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 effects. We investigated the impact of the invasive weed species Senecio jacobaea L. (ragwort) on various aspects of a pasture ecosystem in the Waikato area of New Zealand. S. jacobaea plants often enhanced total pasture production in their immediate vicinity, probably through improving microclimatic conditions, and these effects continued for several months after plant death. Lolium perenne L. (perennial ryegrass) was generally stimulated by S. jacobaea, while Trifolium repens L. (white clover) was inhibited by flowering S. jacobaea plants and various weedy species showed a mixed response. Nitrogen fixation by T. repens (measured by acetylene reduction bioassay) was generally unresponsive to S. jacobaea plants but some temporary enhancement was observed around the edges of individual flowering plants. The soil microbial biomass (biomass of microbes responsible for nutrient cycling) and saprophytic microarthropods were both reduced in the area immediately adjacent to flowering S. jacobaea plants, and this appeared to be negatively related to the enhanced production of pasture (especially L. perenne) in this zone. S. jacobaea plants also exerted some effects on soil macrofauna captured in pitfall traps. Addition of nitrogen- and phosphorus- based fertilisers in general did not alter the nature of the interactions we observed. Based on our results for S. jacobaea it would appear that invasive weeds may have a wide range of consequences in pasture ecosystems and these need to be acknowledged before the overall impact of weeds in pastures can be understood.