The consequences of biotic activities with reference to exotic plant invasions were studied in natural forest and savannah and man-modified ecosystems such as Albizia plantation, rubber plantation, teak plantation and wastelands of the Western Ghats of Tamil Nadu at Kodayar in India. Exotic plants contributed to approximately 17 and 35 per cent of the total importance value indices to the plant community structure in natural and man-modified ecosystems, respectively. Lantana camara and Chromolaena odorata were found to be the dominant exotic perennial weeds in both of the ecosystems studied. The high importance value indices of exotics in the man-modified ecosystems could be attributed to frequent disturbance and resource rich transient open environment. The disturbance regime and the type of exotic plant invasions were analysed. It was found that frequent fire incidence in the ecosystem resulted in monotypic savanna grasslands. C. odorata and L. camara invaded the sites where mechanical disturbance and occasional fires occurred. The density and weed composition varied considerably in different agroecosystems and agroforestry systems practiced by 'Kani' tribals in the region. This may be due to variation in the number of weedings and light availability to the forest floor through the crop canopy cover. During the secondary succession suppression of exotic weeds in 8- and 20-year-old fallows could be attributed to light availability decreased by the larger shrubs and trees.