Invading Herbivory. Effects of the Golden Apple Snail (Pomacea canaliculata) in Asian Wetlands

Sammanfattning: The South American golden apple snail (Pomacea canaliculata) was intentionally introduced to aquacultures in South East Asia to produce snails for human consumption, but the aquatic snails soon escaped and started to consume large amounts of rice seedlings. The aim of this thesis was to quantify the effects of this invasive herbivore on aquatic plants in natural wetlands since previous research has focused on effects of the snail in rice fields. In enclosure experiments the snail had a dramatic negative effect on aquatic plants and the most preferred plants were consumed in a very short time. Further, in surveyed wetlands in Thailand, high densities of the invasive snails corresponded to an almost complete absence of aquatic plants, high nutrient concentrations (nitrogen and phosphorus) and high biomass of phytoplankton. A subsequent experiment unravelled that snail herbivory caused this decline in aquatic plants and the shift towards phytoplankton dominance and turbid water. Hence, the ongoing snail invasion alters both the state and function of invaded wetlands to the detriment of all resident and transient organisms that during any life-stage depend on aquatic plants. The use of wetlands as “biological supermarkets” in rural Asia is also seriously threatened. Here, plants, fish, crustaceans and insects are harvested as food and animal fodder, water is used for human consumption and wastewater is biofiltered. These ecosystem level effects of the snail are of major concern since the snail continues to spread and now threatens wetlands ecosystems not only in Asia but also in Northern Australia, Hawaii and Southern USA. The snails’ rapid spread in South East Asia has, at least partially, been explained by release from natural enemies in the invaded range. However, in aquaria and field experiments at least four fish species, one freshwater crab and one freshwater turtle could consume the snail in Laos. As hunting and fishing pressures are extremely high in parts of South East Asia, the snails’ success may reflect low numbers of predators present rather than the indigenous predators’ potential effectiveness in snail control. This is of interest, since the invasion has lead to heavy and unsustainable use of pesticides in rice fields that kill both invasive snails and non-target organisms in the water. Unfortunately, the snails’ eggs are in refuge to this chemical control as they are deposited out of the water. Further studies may reveal that biotic resistance from indigenous predators to this invasive snail may be one of many economical reasons to conserve biodiversity in South East Asia. In conclusion, my results establish that the golden apple snail invasion is a serious threat not only to rice crops but also to natural wetland functioning. Any long-term strategy to control the snail should therefore involve both rice fields and wetlands since these are intimately connected and exchange both pesticides and golden apple snails.