The plant species richness, evenness, dominance and diversity indices for both P. hysterophorus invaded and uninvaded areas are shown in Table 2. The study results indicate that all P. hysterophorus invaded areas had higher species richness (R = 58) than uninvaded areas (R = 39). and the difference was significant (p = 0.043) at 5% level of confidence. Although plant species in uninvaded were more even (E = 0.644) than invaded areas (E = 0.063), dominance was found to be higher in invaded (D = 0.579) than uninvaded areas (D = 0.052). Consequently, the plant species diversity became greater in uninvaded (H’ = 3.
224) than invaded areas (H’ = 1.312). This finding is in line with what Etana et al., (2011) found during the assessment for impact of P. hysterophorus on herbaceous plant diversity of Awash National Park in Ethiopia.
The difference in species richness is attributed to high level of regeneration following soil disturbance. All invaded areas were characterized by bare grounds and patches (Figure 4 (d) and (e)) opened for secondary colonizers. This observation is supported by the fact that created patches allow penetration of sunlight and water necessary for the growth of plants whose seeds could have been in the soil.
In addition, disturbance could have created environmental heterogeneity because of difference in soil temperature, aeration, moisture and soil nutrients, hence suitable for other organisms like warthogs, elephants that could be dispersing P. hysterophorus seeds and seeds of other plant species. In relation to that, Kyayesimira and Lejju, (2015) studied the vegetation regeneration in formerly degraded hilly areas of Rwampara, South Western Uganda from which they noted that regrowth and emergence of new species occur after disturbance.
These new species are formed either from the soil seed bank or are dispersed into the site from the outside. Furthermore, the soil disturbance could have given P. hysterophorus an opportunity to establish its weed monoculture on bare grounds that gradually weakened even the survival of drought tolerant herbaceous plants in vicinity (Etana et al., 2011).
The same scenario was noted at Black water River State Forest in Florida (Holzmueller and Jose, 2012), where invasion of invasive species (87%) had occurred following disturbance by fire and from that, disturbance was strongly related to invasion.The pre-dominance of invaded areas by P. hysterophorus can be explained by its allelopathic properties. According to laboratory studies of P. hysterophorus leaf extract on germination, leaf area, shoot and root length, shoot and root dry weight of Triticum aestivum, Cicer arietinum, Brassica camestris, Avena fatua, Asphodelus tenufolius, Lolium rigidum, a concentration which is above 25gl- caused adverse impacts on all mentioned growth parameters (Hassan et al., 2018; Arne et al., 2018). From reproductive biology point of view, P. hysterophorus flowers within 28-42 days after seedlings emerge (Steven and Shabbir, 2014), and a single plant produces 25000 seeds (Adkins et al., 2010). Therefore, P. hysterophorus exerts control over the occurrence of other plant species through strong competition for plant growth nutrients, water and sunlight. If all these invasion strategies are coupled with suitable Ugandan climatic conditions for its growth and establishment, P. hysterophorus becomes a persistent strong colonizer and competitor in QENP plant community.
The same strategies make its establishment a success and its control difficult. Therefore, it is envisaged that in QENP, P. hysterophorus may reduce forage available for grazers and browsers and affect animal and human health if allowed to continue spreading and establishing itself. The study recorded lower values of species diversity index in P. hysterophorus invaded area than uninvaded area, which indicates the effects of its dominance that caused unequal abundances of plant species. Furthermore, P. hysterophorus being the most dominant plant species, is responsible for low evenness values of the plant species in this area.