Ecological Foundations to Tropical Rice IPM

Whereas the specific conditions critical for management decisions in rice agriculture vary over a small spatial scale, agroecosystems have a general structure and dynamics that is reasonably consistent for the entire system.  This makes it possible to think in terms of a “general theory” for the structure and dynamics of specific agricultural ecosystems.  IPM is not a “theory” in a strict scientific sense; it is a set of practical guidelines for how to best manage a specific crop.  Learning about rice ecology in the Farmer's Field School, however, is based on well-established scientific theories, supported by good field data. 

The fact that ecological systems, even highly complex ones like tropical irrigated rice, are structured by a very few key variables.  Research over the past 20 years in applied ecology of managed systems shows that ecosystem dynamics, regardless of the system, are organised around a small number of nested cycles, each driven by a few dominant variables (Gunderson et al. 1995, Holling 1992).

“A small number of plant, animal, and abiotic processes structure biomes over scales from days and centimeters to millennia and thousands of kilometers.  Individual plant and biogeochemical processes dominate at fine, fast scales; animal and abiotic processes of mesoscale disturbance dominate at intermediate scales; and geomorphological ones dominate at coarse, slow scales….the physical architecture and the speed of variables are organized into distinct clusters, each of which is controlled by one small set of structuring processes.  These processes organize behavior as a nested hierarchy of cycles of slow production and growth alternating with fast disturbance and renewal.” (Gunderson et al. 1995, pg. 27) .

A few key variables and processes determine the dynamics of the irrigated rice ecosystem.  The basic outlines are as follows. See Settle et al. 1996 for details (click on the hyperlink to download a copy of this paper in pdf format).

Key process #1: energy is stored as organic matter in the soil and brought into the system by microorganisms and detritous-eating insects. (refer to Fig. 1 and Fig. 2)

From the time that water first floods a farmer's field in preparation for planting, organic matter--derived from residues from the previous crop cycle, organic waste in irrigation water and algal growth, provides the energy for an array of micro-organisms.  The energy flow begins with bacteria, being eaten by protozoa and rotifers, and continues upwards to larger zooplankton. In a parallel flow, detritus-eating insects, such as the larvae of flies and beetles, and especially the minute but abundant Collembola, feed directly on decaying organic matter, including material floating on the surface of the water.  This process will be found in all irrigated rice systems.

Figure 1 (above).  Trophic-level energy flow diagram for tropical irrigated rice.  In tropical rice consistently low pest populations result from the fact that natural enemies—especially generalist predators—are not directly dependent on pest populations.  Rather, there are three separate avenues for energy flows to natural enemy populations: 1) from organic matter via micro-organism cycles and filter-feeding insects, 2) from organic matter via detritous-eating insects, and 3) from the rice plant via herbivores.

Figure 2 (below).   Functional-group level energy flow diagram for tropical irrigated rice; a more detailed elucidation of the energy-flow diagram for tropical irrigated rice.