Pomacea maculata (Island Apple Snail) Invasion in Seasonal
Wetlands on Florida Ranchland: Association with Plant-
Community Structure and Aquatic-Predator Abundance
Colleen Smith, Elizabeth H. Boughton, and Steffan Pierre
Southeastern Naturalist, Volume 14, Issue 3 (2015): 561–576
Full-text pdf (Accessible only to subscribers.To subscribe click here.)
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C. Smith, E.H. Boughton, and S. Pierre
22001155 SOUTHEASTERN NATURALIST 1V4o(3l.) :1546,1 N–5o7. 63
Pomacea maculata (Island Apple Snail) Invasion in Seasonal
Wetlands on Florida Ranchland: Association with Plant-
Community Structure and Aquatic-Predator Abundance
Colleen Smith1,3,*, Elizabeth H. Boughton1, and Steffan Pierre2
Abstract - The invasive Pomacea maculata (= Pomacea insularum) (Perry) (Island Apple
Snail) is becoming increasingly abundant in the southeastern US with potentially detrimental
effects on wetland vegetation and water quality. Here, we investigate the association of
plant-community structure and aquatic-predator abundance with Island Apple Snail invasion
in seasonal wetlands on a cattle ranch in south-central Florida. We found a negative
correlation between Island Apple Snail abundance and abundance of its preferred forage
species, raising concerns that the snail may have already altered plant communities in
these seasonal wetlands. We also found a negative correlation between Island Apple Snail
abundance in wetlands and distance to the nearest ditch. In a snail-growth experiment, we
found a negative effect of species evenness and a positive effect of total vegetation cover
on Island Apple Snail weight gain. To understand Island Apple Snail invasion success in
seasonal wetlands, more research is needed on the relative importance of landscape- versus
local-scale wetland characteristics and how resources such as preferred forage versus egglaying
sites affect snail-population growth.
Introduction
Invasive species are a major driver of global environmental change. In addition
to being economically harmful, species invasions reduce native biodiversity and
change community composition, often interacting with other global-change drivers
to do so (Vitousek et al. 1996). The introduction of invasive species into wetlands is
an issue of particular concern to humans because wetlands provide many important
ecosystem services including mitigating the impacts of storms and floods and supplying
water (Costanza et al. 1997), and acting as sources of biodiversity (Zedler
and Kercher 2005). The introduction of invasive species into wetlands has the potential
to disrupt these ecosystem services by changing community composition and
nutrient levels and decreasing wetland diversity (e.g, Fickbohm and Zhu 2006, Silliman
and Bertness 2004). Invasive invertebrates in wetlands can impact multiple
trophic levels in a community through competition, predation, and grazing, and can
even have cascading effects on nutrient levels (e.g., Carlsson et al. 2004).
Invasive apple snails (Pomacea spp., see Appendix 1 for common and scientific
names and authority of all study species) are invertebrates known to change wetland
structure and function, and they have been ranked first in pest potential out of
1MacArthur Agro-ecology Research Center, 300 Buck Island Ranch Road, Lake Placid, FL
33852. 2Department of Biology, University of Central Florida, Orlando FL 32816. 3Current
address - Department of Ecology, Evolution, and Natural Resources, Rutgers University,
New Brunswick, NJ 08901. *Corresponding author - colleen.m.smith@rutgers.edu.
Manuscript Editor: Scott Markwith
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non-native mollusks in the US (Cowie et al. 2009). Native to South America, Pomacea
maculata (Island Apple Snail), is becoming a non-native of growing concern in
Florida, Georgia, and Texas (Rawlings et al. 2007). The first genetically confirmed
specimen of the snail was collected in Florida in 2002, but the snail’s eggs may have
been observed in the state as early as 1991 (Rawlings et al. 2007). Island Apple
Snail was first observed on Buck Island Ranch, a commercial cattle ranch in Lake
Placid, FL, in 2010. The snail’s spread into agricultural areas of Florida may have
detrimental economic implications for both private landowners and public agencies
through vegetation changes, disease spread, and water-quality alterations. The
closely related Pomacea canaliculata (Golden Apple Snail) is a significant pest of
Oryzopsis sativa L. (Rice) crops in southeast Asia (Halwart 1994) and has also been
shown to drastically alter ecosystems, changing a wetland in Thailand from macrophyte-
to algae-dominated (Carlsson et al. 2004). Less is known about the ecology
of the Island Apple Snail in its invaded range, although it is possible that studies of
the Golden Apple Snail have confused the 2 morphologically similar species where
their ranges overlap, for example in southeast Asia (Horgan et al. 2014). Laboratory
studies conducted on the Island Apple Snail have demonstrated that the species is
a generalist herbivore that eats several types of wetland plants and has high rates
of consumption and fecundity (Baker et al. 2010, Barnes et al. 2008, Burlakova et
al. 2008, Morrison and Hay 2011). In some cases, the consumption of vegetation
is so high that it can be detrimental to water quality. In humanmade Typha sp. (cattail)
marshes functioning as storm-water treatment areas (STA) in South Florida,
the snail defoliated enough submerged aquatic vegetation to increase phosphorous
outflow and cause 1 STA to be shut down until the snails could be controlled (Andreotta
et al. 2015).
Along with its potentially harmful impacts on wetland structure and function,
the invasive snail has been found to serve as a food source for endangered
bird species such as Rostrahamus sociabilis (Snail Kite) and Aramus guarauna
(Limpkin) (Cattau et al. 2010, Tanaka et al. 2006). Compared to data from historical
bird surveys, both of these birds have been more frequently observed at
Buck Island Ranch since the arrival of the invasive snail (E.H. Boughton, pers.
observ.). It is not clear if other predators of the Island Apple Snail are present
in Florida, although some mammals (i.e., Procyon lotor [Raccoon] and Lontra
canadensis [North American River Otter, hereafter, Otter]) appear to prey on the
snail. Aquatic predators such as fish and Belostoma spp. (giant water bugs) could
also prey on apple snail young.
Little is known about Island Apple Snail ecology in the ditches and seasonal
wetlands where it is introduced. Most research on the snail to date was conducted
in the laboratory (e.g., Burlakova et al. 2008, Morrison and Hay 2011). Feeding
trials show that the Island Apple Snail is an indiscriminate consumer of aquatic
wetland plants that consumed 46 out of 52 macrophyte species tested (Horgan et
al. 2014). Many of these plant species are present in the ditches and seasonal wetlands
embedded in cattle ranches, an important land use in Florida that composes
~1/6th of the state’s land and ~404,686 ha in the Northern Everglades, where the
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hydrology of the watershed is modified by miles of ditches and canals (Swain et
al. 2013). The successful invasion of apple snails into wetlands and ditches on
cattle ranches is likely affected by connectivity of water sources and wetland abiotic
and biotic attributes.
Because the Island Apple Snail is an herbivore, its invasion success into some
wetlands may be affected by attributes of the wetland-plant community such as
richness. Plant communities that are species rich may have a higher probability of
containing preferred food items. The Island Apple Snail may be a potential prey
item for a multitude of species, so the wetland aquatic-animal community could
also affect snail invasion success. Given that little is known about the factors that
affect Island Apple Snail invasion success, we investigated the association of landscape
characteristics, plant diversity, and potential aquatic-predator abundance
with Island Apple Snail abundance and growth in seasonal wetlands on a cattle
ranch in south-central Florida. We also surveyed 2 large ditches to examine the
plant species utilized by the Island Apple Snail for egg-laying.
Methods
Site selection
We conducted our study at MacArthur Agro-ecology Research Center (MAERC;
a division of Archbold Biological Station) located on Buck Island Ranch, a commercial
cattle ranch in Lake Placid, FL. The ranch contains 2 pasture types, improved
pasture and semi-native pasture, which differ in species composition, grazing
pressure, and history. Both contain seasonal wetlands that typically hold water for
~6 months of the year. Improved pastures are dominated by the introduced forage
grass Paspalum notatum (Bahia Grass), are fertilized annually with nitrogen, and
were fertilized with phosphorous until 1986. They are also grazed more heavily
than semi-native pastures. Semi-native pastures were historically unfertilized, and
their wetlands are more species rich than those in improved pastures (Boughton et
al. 2010).
Island Apple Snail egg masses were first observed on Buck Island Ranch in 2010
on culverts in Harney Pond Canal, which borders about half of the property (Fig. 1).
They were subsequently observed on culverts and emergent vegetation in internal
ditches and then in wetlands. The snails’ egg masses appeared to be especially
abundant in the ranch’s East Marsh, so we chose wetlands in and around this area
of the ranch for both parts of our study (Fig. 1). We chose a total of 12 wetlands, 6
in improved pasture and 6 in semi-native pasture (Table 1). In 2007, all wetlands
involved in the study were fenced to exclude cattle in 2007 as part of a different
study. We selected fenced wetlands to insure that cattle would not interfere with our
experimental enclosures.
To understand the snail’s distribution along the ditches in the East Marsh, we
conducted preliminary ditch surveys by walking along ~3300 m of the 2 primary
ditches in the East Marsh (Fig. 1) and recording the locations of each Island Apple
Snail egg mass observed as well as the plant species or structure (i.e., culvert or
fence post) on which the egg mass occurred. Island Apple Snails lay bright pink
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eggs on emergent vegetation or structures, making the egg masses relatively easy
to spot from ditch banks.
Figure 1. Map of the wetlands surveyed in the East Marsh of Buck Island Ranch. Filled
wetlands indicate that Island Apple Snail was encountered in traps and/or empty shells were
encountered along study transects. Outlined wetlands with no fill indicate that the Island
Apple Snail was not encountered.
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Wetland surveys
We measured relative Island Apple Snail abundance and the relative abundance
of its potential predators using un-baited crayfish traps, which are effective tools
for measuring apple snail abundance when mark–recapture methods are used (Darby
et al. 1997). We were able to conduct trapping for apple snails in 6 wetlands (4
in improved pasture and 2 in semi-native pasture) given time constraints imposed
by the drying down of wetlands at the end of Florida’s wet season in November. We
placed 8 crayfish traps in each of the 6 wetlands for 6 consecutive days between 21
October 2013 and 26 November 2013 at water depths between 15 cm and 45 cm.
We checked traps daily for the presence of the Island Apple Snail as well as any
omnivorous or carnivorous organisms, which we considered the snail’s potential
predators. We marked with identification numbers all Island Apple Snail individuals
caught in traps so that we could calculate recapture probability. However, we
did not recapture any individuals, and thus, we used the number of Island Apple
Snails trapped as a measure of relative abundance. We released trapped individuals
10 steps away from the trap. We also recorded the identity and number of potential
predators (snakes, sirens, turtles, fish, and crayfish) and identified fish and crayfish
to species before we released them.
To obtain additional data on the relative abundance of potential Island Apple
Snail aquatic predators in each of the 12 wetlands, we dip-netted for macroinvertebrates,
crayfish, and fish at 10 random points, evenly distributed among the center,
north, south, east, and west quadrants in each wetland. We dip-netted twice at each
Table 1. Size, water-quality, and dominant vegetation of wetlands used in the surveys and enclosure
experiment. E = enclosure experiment, S = shell survey, T = trapping survey, SN = semi-native pasture,
I = improved pasture, TP = total phosphorous, TN = total nitrogen, ALTPHI = Alternanthera
philoxeroides, AMPMUH = Amphicarpum muehlenbergianum, CENASI = Centella asiatica, CLAJAM
= Cladium jamaicense, DIOVIR = Diodia virginiana, HYMAMP = Hymenachne amplexicaulis,
LEMMIN = Lemna minor, PANHEM = Panicum hemitomon, PASACU = Paspallum acuminatum,
POLPUN = Polygonum punctatum, RHYINU = Rhynchospora inundata, and SALMIN = Salvinia
minima.
Wetland Area Depth TP TN Snails
ID Used for Pasture (m2) (cm) pH (ppm) (ppm) (Y/N) Dominant vegetation
49 S, E SN 2.02 21 4.53 0.06 2.41 N AMPMUH, RHYINU
84 S SN 0.78 25 4.84 0.08 1.89 Y CLAJAM, PANHEM
91 T, S SN 1.27 47 5.17 0.10 2.83 Y PANHEM, PASACU
148 T I 2.50 60 5.22 0.29 4.22 N ALTPHI, SALMIN
151 T I 3.08 53 4.52 0.55 1.02 N PANHEM, ALTPHI
196 S I 1.67 37 6.03 0.23 3.33 N ALTPHI, HYMAMP
210 S, E SN 1.06 45 4.32 0.16 3.08 N CENASI, RHYINU
225 S SN 2.20 62 4.6 0.09 2.84 N PANHEM, DIOVIR,
CENASI
258 T, S, E SN 2.02 48 5.04 0.13 1.76 Y ALTPHI, HYMAMP
271 T, E I 1.85 38 5.38 0.17 3.48 Y ALTPHI, LEMMIN
295 T, S, E I 1.10 46 6.15 0.42 4.33 Y ALTPHI, PANHEM
317 T I 2.31 50 5.98 0.71 3.72 Y LEMMIN, ALTPHI,
POLPUN
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point and recorded the number and identity of all fish, crayfish, and water beetles.
We considered the total number of potential aquatic predators caught by trapping
and by dip-netting in a wetland to be the wetland’s relative abundance of potential
aquatic predators. Because wetlands were fenced, many terrestrial predators were
likely excluded, including Otters and Raccoons, and our sampling technique did not
permit us to measure the abundance of these and potential avian predators such as
Limpkins and Snail Kites.
To measure plant-species richness and community composition in the 12 wetlands,
we recorded the identity of all plant species in 1-m2 quadrats at 15 random
stratified points (stratified by center, north, south, east, and west) in each wetland
between 25 September and 31 October 2013.
To obtain additional data on Island Apple Snail population sizes and predation in
the wetlands, we counted the number of empty shells observed along random transects
that crossed 9 of the wetlands that were sufficiently dry to conduct sampling
by 9 December 2013. For each wetland, we recorded the number of empty Island
Apple Snail shells encountered within 1 m on either side of the transect. Given that
the empty shells we encountered were generally large in size, and that shells were
often grouped around bird nests, these shells were probably the remains of bird
predation. Limpkins are sighted frequently in wetlands at Buck Island Ranch. We
used the number of Island Apple Snail shells encountered in each wetland as an
indirect measure of the snail’s abundance, assuming that birds predate upon Island
Apple Snails primarily in the wetland where their nest is sited and most heavily in
wetlands where the snail is common. This measure of snail abundance is not ideal
because snail shells can persist for a long time, and predatory activity many not
be uniform across wetlands, therefore results based on empty shells must be interpreted
with caution.
In September and October 2006 and again in October 2008, we took pH, phosphorous,
and nitrogen measurements at 3 stratified points. We collected a grab
sample at each of the 3 points using a 1-m-long sampling probe. The grab samples
were collected at the central staff gauge, 10 m from the staff gauge and 2–3 m from
the edge. At each sampling point, acid-washed 100-ml bottles were filled with unfiltered
water and preserved with H2SO4. We measured pH with a protable YSI 556
multi-probe meter.
Data analysis: wetland surveys
We used Spearman’s rank correlation to examine the relationship of habitat factors
(plant richness, relative abundance of fish and insects, relative abundance of
structural plants and preferred plants [defined below], and distance to nearest ditch)
with abundance of trapped Island Apple Snails in each wetland (n = 6).
We used linear regression with a model-selection approach to determine which
factors were associated with the number of empty Island Apple Snail shells encountered
along transects in each wetland (n = 9). We selected the model with the
lowest corrected Akaike information criterion (AICc) value and the highest weight
(Burnham and Anderson 2002). In total, we assessed 8 models including a null
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model. All analyses were conducted in the R statistical program (R Development
Core Team 2008).
The independent variables were plant species richness, distance to the nearest
ditch, wetland size, fish and insect relative abundance, relative frequency of
preferred forage species (Sagittaria lancifolia [Bulltongue Arrowhead], Eicchornia
crassipes [Water Hyacinth], Limnobium spongia [American Spongeplant],
Panicum repens L. [Torpedo Grass], Utricularia spp. [bladderworts], and Bacopa
caroliniana (Walter) B.L. Rob. [Blue Waterhyssop]; Baker et al. 2009, Burlakova
et al. 2008, Morrison and Hay 2011), and relative frequency of structural plants
(Hymenachne amplexicaulis [West Indian Marsh Grass], Water Hyacinth, Myrica
cerifera [Wax Myrtle], Pontederia cordata [Pickerelweed], Bulltongue Arrowhead,
Thalia geniculata [Bent Alligator-flag], American Spongeplant, Cephalanthus occidentalis
[Buttonbush], Eupatorium capillifolium [Dog-fennel], Sabal palmetto
[Cabbage Palm], Sesbania herbacea [Bigpod Sesbania], and Juncus effusus [Common
Rush]). These structural plant species have suitably rigid stems for Island
Apple Snail to use for egg laying and include all plant species on which we encountered
5 or more egg masses during the initial ditch survey, in addition to species
with rigid stems and/or leaves that are common in wetlands but not ditches, and
thus would not have been observed in the ditch survey. We also examined the effect
of pasture type on snail abundance measures using Mann-Whitney U tests.
Enclosure experiment
We conducted enclosure experiments to directly test the role of plant-species
richness on the growth rate of the Island Apple Snail. We placed three 1.6-m tall
x 0.5 m2 circular wire-mesh enclosures in each of 6 wetlands—3 in improved
pasture and 3 in semi-native pasture. We used garden staples to keep enclosures
in the ground and covered the tops of the enclosures with mesh cloth to keep
snails from escaping and to keep birds from consuming snails. For each pasture
type, the number of plant species in enclosures ranged from 1 to 9. Each wetland
contained 3 enclosures, 1 each of low, medium, and high species richness (1–3,
4–6, and 7–9 plant species, respectively). In addition, we established three 0.5-
m2 control plots in each wetland with equivalent plant species richness. We chose
enclosure sites and control plots by surveying wetlands with 0.5-m2 hoops to find
the appropriate richness levels at water depths that ranged from 20 to 40 cm. To
account for differences between enclosures, we measured total plant percent cover
in each and included this as a covariate.
We collected Island Apple Snails for the enclosure experiment from wetlands
and ditches on Buck Island Ranch that were not part of the study. Before adding
snails to enclosures, we weighed individuals and measured shell length and height.
We grouped snails into a large-size class and a small-size class. Snails in the smallsize
class ranged in weight from 2.0 g to 9.1 g, and snails in the large-size class
ranged in weight from 35.8 g to 80.0 g. In October of 2013, we marked 1 large and
1 small snail with nail polish for each enclosure, placed them in the enclosure; left
them for 4–6 weeks, and then removed and weighed them.
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Data analysis: enclosure experiment
We used Spearman’s rank correlation to determine which factors were associated
with Island Apple Snail percent weight change in enclosures. The covariates
were plant-species richness, total plant percent cover, evenness, pH, total P, and
total N. We used Mann-Whitney U tests to analyze the effect of pasture type (improved
pasture or semi-native pasture). In this analysis, we averaged snail response
from each enclosure and considered snail response an independent replicate, resulting
in 12 snail responses collected from 5 wetlands (Table 2). Note that we retrieved
no snails in 1 wetland (wetland 49).
Table 2. Plant-species composition and number of snails retrieved for each enclosure. An “e” in the
“Snails retrieved” column denotes that the snail was found estivating. ID = enclosure ID, n = plant
richness, ALTPHI = Alternanthera philoxeroides, AZOLLA = Azolla sp., CARABL = Carex albolutescens,
COMDIF = Commelina diffusa, DIOVIR = Diodia virginiana, EUPCAP = Eupatorium capilllifolium,
HYMAMP = Hymenachne amplexicaulis, JUNEFF = Juncus effusus, JUSANG = Justicia
angusta, LEEHEX = Leersia hexandra, LEMMIN = Lemna minor, LIMSPO = Limnobium spongia,
LUDREP = Ludwigia repens, PANHEM = Panicum hemitomon, PASACU = Paspalum acuminatum,
PHYNOD = Phyla nodiflora, PONCOR = Pontedaria cordata, POLPUN = Polygonum punctatum,
PROPEC = Proserpinaca pectinata, SACSTR = Sacciolepis striata, SAGLAN = Sagittaria lancifolia,
THAGEN = Thalia geniculata, and UTRFOL = Utricularia foliosa.
Wetland Total % Snails % weight
ID ID n Plant species cover retrieved change
1 49 2 PANHEM, SAGLAN 60 1-e -
2 5 LEEHEX, PANHEM, PONCOR, SACSTR, SAGLAN 67 0 -
3 8 HYMAMP, LUDREP, PANHEM, POLPUN, PROPAL, 89 1-e -
SACSTR, SAGLAN, UTRFOL
4 151 2 ALTPHI, PANHEM 85 1 2.68
5 5 ALTPHI, AZOLLA, LIMSPO, PANHEM, POLPUN 88 1 -0.02
6 8 ALTPHI, AZOLLA, JUNEFF, LEEHEX, LEMMIN, 137 2 0.42
PANHEM, POLPUN, SACSTR
7 210 3 HYMAMP, PASACU, POLPUN 32 1 -0.09
8 6 LEEHEX, DIOVIR, JUSANG, PANHEM, PASACU, 39 1 -0.17
SAGLAN
9 9 HYMAMP, DIOVIR, JUSANG, PASACU, PANHEM, 61 1 -0.06
PONCOR, PROPEC, SAGLAN, UTRFOL
1 0 258 1 LEMMIN 42 2 0.08
1 1 4 ALTPHI, HYMAMP, JUNEFF, POLPUN 77 1 0.08
12 7 ALTPHI, AZOLLA, HYMAMP, JUNEFF, LEMMIN, 82 1 0.05
POLPUN, UTRFOL
1 3 271 1 PANHEM 45 1 0.06
1 4 5 ALTPHI, HYMAMP, LEMMIN, PANHEM, POLPUN 34 1 -0.08
15 7 ALTPHI, CARABL, JUNEFF, LEMMIN, PHYNOD, 83 0 -
POLPUN, THAGEN
1 6 295 3 PANHEM, POLPUN, LEMMIN 70 1 0.03
17 6 ALTPHI, COMDIF, EUPCAP, LEMMIN, PANHEM, 65 2-e -
POLPUN
18 9 SAGLAN, EUPCAP, POLPUN, LEMMIN, PANHEM, 82 1-e -
COMDIF, ALTPHI, CARABL, LEEHEX
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Results
Ditch and wetland surveys
We observed 509 egg masses on 17 plant species over 4 d of sampling between
12 August and 20 August 2013. The plants on which we observed egg
masses tended to be either woody shrubs or herbaceous plants with sturdy stems
(Table 3).
In total, Island Apple Snail occurred in 6 out of the 12 wetlands studied (Table 1,
Fig. 1). We encountered the snail in 4 out of the 6 wetlands in which we conducted
trapping and in 5 out of the 9 wetlands in which we conducted transect surveys
(Table 1).
Wetland survey: trapping
Island Apple Snail abundance obtained by trapping was negatively correlated
with the frequency of preferred plant species (ρ = -0.91, P = 0.01; Fig. 2). There was
no relationship between Island Apple Snail abundance and any other variable—
plant richness (ρ = -0.09, P = 0.87), fish abundance (ρ = 0.46, P = 0.35), insect
abundance (ρ = -0.44, P = 0.38, frequency of structural plants (ρ = 0.43, P = 0.39),
or distance to the nearest ditch (ρ = 0.66, P = 0.15). There was no difference in
snail abundance between wetlands embedded in improved or semi-native pastures
(P = 0.24).
Wetland survey: transects
The model that best explained the abundance of empty Island Apple Snail shells
per wetland contained the main effect of distance to the nearest ditch (Table 4). The
Table 3. The number of egg masses encountered on plant species and manmade structures.
Plant species or structure Common name Number of egg masses
Hymenachne amplexicaulis West Indian Marsh Grass 223
Cephalanthus occidentalis Buttonbush 54
Myrica cerifera Wax Myrtle 54
Juncus effusus Common Rush 40
Cladium jamaicense Saw-grass 20
Culvert 18
Dead woody material 18
Eichhornia crassipes Water Hyacinth 18
Pontederia cordata Pickerelweed 16
Sesbania herbacea Bigpod Sesbania 14
Sabal palmetto Cabbage Palm 11
Sagittaria lancifolia Bulltongue Arrowhead 7
Eupatorium capillifolium Dogfennel 6
Fence 2
Quercus virginiana Live Oak 2
Alternanthera philoxeroides Alligator Weed 1
Andropogon virginicus Broomsedge Bluestem 1
Paspalum notatum Brook Crowngrass 1
Polygonum punctatum Dotted Smartweed 1
Spartina bakeri Sand Cordgrass 1
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coefficient of the main effect of distance from ditch was significant (coefficient =
-0.4952, P < 0.001) and was negatively related to shell abundance. There was no
difference in snail abundance between wetlands embedded in improved or seminative
pastures (P = 0.59).
Enclosure experiment
Snails were kept in enclosures with different plant composition and richness in 6
different wetlands for 4–6 weeks (Table 2). Some wetlands had partially dried down
by the end of this time period, causing snails in 5 enclosures to begin hibernating
belowground. When possible, we obtained these snails by digging, but did not use
these estivating snails’ weight changes as part of the data set because weight loss
occurs during estivation (Table 2, Little 1968). We calculated a total recovery rate
of 38.8%.
Table 4. The top 4 models that explained abundance of Island Apple Snail shells per wetland. The
best model was the direct effect of distance to the nearest ditch. K = number of parameters, Log(£)
= maximum log-likelihood, AICc = corrected Akaike information criterion value, ΔI = difference
between the lowest AIC value and AICi, wI = model weight, D = distance to the nearest ditch, M =
macroinvertebrate abundance, and S = relative frequency of structural plants.
Model df K Log(£) AICc ΔI wI
D 7 2 -49.29 104.57 0 1.00
M 7 2 -58.59 123.18 18.61 9.09 x 10-5
S 7 2 -70.29 146.58 42.01 7.53 x 10-10
Null 8 1 -74.22 151.02 46.45 8.20 x 10-11
Figure 2. Snail abundance obtained from trapping has a negative relationship with the frequency
of its preferred forage species (ρ = -0.91, P = 0.01).
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Of the snails that we collected, percent change in weight was positively related
to the total amount of plant cover and negatively related to evenness (Table 5,
Fig. 3). There was a marginal negative relationship between change in weight and
total N (Table 5). There was no difference in weight change between snails from
the two pasture types (P = 0.39).
Discussion
There is a growing concern about the invasive Island Apple Snail in Florida
because of its potentially detrimental effects on wetland vegetation, water quality,
and disease spread. In one of the only field studies conducted t o date on the Island
Apple Snail in seasonal wetlands, we showed a negative association between snail
abundance and the frequency of its preferred forage species, raising concerns about
the potential for the snail to alter plant communities in seasonal wetlands in the
southeastern US. We also found associations between the snail’s abundance and the
wetland’s distance to the nearest ditch, and between its growth and plant-species
evenness and percent cover. Below we discuss the implications of our findings and
include some caveats.
The results of the ditch survey suggest that the Island Apple Snail oviposits on
thick-stemmed and woody vegetation. We found almost half of all egg-masses on
the invasive West Indian Marsh Grass, which forms dense monocultures in wetlands
and ditches (Diaz et al. 2013). Although lack of data on ditch plant-community
composition did not allow us to assess the snail’s oviposition preferences, the results
of our survey suggest that West Indian Marsh Grass management may also
reduce the abundance of egg-laying substrate for Island Apple Snail. Interestingly,
it is probable that much of the vegetation the snail uses for oviposition are not palatable
to it, perhaps due to the thick cell walls and high lignin content of the emergent
vegetation necessary to support egg masses above water. Burlakova et al. (2008)
found that Island Apple Snails preferred to eat plant species with low dry-matter
content, an indication of low cellulose and lignin content, and they recommended
that plants with high dry-matter content be used for restoration. However, because
it is likely that many of the plants on which Island Apple Snail oviposits have high
dry-matter content, these restoration efforts may not actually aid in the snail’s suppression,
depending on whether food or laying substrate is the limiting factor in a
given wetland community. Future studies should examine what life-history stages
Table 5. Spearman’s rank correlations between the independent variables and Island Apple Snail
percent weight change in the enclosure experiment.
Variable ρ P
Plant richness -0.29 0.35
Total percent cover 0.64 0.03
Evenness -0.61 0.04
pH 0.30 0.34
Total P 0.18 0.59
Total N -0.55 0.06
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2015 Vol. 14, No. 3
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are most important to snail invasion and how abundance of emergent structures affects
restoration efforts.
In the wetland survey, we found a negative relationship between Island Apple
Snail abundance and the frequency of its preferred forage species. This result
Figure 3. Percent weight change of snails in enclosures (A) increases with percent cover
(with outlier [removed from graph for clarity]) ρ = 0.64, P = 0.03 (without outlier ρ = 0.61
and P = 0.051) and (B) decreases with evenness (with outlier [removed from graph for
clarity] ρ = -0.61, P = 0.04 and without outlier ρ = -0.545 and P = 0.087). Percent cover
of vegetation can be greater than 100 because percent cover of submerged and emergent
vegetation in enclosures was measured separately.
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C. Smith, E.H. Boughton, and S. Pierre
2015 Vol. 14, No. 3
suggests that the Island Apple Snail may impact vegetation in seasonal wetlands
by consuming and thus driving down the abundance of certain wetland plants,
especially submerged aquatic vegetation. This result is concerning because many
of these preferred species are native, and because such reductions in vegetation
could increase wetland nutrient levels (Andreotta et al. 2015). However, our
sample size was small (6 wetlands), and trapping is generally influenced both by
time-of-year and site-to-site variation (Elzinga et al. 2001). Importantly, this relationship
is correlative, and we did not measure the direct cause of the negative
relationship. More research on the snail’s effect on plant-community composition
in seasonal wetlands is greatly needed.
We conducted additional surveys of snail-shell abundance in 9 dried-down
wetlands. Our analysis showed that distance to the nearest ditch was negatively
related to the abundance of snail shells. More shells may be found in wetlands
closer to ditches because wetlands closer to ditches experience higher propagule
pressure; snails experience more predation in wetlands near ditches, leaving behind
more shells; or wetlands close to ditches were invaded earlier and more shells accumulated
prior to sampling. It is important to note that snail-shell abundance is an
indirect measure of actual snail abundance and may be affected by wetland vegetation
characteristics that affect apple snail burrowing. More research is needed to
understand how ditch connectivity and other landscape-scale characteristics affect
snail invasion.
Contrary to expectations, plant-species richness in the enclosure experiment, was
not related to Island Apple Snail growth. There was a significant positive relationship
between total plant percent cover and snail growth, and an unexpected negative effect
of plant-species evenness on Island Apple Snail weight change. One explanation
for the negative trend is that, although this snail is a generalist, it exhibits preferences
and performs optimally when consuming certain plant species over others (Morrison
and Hay 2011). Using our experimental design, we were not able to determine which
plant species the Island Apple Snail consumes, making it difficult to assess why there
was a negative trend between evenness and weight gain.
Implications of Island Apple Snail invasion in ranchland wetlands
Our results suggest that managers aiming to prevent introductions of the Island
Apple Snail should focus on wetlands close to propagule sources such as ditches.
Additionally, to mitigate the snail’s impact, special concern should be paid to
conserving the snail’s preferred forage species. The results of this study are preliminary
and more work is needed to understand Island Apple Snail invasion in
seasonal wetlands. Future studies should examine the relative importance of food
versus egg-laying resources for snail-population growth, the effect of the snail on
wetland nutrient levels, and the effect of landscape characteristics and propagule
pressure on the snail’s invasion success.
Acknowledgments
Thank you to Julia Maki, Jessica Franks, and Vi Bui for help with field work. We
would also like to thank 2 anonymous reviewers for thoughtful comments that improved
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C. Smith, E.H. Boughton, and S. Pierre
2015 Vol. 14, No. 3
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the manuscript. C. Smith was funded as a research intern by Archbold Biological Station.
USDA funding (USDA CSREES FLAR-2006-01378) was used to fence the study wetlands
in 2007. This is contribution #162 from the MacArthur Agro-ecology Research Center.
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Appendix 1. The scientific name, authority, and common name of all species discussed.
Scientific name with authority Common name
Alternanthera philoxeroides Mart. (Standl.) Alligator Weed
Amphicarpum muehlenbergianum Chapm. Muhlenberg Maidencane
Andropogon virginicus L. Broomsedge Bluestem
Aramus guarauna L. Limpkin
Azolla sp. Mosquito Fern
Bacopa caroliniana (Walter) B.L. Rob. Blue Waterhyssop
Carex albolutescens Schwein. Greenwhite Sedge
Centella asiatica (L.) Urb. Spadeleaf
Cephalanthus occidentalis L. Buttonbush
Cladium jamaicense Crantz (Kük.) Jamaica Swamp Sawgrass
Commelina diffusa Burm.f. Climbing Dayflower
Diodia virginiana L. Virginia Buttonweed
Eichhornia crassipes (Mart.) Solms Water Hyacinth
Eupatorium capillifolium (Lam.) Small Dogfennel
Hymenachne amplexicaulis (Rudge) Nees West Indian Marsh Grass
Juncus effusus L. Common Rush
Justicia angusta (Chapm.) Small Pineland Water-willow
Leersia hexandra Sw. Southern Cutgrass
Lemna minor L. Common Duckweed
Limnobium spongia (Bosc) Rich. ex Steud. American Spongeplant
Lontra canadensis Schreber North American River Otter
Ludwigia repens J.R. Frost Creeping Primrose-willow
Myrica cerifera (L.) Small Wax Myrtle
Panicum hemitomon Schult Maidencane
Panicum repens L. Torpedo Grass
Paspalum acuminatum Raddi Brook Crowngrass
Paspalum notatum Flüggé Bahia Grass
Phyla nodiflora (L.) Greene Turkey Tangle Fogfruit
Polygonum punctatum Elliott Dotted Smartweed
Pomacea canaliculata Lamarck Golden Apple Snail
Pomacea maculata Perry Island Apple Snail
Pontederia cordata L. Pickerelweed
Proserpinaca pectinata Lam. Combleaf Mermaidweed
Pryocon lotor L. Racoon
Quercus virginiana Mill. Live Oak
Rhynchospora inundata (Oakes) Fernald Narrowfruit Horned Beaksedge
Rostrhamus sociabilis Vieillot Snail Kite
Sabal palmetto (Walter) Lodd. ex.Schult. & Schult. f. Cabbage Palm
Sacciolepis striata Nash American Cupscale Grass
Sagittaria lancifolia L. Bulltongue Arrowhead
Salvinia minima Baker Water Spangles
Sesbania herbacea (Mill.) McVaugh Bigpod Sesbania
Spartina bakeri Merr. Sand Cordgrass
Thalia geniculata L. Bent Alligator-flag
Utricularia foliosa L. Leafy Bladderwort