FOOD-WEB DYNAMICS OF IRRIGATED RICE FIELDS AT 5 ELEVATIONS IN LUZON, PHILIPPINES

The above-water food webs of arthropod communities in irrigated rice f ields on Luzon Island, Philippines, were studied over the growing seas on at five sites (Los Banos, Cabanatuan, Bayombong, Kiangan, Banaue) r anging in elevation from 22 m to 1524 m. Arthropod populations were va cuum-sampled at roughly weekly intervals from the date after seedlings were transplanted to flowering at each site. Site- and time-specific webs were constructed from a 687-taxa cumulative Philippines web and t ime-series of species present. Taxonomic composition, food web structu re, and arthropod phenology were broadly similar across different site s. Arthropod abundance was inversely associated with altitude across t he Ave sites, but numbers of taxa and links and six food web statistic s showed no obvious increasing or decreasing trend with altitude. The rise of taxa, links and mean food chain length over the growing season at each site reflected an increase in plant size with age and, at som e sites, an orderly accumulation of newly arriving herbivore, predator , parasitoid and omnivore species. At each site, herbivores built up f aster than predators and parasitoids, and predators arrived faster tha n parasitoids; the difference between the latest and earliest sampling dates of first arrivals, averaged over the Ave sites, was 38, 63 and 73 days for herbivores, predators and parasitoids, respectively. Site- to-site consistencies in food web properties and first arrivals sugges t that such patterns may be influenced more by crop age than by geogra phy or altitude. Sampled predator, parasitoid and omnivore taxa potent ially encountered only a subset of their lifetime prey and predator sp ecies at any particular time in the rice field. Prey lists cumulated o ver time may underestimate the temporal specificity of predation by po tential biological control agents. Research opportunities linking rice food webs and integrated pest management with East Indies biogeograph y are proposed.