3.1. Plant composition
As the result revealed, 74 plant species belonging to 26 families and 57 genera were identified and documented. This might be because the wetlands are the most important habitats for biodiversity conservation (Menbere and Menbere 2018). Moreover, while wetlands are disturbed, they create many microhabitats and microclimates, which could create suitable media for growth of various species such as upland, wetland, non-native and native plant species so that the species richness could increase up. Further, among 26 families identified, Asteraceae and Poaceae were the most dominant ones, which contributed 20.27% and 16.22% to the total species of the study area. This finding is in line with the report of Moges et al (2017) from Ethiopia, and of Odull and Byaruhanga (2009) from Uganda. Additionally, those Asteraceae and Poaceae families shared 24.56% and 14.04% of the total genera (57) of the plant species, respectively. Next to those families, Lamiaceae, Commelinaceae and Apiaceae were the co-dominant families, and contributed 5.41%, 4.05% and 4.05% to the total plant species, respectively. Those all dominant and co-dominant families have the adaptation potential of both terrestrial and aquatic ecosystems. In fact, based on the assessments made and rated, the Washa and Borale were ecologically mid- and high- impacted sites, respectively (Supplementary file 1). This implies that such impacted wetland sites support dominantly those plant species resisting the ecological disturbance factors. Because of those dominant and co-dominant families, the other 16 families sharing about 62% of the total identified families had the least contribution (1 genus, 1.75%) and (1 species, 1.35%) to the total genera and species, respectively, with the exception of only one family (Asphodelaceae) contributing two species (2.70%). This might also be due to the hydrological and ecological modifications of the study wetlands, resulting in creating suitable microhabitats for growing many upland and native, but less abundant in their coverage, might be owing to the shortage of time for their invading the study area dominantly.
While comparing the two study sites in terms of their dominant families, Borale Wetland supported a larger number of families than that of Washa Wetland (Fig. 3). This might be due to the variations of their ecological status, i.e., Borale site was more disturbed than Washa site. Mulatu et al (2014) and Moges et al (2017) also reported from the southwestern highlands of Ethiopia that the disturbed wetlands support more plant species than the non-disturbed ones. Despite unlike in their extent of their ecological disturbances, the two wetlands had common physiognomies in their climate (moisture and temperature) and agro-ecological zones, resulting in having more than half common families growing in the two sites (~54%) (Table 1, Fig. 3). This might be because climate and altitude are the most determinant factors for such same plant species growth.
Regarding the plant habits, there were only three habits (herbs, shrubs and climbers) identified in the study area. Of those, the majority of the species (~92%) was herb, followed by shrub (~7%). Contrarily, climber/liana (~1%) was found to be the least number of species. This report also agrees with the findings of Mulatu et al (2014) and Moges et al (2017). This might be due to that the wetlands having surface or subsurface water mostly support more herbs than the other plant habits.
3.4.2. Richness, Shannon diversity, evenness and similarity between Washa and Borale sites
Richness is an apt measure of diversity, and the richness of a range of habitats (of both wetlands) is also termed as gamma diversity (Whittaker 1972), but the average richness is called beta diversity (Whittaker 1972, Kent 2012). However, the total number of species per wetland site is called alpha diversity (Du Toit et al 2021). Thus, the species richness (S) is the most repeatedly used index (Magurran 2004) for comparing the diversity between sites (wetlands) (Woldu 1985) and may be used gamma, beta and alpha diversity interchangeable and accordingly for this paper.
Therefore, the gamma diversity of the present study area was 74. The richness per plot (2m2) in the study area was also ranged from eight (8) to twenty (20) with 14.55 average richness (beta diversity). This means that close to 15 plant species were found per 2m2 (~ 8 species per 1m2), showing a large number of species per 1m2 compared to other land use systems. Still, there were the variations of species richness between Washa and Borale wetlands. Accordingly, the alpha diversity of Wash and Borale sites were 51 and 64, respectively. Moreover, the maximum richness values of Washa and Borale wetlands were 17 and 20, respectively, but the minimum richness values of Washa and Borale were equal (8). Additionally, the beta diversity values of Washa and Borale wetlands were 13.2 and 15.9, respectively. These all indicated that Borale site supported more plant species than Washa site. Murillo-Pacheco et al. (2016) also reported similar results that in aquatic plants, there are marked differences in total alpha diversity in the two wetlands among all study sites. This was also confirmed by paired t-test made for the mean richness of the two sites (p = 0.0113), as there was a high significant difference between the two sites.
While also considering the Shannon’s diversity and evenness of the Washa and Borale sites, there were medium diversity, and high uniformity in distributions of the species. However, the Shannon diversity values of Washa (H’ = 2.24) and Borale (H’ = 2.67) were somewhat different. Similarly, the distribution of the plant species or evenness in the Washa (J = 0.87) and Borale (J = 0.97) was still dissimilar. Thus, based on the mean values of the Shannon diversity and evenness indices, there was a very high significant difference between the two wetlands (p = 0.0000). This is because wetlands by nature are dynamic ecosystems; there are always ecological changes or disturbances in wetlands due to anthropogenic activities (Menbere and Menbere 2018). Connell (1987) and Moges et al (2017) also reported that environmental heterogeneity, disturbance and competitive exclusive processes are the determinant factors for either increasing or decreasing the diversity of an ecosystem. As also reported during the ecological assessment of the two study sites, Borale site was highly impacted compared to Washa site due to the anthropogenic activities taking place within and around the two study sites (Supplementary file 1). Generally, diversity patterns seem to be driven by high landscape heterogeneity and wetland management.
Concerning the dominant species found in the two study sites, the species E. marginatum, G. dissectum, C. fischerianus, P. thunbergii, Alchemilla abyssinicum, and L. stolonifera, were the most dominant species in Borale study site. Similarly, the most dominant species in Washa site were E. marginatum, L. stolonifera, G. dissectum, C. fischerianus, P. thunbergii and Eleusine floccifolia. Thus, the species E. marginatum, C. fischerianus, L. stolonifera, P. thunbergii and G. dissectum were the most common dominant species to the two study wetlands. This result indicated that the dominant species in the two wetlands are almost similar, and more than half of them were obligate wetland species. Of course, the SSC between the two sites was about 71% similarity, and confirmed this finding. Barbour et al., (1987) also reported that any two plant communities those have more than 50% similarity shows the same association. Additionally, Kent (2012) stated that when calculated between all pairs of quadrats of a study area, or between two sites, a similarity or dissimilarity matrix is produced. Thus, based on this general rule, the Washa and Borale sites had similarity in their plant composition. However, Murillo-Pacheco et al (2016) from Colombia, and Moges et al (2017) from Ethiopia reported the opposite result that the similarity in composition of aquatic plants among the study sites was low. These also imply that although the SSC showed similarity between the present study sites due to their similar agro-ecological and climatic factors, the two study sites could have significance difference owing to the variations of the extent of ecological disturbances, resulting in creating heterogeneous microhabitats, which in turn, leading to support different species.
3.4.3. Characteristic and common species of the study area
The characteristic species are species that characterize the specific site or habitat being unique to only that of other sites of the study area. Thus, persicaria decipens, Cyperus pauper, Cyperus brevifolius, Commelina diffusa, Anthemis tigreansis, Emilia leptocephala, Galium acrophyum, Senecio myriocephalus, and Taraxacum officinale were the characteristic species to Washa Wetland since they were found only from Washa site. Particularly, the first four plant species were common wetland plants in Washa and other less or non-impacted wetlands despite having many uplands (Table 5). Contrarily, the plant species including T. latifolia, L. tomentosa, Guizotia scrab, Plectranthus punctatus, Rumex abysinicus, Sida schimperiana and some others listed (Table 5) were unique to only Borale Wetland. Many characteristic species of Borale wetland were also uplands, and are considered as good indicators of disturbed sites. Even, some aquatic plants such as T. latifolia and Emilia leptocephala (Mattf.) C.Jeffrey are important characteristic plants being indicators of water pollution or ecological degradation in Borale Wetland. As also reported from Jimma Highlands, Boye Wetland was highly impaired and invaded by T. latifolia (and Emilia leptocephala) (Moges et al 2017).
Additionally, from the total characteristic species (33), 90.9% and 54.54% were native and perennials, respectively. Contrarily, of the total (74), 41 (55.4%) species were common to the two wetlands. Of these common species, 92.68% and 56.1% were native and annual species, respectively (Appendix B). Seventeen species (41.46%) were perennials, and only one (2.44%) was biannual. Regarding these wetland indicator species of the common plants to the two sites, 10 (24.39%), 10 (24.39%), and 19 (51.22%) were wetland species (OB + FW), facultative (F), and upland species (FU + UP), respectively. This also indicates that the majority of the wetland indicator species growing in both wetlands were uplands. This finding is in line with Thompson et al (2007). These all imply that the invasion of alien species in the study sites was very low; however, due to their ecological disturbances, many of the aquatic plants were replaced by native and annual upland invaders. Still, those OB, FW and F characteristic species together represented about 41%, which grow and easily adapt the wetland ecosystems, and which indicate the restoration potential of the study sites.
Seventy four plant species were identified and documented from the two wetlands, located at Central Ethiopia. Asteraceae and Poaceae were dominant families, and contributed the largest number to the total species. The species diversity, richness and evenness of Borale were higher than Wash site, and showed significant differences between the two sites since Borale site was highly disturbed due to anthropogenic activities. Additionally, the study sites largely supported native and annual species, but many of them were uplands, considered as native invaders due to the hydrological and ecological changes in the study sites, creating a good environment for widely growth of upland species by excluding wetland species. Thus, if apt measures are not taken soon, both wetlands, especially Borale site would reach unrestored conditions. Therefore, to ensure a long-term conservation and sustainable use of wetlands, it is essential to develop and implement in-situ restoration and management strategic plans that take both reservoirs and natural wetlands with their catchments into consideration.