Re ECF Project: 2006-08

Project Title: A Study of the Impact of Apple Snails on Macrophytes, Nutrients and Chlorophyll in Local Wetlands

Applicant: The Hong Kong Baptist University (Dr. Jian-wen QIU)

Total Approved Grant: $471,440.73

Duration: 01/08/2007 to 31/07/2009

Project Status/Remarks: Completed

The objectives of the study are:

  1. to quantify the feeding of apple snails on local wild macrophytes;
  2. to examine the plant traits that determine the food preference in apple snails; and
  3. to assess the consequences of such destruction in wetland plant diversity, nutrient level and algal biomass.

Summary of the Findings/Outcomes:
The results showed that Pomacea canaliculata had a clear food preference, and the daily consumption rate could reach 22% of its body mass. This snail fed more on plants with a high nitrogen content, while less on plants with a high dry matter content or a high C/N ratio. Plants with high phenolic content and possibly other secondary plant metabolites were not preferred. When eight species of plants were present simultaneously, the apple snail completely consumed the leaves of several palatable plants, significantly reduced the growth of some other plants, but did not have a great affect the growth of one of these species. Snail grazing did not result in a marked increase in nutrient level, and the effects on chlorophyll a varied between experiments. There were some snail-density dependent changes in phytoplankton concentration and composition, but further experiments are needed to confirm this result. In addition to reducing macrophyte biomass, the apple snail appeared to have controlled the growth filamentous algae. Together with previous reports that apple snails could eat the juveniles and eggs of other freshwater snails, our results indicated that P. canaliculata might have out-competed them by predation on their juveniles or eggs. Alternatively, P. canaliculata might have out-competed them by monopolisation of food resources. Overall, these results indicated the apple snail might cause significant changes in wetland diversity by selective grazing, but such grazing might not trigger the system to change to a turbid water state. Given the multiple effects of P. canaliculata on wetland biodiversity, 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 that could control the development of filamentous algae.