Ecological mechanisms underlying the sustainability of the agricultural heritage rice-fish coculture system

Abstract

For centuries, traditional agricultural systems have contributed to food and livelihood security throughout the world. Recognizing the ecological legacy in the traditional agricultural systems may help us develop novel sustainable agriculture. We examine how rice-fish coculture (RF), which has been designated a "globally important agricultural heritage system," has been maintained for over 1,200 y in south China. A field survey demonstrated that although rice yield and rice-yield stability are similar in RF and rice monoculture (RM), RF requires 68% less pesticide and 24% less chemical fertilizer than RM. A field experiment confirmed this result. We documented that a mutually beneficial relationship between rice and fish develops in RF: Fish reduce rice pests and rice favors fish by moderating the water environment. This positive relationship between rice and fish reduces the need for pesticides in RF. Our results also indicate a complementary use of nitrogen (N) between rice and fish in RF, resulting in low N fertilizer application and low N release into the environment. These findings provide unique insights into how positive interactions and complementary use of resource between species generate emergent ecosystem properties and how modern agricultural systems might be improved by exploiting synergies between species.

Fig. 1.

Rice yield and stability of rice yield (S) in rice monoculture (RM) and rice–fish coculture (RF). (A) Rice yield and S and pesticide use (Inset) in a field survey. (B) Rice yield and S (Inset) in experiment 1. (C) The relationship between S and the use of pesticides as determined in the survey. a.i., active ingredient. Error bars are SE.

Fig. 2.

Rice pests in rice monoculture (RM) and rice-fish coculture (RF) in experiment 1. (A) Density of rice planthoppers. (B) Sheath blight incidence. (C) Rice blast incidence. (D) Weed infestation. Error bars are SE.

Fig. 3.

Fish activity and rice planthopper removal in experiment 1. (A) Frequency at which fish hit rice plants (bars, hits per hill per hour) and total numbers of rice planthoppers (lines, numbers per quadrat) collected from the quadrats B and D (Fig. S5). Shading between the lines indicates the numbers of planthoppers falling into the water, presumably because of fish activity. (B) Frequency of fish (bars) occurring in quadrats A and C (SI Text, section S2, and Fig. S5) in rice–fish coculture (RF) and fish monoculture (FM), and air temperature (line) in experiment 1. Error bars are SE.

Fig. 4.

Temperature of surface water (A) and light intensity under the rice plant canopy (B) in experiment 1. RF, rice–fish coculture; FM, fish monoculture.

Fig. 5.

Ammonium N in water (A) and total N in soil (B) in experiment 1. RF, rice–fish coculture; FM, fish monoculture. Error bars are SE.

Fig. 6.

(A–C) Yield of rice and fish (A), N balance (B), and the fate of N input (C) in experiment 2. Bars in B show balance of N input and output in RM, RF without fish feed (RF feed 0), RF with fish feed (RF feed 1), FM without feed (FM feed 0), and FM with feed application (FM feed 1). A negative value for N in the environment means that some fraction of N in rice or fish was from the environment, and a positive value for N in the environment means that some portion of the input N was not used by rice and fish but remained in the field. Pie charts in C show partitioning of N derived from fish feed in harvested rice, harvested fish, and the environment in RF and FM (e.g., 11.1% and 14.2% of the N supplied by fish feed was estimated to be contained in fish in RF and FM, respectively). The calculations for the balance of N output and input within each system (RM, RF, or FM) are described in SI Text, section S5. The calculations used to determine the fate of N are described in SI Text, section S6. In A, means for rice yield with the same uppercase letter or means for fish yield with the same lowercase letter are not significantly different (P > 0.05). RM, rice monoculture; RF, rice–fish coculture; FM, fish monoculture. Error bars are SE.

Fig. P1.

Positive interactions and complementary use of nitrogen (N) between rice and fish explain why the rice–fish coculture system maintains productivity for long time periods with low input of chemicals. (A) Positive interactions between rice and fish: Fish remove pests from rice through feeding activity, while rice plants moderate the field environment for fish, which in turn promotes fish activity and pest removal. (B) Complementary use of N by rice and fish: Unused fertilizer N promotes plankton in paddy fields that is consumed by fish. The unconsumed fish feed acts as an organic fertilizer, with the N in the unconsumed feed being gradually used by the rice. Thus, rice and fish use different forms of N, resulting in a high efficiency of N utilization in RF.