Thesis (BS Biology) -- University of the Philippines Mindanao, 2009

Weeds are considered pests by many farmers because they reduce crop yield. Some weed species however, perform useful ecological functions such as providing a habitat for biocontrol agents, nourishing useful microorganisms in the soil and preventing erosion. Appropriate weed management should therefore aim to maintain a certain level of useful weed population and diversity instead of totally eliminating weeds. To determine if this objective can be achieved under two contrasting weed management practice. I report a two-year study on the effects of manual and chemical weed management on the broadleaf weeds in a banana plantation in Davao City, Philippines. The study regularly examined seed banks and monitored the population and biomass of five dominant weed species. In addition, the reproductive biology of the weeds was examined. I found no significant difference in the seed bank counts between manual and chemical plots. The chemical weed management treatment consisting of paraquat and glyphosate, however, consistently and significantly reduced the field counts and biomass of broadleaf weeds compared to the manual treatment. The broadleaf species diversity increased in the chemical plots but decreased in the manual plots. Dominance index generally decreased due to the appearance of previously undetected Drymaria cordata and Acalypha indica, which competed with the dominant and noxious weeds ageratum conyzoids, Bidens pilosa and cleome rutidosperma in the later part of the observation period. Considering the unique characteristics of the weed species, I discussed the ecological significance of the shifts in weed composition resulting from manual and chemical weeding. I conclude that herbicide treatment was ecologically advantageous compared to manual weed control as it has more effectively reduced weed abundance and dominance of noxious weed without concomitantly decreasing weed diversity.