and trematode parasite
presence confirmed the findings of previous studies that \emph{Parorchis
acanthus} can protect foundational plants by generating a trait-mediated
trophic cascade, and showed that the emergence of these ameliorating
effects occurs at intermediate levels of grazer host density. Our
results underscore the context-dependent nature of trophic facilitation
by parasites and inform a more robust understanding of when and where
parasitism may promote ecosystem resistance to overgrazing in this
system.

At an average infection prevalence, the trait-mediated impacts of
trematode parasitism on snail grazing increased the grazer density
threshold at which there was a net loss of cordgrass aboveground biomass
by nearly 25\% (Fig. 1). Additionally, the ameliorating trait-mediated
indirect effects of \emph{P. acanthus} on marsh aboveground growth
emerged at intermediate densities of hosts (50 snails/0.5
m\textsuperscript{2} or 100 snails/m\textsuperscript{2}). At low grazer
host densities, parasite reduction of per-capita grazing rates did not
translate to detectable increases in aboveground plant biomass, likely
because the top-down impacts of these snails only begin to emerge at
densities of 60--144 snails per m\textsuperscript{2} (Silliman \& Zieman
2001, Silliman \& Bertness 2002). While we had predicted that at high
snail densities, the negative impacts of snail grazing on cordgrass
would overwhelm any positive impacts of parasite behavior modification
at the chosen prevalence, we did not find evidence for this at the
highest levels of snail density used in our experiment. Such a threshold
density almost certainly does occur in nature
(\emph{Littoraria}densities within consumer fronts can be as high as
2000 snails/m\textsuperscript{2}) but this threshold may be considerably
higher than highest density of snails used in our experiment (100
snails/0.5 m\textsuperscript{2} or 200 snails/m\textsuperscript{2}).

While our results indicate that the ameliorating trait-mediated impacts
of parasitism by \emph{P. acanthus} are mediated by grazer host density,
the particular density at which these parasites have the potential to
positively impact marsh plants is likely highly variable. For instance,
a previous study in this system revealed that parasitism by \emph{P.
acanthus} led to significant reductions in snail grazing that translated
to cascading effects on aboveground biomass at 10\% and 30\% infection
prevalence and snail densities of \textasciitilde{}143
snails/m\textsuperscript{2} (Morton \& Silliman 2020). While we found
linear relationships between snail density and grazing pressure in the
present study, previous work has shown that this functional relationship
can be linear (Silliman \& Zieman 2001), logarithmic (Atkins et al.
2015), or exponential (Renzi \& Silliman 2021), depending on the
specific context. This variation is likely mediated by local differences
in abiotic stress, nutrient regime, vegetation characteristics,
predation risk, benthic productivity, snail size structure (Atkins et
al. 2015), and the strength of positive species interactions that
enhance cordgrass growth (Bertness \& Miller 1984, Bertness 1985,
Gittman \& Keller 2013, Atkins et al. 2015, Renzi \& Silliman 2021).

Previous investigations in this system found that trait-mediated
reductions in the top-down impacts of grazers on cordgrass biomass
by\emph{P. acanthus} was roughly proportionate to the level of infection
prevalence (Morton 2018, Morton \& Silliman 2020). However, the
magnitude of the effect of trematodes on per capita snail grazing may
vary with host condition, environmental factors or host genetics (Leung
et al. 2010, Thomas et al. 2011). Additionally, different parasite
species may yield varying trait-mediated indirect effects depending on
the mechanisms through which they impact host physiology and modify
snail feeding. The trematode species used in this study, \emph{P.
acanthus} , was chosen for its relative abundance at marsh die-off areas
within our study site, but there are at least four trematode species
that infect \emph{Littoraria} which can occur at varying prevalence and
may have different effects on snails (Holliman 1961, Coil \& Heard 1966,
Heard 1968, 1970). As a result, the density threshold at which snails
exert strong control over cordgrass growth may vary depending on the
trematode component community.

Trematode infection prevalence in \emph{Littoraria} shows great spatial
variation at both local and regional scales (Rossiter 2013, Morton \&
Silliman 2020). While many of the specific processes that mediate
infection prevalence in \emph{Littoraria} have yet to be experimentally
tested, snail size, spatial variation in definitive host (bird) density,
and tidal flooding regime, appear to be important determinants (Morton
\& Silliman 2020). Snail density itself may mediate infection dynamics
in the field. Large densities of grazer hosts could result in lower
individual encounter rates with mobile trematode miracidium, diluting
infection risk and depressing prevalence (Mooring \& Hart 1992, Buck \&
Lutterschmidt 2017). To date, observational and manipulative studies in
this system have examined both parasite infection dynamics and parasite
trait-mediated indirect effects across a fairly narrow range of host
densities (Morton \& Silliman 2020). Examining trematode infection
dynamics across a wider range of \emph{Littoraria} densities is
necessary to determine whether dilution effects mediate snail infection
at high densities.

The effects of both grazer density and parasitism on aboveground
production were clear from our field experiment, but we detected no
effect of either of these factors on cordgrass root or rhizome biomass
(Fig. S2). Belowground growth of marsh plants can be affected through
direct consumption by herbivores, including snow geese (Smith \& Odum
1981) and burrowing crabs (Coverdale et al. 2012, Gittman \& Keller
2013, Daleo et al. 2015). While \emph{Littoraria} does not directly
graze on belowground plant tissues, snail grazing could have indirect
impacts on cordgrass belowground biomass through wholesale destruction
of cordgrass aboveground tissues. However, removal of cordgrass roots
and rhizomes is largely mediated by processes of microbial decay that
likely operate on timescales greater than the duration of our experiment
(Benner et al. 1987, Blum 1993, Blum \& Christian 2004). As a result,
determining how grazers and their parasites mediate cordgrass
belowground biomass requires future investigation.

Parasite alteration of host behavior, whether it be manipulation, an
antiparasitic tactic of the host, or merely a byproduct of infection, is
likely common, though we still know little about how different parasites
affect their hosts---even ecologically influential ones (Poulin et al.
1998, Morton 2018, Buck 2019). Additionally, the ecological reach of
parasites through trait-mediated indirect effects is not reducible to
the prevalence of behavior-altering parasites in a host population. Our
results underscore the importance of host density as one determinant of
the ecological reach of parasite-induced trait-mediated indirect
effects. However, there are numerous factors that may ultimately
influence such effects. Host populations, behaviors, and the
environments in which those hosts exert influence through their behavior
can vary widely in space and time. Fully incorporating parasites into
our understanding of how ecosystems work necessitates that we not only
quantify the often-subtle ways that parasites alter the behavior of
hosts, but that we determine under what conditions these alterations can
add up to have a wider ecological influence.

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Bertness MD (1985) Fiddler crab regulation of Spartina alterniflora
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\emph{}

\emph{Figures}\selectlanguage{english}
\begin{figure}[H]
\begin{center}
\includegraphics[width=0.70\columnwidth]{figures/image1/image1}

\end{center}
\end{figure}

\textbf{Figure 1:} Plot-level measurements of change in aboveground
biomass according to parasite infection and snail density treatments.
Dashed line at zero indicates no change in aboveground biomass, positive
values indicate an increase in biomass during the study, and negative
values indicate a loss in biomass. Lines depict modeled relationships,
and associated shading represent 95\% confidence intervals. The multiple
R\textsuperscript{2} for this relationship was 0.83, with an associated
correlation coefficient of 0.912.

\selectlanguage{english}
\FloatBarrier
\end{document}

