Impact of invasive apple snails on the functioning and services
of natural and managed wetlands
Finbarr G. Horgan
, Alexander M. Stuart
, Enoka P. Kudavidanage
International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines
Department of Natural Resources, Sabaragamuwa University, Belihuloya, Sri Lanka
Received 18 April 2012
Accepted 7 October 2012
Available online 11 November 2012
At least 14 species of apple snail (Ampullariidae) have been released to water bodies outside their native
ranges; however, less than half of these species have become widespread or caused appreciable impacts.
We review evidence for the impact of apple snails on natural and managed wetlands focusing on those
studies that have elucidated impact mechanisms. Signiﬁcant changes in wetland ecosystems have been
noted in regions where the snails are established: Two species in particular (Pomacea canaliculata and
Pomacea maculata) have become major pests of aquatic crops, including rice, and caused enormous
increases in molluscicide use. Invasive apple snails have also altered macrophyte community structure
in natural and managed wetlands through selective herbivory and certain apple snail species can
potentially shift the balance of freshwater ecosystems from clear water (macrophyte dominated) to
turbid (plankton dominated) states by depleting densities of native aquatic plants. Furthermore, the
introductions of some apple snail species have altered benthic community structure either directly,
through predation, or indirectly, through exploitation competition or as a result of management actions.
To date much of the evidence for these impacts has been based on correlations, with few manipulative
ﬁeld or mesocosm experiments. Greater attention to impact monitoring is required, and, for Asia in
particular, a landscape approach to impact management that includes both natural and managed-rice
wetlands is recommended.
Ó2012 Elsevier Masson SAS. All rights reserved.
Wetlands rank among the most productive ecosystems on the
planet, providing a range of ecosystem services and economic
beneﬁts: they defend coastal and riverside areas against storms and
ﬂoods, purify water, control erosion, retain pollutants, and they
maintain a high diversity of animal and plant species, often func-
tioning as nurseries for ﬁsh and shellﬁsh, or as nesting sites for
waterfowl (Mitsch and Gosselink, 2007). Wetlands provide major
sources of human nutrition in the form of aquatic species that are
hunted, ﬁshed or farmed in natural areas, but also from intensive
and semi-intensive agriculture in managed or artiﬁcial wetlands
such as rice ﬁelds. Macrophyte community structure (relative
abundance and diversity) represents a key determinant of wetland
form and function. Macrophytes purify water (by oxygenation and
the conversion of toxic ammonia to usable nitrates), recycle nutri-
ents, provide refuges, microhabitats and food for aquatic organisms,
and provide physical structures (stems and leaves) that determine
water ﬂow patterns, sedimentation levels, and light and tempera-
ture gradients through the water body (De Nie, 1987; Petr, 20 00). In
agricultural wetlands, aquatic macrophytes (other than the crop
species) are often regarded by farmers and agronomists as nuisance
weeds that compete with the crop for resources (Ampong-Nyarko
and De Datta, 1991). However, several crop-associated macro-
phytes are used as supplementary food for people and livestock, and
as natural medicines (Cruz-Garcia and Price, 2011). For example,
Kosaka et al. (2006) identiﬁed 11 species that are used as human
food, two as animal fodder, and ﬁve medicinal plants from among
184 rice-associated weeds in Laos. Some macrophytes, including the
free-ﬂoating fern Azolla, are encouraged in rice ﬁelds to increase
nitrogen-ﬁxation (Mandal et al., 1999). Changes in aquatic macro-
phyte communities can have marked effects on water turbidity and
chemistry, particularly in shallow ponds and lakes (De Nie, 1987;
Petr, 2000; Carlsson et al., 2004; Hargeby et al., 2004). Such
changes are sometimes the effects of over-exploitation of native
herbivores, the introduction of predators (cascades), or the invasion
of wetlands by exotic herbivorous ﬁsh, crustaceans or mollusks
(Hansson et al., 1987; Scheffer et al., 1993; Gunderson, 2000;
Carlsson et al., 2004).
*Corresponding author. Tel.: þ63 2 5805600x2708.
E-mail address: firstname.lastname@example.org (F.G. Horgan).
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Acta Oecologica 54 (2014) 90e100
Several species of aquatic snail have invaded wetlands outside
their original distribution ranges. These invasions are often associ-
ated with enormous increases in secondary production and signif-
icant alterations in wetland conditions (habitat, structure of benthic
communities, water turbidity) (Hall et al., 2003, 2006; Dana and
Appleton, 2007; França et al., 2007; Arango et al., 2009). Many
invasive snails feed predominantly on detritus, periphyton, lower
aquatic algae and other microscopic organisms; but one group in
particular ethe apple snails (Ampullariidae) eare recognized for
their tendency to feed predominantly on fresh macrophyte material
(Estebenet and Martin, 2002a,b; Estebenet, 1995). Several species of
apple snails have invaded regions outside their native distribution
ranges (Table 1). Their diets together with their large body mass and
generally high reproductive output (Table 2) allow some of these
snails to effect rapid changes in macrophyte community structure,
shift the nutrient balance and turbidity states of water bodies
and cause huge losses to agricultural productivity and proﬁtability
(i.e., Pomacea canaliculata (Lamarck): Litsinger and Estano, 1993;
Halwart, 1994; Naylor, 1996; Carlsson et al., 2004; Kwong et al.,
2010). Because apple snails are predominantly tropical and
subtropical, their negative impacts are mostly sustained in devel-
oping nations where rice is the main staple food and rice farming is
the principal agricultural activity (Khush, 1997). Apple snails
directly impact peoples’livelihoods in developing nations because
of a relatively high reliance on local agriculture and natural
ecosystems for food and materials (Cruz-Garcia and Price, 2011).
In this review, we report on the diversity of apple snails intro-
duced to tropical and subtropical regions, examining their impact
on aquatic macrophyte and benthic communities and consequently
on wetland function. Although apple snails are often used for food
and medicine, this review focuses mainly on their potential nega-
tive impacts cognizant that any beneﬁts from introduced snails,
particularly in South East Asia, could have been gained through
better management of native snail species (i.e., Jahan et al., 2001;
Thaewnon-ngiw et al., 2003). Since 2000, research attention has
generally shifted away from exploring the impact and management
Species of apple snail found outside their native distribution range.
Species and common name Reason for introduction Native range
Marisa cornuarietis (Linnaeus,
1758) eGiant rams-horn snail
Biological control of schistosomiasis
vectors and weeds, pet trade
South America (Colombia
and Venezuela) 
Costa Rica, Cuba, Dominican Republic, Egypt,
French Guyana, Guadeloupe, Guyana, Panama,
Puerto Rico, Sudan, Surinam, Tanzania, USA
(Florida, Texas) ; Martinique, St Kitts ;
New Zealand 
Pila conica (Wood, 1828) eBlack
Food South-East Asia (Philippines)  Guam, Palau, USA (Hawaii) ; India 
Pila globosa (Swainson,
1822) eIndian apple snail
Food, commerce, potential biological
control of schistosomiasis vectors
Bangladesh, India (North)  India (Kerala) 
Pila leopoldvillensis (Putzeys,
1889) eGiant African apple snail
Food Africa  Philippines ; Taiwan 
Pomacea bridgesii (Reeve,
1856) eSpike-topped apple snail
Pet trade Bolivia, Brazil  Chile ; India (West Bengal) ; Sri Lanka 
Pomacea canaliculata (Lamarck,
1822) eChanneled apple snail
Food and commerce, pet trade Argentina, Uruguay, Paraguay,
Brazil, and Possibly Bolivia 
Cambodia, Chile, China, Dominican Republic,
Egypt, Guam, Indonesia, Japan, Lao, Malaysia,
Myanmar, Papua New Guinea, Philippines,
Singapore, South Africa, South Korea, Taiwan,
Thailand, Vietnam, USA (Arizona, California,
Florida, Hawaii) ; USA (Texas, North Carolina)
, Pakistan , Russia (Siberia) ;
Pomacea diffusa (Blume,
1957) eSpike-topped apple snail
Pet trade South America (Amazon Basin)  Australia (western) ; Brazil (Pará,
Pernambuco, Rio de Janeiro), Colombia,
French Guiana, Panama, Sri Lanka, USA
(Florida, Hawaii), Venezuela ; New Zealand
; Puerto Rico, USA (Alabama) ;
Pomacea glauca (Linnaeus, 1758) Biological control of schistosomiasis
vectors and weeds, pet trade
South America (Northern) and
; Philippines 
Pomacea haustrum (Reeve,
1858) eTitan apple snail
Pet trade Bolivia, Brazil (Amazon
region), Peru 
USA (Florida) 
Pomacea lattrei (Reeve, 1856)
Aquaculture Guatemala Panama 
Pomacea lineata (Spix, in
Pet trade Brazil and Guyanas  South Africa 
Pomacea maculata (D’Orbigny,
1835) eIsland apple snail
Food, commerce, pet trade Lower Paraná, Uruguay and
La Plata Basins 
Cambodia, Singapore, South Korea, Thailand,
USA (Alabama, Florida, Georgia, Louisiana,
South Carolina, Texas), Vietnam ;
Malaysia (Borneo), Taiwan ; Puerto Rico,
USA (Arizona) ; Spain 
Pomacea paludosa (Say,
1829) eFlorida apple snail
Pet trade Cuba, USA (Florida)  USA (Alabama, North Carolina) ;
Puerto Rico 
Pomacea scalaris (d’Orbigny, 1835) Food, commerce Argentina, Bolivia, Brazil  Taiwan 
Numbers in parentheses indicate source references e1, Cowie and Hayes (2012);2,Cowie (2001);3,Chapman et al. (1974);4,Cowie (2002);5,Jahan et al. (2001);6,
Thomas (1975);7,Barcelo and Barcelo (1988);8,Cowie and Thiengo (2003);9,Letelier et al. (2007); 10, Raut and Aditya (1999); 11, Marambe et al. (2003); 12, Rawlings et al.
(2007); 13, Baloch et al. (2012); 14, Yanygina et al. (2010); 15, Hayes et al. (2008); 16, Colloer et al. (2011); 17, United States Geological Survey (2012); 18, Mochida (1987); 19,
Angehr (1999); 20, Dana and Appleton (2007); 21, European Food Safety Authority (2012); 22, Hayes (personal communication); 23, Williams et al. (2001).
Pomacea luzonica is a synonym of P. conica that appears in some publications.
Pomacea bridgesii and P. diffusa are often confused during identiﬁcation; All reported P. bridgesii introductions have not yet been veriﬁed.
This species might not be native to the Caribbean.
Pomacea haustrum is possibly a synonym of P. maculata (Hayes, personal communication).
Reports of introduction of P. lattrei have not been conﬁrmed. The species may be a synonym of P. ﬂagellata (Say, 1829) (Hayes, personal communication).
Unconﬁrmed report of P. lineata in South Africa is likely P. canaliculata (Hayes, personal communication).
Pomacea insularum and P. gigas are synonyms of P. maculata (Hayes et al., in press).
F.G. Horgan et al. / Acta Oecologica 54 (2014) 90e100 91
of apple snails in agriculture (rice, Oryza sativa L. and taro, Colocasia
esculenta L.) or as biological control agents of disease vectors and
weeds, and toward understanding their dynamics, behavior and
impact on natural and semi-natural wetland ecosystems. This new
body of research has tremendously increased our capacity to predict
the outcomes of apple snail invasions and to improve management
and amelioration strategies. In particular, over the last ﬁve years,
a large number of studies have focused on the interactions between
apple snails and aquatic macrophytes, and have rapidly advanced
knowledge in this area. Here we concentrate on the environmental
impacts of apple snails and look at the state of evidence that
connects invasions to reported impacts, identifying knowledge gaps
and highlighting studies that have elucidated impact mechanisms.
We also examine the links between agricultural and natural
wetlands and suggest that these might best be regarded as part of
a dynamic invasion landscape each with different roles in deter-
mining the distribution and impact of the snails. Finally, we make
recommendations concerning future research required to improve
our understanding of the ecological impacts of invasive apple snails.
2. Apple snail invaders of wetland ecosystems
Apple snails are freshwater snails that naturally occur
throughout the humid tropics and sub-tropics. The genus Pomacea
with the largest number of species (about 50), and the genus Marisa
are mainly of South and Central American origin. The genus Pila,
with over 30 species, occurs throughout South and South-East Asia,
and in Africa (Cowie, 2002; Hayes et al., 2008). Over the last 60
years, several members of these three genera have been introduced
to new regions (Table 1). Many of these introductions have been
linked to the pet trade, but there have also been several deliberate
introductions to promote aquaculture and cottage industry, or for
the biological control of other aquatic snails and weeds. Several
authors have described the unfortunate oversights that led to the
introduction of a number of Pomacea species to South East Asia,
where two species, P. canaliculata and Pomacea maculata D’Orbigny
(synonym: Pomacea insularum), are considerable pests of rice and
aquatic vegetables (Cowie, 2002; Qiu and Kwong, 2009). Table 1
summarizes reported introductions of 14 apple snail species to
new regions, including some native transfers (where species are
found at new locations within their native region but outside their
normal distribution; i.e., Pomacea paludosa (Say)) and unveriﬁed
reports of introductions, (i.e., Pomacea bridgesii (Reeve), Pomacea
lattrei (Reeve)). The table does not include species that have invaded
new habitat in their native ranges (i.e., Pomacea dolioides (Reeve) in
pre-germinated rice ﬁelds in Surinam: Cowie, 2002; Wiryareja and
Tjoe-Awie, 2006), although the dynamics of ‘new habitat’and
‘regional’invasions are potentially similar. Some of the reports
concern the known introduction of species for speciﬁc objectives
(i.e., biological control or gastronomy) with no further records of
establishment or spread (i.e., Pila leopoldvillensis (Putzeys) in the
Philippines and Pomacea diffusa Blume in Australia and New Zea-
land: Barcelo and Barcelo, 1988; Halwart, 1994; Hayes et al., 2008;
Colloer et al., 2011).
Difﬁculties in identifying apple snail species have led to
several reports of distribution and impact that are associated
with incorrectly identiﬁed or unveriﬁed species. In this review, we
present species names as they appear in original reports, but
Invasive status of introduced apple snails with available information on life-history traits and diet.
Species Status outside its native range Clutch size
Time to hatch (reported
range in days)
P. canaliculata Widespread in tropics and
14e500  7e28  80  51/67 þbryozoans, ﬁlamentous algae,
P. maculata Widespread in tropics and
522e4751  7e21  80 
46/52 þsnail eggs, periphyton
M. cornuarietis Became widespread in the
Caribbean, possibly declining
in some other regions 
50e210  8e24  60  23/26 þsnail eggs, animal matter
P. conica Restricted to a few Hawaiian
and Paciﬁc islands 
28e313  17e28  32  Macrophytes [26,27]
P. diffusa Localized  or declining  na na na 1/8 
P. glauca Localized , declining or
30e90  14e17  na Macrophytes 
P. haustrum Localized or declining  ca. 236  9e30  na 5/8 þﬁlamentous algae, animal
matter, snail eggs [5,14,29]
P. paludosa Localized  or not
3e141  15e20  70  5/8 
P. scalaris Localized  9e302  na 40  Macrophytes 
P. globosa No recent records 200e300  10e30  na Macrophytes 
P. leopoldvillensis Possibly never established
na na na Macrophytes, snails, insect larvae and
other arthropods 
P. bridgesii Unveriﬁed records 50e200  15e24  57  Macrophytes, snail eggs, animal
P. lattreieP. ﬂagellata Unveriﬁed records  na na 27  Macrophytes 
P. lineata Unveriﬁed records
400e600  ca. 15  na Varied sources from several
trophic levels 
Numbers in parentheses indicate source references - 1, Rawlings et al. (2007);2,Hayes et al. (2008);3,Cowie (2002);4,Estebenet and Cazzaniga (1998);5,Cazzaniga and
Estebenet (1984);6,Lach et al. (2000);7,Carlsson and Lacoursière (2005);8,Carlsson and Brönmark (2006);9,Wood et al. (2006); 10, Boland et al. (2008); 11, Qiu and Kwong
(2009); 12, Fang et al. (2010); 13, Wong et al. (2010); 14, Morrison and Hay (2011a); 15, Morrison and Hay (2011b); 16, Qiu et al. (2011); 17, Wang and Pei (2012); 18, Barnes
et al. (2008); 19, Howells et al. (2006); 20, Burlakova et al. (2009); 21, Baker et al. (2010); 22, Burks et al. (2010); 23, Seaman and Porterﬁeld (1964); 24, Hofkin et al. (1991);
25, Tran et al. (2008); 26, De Lara (2003); 27, Levin et al. (2006); 28, Pointer et al. (1991); 29, Guimarães (1983); 30, Hayes epersonal communication; 31, Wu et al. (2011);
32, Jahan et al. (2001); 33, Van Coillie et al. in Cazzaniga and Estebenet (1984); 34, Coelho (2012); 35, Aditya and Raut (2001); 36, Aditya and Raut (2002); 37, Reed and Janzen
(1999); 38, Angehr (1999); 39, Lopes (1956); 40, Fellerhoff (2002);na¼no data available.
Ranges are given to indicate plasticity and maximum values, but temperatures and conditions differed across studies; Maximum shell height refers to the maximum
recorded in published scientiﬁc papers.
In some, perhaps unusual cases, snails may attain >155 mm in height (Cowie, 2002).
Apparently introduced to the Philippines in the 1980s (Mochida, 1987), but there have been no records since, records of P. glauca may be misidentiﬁcations.
Reported from South Africa with no further records.
F.G. Horgan et al. / Acta Oecologica 54 (2014) 90e10092
indicate probable misidentiﬁcations and in some cases use cor-
rected species names (based on recent updates in taxonomy,
improved distribution records, and advice from snail taxonomists,
in particular K.A. Hayes). Confusion in identifying invading apple
snails and in determining their spread has been due to two main
factors: (1) that many introductions have been illegal and consisted
of unidentiﬁed species gathered from the wild or from mixed
sources; and (2) that apple snails are anatomically very similar and
species identities are often difﬁcult to determine. Recent studies
have used molecular approaches and developed molecular tools to
help clarify some of the issues (Rawlings et al., 2007; Hayes et al.,
2008, in press; Matsukura et al., 2008; Tran et al., 2008; Dong
et al., 2011; Cooke et al., 2012); For example, through analysis of
subunit I of the cytochrome c oxidase gene (COI) (mitochondrial
DNA [mtDNA]) in snails from 164 Asian locations and 57 South
American locations, Hayes et al. (2008) determined that there were
multiple apple snail introductions to Asia with two species,
P. canaliculata and P. maculata, now widespread, and two, P. diffusa
and Pomacea scalaris (D’Orbigny) restricted to a few sites. Mean-
while, Rawlings et al. (2007) used two portions of mtDNA,
including COI, to identify ﬁve species in the continental USA. These
authors note that some introduced apple snail species may be in
decline in the USA due to competition with recently introduced and
more aggressive congeners. Because species have been confused in
the past, it is difﬁcult to clearly attribute impacts to particular
species. For example, P. canaliculata has spread throughout most of
South-East Asia where it causes severe damage to rice during crop
establishment. However, P. maculata invaded many of the same
regions as P. canaliculata at about the same time. There is little
direct evidence for P. maculata impact on rice ﬁelds and other
aquatic ecosystems in the region because all early reports referred
only to P. canaliculata. Where possible, Table 1 indicates the
commonly misidentiﬁed species; however, as taxonomy continues
to be revised, some of the species in Table 1 may be merged with
other taxa, and some reported introductions may be corrected in
the future ein particular the introductions of P. bridgesii,P. lattrei
and Pomacea lineata (Spix, in Wagner) require veriﬁcation.
Whereas much of the information presented in later sections of
this review relates to potential impacts of apple snails in general, we
have included speciﬁc examples where possible, mindful that
different species will have different impacts: Impact and invasive-
ness are determined by the biological and behavioral traits of each
snail species as well as their interactions with other components of
the invaded habitat, including release from natural enemies. Table 2
presents some aspects of the biology of apple snails that could be
linked to their invasiveness and impact. It is noteworthy that two of
the most destructive species, P. canaliculata and P. maculata,are
among the largest and most fecund. Even though egg clusters can be
several times larger in P. maculata than in P. canaliculata, the
hatchlings of the latter species are considerably bigger (Barnes et al.,
2008) such that reproductive effort as a whole may be a better
determinant of invasiveness. An investment in larger eggs could
increase the competitive ability of hatchlings and improve a species’
capacity to successfully establish in new habitats. The beneﬁts and
trade-offs related to larger hatchlings or higher fecundities for
invasive snails are unknown. Wu et al. (2011) suggest that large
clutches, larger hatchlings and faster development have deter-
mined the wider distribution of P. canaliculata compared to
P. scaralis in Taiwan; however, in the case of Pila conica (Wood) and
P. canaliculata, the hatchlings of the former are almost twice as large
as those of P. canaliculata, yet evidence suggests that P. conica has
declined or been eliminated from habitats recently invaded by
P. canaliculata (De Lara, 2003; Tran et al., 2008). Furthermore, under
similar laboratory conditions, the onset of oviposition was three
times faster in P. canaliculata than in P. conica, but clutches were
three times as large in the latter species (De Lara, 2003). Other
factors that may inﬂuence the success of apple snails during inva-
sion include their rate of development (time to egg hatch, time to
sexual maturity), longevity, and body size. Details of some of these
parameters are presented in Table 2; however, apple snails
demonstrate tremendous plasticity in life-history traits, and these
parameters are likely to change with food availability, ambient
temperatures and population densities (P. canaliculata:Estebenet
and Martin, 2002a,b; Tamburi and Martín, 2009;P. maculata:
Burlakova et al., 2010). Plasticity itself could be a trait that
contributes to apple snail invasiveness. Information about key life-
history traits at comparative temperatures and population densities
is not available for most invasive apple snails (Table 2). Further-
more, the broad diet ranges of each species have not been compared
during controlled experiments (but see Morrison and Hay, 2011a)
(Table 2). Nevertheless, according to Table 2, the three most
successful invaders each consumed over 76% of the species offered
during feeding trials (P. canaliculata, 76%, P. maculata, 88%, and
Marisa cornuarietis (Linnaeus), 88%). During trials conducted by
Morrison and Hay (2011a) using eight plant species, the diets and
conversion efﬁciencies were similar among M. cornuarietis,
P. canaliculata,P. maculata,Pomacea haustrum (Reeve) and
P. paludosa. Meanwhile P. diffusa had a limited diet range and poor
conversion efﬁciency (Table 2). As knowledge of the life-histories
and behaviors of these species accumulates, apple snails could
become a model to assess key traits associated with invasiveness.
3. Effects on macrophyte community structure
Apple snails will feed on most classes of macrophyte
(submerged, ﬂoating and emergent) but have marked preferences
for certain plant species and often perform poorly (feed less, grow
slower and reproduce less) on unpalatable plants (Estebenet, 1995;
Lach et al., 2000; Carlsson and Brönmark, 2006; Boland et al., 2008;
Gettys et al., 2008; Burlakova et al., 2009; Qiu and Kwong, 2009;
Wong et al., 2009; Baker et al., 2010; Morrison and Hay, 2011a;
Wang and Pei, 2012). However, there is still little evidence
from ﬁeld experiments in natural wetlands or agricultural systems
of preferential grazing on one or other plant species (Table 3):
Burlakova et al. (2010) found clear habitat preferences by
P. maculata for macrophyte-dominated zones in artiﬁcial ponds in
Texas, USA, where the snails were more abundant on Alligator weed
(Alternanthera philoxeroides Griseb) than in deeper water zones.
Alligator weed was also the preferred food in parallel laboratory
experiments (Burlakova et al., 2009). Carlsson and Brönmark (2006)
noted that Ludwigia adscendens L. Hara and Salvinia cucullata Rox ex
Bary, which were severely impacted by P. canaliculata in caged
feeding trials, had almost disappeared from natural ponds near
Vientiane, Laos. Furthermore, Wang and Pei (2012) observed that
P. canaliculata reduced the biomass and density of Hydrilla verti-
cillata L. f. and Potamogeton crispus L., but had no effect on Vallisneria
spiralis L. and Acorus calamus L. in a riverine restoration plot in
China. The case of M. cornuarietis and Pomacea glauca L. as biological
control agents reducing weeds in the Caribbean and Africa is well
documented (Jobin, 1970; Jobin et al., 1973; Nguma et al., 1982;
Pointer et al., 1991; Pointer and Augustin, 1999; Pointer, 2001). In
the Philippines, evidence from experimental rice plots that
manipulated snail densities, and from farmers’ﬁelds, suggests that
P. canaliculata can reduce weed populations and increase rice yield
(Joshi et al., 2006). Similarly Luna Maldonado and Nakaji (2008)
indicate that Cyperus difformis L. had disappeared, and several
other weed species declined between 1996 and 1997 when apple
snails invaded a farm in Japan. Much of these observations are
poorly supported with data (Table 3) and more direct evidence from
replicated ﬁeld sites would be welcome.
F.G. Horgan et al. / Acta Oecologica 54 (2014) 90e100 93
Reported impacts of invasive apple snails on native animal and plant communities, nutrient cycling and the spread of disease.
Species Region Habitat Reported impact Evaluation method Source
M. cornuarietis USA (Texas) Lake (Landa) Some dense macrophyte patches were
denuded, reducing the need for mowing
of plants by park employees
Field observations Horne et al. (1992)
P. canaliculata and/or
possibly P. maculata
Laos Ponds Reduced densities of Ludwigia adscendens
L. Hara and Salvinia cucullata Rox ex Bary
linked to feeding
trials in enclosures
P. canaliculata South China Riverine
Altered macrophyte community and
reduced water quality due to detritus
and plant waste materials
linked to feeding
trials in enclosures
Wang and Pei (2012)
P. canaliculata South China Cultivated
Damaged several cultivated semi-aquatic
Qiu and Kwong (2009)
Pomacea spp. Hawaii, Hong Kong,
Damaged several cultivated semi-aquatic
Literature search Cowie (2002)
P. canaliculata Japan Rice ﬁelds Reduced density of Cyperus difformis
L. and other macrophytes
Comparison of pre
and Nakaji (2008)
P. canaliculata Philippines Rice ﬁelds Reduced densities of non-crop
macrophytes and increased rice
and untreated plots,
under different snail
Joshi et al. (2006)
P. canaliculata Philippines Rice ﬁelds Destroyed up to 96% of rice hills when
seedlings were 20 days old; US$1200
million losses in rice production between
1980 and 1990
Comparisons of plots
with known snail
Litsinger and Estano
P. canaliculata Japan Rice ﬁelds Reduced macrophyte richness and
Survey of ﬁelds with
and without snail
Hidaka et al. (2007)
Macrophytes and snails
M. cornuarietis Puerto Rico Farm ponds Reduced macrophyte and Biomphalaria
glabrata (Say) densities
Periodic surveys Jobin (1970) and
Jobin et al. (1973)
P. glauca Guadeloupe Lake
Reduced densities of Pistria stratiotes
L. and B. glabrata
over several years
Pointer et al. (1991)
M. cornuarietis Guadeloupe Lakes and ponds Reduced densities (eliminated) of
P. stratiotes and B. glabrata
over several years
Pointer and David (2004)
M. cornuarietis Tanzania Reservoir Reduced densities of Bulinus tropicus
(Krauss), Biomphalaria pfeifferi (Krauss)
and Lymnaea natalensis (Krauss) and
eliminated Cyperus spp.
Monitoring Nguma et al. (1982)
P. canaliculata Hong Kong General Reduced density of Austropeplea
ollula (Gould) and Biomphalaria
Not presented Fang et al. (2010)
P. canaliculata and/or
possibly P. maculata
Thailand Ponds and
Reduced bryozoan abundance and
changed community composition
linked to feeding
trials in enclosures
Wood et al. (2006)
P. maculata USA (Florida) Ponds and
Reduced densities of Pomacea haustrum
(Reeve) in some areas
Not presented Morrison and
P. canaliculata Philippines Rice ﬁelds Reduced densities of Pila conica (Gray)
that was once used for human food
Not presented Basilio (1991)
P. canaliculata Philippines Rice ﬁelds Reduced densities of Misgurnus
anguillicaudatus (Cantor) that was
once used for human food
Not presented Halwart (1994)
P. canaliculata Hong Kong Stream Caused a delay in reproduction and
longer recruitment period of Radix
P. canaliculata Hong Kong Lotic and
Massive increase in secondary production
(greater than published estimates for
any other freshwater snails)
method with samples
Kwong et al. (2010)
P. canaliculata Hong Kong Pond
Reduced biomass of ﬁlamentous green
algae (Spirogyra sp.); reduced macrophyte
biomass, increased phytoplankton biomass
and altered phytoplankton community
enclosure with and
Fang et al. (2010)
P. canaliculata Thailand Wetlands Reduced densities of aquatic macrophytes,
increased nutrient concentrations in the
water, increased phytoplankton biomass
and shifted water toward a turbid state
from survey of 13
Carlsson et al. (2004)
F.G. Horgan et al. / Acta Oecologica 54 (2014) 90e10094
Results from a number of recent laboratory studies will help
predict the impact of apple snails on invaded macrophyte
communities based on plant characteristics underlying feeding
preferences. It is not surprising that apple snails (P. canaliculata and
P. maculata) show preferences for several species with low chemical
defenses (i.e., phenols) (Boland et al., 2008; Qiu and Kwong, 2009;
Wong et al., 2010), including plants whose defenses have been
induced (P. canaliculata:Morrison and Hay, 2011b). However, there
are exceptions to this general rule, suggesting that apple snails have
adapted to avoid or overcome the defenses of some common plants
from their native ranges (but see Morrison and Hay, 2011c). For
example both P. canaliculata and P. maculata can feed liberally on
Myriophyllum spp. in spite of the high chemical defenses of these
plants (Boland et al., 2008). Plants with low physical defenses
(including low dry-matter content) (Litsinger and Estano, 1993;
Burlakova et al., 2009; Wong et al., 2010) and high nitrogen, high
phosphorous and high chlorophyll contents (Sharfstein and
Steinman, 2001; Qiu and Kwong, 2009; Wong et al., 2010) were
generally preferred in feeding trials with P. canaliculata and
P. maculata. Preference for plants with these traits appears to be
a general phenomenon, such that feeding preferences and perfor-
mance are similar across several apple snail species (Morrison and
Hay, 2011a,c). The inﬂuence of these plant traits on herbivory will
lead to higher vulnerability of submerged plants compared to
emergent plants (Gettys et al., 2008; Wang and Pei, 2012) and of
crop plants compared to weeds (Qiu and Kwong, 2009; Burlakova
et al., 2009). Furthermore, young plants may be generally more
vulnerable than older plants, but this has not been examined in
much detail outside the rice system and research has mainly been
conﬁned to P. canaliculata (Litsinger and Estano, 1993). If seedlings
are generally vulnerable, then plants with modular growth, such as
several important invasive aquatic plants (i.e., alligator weed, water
hyacinth, Eichhornia crassipes (Mart.) Solms, and water lettuce,
Pistia stratiotes L.), might become dominant over time. Of signiﬁ-
cant importance is the observation by Morrison and Hay (2011c)
that snails (P. canaliculata,P. maculata,P.haustrum and
P. paludosa), when presented with plants from a snail-invaded
region (North America) and from the snails’native region (South
America) as congeneric pairs in choice tests, showed marked
preferences for the North American species. The authors suggest
that the native North American plants were naïve to snail herbivory
and therefore lacked adequate chemical (or physical) defenses to
deter the invasive snails. This suggests that habitat invaded by
apple snails will be less resistant to subsequent invasion by exotic
aquatic plant species thus supporting the ‘invasional meltdown’
model of Simberloff and Von Holle (1999).
Given the intense grazing pressure and high biomass of invasive
snails recorded from several ﬁeld studies (Horne et al., 1992;
Carlsson et al., 2004; Hall et al., 2006; Kwong et al., 2010), and
the nature of apple snail feeding preferences, we may predict that
apple snails will shift macrophyte communities toward dominance
by chemically and physically (high dry weight) defended plants. In
most cases these will be emergent plants and the more palatable
submerged and ﬂoating species are expected to decline. However,
recruitment of emergent ﬂora will also be affected by snails if
seedlings and younger plants are vulnerable to snail herbivory (as
in rice). Dry downs in the Florida everglades are noted for their role
in augmenting macrophyte diversity (Karunaratne et al., 2006;
Darby et al., 2008). Such periods of low water restrict snail
activity (i.e., P. paludosa movement is restricted when water levels
fall below 10 cm eDarby et al., 2002) and could improve recruit-
ment of emergent ﬂora, allowing a dominance of emergent plants
after water levels increase again and when the seedlings are
sufﬁciently robust to avoid snail damage. In effect, this is the
mechanism underlying current management of snails in rice where
ﬁelds are periodically drained to prevent apple snail movement and
feeding during the vulnerable seedling stages (Litsinger and Estano,
1993; Joshi, 2007). Since most apple snails also require emergent
plants as sites for egg laying (Horn et al., 2008; Kyle et al., 2011) and
because the hatchlings of some species may prefer epiphytic
periphyton over benthic periphyton (i.e., P. paludosa:Sharfstein and
Steinman, 2001; Shuford et al., 2005), habitats with high densities
of emergent vegetation may represent signiﬁcant sources for snail
colonization of adjacent ponds and ﬂooded ﬁelds. Because macro-
phyte species richness (and presumably diversity) enhances
wetland ecosystem function (i.e., algal and total plant biomass, and
nutrient retention eEngelhardt and Ritchie, 2001), then the
alteration of macrophyte communities by apple snails toward
dominance by a few functional groups (particularly emergent
species) is predicted to decrease the efﬁciency of wetlands. When
exacerbated by shifts in water state (see below), macrophyte
diversity loss is expected to severely alter functioning and decrease
the ecosystem services provided by wetlands.
4. Effects on benthic communities
There are several reports of invasive apple snails adversely
affecting populations of other benthic organisms (Table 3). Apple
snails can feed on living invertebrates such as worms, micro-
crustaceans, and other snails (M. cornuarietis:Stryker et al., 1991;
Hofkin et al., 1991;P. bridgesii:Aditya and Raut, 2001, 2002;
P. canaliculata:Cazzaniga, 1990; Estebenet and Cazzaniga, 1992;
Wood et al., 2006 (possibly also includes P. maculata); Kwong
et al., 2009;P. haustrum Reeve: Guimarães, 1983) including
conspeciﬁc eggs and hatchlings (i.e., P. maculata:Horn et al., 2008);
however, under conditions where macrophytes, algae and decaying
Table 3 (continued )
Species Region Habitat Reported impact Evaluation method Source
Pomacea spp. Panama Lake (Gatun) High snail densities supported breeding
populations of snail kite (Rostrhamus
sociabilis plumbeus Ridgway) that were
previously only vagrant at the lake.
Field observations Angehr (1999)
P. maculata USA (Florida) Wetland
Alterations in adult snail kite feeding
behavior and energetic deﬁciencies in
of kites in invaded
Cattau et al. (2010)
Human and animal health
P. canaliculata Indo-Paciﬁc region General Carry and spread angiostrongyliasis
and other human and animal diseases
Not presented Lv et al. (2009),
F.G. Horgan et al. / Acta Oecologica 54 (2014) 90e100 95
plant materials are sufﬁciently abundant, predation of aquatic
fauna is probably low. For example, Kwong et al. (2010) indicated
that less than 0.1% of the gut contents of P. canaliculata contained
invertebrate body parts. Nevertheless, the impact of some apple
snail populations could be considerable given their invasive
dominance of benthic communities (Kwong et al., 2010). Kwong
et al. (2009) found P. canaliculata caused mortality of the early
stages of ﬁve aquatic snails, and of adults of three of the species (all
pulmonates), but did not consume adults of two prosobranchs. This
pattern was mirrored in observations on the effects of
M. cornuarietis in a reservoir in Tanzania: The introduced snail
‘eliminated’three pulmonate species, but had no apparent affect on
Melanoides tuberculata Müller, a prosobranch (Nguma et al., 1982).
Such ﬁeld observations are rare and often include more than one
introduced biological control snail species, whereas most of the
direct observations of predation by apple snails on the eggs,
hatchlings and adults of other aquatic snails (i.e., P. bridgesii,
P. canaliculata and P. haustrum) have come from experiments in
conﬁned aquaria (i.e., Guimarães, 1983; Cazzaniga, 1990; Aditya
and Raut, 2002). It is likely that apple snails in general only inci-
dentally consume invertebrates. P. bridgesii and P. diffusa may be
exceptions. P. diffusa consumed less plant material in comparative
studies with other apple snails (Morrison and Hay, 2011a) and
P. bridgesii (unveriﬁed species identiﬁcation) exhibited distinct
scavenging behavior during feeding trials in India (Aditya and Raut,
2001). M. cornuarietis is also frequently cited as a predatory species
although authors differ on whether this predation is accidental
(Jobin, 1970; Jobin et al., 1973) or not (Demian and Lutfy, 1966;
Demian and Kamel, 1973). Some reports suggest that the species
may become increasingly predatory as it gains experience through
encounters with prey items (Jobin et al., 1973).
Cases of invasive apple snails having depleted or ‘eliminated’
other aquatic snails are often associated with notable declines in
aquatic vegetation. This suggests that exploitation competition is
a probable mechanism underlying species loss following invasion.
For example at Grand-Etang Lake in Guadeloupe, P. glauca depleted
mats of the invasive aquatic macrophyte P. stratiotes reducing food
availability for the schistosome-vector Biomphalaria glabrata Say e
but it was the later introduction of M. cornuarietis that ﬁnally
‘eliminated’the pulmonate from the lake (Pointer et al., 1991).
Similarly, Jobin et al. (1973) suggest that the elimination of
B. glabrata from some farm ponds in Puerto Rico, but not from
others, was primarily dependent on the amount of vegetation in the
ponds. Monitoring over several years at ponds and lakes in
Martinique and Guadeloupe indicated that, besides B. glabrata,
M. cornuarietis had no adverse effects on other, native pulmonates
(Pointer and Augustin, 1999; Pointer, 2001; Pointer and David,
2004). Some native pulmonates may alter their foraging or repro-
ductive behaviors to coexist with the invading apple snail as occurs
with Radix plicatulus Benson populations that coexist with
P. canaliculata in Hong Kong (Lam, 1994). In some cases the elimi-
nation of benthic pulmonates by apple snails could be through
exploitation competition alone (Pointer et al., 1991; Kruatrachue
and Upatham, 1993). The elimination or density reduction of
similar snails such as Pila spp. in Asia, P. conica on some islands of
Hawaii (Tran et al., 2008) and other apple snails in North America,
following new introductions of apparently more aggressive species
(Rawlings et al., 2007), also suggests that interspeciﬁc competition
can be intense. The niches of these snails are likely to overlap
considerably but their competitive abilities will differ. For example,
Conner et al. (2008) found that one P. maculata adult is sufﬁcient to
reduce growth and survival of P. paludosa to the same extent as four
Competition leading to the elimination of dissimilar snails, such
as pulmonates, is more difﬁcult to explain. As mentioned above,
exploitation competition with P. glauca is implicated in reducing
densities of B. glabrata in Guadeloupe prior to the release of
M. cornuarietis (Pointer et al., 1991) and by M. cornuarietis itself in
farm ponds in Puerto Rico (Jobin, 1970). However, in the latter case
Jobin (1970) indicates that the longevity of the apple snail was
largely responsible for the outcome of the interaction. It is perhaps
unlikely that elimination of pulmonates could result from compe-
tition with such dissimilar species as apple snails, without the
added mortality due to direct (accidental or selective) predation.
Therefore, the observed reduction of native snail densities could
result from three distinct processes: a) intraspeciﬁc competition
and dominance of a restricted niche; b) direct predation on eggs,
hatchlings or juveniles of a naïve native species; or c) a two tiered
process whereby the snails deplete vegetation thereby reducing
refuges of native species and restricting the invasive and native
species together into close proximity where predation of the latter
is more intense. Evidence gathered so far cannot distinguish these
three hypotheses. Further research is required to elucidate the
precise mechanisms behind declining native benthic species in
areas where apple snails have invaded.
Large numbers of empty shells are an often noted feature of
apple-snail invaded habitats (i.e., M. cornuarietis:Horne et al.,
1992), Where apple snails are consumed by limpkins, Aramus
guarauna (Linnaeus), or snail kites, Rostrhamus sociabilis (Vieillot),
accumulations of empty shells occur near perches or nest sites
(Beissinger, 1983; Reed and Janzen, 1999; Macek et al., 2009).
Empty shells can provide an abundant substrate for the attachment
or protection of aquatic organisms. For example, a range of epi-
bionts, symbionts and commensals occur on or in the shells of
living and dead apple snails (Damborenea et al., 2006; Vega et al.,
2006; Pedroso Dias et al., 2008). Most of these records are from
regions where the snails are native and occur at low densities.
Apple-snail shells are sometimes covered with epiphytic algae even
in invaded habitats; however, under high snail densities shells
often appear clean, presumably due to intense grazing by conspe-
ciﬁcs, including occasional rasping of the shell apex (personal
observations). Therefore, whereas shells could provide shelter for
certain small bodied organisms, the suitability of the microhabitat
created by shells will likely depend on snail density in the habitat.
5. Effects on nutrient cycling in shallow ponds and lakes
The theory of alternative lake equilibriums holds that shallow
ponds and lakes exist in either a clear water state with a high
coverage of macrophytes, or a turbid water state with high phyto-
plankton biomass (Scheffer et al., 1993). These equilibrium states
are stabilized by interspeciﬁc interactions between herbivores and
macrophytes, often under the inﬂuence of regulators at higher
trophic levels (i.e., Hansson, 1992; Hansson et al., 1998). Several
mechanisms are involved: in the clear water state, macrophytes
control phytoplankton development by reducing light penetration
or through negative allopathy (biochemical interactions between
the plants and phytoplankton: Van Donk and Van de Bund, 2002)
and by preventing the re-suspension of sediment in the water
column. In the turbid water state, herbivores reduce macrophage
coverage and predators reduce zooplankton density, releasing
nutrients to the water and thereby promoting the development of
phytoplankton (Scheffer et al., 1993). Ponds can shift between these
states when nutrient pulses from agricultural run-off lead to
eutrophication (Simpson et al., 1994a,b). However, changes in the
composition of predator or herbivore assemblages can also cause
shifts in water state through cascade effects on phytoplankton
biomass mediated through macrophyte community structure.
Invasive freshwater snails can affect nutrient cycles in shallow
ponds and streams in several ways: Many invasive snails are
F.G. Horgan et al. / Acta Oecologica 54 (2014) 90e10096
detritus or algal feeders associated with massive increases in
secondary productivity releasing large amounts of nutrients to the
water, reducing nitrogen-ﬁxing algae and bacteria and thereby
stimulating macrophyte growth, especially the growth of free-
ﬂoating species (Brönmark, 1985; Pinowska, 2002; Hall et al.,
2003, 2006; França et al., 2007; Arango et al., 2009; Li et al.,
2009); In contrast, by feeding directly on macrophytes, high
densities of apple snails could alter the nutrient dynamics of water
bodies while at the same time signiﬁcantly reducing the biomass
and altering the community composition of macrophytes.
In a survey of natural wetlands invaded by P. canaliculata (and
possibly P. maculata) in Thailand, Carlsson et al. (2004) found strong
positive correlations between snail densities and the concentra-
tions of nitrogen, phosphorus and chlorophyll a (a measure of
phytoplankton) at wetland sites. Macrophyte species richness and
coverage were severely depleted where the snail densities excee-
ded 2 m
. Furthermore, the authors reported that algal blooms had
occurred at several sites following the introduction of the snail(s).
In effect, according to Carlsson et al. (2004), the snails had tipped
the balance from macrophyte-dominated to phytoplankton-
dominated primary production and from clear to turbid water.
Although the survey cannot rule out the effects of other unrecorded
factors on these observed relationships, the results strongly
support the idea that unregulated herbivory by apple snails had led
to a chain of reactions that altered the water state. In contrast, Fang
et al. (2010), in the only other study of effects of invasive apple
snails on nutrient cycling, found P. canaliculata to have no effect on
nitrogen and phosphorous concentrations in 1 m
the presence of Myriophyllum aquaticum (Vell.) Verdc., phyto-
plankton biomass increased with increasing snail densities and the
community became dominated by Cryptophyceae, whereas with
E. crassipes, phytoplankton biomass was independent of snail
density and was co-dominated by Cryptophyceae, Chlorophyceae
and Bacillariophyceae (Fang et al., 2010). Although the experiments
were conducted over a relatively short time period (about 1
month), the authors suggest that herbivory by apple snails does not
necessarily lead to a shift from clear to turbid water. Further mes-
ocosm studies could help clarify the potential effects of apple snails
on nutrient dynamics in tropical wetlands and determine factors
governing the severity of their impact.
6. Effects in agricultural wetlands
Rice, taro and several aquatic vegetables are produced in arti-
ﬁcial or managed wetlands. During periods when the crops are
ﬂooded, water moves over the ﬁelds, generally in a systematic
manner determined by proximity to irrigation sources, topography
and the opening or closing of slews and gates (Goodell, 1984).
Agricultural wetlands are characterized by the dominance of
a single or few macrophyte crops, the biomass of which increases
gradually over the growing season, often negatively affecting
associated ﬂora and benthic fauna and by short pulses of nutrients
in the form of chemical or organic fertilizers (Simpson et al., 1994a).
Apple snails have successfully invaded these agricultural wetlands
causing considerable economic losses by feeding on young rice
plants and other aquatic crops. For example, Naylor (1996) esti-
mated that P. canaliculata had caused accumulated losses of over
US$1200 million to rice farmers in the Philippines during the ﬁrst
ten years after its introduction. Apart from direct losses caused to
rice production, the invasive apple snails have led to phenomenal
increases in pesticide use. For example in the Philippines, mollus-
cicide purchases increased from less than 10 kg in the early 1980s
(before the snails were introduced) to about 242,000 kg in 1997
(Adalla and Magsino, 2006). Massive molluscicide inputs at rice
planting stages will undoubtedly have major effects on native snails
and other fauna and may be the direct cause of declines in several
native benthic species. For example, niclosamide and meth-
aldehyde reduce the richness and biomass of decomposer ﬂy
communities and could directly affect ecosystem functioning by
delaying carcass decomposition (Horgan unpublished).
In some cases, the successful invasion of P. canaliculata and
P. maculata into agricultural wetlands may be due to the higher
nutritional value and lower defenses of agricultural crops such as
Amaranthus gangeticus L. (Amaranth), Apium graveolens dulce D.C.
(Chinese celery), Ipomea aquatica Forsk. (water spinach) and
Nasturtium ofﬁcinale R. Br. (watercress) (Qiu and Kwong, 2009).
However, rice and taro, are apparently less palatable (Qiu and
Kwong, 2009). This suggests that the extent and availability of
habitat as well as currents in irrigation channels that direct snails
toward the ﬁelds are principal determinants of apple snail presence
and densities in crops. Fertilizers cause punctual changes in soil and
ﬂoodwater chemistry: Broadcast applications of nitrogen fertilizer
increase photosynthesis levels and pH in the ﬂoodwater, increase
primary productivity (including periphyton and phytoplankton
productivity), reduce the biomass of nitrogen-ﬁxing algae, and
reduce concentrations of dissolved CO
(Mikkelsen et al., 1978;
Simpson et al., 1994b). The resulting high O
demand can create
anoxic conditions in the ﬂoodwater at night (Saito and Watanabe,
1978). These pulses in primary productivity are rapid and of short
duration, but can cause lasting changes in benthic communities
(Simpson et al., 1994a,b). Populations of benthic organisms that use
gills for respiration and are dependent on dissolved O
noides spp.) are expected to decline under such conditions. Some
apple snails may use aerial respiration to survive nutrient pulses,
and could consequently beneﬁt from sudden increases in primary
productivity and the palatable new growth of macrophytes and
algae. Although high nitrogen appears to negatively affect
P. canaliculata causing high mortality during the ﬁrst few days after
application (De La Cruz et al., 2001), it is still unknown whether
assimilated nitrogen in plants could later beneﬁt the snails. High
nitrogen levels in the soil may also increase plant tolerance to snail
damage and allow plants to more rapidly escape from the vulner-
able seedling stages (personal observations). These pulse dynamics,
together with highly variable water depths that are determined by
irrigation, and consequent synchronized emergence of estivating
adult and juvenile snails probably contribute to the sudden, large,
and short-term increases in apple snail populations in managed
habitats (i.e., P. maculata:Burlakova et al., 2009). In rice ﬁelds, once
the vulnerable crop stages have passed (i.e., >30 days in rice), apple
snails (i.e., P. canaliculata) can be considered beneﬁcial bio-weeders
(Joshi et al., 2006; Hidaka et al., 2007). However, such periods,
where the snails appear innocuous to farmers, may be important
recruitment periods where emergent plants in the form of rice and
taro are used for egg laying and abundant epiphytic algae sustain
7. Recommendations etoward a unifying framework for
research in natural and agricultural ecosystems
It appears from Tables 1e3and the related discussion that,
although 14 species of apple snails have been introduced to new
habitats, only four species (M. cornuarietis,P. canaliculata,P. glauca
and P. maculata) have attained high densities over large or localized
areas with noted impacts. There have also been reports of damage
to rice by each of these species (Joshi and Sebastian, 2006; Cowie,
2002). Research on the impact of apple snails in agricultural and
natural wetland ecosystems rarely overlaps. Of the several docu-
ments generated over the past 30 years since their initial cata-
strophic outbreaks in South East Asia, only very few have compared
aspects of snail ecology in both natural and agricultural habitats
F.G. Horgan et al. / Acta Oecologica 54 (2014) 90e100 97
(but see Burlakova et al., 2009). Wetland habitats are not isolated
but gradually merge into one another. Water moves from natural to
managed habitats and vice versa and this can transport snails
(Lewis and Magnuson, 2000; Ichinose and Yoshida, 2001).
Furthermore, in Asia, irrigated rice is the most extensive wetland
habitat (Khush, 1997), such that activities in rice ﬁelds will affect
many adjacent and connected natural wetlands. Landscape
approaches that focus on watershed regions are therefore neces-
sary for an improved understanding and management of apple
snails. Of particular interest is the possible recruitment of snails in
agricultural habitats as sources of invaders for natural wetlands. In
effect, rice ﬁelds may represent efﬁcient nurseries for the snails,
that also reduce potential predators and competitors where
molluscicides and other pesticides are applied. The effects of high
nutrient levels in wetland systems have been well documented, and
the role of snail herbivores in shifting water toward turbidity seems
probable. Whereas few mesocosm studies have been conducted to
represent natural habitat, experiments in the management of snail
damage in rice ﬁelds do represent simpliﬁed habitats and indicate
how apple snails can alter macrophyte communities. What is still
poorly understood is whether snails are likely to avoid high
nutrient waters, which would represent a beneﬁcial negative
feedback mechanism, or whether the snails, because of their robust
habits, continue to drive wetlands toward a more heavily perturbed
state. Preliminary observations from agricultural systems suggest
that apple snails are attracted to high nutrient ﬁelds and patches
(Horgan and Stuart, unpublished), suggesting that chemical fertil-
izers increase snail biomass and accelerate invasion and spread.
Further research on the dynamics and movement of water, nutri-
ents and snails in agricultural and natural wetlands would help
determine the consequences of apple snail invasions at landscape
and regional levels.
Financial support for this research was through a Global Rice
Science Partnership (GRiSP) fund. We thank two anonymous
reviewers for several helpful suggestions and comments that
improved the manuscript and Ken Hayes (Center for Conservation
Research and Training, University of Hawaii) for reviewing the
species names used in the manuscript and providing up to date
information on snail taxonomy and distribution.
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