Range expansion by alien weeds in the coastal farmlands of Guyana.
Experimental field studies, undertaken on 6 alien species found in and around cultivated fields, clarified their relative importance as weeds, and suggest the species may represent 4 ecological groups differing in their abilities to establish and maintain colonies in cultivated fields. Species of Group 1 (Macroptilium lathyroides, Emilia fosbergii) readily established many large colonies and are likely to become more widespread if the current practice of not controlling weeds during the dry season and periods when cultivation is interrupted continues. The single species of Group 2 (Echinochloa colonum) established few large colonies during the dry season and was not able to spread to new sites during fallowing. However, rapid maturity and continuous seed production during the dry season, like the species of Group 1, may enable the species to maintain its current importance as a weed in biannually cultivated fields. Species of Group 3 (Asclepias curassavica, Malachra alceifolia) established fewer and smaller colonies than the previous 2 groups during the dry season, and appeared to be hampered by drought, predation and competition. No individuals of Group 4 (Paspalum virgatum) matured during this period. However, both groups maintained colonies and spread to new sites during fallowing, and their importance as weeds in cultivated fields may increase if frequent interruptions occur.