The distribution of lantana camara and its impact on plant species diversity in Queen Elizabeth National Park
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Invasions by introduced species are the third biggest threat to biodiversity in Uganda today after habitat loss and unsustainable utilization of natural resources. Invasive plants can transform ecosystems by establishing populations with high growth rates that displace the native biota or, thereby potentially transforming ecosystem structure and functioning. The vegetation of the northern, central and southern parts of Queen Elizabeth National Park (QENP) has, since the mid 1970s, been progressively invaded by the alien invasive shrub Lantana camara (L.). In this study, the plant community variations associated with L. camara invasion were examined. Non-invaded plots of native vegetation cover in QENP were compared with adjacent plots dominated by L. camara for species abundance and diversity at three selected sites of bushland, grassland and woodland. Ten experimental plots (20 m x 20 m) and ten control plots of same size were laid at each site along a transect in each case. Within each plot, one sub-plot of 2m x 2m was marked at the first right hand corner for estimating foliage cover of herbs and grass species. All plant species in the selected plots were identified to species level and their abundance estimated. Environmental data on disturbance factors was recorded for each site. Frequency of each species was determined by noting its presence or absence in a given plot. Foliage cover of each plant species was determined by visual estimation of the area of its crown in proportion to the ground area. Shannon diversity index was used to determine and compare the plant diversity between L. camara invaded and non-invaded plots using foliage cover as measure of abundance. The differences in the species diversity indices between the study areas, experimental and control sites were evaluated using a paired student’s t-test in Stata 9.2. The average cover of L. camara for the three sites was 37.3% with grassland having the highest (43.8%). Much of the L. camara was associated with disturbance factors like cultivation and sand mining, originating from the community settlements. The abundance of L. camara in grassland habitat increased significantly as one moved away from the community settlement into the park (L. camara cover = 18.3 + 0.11 x Distance, R2 = 0.52, P < 0.05, d.f. = 9). There was little herb and grass cover under thickets of L. camara as opposed to plant communities where the shrub was absent. The cover of grass species, including Brachiaria brizantha, Cynodon dactylon, Hyparrhenia rufa and Sporobolus pyramidalis was not correlated with cover of L. camara. Lantana camara in the bushland and savanna woodland had an insignificant positive growth relationship with some common shrubs and thicket species such as Capparis tomentosa (rs = 0.51, P > 0.05, n = 14), Dichrostachys cinerea (rs = 0.19, P > 0.05, n = 15), Flueggea virosa (rs = 0.46, P > 0.05, n = 18), Euclea natalensis A.D.C and Cissus rotundifolia Forssk. In grassland, the mean shrub cover, excluding that of L. camara was significantly higher in the invaded plots compared to non-invaded plots (P < 0.05). Mean herb cover in savanna woodland habitat was lower in invaded plots compared to non-invaded plots (P < 0.05). In grassland, mean shrub frequency, excluding that of L. camara was significantly higher in the invaded plots compared to non-invaded plots (P < 0.05). Grass and herb species diversity was higher in invaded plots than in the non-invaded plots whereas in savanna woodland vegetation herb diversity was lower in invaded plots. Tree canopy cover and abundance of L. camara in savanna woodland had an insignificant moderate positive correlation (rs = 0.57, P > 0.05, n = 10). It is here concluded that L. camara is abundant and almost equally distributed in the grassland, bushland and savanna woodland vegetation types of QENP. Grassland habitat is however most susceptible to L. camara invasion. Secondly, two types of disturbances seem to play an important role in the abundance and distribution of L. camara in QENP. These can be categorized into disturbance due to activities on the land such as cultivation as well as disturbance of L. camara itself through harvesting by the local community. Lantana camara has a negative impact on the abundance of herbs and grass species and a negative effect on the diversity shrub species. Analysis of species composition also reveals that invaded plots are more floristically diverse but with most species being of weedy nature hence not desirable. Lastly, there is no significant effect of tree canopy cover on abundance of L. camara in savanna woodland vegetation.