Weeds severely limit agricultural production in many Third World tropical countries where productivity is already marginal due to unfavourable environmental, biological, economic and social factors. Lack of both ecological and distributional information at the scale of the weed assemblage, and lack of detailed data on the life histories and responses of the constituent weedy species hinders development of cheap and effective weed management strategies not contingent on intensive use of herbicides. / Plant geographers can contribute to the solution of tropical weed problems by investigating factors which mediate the dynamics of species: regional and local ranges. The research in this thesis departs from other plant geographic studies on tropical week assemblages (e.g. Kellman, 1973) by undertaking experimental field studies which are designed to uncover aspects of the differential abilities of species to make the transition from localized immigrant population to widespread and abundant weed. / The research focusses on six alien species invading agricultural and non-agricultural land in a lowland tropical region: coastal Guyana. Experiments are directed towards propositions concerning the basis for differential colonizing success of two common (Echinochloa colonum (L) Link, Macroptilium lathyroides L.) and four uncommon (Asclepias curassavica L., Emilia fosbergii D. H. Nicholson, Malachra alceifolia Jacq., Paspalum virgatum L.) species of cultivated fields in an area of prime agricultural land. Experimental colonizing trials, which equalize the opportunities of each species to colonize, provide a reference point against which to assess inter- and intra-species differences. / Results of colonizing trials suggest that the species represent four ecological groups. These are Macroptilium and Emilia (Group 1), Echinochloa (Group 2), Malachra and Asclepias (Group 3), and Paspalum (Group 4). / Species of Group 1 have the highest potential for establishment in cultivated fields within a single growing season. Emilia, though currently not a prominent species in the study area, should be controlled early. Heavy insect predation and consequent slow maturity of Asclepias reduce the likelihood of its rapid establishment in the area. Malachra and Paspalum, though not predated, have slow generation times, and hence are also unlikely to rapidly invade frequently cultivated fields. However, the high incidence of interruption of cultivation in the coastal farmlands is likely to promote their range expansion.

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