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Impact of invasive apple snails on the functioning and services of natural and managed wetlands

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  • EcoLaVerna Integral Restoration Ecology; Centre for Pesticide Suicide Prevention - University of Edinburgh
  • Pesticide Action Network UK

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Original article
Impact of invasive apple snails on the functioning and services
of natural and managed wetlands
Finbarr G. Horgan
a
,
*
, Alexander M. Stuart
a
, Enoka P. Kudavidanage
b
a
International Rice Research Institute, DAPO Box 7777, Metro Manila, Philippines
b
Department of Natural Resources, Sabaragamuwa University, Belihuloya, Sri Lanka
article info
Article history:
Received 18 April 2012
Accepted 7 October 2012
Available online 11 November 2012
Keywords:
Aquatic macrophytes
Asia
Benthic invertebrates
Community structure
Nutrient cycle
Rice
Stable states
abstract
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. Signicant 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
eld 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.
1. Introduction
Wetlands rank among the most productive ecosystems on the
planet, providing a range of ecosystem services and economic
benets: they defend coastal and riverside areas against storms and
oods, purify water, control erosion, retain pollutants, and they
maintain a high diversity of animal and plant species, often func-
tioning as nurseries for sh and shellsh, 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, shed or farmed in natural areas, but also from intensive
and semi-intensive agriculture in managed or articial wetlands
such as rice elds. 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 ow 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) identied 11 species that are used as human
food, two as animal fodder, and ve medicinal plants from among
184 rice-associated weeds in Laos. Some macrophytes, including the
free-oating fern Azolla, are encouraged in rice elds to increase
nitrogen-xation (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 sh, 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: f.horgan@irri.org (F.G. Horgan).
Contents lists available at SciVerse ScienceDirect
Acta Oecologica
journal homepage: www.elsevier.com/locate/actoec
1146-609X/$ esee front matter Ó2012 Elsevier Masson SAS. All rights reserved.
http://dx.doi.org/10.1016/j.actao.2012.10.002
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 protability
(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 peopleslivelihoods 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 benets 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
Table 1
Species of apple snail found outside their native distribution range.
Species and common name Reason for introduction Native range
a
Invaded range
a
Marisa cornuarietis (Linnaeus,
1758) eGiant rams-horn snail
Biological control of schistosomiasis
vectors and weeds, pet trade
South America (Colombia
and Venezuela) [1]
Costa Rica, Cuba, Dominican Republic, Egypt,
French Guyana, Guadeloupe, Guyana, Panama,
Puerto Rico, Sudan, Surinam, Tanzania, USA
(Florida, Texas) [1]; Martinique, St Kitts [2];
New Zealand [3]
Pila conica (Wood, 1828) eBlack
apple snail
b
Food South-East Asia (Philippines) [1] Guam, Palau, USA (Hawaii) [1]; India [4]
Pila globosa (Swainson,
1822) eIndian apple snail
Food, commerce, potential biological
control of schistosomiasis vectors
Bangladesh, India (North) [5] India (Kerala) [6]
Pila leopoldvillensis (Putzeys,
1889) eGiant African apple snail
Food Africa [7] Philippines [7]; Taiwan [1]
Pomacea bridgesii (Reeve,
1856) eSpike-topped apple snail
c
Pet trade Bolivia, Brazil [8] Chile [9]; India (West Bengal) [10]; Sri Lanka [11]
Pomacea canaliculata (Lamarck,
1822) eChanneled apple snail
Food and commerce, pet trade Argentina, Uruguay, Paraguay,
Brazil, and Possibly Bolivia [1]
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) [1]; USA (Texas, North Carolina)
[12], Pakistan [13], Russia (Siberia) [14];
Pomacea diffusa (Blume,
1957) eSpike-topped apple snail
c
Pet trade South America (Amazon Basin) [1] Australia (western) [15]; Brazil (Pará,
Pernambuco, Rio de Janeiro), Colombia,
French Guiana, Panama, Sri Lanka, USA
(Florida, Hawaii), Venezuela [1]; New Zealand
[16]; Puerto Rico, USA (Alabama) [17];
Pomacea glauca (Linnaeus, 1758) Biological control of schistosomiasis
vectors and weeds, pet trade
South America (Northern) and
Caribbean
d
[8]
Caribbean
d
[2]; Philippines [18]
Pomacea haustrum (Reeve,
1858) eTitan apple snail
e
Pet trade Bolivia, Brazil (Amazon
region), Peru [12]
USA (Florida) [12]
Pomacea lattrei (Reeve, 1856)
f
Aquaculture Guatemala Panama [19]
Pomacea lineata (Spix, in
Wagner, 1927)
g
Pet trade Brazil and Guyanas [8] South Africa [20]
g
Pomacea maculata (DOrbigny,
1835) eIsland apple snail
h
Food, commerce, pet trade Lower Paraná, Uruguay and
La Plata Basins [15]
Cambodia, Singapore, South Korea, Thailand,
USA (Alabama, Florida, Georgia, Louisiana,
South Carolina, Texas), Vietnam [1];
Malaysia (Borneo), Taiwan [15]; Puerto Rico,
USA (Arizona) [17]; Spain [21]
Pomacea paludosa (Say,
1829) eFlorida apple snail
Pet trade Cuba, USA (Florida) [8] USA (Alabama, North Carolina) [22];
Puerto Rico [23]
Pomacea scalaris (dOrbigny, 1835) Food, commerce Argentina, Bolivia, Brazil [15] Taiwan [15]
a
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).
b
Pomacea luzonica is a synonym of P. conica that appears in some publications.
c
Pomacea bridgesii and P. diffusa are often confused during identication; All reported P. bridgesii introductions have not yet been veried.
d
This species might not be native to the Caribbean.
e
Pomacea haustrum is possibly a synonym of P. maculata (Hayes, personal communication).
f
Reports of introduction of P. lattrei have not been conrmed. The species may be a synonym of P. agellata (Say, 1829) (Hayes, personal communication).
g
Unconrmed report of P. lineata in South Africa is likely P. canaliculata (Hayes, personal communication).
h
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 ve 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 DOrbigny
(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 unveried
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 elds in Surinam: Cowie, 2002; Wiryareja and
Tjoe-Awie, 2006), although the dynamics of new habitatand
regionalinvasions are potentially similar. Some of the reports
concern the known introduction of species for specic 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).
Difculties in identifying apple snail species have led to
several reports of distribution and impact that are associated
with incorrectly identied or unveried species. In this review, we
present species names as they appear in original reports, but
Table 2
Invasive status of introduced apple snails with available information on life-history traits and diet.
a
Species Status outside its native range Clutch size
(reported range)
b
Time to hatch (reported
range in days)
b
Max shell
height (mm)
b
Diet (macrophytes
consumed/tested þobservations)
P. canaliculata Widespread in tropics and
subtropics [1,2]
14e500 [3] 7e28 [3] 80 [4] 51/67 þbryozoans, lamentous algae,
periphyton [5e17]
P. maculata Widespread in tropics and
subtropics [1,2]
522e4751 [18] 7e21 [19] 80 [18]
c
46/52 þsnail eggs, periphyton
[10,14,20e22]
M. cornuarietis Became widespread in the
Caribbean, possibly declining
in some other regions [1]
50e210 [3] 8e24 [3] 60 [23] 23/26 þsnail eggs, animal matter
[5,14,23,24]
P. conica Restricted to a few Hawaiian
and Pacic islands [25]
28e313 [26] 17e28 [26] 32 [26] Macrophytes [26,27]
P. diffusa Localized [1] or declining [25] na na na 1/8 [14]
P. glauca Localized [28], declining or
not established
d
30e90 [3] 14e17 [3] na Macrophytes [28]
P. haustrum Localized or declining [1] ca. 236 [3] 9e30 [3] na 5/8 þlamentous algae, animal
matter, snail eggs [5,14,29]
P. paludosa Localized [30] or not
established [27]
3e141 [3] 15e20 [3] 70 [1] 5/8 [14]
P. scalaris Localized [31] 9e302 [31] na 40 [31] Macrophytes [31]
P. globosa No recent records 200e300 [3] 10e30 [3] na Macrophytes [32]
P. leopoldvillensis Possibly never established
d
na na na Macrophytes, snails, insect larvae and
other arthropods [33]
P. bridgesii Unveried records 50e200 [34] 15e24 [34] 57 [34] Macrophytes, snail eggs, animal
matter [35,36]
P. lattreieP. agellata Unveried records [30] na na 27 [37] Macrophytes [38]
P. lineata Unveried records
e
400e600 [39] ca. 15 [3] na Varied sources from several
trophic levels [40]
a
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 Portereld (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.
b
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 scientic papers.
c
In some, perhaps unusual cases, snails may attain >155 mm in height (Cowie, 2002).
d
Apparently introduced to the Philippines in the 1980s (Mochida, 1987), but there have been no records since, records of P. glauca may be misidentications.
e
Reported from South Africa with no further records.
F.G. Horgan et al. / Acta Oecologica 54 (2014) 90e10092
indicate probable misidentications 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 unidentied species gathered from the wild or from mixed
sources; and (2) that apple snails are anatomically very similar and
species identities are often difcult 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 (DOrbigny) restricted to a few sites. Mean-
while, Rawlings et al. (2007) used two portions of mtDNA,
including COI, to identify ve 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 difcult 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 elds and other
aquatic ecosystems in the region because all early reports referred
only to P. canaliculata. Where possible, Table 1 indicates the
commonly misidentied 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 verication.
Whereas much of the information presented in later sections of
this review relates to potential impacts of apple snails in general, we
have included specic 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 benets 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 inuence 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 efciencies 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 efciency (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, oating 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 eld 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 articial 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 farmerselds, 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 eld sites would be welcome.
F.G. Horgan et al. / Acta Oecologica 54 (2014) 90e100 93
Table 3
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
Macrophytes
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
Field observations
linked to feeding
trials in enclosures
Carlsson and
Brönmark (2006)
P. canaliculata South China Riverine
restoration plot
Altered macrophyte community and
reduced water quality due to detritus
and plant waste materials
Field observations
linked to feeding
trials in enclosures
Wang and Pei (2012)
P. canaliculata South China Cultivated
aquatic crops
Damaged several cultivated semi-aquatic
vegetables
From newspaper
reports
Qiu and Kwong (2009)
Pomacea spp. Hawaii, Hong Kong,
Japan, Philippines
Cultivated
aquatic crops
Damaged several cultivated semi-aquatic
vegetables
Literature search Cowie (2002)
P. canaliculata Japan Rice elds Reduced density of Cyperus difformis
L. and other macrophytes
Comparison of pre
and post-invasion
macrophyte
communities
Luna Maldonado
and Nakaji (2008)
P. canaliculata Philippines Rice elds Reduced densities of non-crop
macrophytes and increased rice
grain yield
Comparison of
molluscicide treated
and untreated plots,
and farmerselds
under different snail
management regimes
Joshi et al. (2006)
P. canaliculata Philippines Rice elds 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
densities
Litsinger and Estano
(1993),Halwart
(1994),Naylor (1996)
P. canaliculata Japan Rice elds Reduced macrophyte richness and
coverage
Survey of elds 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
(Grand-Etang)
Reduced densities of Pistria stratiotes
L. and B. glabrata
Surveys conducted
over several years
Pointer et al. (1991)
M. cornuarietis Guadeloupe Lakes and ponds Reduced densities (eliminated) of
P. stratiotes and B. glabrata
Surveys conducted
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)
Benthic communities
P. canaliculata Hong Kong General Reduced density of Austropeplea
ollula (Gould) and Biomphalaria
straminea (Dunker)
Not presented Fang et al. (2010)
P. canaliculata and/or
possibly P. maculata
Thailand Ponds and
waterways
Reduced bryozoan abundance and
changed community composition
Field observations
linked to feeding
trials in enclosures
Wood et al. (2006)
P. maculata USA (Florida) Ponds and
waterways
Reduced densities of Pomacea haustrum
(Reeve) in some areas
Not presented Morrison and
Hay (2011a)
P. canaliculata Philippines Rice elds Reduced densities of Pila conica (Gray)
that was once used for human food
Not presented Basilio (1991)
P. canaliculata Philippines Rice elds 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
plicatulus (Benson)
Comparison of
life-histories in
invaded and
non-invaded sites
Lam (1994)
Food chain
P. canaliculata Hong Kong Lotic and
lentic wetlands
Massive increase in secondary production
(greater than published estimates for
any other freshwater snails)
Sizeefrequency
method with samples
collected throughout
a year
Kwong et al. (2010)
P. canaliculata Hong Kong Pond
enclosures
Reduced biomass of lamentous green
algae (Spirogyra sp.); reduced macrophyte
biomass, increased phytoplankton biomass
and altered phytoplankton community
structure
Comparison of
enclosure with and
without snails
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
Regression analyses
from survey of 13
wetland sites
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 inuence 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
conned 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 signi-
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 snailsnative 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 eld 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 oating species are expected to decline. However,
recruitment of emergent ora 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 ora, allowing a dominance of emergent plants
after water levels increase again and when the seedlings are
sufciently robust to avoid snail damage. In effect, this is the
mechanism underlying current management of snails in rice where
elds 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 signicant sources for snail
colonization of adjacent ponds and ooded elds. 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 efciency 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
conspecic 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
fragments
Alterations in adult snail kite feeding
behavior and energetic deciencies in
juvenile birds
Comparison of
time-activity budgets
of kites in invaded
and non-invaded
wetlands
Cattau et al. (2010)
Human and animal health
P. canaliculata Indo-Pacic region General Carry and spread angiostrongyliasis
and other human and animal diseases
(not exclusive)
Not presented Lv et al. (2009),
Hollingsworth and
Cowie (2006)
F.G. Horgan et al. / Acta Oecologica 54 (2014) 90e100 95
plant materials are sufciently 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 ve 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
eliminatedthree pulmonate species, but had no apparent affect on
Melanoides tuberculata Müller, a prosobranch (Nguma et al., 1982).
Such eld 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
conned 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 (unveried species identication) 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 nally
eliminatedthe 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 interspecic 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 sufcient to
reduce growth and survival of P. paludosa to the same extent as four
conspecic adults.
Competition leading to the elimination of dissimilar snails, such
as pulmonates, is more difcult 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) intraspecic 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-
cics, 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 interspecic interactions between herbivores and
macrophytes, often under the inuence 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-xing algae and bacteria and thereby
stimulating macrophyte growth, especially the growth of free-
oating 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 signicantly 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
2
. 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
3
mesocosms. In
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-
cial or managed wetlands. During periods when the crops are
ooded, water moves over the elds, 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 ora 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 rst
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 y
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 ofcinale 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 elds are principal determinants of apple snail presence
and densities in crops. Fertilizers cause punctual changes in soil and
oodwater chemistry: Broadcast applications of nitrogen fertilizer
increase photosynthesis levels and pH in the oodwater, increase
primary productivity (including periphyton and phytoplankton
productivity), reduce the biomass of nitrogen-xing algae, and
reduce concentrations of dissolved CO
2
(Mikkelsen et al., 1978;
Simpson et al., 1994b). The resulting high O
2
demand can create
anoxic conditions in the oodwater 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
2
(i.e. Mela-
noides spp.) are expected to decline under such conditions. Some
apple snails may use aerial respiration to survive nutrient pulses,
and could consequently benet 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 rst few days after
application (De La Cruz et al., 2001), it is still unknown whether
assimilated nitrogen in plants could later benet 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 elds, once
the vulnerable crop stages have passed (i.e., >30 days in rice), apple
snails (i.e., P. canaliculata) can be considered benecial 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
hatchlings.
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 elds 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 elds may represent efcient 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 elds do represent simplied 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 benecial 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 elds 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.
Acknowledgments
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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F.G. Horgan et al. / Acta Oecologica 54 (2014) 90e100100
... These two studies both suggested that the costs of aquatic invasions are an order of magnitude lower than costs of terrestrial invasions, explainable by the greater difficulty and fewer assessments which have caused an observable research bias. One possible reason for this could be that the current economic losses associated with aquatic invasions mainly came from the agricultural damages and losses (e.g., rice and taro) caused by apple snails, which has attracted a wide research attention in the past (Horgan et al., 2014). Moreover, the considerable knowledge gap concerning terrestrial gastropod invaders is possibly caused by current cost estimations largely ignoring the diminished economic value associated with a declining biodiversity (e.g. the population shrinkage or extinction of endemic gastropod species), which have been difficult to quantify (Lodge et al., 2016;Renault et al., 2022). ...
... Nevertheless, the reports for economic losses of apple snails in Asia may be insufficient, given the widespread introductions throughout South-Eastern Asia but cost reports restricted in four countries (i.e., Philippines, Thailand, Vietnam, China). Moreover, the invasions of apple snails also dramatically impacted non-agricultural wetlands in Asia and other regions, and ecological impacts which are also difficult to quantify (Horgan et al., 2014). ...
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Invasive alien gastropods are a particularly pervasive taxonomic group worldwide, often causing substantial impacts on aquatic and terrestrial ecosystems. Yet, much remains unknown about this invasive group’s economic costs to human society. Here, we used the InvaCost database to evaluate the taxonomic, spatial, and temporal patterns of economic costs associated with invasive gastropods on the global scale. In total, 13 species of invasive gastropods caused a cumulative global cost of US$ 3.94 billion over the period 1966–2020, with a majority being attributed to aquatic species ($ 3.72 billion, 94.4 %, concentrated mostly in Asia) and only $ 0.22 billion (5.6 %, concentrated mostly in Europe) to terrestrial species. Among different regions, Asia (3.71 billion) reported the greatest costs, compared to far lower costs reported in Europe (214.50 million), North America (13.80 million), Oceania (2.69 million), South America (
... The Invasive Species Specialist Group has listed P. canaliculata as one of the world's 100 worst invasive species [8]. In wetland agriculture, the apple snails attack the seedlings and other flooded crops, and the estimated economic loss has reached billions of dollars [9]. In natural wetlands, the apple snails have become pests as they can change the structure of the macrophyte community and disrupt ecosystem services [10]. ...
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The successful establishment of invasive populations is closely linked to environmental factors. It is unclear whether coexisting species in the native area follow the same genetic pattern in the invaded continents under the local climate factors. Two coexisting morphologically similar snails (Pomacea canaliculata and P. maculata), native to tropical and sub-tropical South America, have become invasive species for agriculture production and wetland conservation across five continents over 40 years. We analyzed the correlation between the genetic diversity of the two snails and the climate factors or habitat changes. Based on the 962 sequences from the invaded continents and South America, the nucleotide diversity in the agricultural habitat was low for P. canaliculata, whereas it was high for P. maculata, compared with that in the non-agricultural habitat. The two snails showed a divided population structure among the five continents. The P. canaliculata population in the invaded continents has remained stable, whereas the P. maculata population expanded suddenly. Seven main haplotype networks and two ancestral haplotypes (Pc3, Pm1) were found in the P. canaliculata and P. maculata populations. The haplotypes of the two snails were related to local climate factors. The overall fixation index of P. canaliculata and P. maculata was 0.2657 and 0.3097 between the invaded continents and South America. The population expansion of the two snails fitted the isolation-by-distance model. We discovered nine new sequences from the sampling locations. Overall, the genetic diversity and genetic differentiation of the two invasive snails were closely related to geographic separation, agricultural habitat, and climate factors.
... It is reported to be potential pest for 15 important economic agricultural crops in China [14]. Also, P. canaliculata is able to spread rapidly into wetland [15] and other freshwater ecosystems, resulting in severe habitat modification and a decrease of local biodiversity [16]. ...
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Background: The invasive alien species may lead to great environmental and economic crisis due to its strong capability of occupying the biological niche of native species and altering the ecosystem of the invaded area. However, its potential to serve as the vectors of some specific zoonotic pathogens, especially parasites, has been neglected. Thus, the damage that it may cause has been hugely underestimated in this aspect, which is actually an important public health problem. This paper aims to discuss the current status of zoonotic parasites carried by invasive alien species in China. Main body: This review summarizes the reported zoonotic parasites carried by invasive alien species in China based on the Database of Invasive Alien Species in China. We summarize their prevalence, threat to human health, related reported cases, and the roles of invasive alien species in the life cycle of these parasites, and the invasion history of some invasive alien species. Furthermore, we sum up the current state of prevention and control of invasive alien species in China, and discuss about the urgency and several feasible strategies for the prevention and control of these zoonoses under the background of booming international communications and inevitable globalization. Conclusions: Information of the zoonotic parasites carried by invasive alien species neither in China or worldwide, especially related case reports, is limited due to a long-time neglection and lack of monitoring. The underestimation of their damage requires more attention to the monitoring and control and compulsory measures should be taken to control the invasive alien species carrying zoonotic parasites.
... Snail Kites are not threatened, globally or in Ecuador (BirdLife International, 2016;Freile et al., 2019), and increasing and expanding populations have been reported across its range (Ferguson-Lees and Christie, 2001;Bierregaard, Jr. and Kirwan, 2013). Ecuadorian populations may be increasing due to habitat modification to grow rice and high densities of freshwater Pomacea snails (Horgan et al., 2014b). However, growing snail populations are due to an invasive species, Pomacea canaliculata, considered a pest due to its severe impacts on agriculture, human health, and the environment (Horgan et al., 2014a). ...
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Helminths are not usually considered important pathogens for birds of prey. There is a single published report of mortality in raptors due to an air sac trematode infection. We report a well-documented death case from massive infection by an air sac trematode of the family Cyclocoelidae in a wild-caught, juvenile male Snail Kite (Rostrhamus sociabilis) in Ecuador. The necropsy of a Snail Kite revealed more than 200 trematodes among air sacs, lungs, heart, gizzard, proventriculus, and liver. Within air sacs and lungs, mature flukes were associated with sacculitis, bronchitis, pneumonia, and atelectasis. Using an integrative taxonomic approach with morphological and molecular data, we identified the parasites as Bothrigaster variolaris (Trematoda: Cyclocoelidae: Ophthalmophaginae). This case provides the first evidence for the pathologic presence of air sac trematodes associated with morbidity in birds of prey in South America. Our results suggest that cyclocoelids may cause debilitation and significant clinical lesions in birds of prey, with potentially fatal consequences.
... The apple snail Pila globosa, an ectothermic semisessile amphibious mollusc model with strong adaptation capacity against adverse environmental conditions, is cosmopolitan in distribution. Its habitat ranges from large to small freshwater bodies and it survives under high and low temperature in summer and winter seasons by adapting hibernation and aestivation strategies beneath the soil, respectively (Horgan et al., 2014). Its physiological adaptive nature ensures its survivability under adverse environmental conditions (Ajesh and Srijeeth 2014). ...
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Information on the oxidative stress physiology parameters (OSPP) in general and as a function of the fluctuation of Mg, Ca and organic carbon present in soil and aquatic physico-chemical factors such as pH, temperature and salinity in particular are scanty in the amphibious snail Pila globosa. A spatio-temporal analysis of redox metabolism (as OSPP) followed by discriminant function analysis of the obtained data were performed in P. globosa sampled from the east-coasts of Odisha state, India (mostly along the Bay of Bengal) for environmental health assessment purposes. Results revealed that the OSPP are susceptible to seasonal synergistic variation of soil and physico-chemical factors. Overall, lipid peroxidation, total antioxidant capacity, activities of catalase, glutathione reductase had positive correlation whereas ascorbic acid, the reduced glutathione and the activity of superoxide dismutase had non-significant correlation with the soil Mg, Ca, organic carbon, and pH, temperature and salinity of water. In the summer season, the snails had a marked 51.83% and 26.41% higher lipid peroxidation level and total antioxidative activity as compared to the other seasons. Spatial variation of OSPP indicates that snails residing away from the Bay of Bengal coast had at least 4.4% lower antioxidant level in winter and 30% higher lipid peroxide levels in summer as compared to the rest of the sampling sites. Results on OSPP in P. globosa may be useful for monitoring the ecotoxic effects of environment using molluscs in general and P. globosa in particular.
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This datasheet on Pomacea maculata covers Identity, Overview, Distribution, Dispersal, Hosts/Species Affected, Vectors & Intermediate Hosts, Diagnosis, Biology & Ecology, Environmental Requirements, Natural Enemies, Impacts, Uses, Prevention/Control, Further Information.
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Aquatic gastropods, both freshwater and marine, often crawl along the water surface with the sole of the foot facing upward. Differential regulation of the activity of cilia on the sole turns the gliding mechanism into pedal surface collecting, by which food particles floating on the water are collected on the sole. Over the last 300 years, surface crawling and pedal surface collecting have been studied in numerous species, but the accumulated knowledge has fallen victim to scientific amnesia. Today, pedal surface collecting usually is considered a unique behaviour that evolved in the family Ampullariidae (Caenogastropoda), which includes the genus Pomacea with some of the globally worst invasive pests. Consequently it may appear feasible to tackle invasive Pomacea species specifically by delivering molluscicides via the water surface. Based on a review of our forgotten literature, I here argue that such an approach would have potentially devastating, unintended effects on native gastropod faunas.
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Previous studies have found that temperature influences molluscicidal the activity of pedunsaponin A (PA), which may be related to the expression of Hsp70, a cold-tolerance gene in Pomacea canaliculata. We determined the temperature effect of PA and the relationship between Hsp70 and temperature sensitivity of P. canaliculata poisoned by PA. Toxicity tests resulted in LC50 values of 17.7239 mg⋅L⁻¹ at 10 °C, which decreased to 2.5774 mg⋅L⁻¹ at 30 °C, implying a positive correlation between toxicity of PA and temperature. After Hsp70 being interfered, the mortality rate of P. canaliculata treated with PA for 72 h was 70%, which was significantly higher than that of snails treated with PA for 72 h without interfering (56.7%). Meanwhile, immune enzyme activities such as SOD, ACP and AKP were significantly increased in the interfered group and expression level of PcAdv in the gill was also significantly increased. These results suggest that deletion of Hsp70 promotes the activation of some immune enzymes of P. canaliculata and elevates the content of target proteins to cope with the dual stresses of low temperatures and molluscicides. These findings indicate that the Hsp70 plays an important role in influencing the temperature sensitivity of P. canaliculata when treated with PA.
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Aquatic macrophytes supply an important link in freshwater systems through their multiple functions and ecosystem services. However, invasive generalist herbivores threatened their diversity and abundance. Pomacea canaliculata, one species of invasive freshwater apple snail, shows generalist macrophyte tendencies. Information about its natural diet would allow better anticipate its impacts but diet studies determining the macrophytes species eaten in natural aquatic environments are lacking. We evaluated the feasibility of identifying and quantifying macrophyte remains in P. canaliculata digestive contents. We performed single species and multiple species trials with five pre-weighed aquatic macrophytes to record the amount ingested of each of them and then dissected the snails to analyse their digestive contents microscopically. Microscopic traits provided ways to characterize macrophytes and distinguish them in combined species trials. Identifiable macrophyte remains highly varied in both single species (4.5-58.0%) and multiple species trials (5.7-35.1%); the total percentage of identifiable remains in the latter was 51-52%. The amount of identifiable remains in the multiple species trial was in general correlated with the amount ingested of each macrophyte although in one case it was strongly overestimated due to leaf fragmentation. The microscopic identification and quantification of macrophyte remains in the digestive contents appears feasible, even with several species that differ in abundance, if feeding trials accompany gut content analyses.
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AbstrakSawi hijau (Brassica rapa L.) merupakan sayuran yang memiliki nilai ekonomis karena banyak digemari oleh masyarakat, sehingga budi dayanya membutuhkan pemupukan yang menunjang pertumbuhannya. Penggunaan pupuk organik cair memiliki keunggulan cepat diserap oleh tanaman. Azolla pinnata dan telur keong mas (Pomacea cannaliculata) dapat menjadi kandidat pupuk cair karena mengandung unsur hara tinggi berupa sumber nitrogen, fosfat, dan kalium. Penelitian ini bertujuan untuk mengetahui pengaruh penggunaan pupuk A. pinnata dan P. cannaliculata berbeda konsentrasi terhadap pertumbuhan dan produktivitas sawi hijau. Metode yang digunakan adalah Rancangan Acak Lengkap (RAL) faktorial 3 x 3 yang terdiri dari 9 perlakuan dan 6 kali ulangan. Konsentrasi pupuk A. pinnata adalah 0 mL/L, 100 mL/L, dan 150 mL/L, sedangkan konsentrasi pupuk P. cannaliculata adalah 0%, 15%, dan 45%. Parameter pengamatan berupa tinggi tanaman, jumlah daun, berat basah, berat kering, panjang daun, lebar daun dan klorofil total. Dana dianalisis menggunakan analisis variansi dan Duncan Multiple Range Test. Hasil penelitian menunjukkan kombinasi pupuk Azolla pinata dan P. cannaliculata tidak memberikan pengaruh nyata terhadap tinggi tanaman, jumlah daun, berat basah, berat kering, lebar daun, panjang daun, dan kadar klorofil total. Namun demikian, pemberian pupuk secara terpisah berpengaruh positif terhadap pertumbuhan sawi hijau, dengan penggunaan yang disarankan 100 mL/L air A. pinnata dan 45% P. canaliculata.Abstract Green mustard (Brassica rapa L.) is a vegetable has economic value because it is favored by the community, so its cultivation requires fertilization to support its growth. The use of liquid organic fertilizer has the advantage of being quickly absorbed by plants. Azolla pinnata and golden snail eggs (Pomacea cannaliculata) can be candidates for liquid fertilizer because they contain high nutrients in the form of nitrogen, phosphate and potassium. The study aims to determine the effect of A. pinnata and gold snail eggs as fertilizers with different concentrations on the growth and productivity of green mustard. The research used a factorial Completely Randomized Design (CRD) of 3 x 3 consisting of 9 treatments and 6 replications. Concentrations of A. pinnata were 0 mL/L, 100 mL/L, and 150 mL/L, while P. cannaliculata were 0%, 15%, and 45%. Parameters observed included plant height, number of leaves, wet weight, dry weight, chlorophyll content, leaves width and length. Data obtained were analyzed using the analysis of variance and Duncan Multiple Range Test. Result showed that combination of Azolla pinnata and P. canaliculata had no significant effect on plant height, the number of leaves, wet weight, dry weight, leaf width, leaf length, and total chlorophyll content. However, the application of fertilizer separately showed a positive effect on the growth of the plant, with the recommended use of A. pinnata at 100 mL/L of water and P. canaliculata at 45%.
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Se reportan las características de los desoves de "churo", Pomacea maculata, en acuarios de vidrio en Iquitos-Perú. Se utilizaron seis acuarios de vidrio de 70 x 40 x 40 cm. con un tirante de agua de 15 cm. En cada acuario se colocaron cuatro "churos" adultos, proporcionándose alimento peletizado con un tenor de 30% de proteina total y además hierbas acuáticas: "putu-putu", Eichornia crassipes, "lenteja de agua", Salvinia auriculata y "huama", Pistia stratiotes. La oviposición se realizó en las paredes internas de los acuarios a una altura de 15 a 30 cm sobre el nivel del agua. Los desoves presentan una amplia variabilidad, tanto en longitud, como en ancho y espesor (coeficiente variabilidad (c.v) = 15 a 20 %). El desarrollo ontogénico ocurre entre los 12 y 16 días, con una media de 14 días, obteniéndose entre 77 y 483 crías por desove, con un peso individual de 0,028 gramos. El número de crías representa aproximadamente del 44 al 50 % de los huevos en cada puesta.
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Freshwater samples and gastropod mollusks (Pomacea canaliculata) were collected at 5 sampling stations located along the lower Río de la Plata basin, Argentina, to assess the extent of tributyltin (TBT) contamination. Determined data revealed the presence of TBT and some of its breakdown products (dibutyltin: DBT, and monobutyltin: MBT) in all freshwater samples and also in soft tissues of P. canaliculata gastropods. Chronic bioassays (6 months) were performed using female gastropods that had been reared under laboratory conditions and exposed to a similar TBT concentration than the value determined in freshwater samples (1 µg L⁻¹). The aims of this study were to evaluate the extent of TBT accumulation, the tissue distribution, and the effects on selected biomarkers (activity of superoxide dismutasa: SOD, activity of catalase: CAT, levels of total glutathione: t-GSH, lipid peroxidation, and activity of acetylcholinesterase: AChE). Gonads presented the highest accumulation, followed by the cephalopedal region, albumin gland, and finally hepatopancreas. Both metabolites, DBT and MBT, were also found. All exposed female animals presented development of a penis reflecting the potential of TBT as an endocrine disrupting chemical for this gastropod species. Results on the selected biomarkers confirmed additional adverse effects induced by TBT. An increase in CAT activity and changes in t-GSH levels are indicative of alterations on the cellular redox status. The inhibition of AChE could reflect signs of neurotoxicity. Altogether, these results reveal a negative impact on the health of this gastropod population.
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Climate change has dramatically altered freshwater ecosystems and will continue to affect them further. As water-level fluctuations increase in frequency and intensity, the tolerance of aquatic organisms to abiotic stressors will become critical determinants of survival. Apple snail species in the genus Pomacea (Ampullariidae) live in freshwater throughout the tropics and subtropics, but deposit egg clutches on terrestrial substrates. Two species, Pomacea canaliculata and Pomacea maculata, are amongst the world's most invasive species and are expanding their invasive ranges. To evaluate their survival during extreme dry-down conditions in their native range, we collected 156 apple snails (Pomacea spp.) from 4 permanent water bodies in Uruguay and buried them in dry sand to simulate drought. We placed the snails outside, sheltered from sun and rain (ambient temperature: 15.2-28.17C, ambient humidity: 44.9-100.6% below sand surface, 34.2-98.7% in outside air), and checked survival daily. Genetic identification confirmed the presence of P. canaliculata, P. maculata, and a morphologically indistinguishable undescribed congeneric among these snails. Thirty-six percent of snails survived the entire period (47 d) buried in the sand. Fewer males survived than females. Median lethal exposure time (LT50) was 33 d for the total group of Pomacea species. After re-immersion in water, 93% of survivors resumed normal activities, including feeding, within 24 h and mated during later observations, indicating resistance to drying conditions and resilience during recovery. Offspring from 1 clutch laid after recovery survived >1 y after this experiment. We found high tolerance to prolonged drying and rapid recovery after re-immersion for 3 Pomacea species. As drought frequency increases with climate change, tolerance to these conditions and rapid restoration of normally high reproductive rates after dry-down may confer an advantage to these species, contribute to long-term persistence in their native range, and facilitate continued invasions.
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Agricultural practices have been intensified in recent decades in Argentina. The Pampa plain is the main agricultural region of the country. The effect of increased application of agrochemicals on the invertebrate fauna of the Pampasic streams remains unreported. In the present study, we compared the abundance and composition of invertebrate assemblages in seven Pampasic streams with contrasting soil use in their basins. Two streams run through intensively cropped plots, two drain basins with livestock fields, while the other three are located within a biosphere Reserve. Higher nutrient and insecticide concentrations were measured in the streams draining cropped basins, related with pesticide applications and crop fertilization. The invertebrate assemblage composition of the cropped streams differed significantly from the others and between the two. Present evidence suggests that the impact of agrochemicals results in a change in composition with decreased abundance or absence of sensitive species such as Hyalellidae, Caenidae, Baetidae and Curculionidae and increased abundance of more tolerant taxa: Ostracoda, Glossiphoniidae (Hirudinea), Ancylidae (Gundlanchia), Ampullariidae (Pomacea canaliculata), Sphaeriidae and Dugesiidae. Available information suggests that macrophyte cover and composition also influence the invertebrate assemblages of the Pampasic streams.
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Apple snails (Ampulariidae) occur throughout tropical and subtropical rice-growing regions. Native apple snails rarely damage rice; however, in hot and humid tropical regions, some native species will damage wet-direct-seeded rice (i.e., Pomacea spp. in Suriname and Brazil). Similarly, exotic apple snails in wet, temperate regions can damage direct-seeded rice (i.e., Pomacea canaliculata in Japan). However, if left unmanaged, exotic apple snails in warm tropical regions (i.e., P. canaliculata and P. maculata in South East Asia) can cause significant economic losses even to transplanted rice (which is more robust that direct-seeded rice). The negative impact of apple snails on rice yield can be reduced by reducing seedling vulnerability or controlling snail population densities. Reducing vulnerability is a more sustainable solution to apple snails but requires new methods such as seedling broadcasting and machine transplanting to decrease labor costs. To avoid further spread of apple snails, the implementation of effective quarantine directives is recommended for tropical countries that are vulnerable to exotic apple snails.
Chapter
This 19-chapter book discusses the biology (including reproduction, life history, feeding preferences and sexual behaviour) of molluscs as pests of horticultural, field and fodder crops, and outlines the development of appropriate mechanisms for the control of these pests (mainly biological, cultural and chemical). Two chapters review progress towards the development of chemical control strategies, one addressing the toxicology of chemicals, the other the deployment of molluscicides in baits. These chapters also highlight the statistical and biological procedures for screening and evaluating molluscicides which are not a component of the standard procedure of mollusc control. A series of chapters focus on specific crop situations, providing a synopsis of the current pest status of gastropod species or species groups.
Article
The assessment of the risk posed by invasive alien species (IAS) to the environment is a component of increasing importance for Pest Risk Analysis. Standardized and comprehensive procedures to assess their impacts on ecosystem services have been developed only recently. The invasive apple snails (Pomacea canaliculata and P. maculata) are used as a case study to demonstrate the application of an innovative procedure assessing the potential impact of these species on shallow freshwater ecosystems with aquatic macrophytes in Europe. The apple snail, Pomacea maculata, recently established in the Ebro delta in Spain resulting in a serious threat to rice production and wetlands, having also a high risk to spread to other European wetlands. Here, the population abundance of apple snails is regarded as the main driver of ecosystem change. The effects of ecosystem resistance, resilience and pest management on snail population abundance are estimated for the short (5 years) and the long (30 years) term. Expert judgment was used to evaluate the impacts on selected ecosystem services in a worst-case scenario. Our study shows that the combined effects of apple snails are estimated to have profound effects on the ecosystem services provided by shallow, macrophyte-dominated ecosystems in Europe. This case study illustrates that quantitative estimates of environmental impacts from different IAS are feasible and useful for decision-makers and invasive species managers that have to balance costs of control efforts against environmental and economic impacts of invasive species.