Reproductive Traits That Foster the Invasion of Four Alien Plant Species in Botswana

Abstract

Studies focusing on some reproductive traits that foster the invasion of Gomphocarpus fruiticosus, Argemone mexicana, Sesbania bispinosa, and Verbesina encelioides were carried out in Gaborone, Botswana. The specific objectives of these studies were to: (i) determine density of stems; (ii) determine the number of inflorescences/flowers and fruits produced per plant; (iii) determine the number of seeds per fruit, plant, and hectare; (iv) determine the 1000-seed weight and single-seed mass; (v) test different pre-sowing treatments on the germination of seeds; and (vi) assess the density of soil seed banks of the species. Results on density of the four study species as well as numbers of inflorescences, flowers, fruits, and seeds plant−1, m−2, and ha−1 are presented. Among the study species, the numbers of seeds kg−1 ranged between 129 870 (S. bispinosa) and 458 715 (A. mexicana), while the numbers of seeds ha−1 ranged between 62 330 000 (G. fruiticosus) and 8 890 070 000 (V. encelioides). Fresh seeds of the study species failed to germinate even when they were subjected to different pre-sowing seed treatments, except S. bispinosa, which responded well to the scarification pre-sowing treatments, germinating up to 98%. These results indicate that the seeds possess deep dormancy, a strategy that spreads germination of the study species over appropriate time and space. The study species also accumulated large numbers of viable seeds in the litter and topsoil layers, ranging between 3 400 000 (A. mexicana) and 14 380 000 (S. bispinosa) seeds ha−1, suggesting that the study species accumulate persistent seed reserves in the soil, which will guarantee recruitment of seedlings in the event of natural and/or anthropogenic disturbances. These results indicate that the study species are well equipped with a number of reproductive traits that foster their rapid invasion of new areas, making them powerful noxious invaders, as can be evidenced from their rampant invasion in the study site. Therefore, any effort in the management, control, or elimination of these species that does not take these reproductive traits into account is bound to fail. Recommendations that could help in addressing the invasion of the study species in Botswana are forwarded.