Functions of indigenous animals in paddy fields: an in situ experiment on their effects on water quality, phytoplankton, weeds, soil structure, and rice growth

Paddy fields are used for growing semiaquatic rice and are also important habitats for a diversity of aquatic animals, which may be beneficial for rice production. However, studies on changes in environmental conditions such as water quality and community structure, and eventually in rice yield, made by animals in paddy fields are rare or have not used indigenous animals at natural densities in situ. We separately introduced the common paddy field species of loaches, tadpoles, and snails into 12 in situ enclosures at naturally occurring densities and examined their effects on paddy field environment and rice growth. Our results showed that rice growth did not increase in the presence of animals but was negatively correlated with weed biomass. Loaches increased turbidity and decreased the concentration of phosphate in surface water, probably because of their high bioturbation rates. Snails decreased the dissolved oxygen concentration in surface water. Total phytoplankton and weed biomass as well as soil density were not affected by the animals. These results show that nurturing animals in paddy fields could change the environment but does not cause higher rice production. The value of nurturing high animals in paddy fields may be found in other aspects besides rice growth.