Complex interactions among fish, snails and macrophytes: implications for biological control of an invasive snail

The golden apple snail (Pomacea canaliculata), a native of freshwater wetlands of South America, has invaded many Asian countries and grazed heavily in agricultural and wild areas. Common carp (Cyprinus carpio) has been proposed as a biological control agent against this snail, but little is known about its impact on non-target aquatic plants and animals. In a 8-week enclosure experiment, we quantified the impact of common carp on three species of aquatic macrophytes and nine species of snails, including the apple snail, in a shallow pond. The results showed that the apple snail or carp alone significantly reduced the plant biomass, although the apple snail had a stronger overall herbivorous effect than the carp. The carp completely removed juvenile apple snails, but had only a weak predatory effect on larger apple snails and no effect on the adults’ oviposition frequency. Furthermore, the carp significantly reduced the populations of most species of other snails that occurred naturally in the pond. Our results thus indicate that common carp can be an effective biological control agent against the invasive apple snail, but caution should be taken about its potential to reduce wetland floral and faunal diversity.     

Impact of invasive apple snails in Hong Kong on wetland macrophytes,nutrients, phytoplankton and filamentous algae

1. Grazing by invasive species can affect many aspects of an aquatic system, but most studies have focused on the direct effects on plants. We conducted mesocosm and laboratory experiments to examine the impact of the invasive apple snail Pomacea canaliculata on macrophytes, filamentous algae, nutrients and phytoplankton. 2. In a freshwater pond, we confined 500 g of Myriophyllum aquaticum or Eichhornia crassipes with 0, 2, 4 or 8 apple snails in 1 m × 1 m × 1 m enclosures for approximately 1 month. Apple snails grazed heavily on both species of macrophytes, with higher overall weight losses at higher snail densities. The damage patterns differed between the two macrophytes. In M. aquaticum, both leaves and stems suffered from substantial herbivory, whereas in E. crassipes, only the roots suffered significant weight reduction. 3. In addition to grazing on macrophytes, apple snails appeared to have controlled the growth of filamentous algae, as these did not develop in the snail treatments. The ability of P. canaliculata to control filamentous algae was supported by a laboratory experiment where the consumption was as high as 0.25 g g⁻¹ snail DW d⁻¹. Because of a lack of native herbivorous snails in the pond, the growth of filamentous algae (mainly Spirogyra sp.) reached 80.3 g m⁻², forming a spongy pond scum in the no‐apple snail control. Together with previous reports that apple snails could eat the juveniles and eggs of other freshwater snails, our results indicated that P. canaliculata could have out‐competed native herbivorous snails from the pond by predation on their juveniles or eggs. Alternatively, P. canaliculata might have out‐competed them by monopolisation of food resources. 4. Nitrogen and phosphorous concentrations remained low throughout both experiments and were not correlated with apple snail density. The treatment effects on chlorophyll a (Chl a) and phytoplankton composition varied in the two experiments. In the M. aquaticum experiment, with increasing snail density, Chl a increased, and the phytoplankton community became dominated by Cryptophyceae. In the E. crassipes experiment, Chl a level was independent of snail density, but with increasing snail density, the phytoplankton community became co‐dominated by Cryptophyceae, Chlorophyceae and Bacillariophyceae. 5. Given the multiple effects of P. canaliculata on wetland biodiversity and function, management strategies should be developed to prevent its further spread. In invaded wetlands, strategies should be developed to eradicate the apple snail and re‐introduce native snails which can control the development of filamentous algae.     

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

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.     

Biological control of invasive apple snails by two species of carp: Effects on non-target species matter

Molluscivorous fish, especially carp, have been adopted as bio-control agents of the invasive apple snail Pomacea canaliculata, but previous studies have focused on their effectiveness, with little attention paid to their undesirable effects on non-target plants and animals. We conducted an 8-week mesocosm study to compare the effectiveness of two indigenous fish, common carp (Cyprinus carpio) and black carp (Mylopharyngodon piceus), in removing P. canaliculata, and their potential side effects on macrophytes and non-target mollusks in a freshwater wetland. Three species of macrophytes and a community of mollusks in the wetland sediment were enclosed in 1 × 1 × 1 m enclosures either with apple snails (AS), with apple snails and common carp (AS + CC), with apple snails and black carp (AS + BC), or without apple snails and fish. Both species of carp were effective predators of P. canaliculata, removing most of the individuals in the enclosures except a few that were too big to fit into their mouth. By reducing apple snail population, black carp reduced grazing of apple snail on macrophytes. In contrast, although common carp controlled apple snail population, it did not reduce overall loss in plant biomass as the fish might also fed on macrophytes. Both species of carp preyed on non-target mollusks. Application of bio-control agents in invasive species management needs to consider their effects on both the pest and non-target plants and animals. Adoption of common and/or black carp to control apple snail populations thus depends on the weight given to their effectiveness and subtle different effects on non-target organisms by wetland management authority.     

Size-dependent effects of an invasive herbivorous snail (Pomacea canaliculata) on macrophytes and periphyton in Asian wetlands

1. The invasive golden apple snail (Pomacea canaliculata), native to South America, is a serious pest on rice seedlings in south‐east Asia and has also been shown to consume large amounts of macrophytes in natural wetlands, with large effects on ecosystem functioning. Earlier studies suggest that the snail undergoes an ontogenetic diet shift, feeding on algae and detritus as juveniles and shifting to aquatic macrophytes as adults. 2. Here, we study the effects of snail populations with a size‐structure typical of either populations at an invasive front or the size‐structure of established populations. In an enclosure experiment performed in a wetland in Laos, we compared treatments with small snails only (3 mm; invasive treatment) to treatments with small, medium sized (10 mm) and adult (>25 mm) snails (established treatment). The effects of snail grazing on three aquatic macrophyte species and periphytic algae were quantified. 3. We found that snails of all sizes had a strong negative effect on the biomass of all macrophyte species and periphytic algae. There was no evidence of an ontogenetic diet change, i.e. snails in both the invasive and established treatments affected macrophyte biomass. Foraging was size‐dependent in that small snails had higher relative foraging capacity (g plant consumed per g of snail) compared with medium and adult snails. Small snails, therefore, depressed growth of medium snails at increasing densities through exploitative competition for preferred resources, while adult snails did not grow at all in the presence of small snails. 4. Density dependence is common in freshwater invertebrates, including gastropod populations, but differences in size dependent foraging‐ and competitive‐ability have rarely been demonstrated in this group of organisms. Knowledge about intra‐specific differences in ecological performance may, however, both deepen our understanding of the processes that underlie population dynamics in invertebrates such as gastropods, and help develop control strategies for invasive golden apple snails.     

Food Preference and Reproductive Plasticity in an Invasive Freshwater Snail

The freshwater apple snail Pomacea canaliculata has become a major crop pest in southeast Asia and Hawai'i and threatens natural wetland habitats in these regions and elsewhere. Deliberately introduced as a potential human food resource, it has also been proposed as a possible biocontrol agent against aquatic weeds. Various factors may facilitate its rapid invasion of new areas; we focus on two: growth rate and food preference. Our field observations and laboratory experiments suggest that in Hawai'i P. canaliculata reaches reproductive maturity in 10 months or more, less time than in its native temperate and seasonal Argentina, where it takes 2 years, but longer than in parts of southeast Asia, where it may take as little as 2 months. This increased growth rate, and thence reproductive rate, probably facilitate rapid population growth. Although P. canaliculata is usually considered an indiscriminate generalist macrophytophagous feeder, laboratory experiments indicated preferences among the dominant plants at our field site and growth rate differences when constrained to feed only on one of these plants. Water hyacinth (Eichhornia crassipes), a major invasive weed, was not preferred in food choice experiments, and snails offered only water hyacinth on which to feed did not differ in growth rate from unfed snails. Another important invasive weed, water lettuce (Pistia stratiotes), was also not preferred, but snails fed on it did grow, though not as quickly as those fed on green-leaf lettuce. Among the food plants offered in the experiments the native Vigna marina was the most preferred. Therefore, although a generalist, P. canaliculata exhibits some discrimination among food plants. We recommend that it not be introduced for use as a biological control agent for aquatic weeds.     

Palatability of macrophytes to the invasive freshwater snail Pomacea canaliculata: differential effects of multiple plant traits

1. By selective grazing, invasive grazers can alter macrophyte‐herbivore relationships in shallow freshwater bodies. Evaluating the palatability of macrophytes and understanding the determinants of plant palatability can help predict grazing impact. In no‐choice feeding assays, we tested the palatability of 21 species of freshwater macrophytes to the invasive freshwater apple snail Pomacea canaliculata. 2. Daily feeding rate varied greatly with plant species, ranging from 1.1 to 22% of snail body mass. We assessed six plant properties and examined their correlation with feeding rate. Total nitrogen content was positively related, and C:N ratio and dry matter content (DMC) negatively related, to snail feeding rate. There was no significant correlation between snail feeding rate and plant phenolic content, but the feeding rate on Myriophyllum aquaticum (the plant with the highest phenolic content) was very low. 3. We repeated the feeding assays for 15 species that were not palatable as fresh leaves with reconstituted plant tissues formed by mixing ground up dried leaves with agar. The feeding rate still differed greatly among macrophyte species. Phragmites australis and Vallisneria natans (two species with the highest DMC) were eaten much more as reconstituted plant than as fresh leaves, indicating that structure (i.e. DMC) may be important in their defence against snail herbivory. For two plants (M. aquaticum and Alternanthera philoxeroides) that had moderate amounts of nitrogen/phosphorus but were consumed very little as fresh and reconstituted tissues, we incorporated their extracts into a palatable agar‐based food. The extracts from both species greatly reduced snail feeding rate, indicating the presence of chemical defences in these two species. 4. These results indicated that feeding was affected by several plant traits. The snail favoured plants with a high nitrogen content and avoided plants with a high DMC. Only a few plants possessed chemical feeding deterrents that were effective against this snail. Given the invasive spread of P. canaliculata in Asia, ecologists and managers should consider plant palatability when selecting plants for use in wetland restoration and when predicting the impact of further invasion by this species.     

Feeding and growth of native,invasive and non-invasive alien apple snails (Ampullariidae) in the United States: Invasives eat more and grow more

The United States hosts one native and five non-native species of aquatic apple snails (Ampullariidae). All are currently found in or around the Everglades in Florida. Two of these introduced species have devastated wetlands in Southeast Asia, but little is known about how they may impact the Everglades. To evaluate potential impacts of introduced apple snails relative to the native species, we investigated plant species preference, consumption rates, growth rates, and growth efficiencies in five introduced and the single native species across eight native macrophytes common in the Everglades. Three of the non-native snails are invasive, one has shown no tendency to expand, and one appears to have minimal direct impact on macrophytes due to its diet. All snails exhibited similar feeding preferences, with Utricularia sp. being the most preferred, Bacopa caroliniana, Sagittaria latifolia, and Nymphaea odorata being of intermediate preference, and Eleocharis cellulosa, Pontederia cordata, Panicum hemitomon and Typha sp. being least preferred (avoided as foods). Consumption and growth was minimal for P. diffusa on all macrophytes. On Utricularia sp. and Bacopa caroliniana, the invasive species Pomacea insularum and P. canaliculata tended to eat more, grow more, and have higher conversion efficiencies than the native P. paludosa or the non-invasive P. haustrum. These contrasts were more often significant for P. insularum than for P. canaliculata. The greater rates of expansion by the invasive species may derive from their enhanced feeding and growth rates.     

Invasive macrophyte effects on littoral trophic structure and carbon sources

Limited data from terrestrial ecosystems suggest that invasive species can affect energy flow and nutrient cycling in invaded systems. This is likely also true for aquatic ecosystems, yet little information is available on food web effects of invasive macrophytes. This study examined the effects of dominant invasive Eurasian watermilfoil on lake trophic structure and energy flow. Stable isotopes of carbon and nitrogen were used to compare trophic structure in invaded and uninvaded lakes and macrophyte stands. Contribution of native and invasive macrophytes, their epiphyton and detritus to the upper trophic level of lacustrine food webs was partitioned using mixing models. Carbon isotope values of macroinvertebrate consumers were similar to macrophyte-associated production in stands from which they were collected. However, contribution of Eurasian watermilfoil and its epiphyton to higher trophic level was negligible, and littoral fish derived most of their energy from sources associated with native macrophytes, despite their lower abundance. This means that littoral fish may depend on the remaining patches of native macrophytes in lakes invaded by non-native plants. Considering previous findings, these results show that the assessment of ecosystem-level processes is needed to understand the entire range of impacts of invasive species.     

Effects of macrophytes on feeding and life-history traits of the invasive apple snail Pomacea canaliculata

1. Biological invasions have become a serious threat to ecosystems worldwide. Various factors can contribute to the success of biological invasion. We examined how different macrophyte food affected feeding and life‐history traits of the invasive herbivorous snail Pomacea canaliculata, and whether differences in snail life‐history traits could explain its successful infestation of agricultural and non‐agricultural wetlands in Asia. 2. We tested five cultivated and five wild semi‐aquatic macrophytes. Snail daily feeding rate varied substantially with plant species, ranging from 1.3% to 22% of its body mass. Snails fed with four (Amaranthus gangeticus, Apium graveolens dulce, Ipomoea aquatica and Nasturtium officinale) of the five cultivated macrophyte species exhibited high survivorship, fast growth and high fecundity. Snails fed with Colocasia esculenta, however, grew poorly, did not reproduce and eventually died. 3. Of the five wild species (Eichhornia crassipes, Ludwigia adscendens, Murdannia nudiflora, Myriophyllum aquaticum and Polygonum hydropiper), M. nudiflora supported a high snail survival, but snails had slower growth and lower fecundity than those reared on the four palatable cultivated species. Snails fed with L. adscendens grew substantially slower than those fed with M. nudiflora, and produced only a small clutch of eggs. Snails fed with E. crassipes, M. aquaticum and P. hydropiper had very low survivorship, grew very little and did not reproduce. 4. We determined six plant properties and their correlation with the feeding, growth and reproduction of the apple snails. Cultivated macrophytes in general had a higher nutritional value and lower physical and chemical defences. Phenolic content was negatively correlated with snail feeding rate, while plant nitrogen and phosphorus contents were positively correlated with snail egg production and growth, respectively. 5. These results indicate that, due to their higher nutritional value and lower chemical and physical defences, cultivated macrophytes are in general desirable for the apple snail which may partly explain its successful invasion into wet agricultural areas in Asia. This snail may also selectively graze poorly defended wild macrophytes in non‐agricultural wetlands, leading to changes in floral diversity and wetland functioning. Management of this and other apple snails with similar life‐history traits should thus focus on the prevention of their further spread.     

Interactive effects of light and snail herbivory rather than nutrient loading determine early establishment of submerged macrophytes

Submerged macrophytes play a key role in maintaining a clear‐water phase and promoting biodiversity in shallow aquatic ecosystems. Since their abundance has declined globally due to anthropogenic activities, it is important to include them in aquatic ecosystem restoration programs. Macrophytes establishment in early spring is crucial for the subsequent growth of other warm‐adapted macrophytes. However, factors affecting this early establishment of submerged macrophytes have not been fully explored yet. Here, we conducted an outdoor experiment from winter to early spring using the submerged macrophytes Potamogeton crispus and Vallisneria spinulosa to study the effects of shading, nutrient loading, snail herbivory (Radix swinhoei), and their interactions on the early growth and stoichiometric characteristics of macrophytes. The results show that the effects strongly depend on macrophyte species. Biomass and number of shoots of P. crispus decreased, and internode length increased during low light conditions, but were not affected by nutrient loading. P. crispus shoot biomass and number showed hump‐shaped responses to increased snail biomass under full light. In contrast, the biomass of the plant linearly decreased with snail biomass under low light. This indicates an interaction of light with snail herbivory. Since snails prefer grazing on periphyton over macrophytes, a low density of snails promoted growth of P. crispus by removing periphyton competition, while herbivory on the macrophyte increased during a high density of snails. The growth of V. spinulosa was not affected by any of the factors, probably because of growth limitation by low temperature. Our study demonstrates that the interaction of light with snail herbivory may affect establishment and growth of submerged macrophytes in early spring. Macrophyte restoration projects may thus benefit from lowering water levels to increase light availability and making smart use of cold‐adapted herbivores to reduce light competition with periphyton.     

Experimental Test of Preferences for an Invasive Prey by an Endangered Predator: Implications for Conservation

Identifying impacts of exotic species on native populations is central to ecology and conservation. Although the effects of exotic predators on native prey have received much attention, the role of exotic prey on native predators is poorly understood. Determining if native predators actively prefer invasive prey over native prey has implications for interpreting invasion impacts, identifying the presence of evolutionary traps, and predator persistence. One of the world’s most invasive species, Pomacea maculata, has recently established in portions of the endangered Everglade snail kite’s (Rostrhamus sociabilis plumbeus) geographic range. Although these exotic snails could provide additional prey resources, they are typically much larger than the native snail, which can lead to lower foraging success and the potential for diminished energetic benefits in comparison to native snails. Nonetheless, snail kites frequently forage on exotic snails. We used choice experiments to evaluate snail kite foraging preference in relation to exotic species and snail size. We found that snail kites do not show a preference for native or exotic snails. Rather, snail kites generally showed a preference for medium-sized snails, the sizes reflective of large native snails. These results suggest that while snail kites frequently forage on exotic snails in the wild, this behavior is likely driven simply by the abundance of exotic snails rather than snail kites preferring exotics. This lack of preference offers insights to hypotheses regarding effects of exotic species, guidance regarding habitat and invasive species management, and illustrates how native-exotic relationships can be misleading in the absence of experimental tests of such interactions.     

Home-field advantage: native signal crayfish (Pacifastacus leniusculus) out consume newly introduced crayfishes for invasive Chinese mystery snail (Bellamya chinensis)

The introduction of non-indigenous plants, animals and pathogens is one of today’s most pressing environmental challenges. Freshwater ecologists are challenged to predict the potential consequences of species invasions because many ecosystems increasingly support novel assemblages of native and non-native species that are likely to interact in complex ways. In this study we evaluated how native signal crayfish (Pacifastacus leniusculus) and non-native red swamp crayfish (Procambarus clarkii) and northern crayfish (Orconectes virilis) utilize a novel prey resource: the non-native Chinese mystery snail (Bellamya chinensis). All species are widespread in the United States, as well as globally, and recent surveys have discovered them co-occurring in lakes of Washington State. A series of mesocosm experiments revealed that crayfish are able to consume B. chinensis, despite the snail’s large size, thick outer shell and trapdoor defense behaviour. Crayfish exhibited size-selective predation whereby consumption levels decreased with increasing snail size; a common pattern among decapod predators. Comparison of prey profitability curves—defined as the yield of food (weight of snail tissue) per second of feeding time (the time taken to crack the shell and consume the contents)—suggests that small and very large snails may represent the most profitable prey choice. By contrast, previous studies have reported the opposite pattern for crayfish consumption on thin-shelled snails. For all snail size classes, we found that native P. leniusculus and invasive O. virilis consumed greater numbers of snails than invasive P. clarkii. Moreover, P. leniusculus consistently handled and consumed snails at a faster pace compared to both invasive crayfishes across the range of snail sizes examined in our study. These results suggest not only that B. chinensis is a suitable food source for crayfish, but also that native P. leniusculus may ultimately out-consume invasive crayfishes for this new prey resource.     

Experimental test of the Invasional Meltdown Hypothesis: an exotic herbivore facilitates an exotic plant,but the plant does not reciprocally facilitate the herbivore

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Invasive Crayfish Threaten the Development of Submerged Macrophytes in Lake Restoration

Submerged macrophytes enhance water transparency and aquatic biodiversity in shallow water ecosystems. Therefore, the return of submerged macrophytes is the target of many lake restoration projects. However, at present, north-western European aquatic ecosystems are increasingly invaded by omnivorous exotic crayfish. We hypothesize that invasive crayfish pose a novel constraint on the regeneration of submerged macrophytes in restored lakes and may jeopardize restoration efforts. We experimentally investigated whether the invasive crayfish (Procambarus clarkii Girard) affects submerged macrophyte development in a Dutch peat lake where these crayfish are expanding rapidly. Seemingly favourable abiotic conditions for macrophyte growth existed in two 0.5 ha lake enclosures, which provided shelter and reduced turbidity, and in one lake enclosure iron was added to reduce internal nutrient loading, but macrophytes did not emerge. We transplanted three submerged macrophyte species in a full factorial exclosure experiment, where we separated the effect of crayfish from large vertebrates using different mesh sizes combined with a caging treatment stocked with crayfish only. The three transplanted macrophytes grew rapidly when protected from grazing in both lake enclosures, demonstrating that abiotic conditions for growth were suitable. Crayfish strongly reduced biomass and survival of all three macrophyte species while waterfowl and fish had no additive effects. Gut contents showed that crayfish were mostly carnivorous, but also consumed macrophytes. We show that P. clarkii strongly inhibit macrophyte development once favourable abiotic conditions for macrophyte growth are restored. Therefore, expansion of invasive crayfish poses a novel threat to the restoration of shallow water bodies in north-western Europe. Prevention of introduction and spread of crayfish is urgent, as management of invasive crayfish populations is very difficult.     

Pink eggs and snails: field oviposition patterns of an invasive snail, Pomacea insularum, indicate a preference for an invasive macrophyte

Oviposition of non-calcareous or thinly shelled eggs represents an important life stage of many insects, amphibians, and several gastropods. A recently identified invasive species of apple snail, Pomacea insularum, exhibits alarming invasive characteristics of high reproductive rates and generalist consumption patterns. This snail takes the opposite approach to egg laying compared to most aquatic insects as adult snails crawl out of the water to place clutches on emergent, or terrestrial, substrates. As fecundity best indicates invasive potential for mollusks, control or management efforts need to understand reproductive behavior in P. insularum to predict, and hopefully impede, its spread throughout the Gulf Coast of the United States. Specific characteristics of wetlands and shallow lakes may facilitate the invasion process of P. insularum by providing females with conditions that permit successful oviposition. In order to investigate this possibility, we studied P. insularum oviposition behavior in an invasive population at two times during the reproductive season in Texas, USA. Based on a subsequent survey (August 2009), plants comprised 78% of the available habitat. Wild taro (Colocasia esculenta) and alligator weed (Alternanthera philoxeroides) represented 48 and 43% of that proportion, respectively. During 2008–2009, no new concrete or metal structures appeared in our sampling reach and consistent amounts of plant stands and woody debris remained dominant. Given this distribution, P. insularum laid disproportionately more clutches on wild taro compared to its availability and less on alligator weed and bulrush (Schoenoplectus californicus) than expected. Owing to limited metal and concrete substrates, we found a higher proportion of clutches on these artificial substrates than expected in both May and August 2008. However, artificial substrates comprised less than 2% of available substrates in the bayou. Our results suggest that wetlands and shallow lakes surrounded by large emergent macrophytes, particularly wild taro, likely provide ideal oviposition sites for P. insularum, promote egg supply, and possibly facilitate invasion into new aquatic ecosystems.     

Effects of invasive macrophyte on trophic diversity and position of secondary consumers

Invasive species are one of the widespread stressors of aquatic ecosystems. Several studies document food web effects of invasive fish, but little information is available on the effects of invasive macrophytes. We studied differences in food chain length as well as trophic position and trophic diversity of fish and odonates in lakes dominated by native plants or invasive Eurasian watermilfoil. Trophic position and food chain length were determined using baseline-adjusted δ¹⁵N isotope signatures. Trophic diversity, or isotope niche width, was estimated from convex hull area analysis. Results show that trophic position of secondary consumers was not affected by the invasive macrophyte, whereas trophic diversity was greater in watermilfoil-dominated lakes. The direction of isotopic niche expansion was different in fish and odonates, suggesting potential decoupling in predator–prey interactions. This study shows that dominant non-native macrophytes may cause significant changes in food web structure of invaded ecosystems. Trophic diversity may be a more sensitive indicator of environmental stress than trophic position and has the potential to be used for assessment of invasive species impacts and restoration success.     

Herbivore preference for native vs. exotic plants: generalist herbivores from multiple continents prefer exotic plants that are evolutionarily naïve

Enemy release and biotic resistance are competing, but not mutually exclusive, hypotheses addressing the success or failure of non-native plants entering a new region. Enemy release predicts that exotic plants become invasive by escaping their co-adapted herbivores and by being unrecognized or unpalatable to native herbivores that have not been selected to consume them. In contrast, biotic resistance predicts that native generalist herbivores will suppress exotic plants that will not have been selected to deter these herbivores. We tested these hypotheses using five generalist herbivores from North or South America and nine confamilial pairs of native and exotic aquatic plants. Four of five herbivores showed 2.4-17.3 fold preferences for exotic over native plants. Three species of South American apple snails (Pomacea sp.) preferred North American over South American macrophytes, while a North American crayfish Procambarus spiculifer preferred South American, Asian, and Australian macrophytes over North American relatives. Apple snails have their center of diversity in South America, but a single species (Pomacea paludosa) occurs in North America. This species, with a South American lineage but a North American distribution, did not differentiate between South American and North American plants. Its preferences correlated with preferences of its South American relatives rather than with preferences of the North American crayfish, consistent with evolutionary inertia due to its South American lineage. Tests of plant traits indicated that the crayfish responded primarily to plant structure, the apple snails primarily to plant chemistry, and that plant protein concentration played no detectable role. Generalist herbivores preferred non-native plants, suggesting that intact guilds of native, generalist herbivores may provide biotic resistance to plant invasions. Past invasions may have been facilitated by removal of native herbivores, introduction of non-native herbivores (which commonly prefer native plants), or both.     

Lethal and non-lethal effects of multiple indigenous predators on the invasive golden apple snail (Pomacea canaliculata)

1. We investigated the individual and combined effects of two predators (the climbing perch, Anabas testudineus, and the wetland crab, Esanthelphusa nimoafi) indigenous to wetlands in Laos, on the behaviour and survival of the invasive South American golden apple snail (Pomacea canaliculata). The snail is considered a pest, consuming large amounts of rice and other aquatic vegetation in the region. 2. Snail avoidance reactions to released predator chemical cues were investigated in aquaria while the effects of predators on a mixed snail population were studied in field enclosures that contained native aquatic plants (Salvinia cucullata, Ludwigia adscendens and Ipomoea aquatica). 3. In the aquaria experiment, neonate (2–3 mm) and medium‐sized snails (8–10 mm) responded to fish chemical cues by going to the surface, whereas adult snails (35–40 mm) went to the bottom. In contrast, no size class of snails reacted to chemical cues released by crabs. 4. In the field experiment, fish reduced the abundance of neonate snails, and crabs reduced the abundance of all size classes. The effect of the combined predators could not be predicted from the mortality rate observed in single predator treatments. The survival of neonate and medium‐sized snails was greater and of adults less than expected. The presence of predators did not affect egg production. Snails consumed significant amounts of plants despite the presence of predators. 5. Our findings suggest that some indigenous Asian predators have lethal and sublethal effects on P. canaliculata that depend on snail size and predator type. When in the presence of several predators the response of snails to one predator may either increase or decrease the vulnerability of snails to the others.     

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