ECOLOGICAL EFFECTS OF THE INVASIVE WEED SPECIES SENECIO-JACOBAEA L (RAGWORT) IN A NEW-ZEALAND PASTURE

Invasive pasture weeds are often believed to have detrimental effects in managed pastures, but little is known as to the nature of these eff ects. We investigated the impact of the invasive weed species Senecio jacobaea L. L. (ragwort) on various aspects of a pasture ecosystem in the Waikato area of New Zealand. S. jacobaea plants often enhanced tot al pasture production in their immediate vicinity, probably through im proving microclimatic conditions, and these effects continued for seve ral months after plant death. Lolium perenne L. (perennial ryegrass) w as generally stimulated by S. jacobaea, while Trifolium repens L. (whi te clover) was inhibited by flowering S. jacobaea plants and various w eedy species showed a mixed response. Nitrogen fixation by T. repens ( measured by acetylene reduction bioassay) was generally unresponsive t o S. jacobaea plants but some temporary enhancement was observed aroun d the edges of individual flowering plants. The soil microbial biomass (biomass of microbes responsible for nutrient cycling) and saprophyti c microarthropods were both reduced in the area immediately adjacent t o flowering S. jacobaea plants, and this appeared to be negatively rel ated to the enhanced production of pasture (especially L. perenne) in this zone. S. jacobaea plants also exerted some effects on soil macrof auna captured in pitfall traps. Addition of nitrogen- and phosphorus- based fertilisers in general did not alter the nature of the interacti ons we observed. Based on our results for S. jacobaea it would appear that invasive weeds may have a wide range of consequences in pasture e cosystems and these need to be acknowledged before the overall impact of weeds in pastures can be understood.