Evidence for allelopathy as a mechanism of community composition change by an invasive exotic shrub, Chrysanthemoides monilifera spp. rotundata

Abstract

Chemical interference is increasingly suggested as a mechanism facilitating exotic plant invasion and plant community composition. In order to explore this further, we employed a comprehensive extract-bioassay technique that facilitated detection and demarcation of phytotoxicity, direct allelopathy and indirect allelopathy of bitou bush (Chrysanthemoides monilifera spp. rotundata) compared to an indigenous dominant of the invaded system, acacia (Acacia longifolia var. sophorae). Extracts of the leaves and roots of both species exhibited phytotoxic effects against five indigenous plant species. Evidence for allelopathy between co-evolved indigenous plants was detected between acacia and Isolepis nodosa. Allelopathy between bitou bush and four indigenous plant species was also detected. Therefore we propose that both the acacia and bitou bush have the potential to chemically inhibit the establishment of indigenous plants. Eventual dominance of bitou bush is predicted, however, based on more ubiquitous effects on seedling growth.