Analysis of Invertebrate Biodiversity in a Philippine Farmer's Irrigated Rice Field

Abstract

Rank-abundance curves, indices of community structure, and surrogate-based methods of biodiversity assessment were applied to invertebrate time-series data collected from a farmer's irrigated rice ( Oryza sativa L.) field on Luzon Island, Philippines. Canopy and floodwater invertebrates were vacuum- and strainer-sampled, respectively, at roughly weekly intervals after rice seedlings were transplanted up to harvest for a total of 8 sampling dates. The cumulative samples included 202 taxa and 9,570 individuals for the plant canopy and 180 taxa and 84,905 individuals for the floodwater. Rank-abundance curves revealed the following findings: (1) a 10,000-fold range in invertebrate abundance with many abundant taxa remaining abundant over all crop stages; (2) lower evenness (equitability) of invertebrate abundances in the floodwater than the plant canopy, due in large part to the numerical dominance (75%) of 2 crustaceans [ Heterocypris luzonensis Neale, Eucyclops serrulatus (Fischer)]; (3) the preponderance of natural enemies as the largest guild in number of taxa in both the canopy and floodwater, followed by herbivores, detritivores, and tourists (nonpredatory taxa with no known interaction except as prey to rice field predators); and (4) herbivore- and detritivore-dominated faunas typified early crop periods in the canopy and floodwater, respectively, followed by predator-dominated faunas in both systems at mid- and late-crop stages. Rates of community turnover generally increased with crop age in both the canopy and floodwater faunas with the former increasing faster than the latter. Of the 82 terrestrial taxa that were tested as surrogates of all canopy taxa, mymarid wasps ( Anagrus sp.) gave the best fit to the data; however, this taxon's sensitivity to single sampling dates made it and the other canopy taxa unsuitable surrogates of all canopy taxa. Conversely, of the 24 floodwater taxa tested, hydrophilid beetles and 4 other taxa (ostracods, gastropods, corixids and chironomids) passed our sensitivity test and were judged as acceptable surrogates for all floodwater taxa in this dry season rice ecosystem. We propose guidelines linking biodiversity concepts (rank-abundance curves, guilds, community metrics) and assessment methods (surrogate taxa) to agroecology for the purpose of enhancing understanding of tropical rice ecosystems for research and training programs.