Abstract. We investigated the effects of an exotic snail, the golden apple snail (Pomacea canaliculata) on biodiversity and ecosystem functioning in tropical wetland ecosystems. This large snail (up to 80-mm shell height) has invaded large parts of Southeast Asia during recent decades. A survey of natural wetlands in Thailand showed that high densities of the snail were associated with almost complete absence of aquatic plants, high nutrient concentrations, and high phytoplankton biomass, that is, a complete shift in both ecosystem state and function. A field experiment demonstrated that grazing by the snail can cause the loss of aquatic plants, a change toward dominance of planktonic algae, and thereby a shift toward turbid water. Estimates of biologically fixed nutrients released through snails grazing on aquatic plants revealed that phosphorus releases were sufficient to explain the recorded increase in phytoplankton biomass. Hence, our study demonstrates how an herbivore may trigger a shift from clear water and macrophyte dominance to a turbid state dominated by planktonic algae. This shift and the continuing aggressive invasion of this exotic species are detrimental to the integrity and functioning of wetland ecosystems, and to the services these provide in Southeast Asia.

Only a small proportion of exotic species invasions give rise to hyper‐successful nuisance species, but those that do have dramatic negative impacts on ecosystems, such as the displacement of native species and disruption of native food webs. For a native predator, such changes may mean a major transformation in its resource base and a decline in its fitness. However, native predators may adapt to become more effective at feeding on exotic prey, either rapidly, via existing phenotypic plasticity, or more slowly, via natural selection. Despite a rapidly growing number of publications on the importance of species invasions as a driver of contemporary evolution in both invading and native species, we know little about how the arrival of exotic prey affects native predators. We propose that native predators could be important in regulating the long‐term dynamics of invading species and, consequently, that the overexploitation of predators could facilitate biological invasions.

1. Abundant native predators, parasites and pathogens that switch to consuming a hypersuccessful exotic species may be able to control the invasive population. Native predators may, however, need time to adapt to feed effectively on an exotic resource. In this case, mortality on an exotic population from native predators could increase over time even without a numerical increase in the predator population. 2. We measured mortality of zebra mussels (Dreissena polymorpha) in the Hudson River both in controls open to predation and in exclosures that excluded large predators to estimate mortality of zebra mussels from large predators and other causes. 3. We found that predation by the blue crab (Callinectes sapidus), and perhaps other predators, causes high mortality on zebra mussels in the Hudson River estuary. This predation apparently led to increased mortality and altered population structure in the invader over time. 4. Long-term data from the Hudson River suggest that components of the invaded ecosystem, like rotifers, are recovering through predator-caused release from zebra mussel grazing. Increased mortality on hyper-successful exotic populations over time may be a common phenomenon with both ecological and management implications.

The effect of naturally found densities of the exotic and herbivorous golden apple snail (Pomacea canaliculata) on three dominant aquatic plants -duckweed (Lemna minor), water hyacinth (Eichhornia crassipes) and morning glory (Ipomoea aquatica) -was assessed in a wetland survey and quantified in a field experiment in Laos in southeast Asia. Snail grazing reduced plant biomass, but plant species were differently affected by grazing. Duckweed had almost disappeared after 6 and water hyacinth after 21 days, whereas morning glory remained at 80% of initial biomass after 32 days. Snail growth was lowest on morning glory and, when all plant species were simultaneously presented to snails, this plant was not preferred. We suggest that the negative effect the golden apple snail had on the growth of these plant species in field enclosures is present in the natural environment as well. This new and intense herbivory could have serious negative effects on invaded freshwater ecosystems in this region.