Effects Of Apple Snail Pomacea Canaliculata Grazing On Submerged Macrophytes

With the increased impact of human activities, the problem of biological invasion becomes more and more serious. According to the literatures, there are 529 species of alien species in China, which brings a serious damage to agriculture, forestry, animal husbandry and fishery production, transportation, shipping, human health and so on.Apple snail Pomacea canaliculata is a freshwater gastropod, which has been regarding as one of the major harmful exotic species in aquatic ecosystems of Southern China, reducing crops production severely such as rice, melon and fruit and destroying aquatic macrophytes. At present, more and more lake water environment has become deteriorated and the problem of lake eutrophication gets worse. Surely, it is emergent to restore eutrophic lakes. By repeated practices, the treatments of shallow lake restoration have changed from simple physical or chemical methods to comprehensive treatments, which regard ecological restoration as priority. Actually, it is important to establish aquatic vegetation for ecological restoration. Vallisneria natans, Myriophyllum spicatum and Hydrilla verticillata are used to restorate eutrophic shallow lakes frequently, with a strong effect to purify water. In most cases,in restoration of shallow lakes, aquatic vegetation are susceptible to interfered by snails, fish and other creatures. In order to explore the size-dependent effects of P.canaliculata on submerged macrophytes and the effects of adult snails grazing on the community structure of submerged macrophytes, composed by V. natans, M. spicatum and H. verticillata, the research would to be divided into two parts.The first part, in order to explore size-dependent effects of P. canaliculata on V.natans, M. spicatum and H. verticillata with no selectivity and on the community structure of submersed macrophytes with selectivity, the microcosms was constructed,with small(shell height, SH<3mm), medium(10<SH<15mm) and large(30<SH<35mm)size snails. The results showed that when the three species of submerged macrophytes existed all at once, large snails and medium snails were prior to graze V. natans, and next was M. spicatum. But small snails had no appetite to all macrophytes. In each noselective experiment, medium snails could graze from multiple starting point and ate more plant than large one, although the biomass of medium snails was the same as large snails. But both of them herded V. Natans most. This study indicates that P.canaliculata of different size has different appetite to the three species of submerged macrophytes and imposes different effects on biomass and composition structure of submerged macrophytes. Therefore, in management of shallow lakes ecosystem and practice of ecological restoration, it need to strengthen the control of apple snails invasion, especially snails with shell height >10mm.The second part, in order to explore the effects of P. canaliculata(30-35mm)on community structure of submersed macrophytes, the outdoor experiment was conducted to investigate the effects of the snail grazing on the mixed community of V.natans, M. spicatum and H. verticillata, with four density treatment groups(0, 15, 29,58 ind/m2). In this experiment, the total biomass of submerged macrophytes was expressed as wet weight and their organs were expressed as dry weight respectively.The results showed that the gazing of the snail could reduce total biomass of submerged macrophytes significantly, which exerted a different effect on the three species of submerged macrophytes. Especially, the densities of P. canaliculata had a strong negative effect on the biomass of V. natans and M. spicatum, primarily on their above ground part for V. natans. For M. spicatum, the effect on leaves was more pronounced than stems at low density of snails. Furthermore, with the increase of snail densities, the negative effect on biomass of V. natans & M. spicatum was reinforced gradually. But P. canaliculata produced a positive effect on H. verticillata.At low snail densities, the growth of H. verticillata was enhanced, because the consumption demand of all snails was relatively lower and it was easy to be met by V.natans & M. spicatum. But such effect was reduced at higher density treatments, since snails were forced to graze H. verticillata, when the biomass of V. natans & M.spicatum was reduced. In addition, snails densities had a positive effect on increasing the content of total nitrogen, total phosphorus and water particle organic matter.Actually all of them affected the submerged macrophytes community structure indirectly. This study indicates that P. canaliculata can impose a significant effect onthe biomass and species composition of submerged macrophytes, thus bringing a serious damage to aquatic ecosystem functioning. Therefore, invasion and population development of P. canaliculata need to be controlled while managing lake and wetland ecosystems.