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Invasive plant species are the second threat to global biodiversity loss next to land-use changes. Prosopis juliflora (Sw.) D.C. (Fabaceae) (Henceforth referred to as Prosopis) is among invasive plant species native to South America, the Caribbean, and Central America. Prosopis has been introduced consciously to Ethiopia particularly in Afar in the late 1970s and 1980s. In the arid and semi-arid tropical region, though introduced Prosopis has been serving as fuel wood and dry season fodder for the rural population, the threat posed by it in terms of invasion of fertile agricultural lands and loss of bio-diversity is looming enormous. In lowlands of Ethiopia, arid and semi-arid rangelands are subjected to different human and natural impacts. Undesirable weeds and woody plants are encroaching rangelands that have become threat to pastoral production systems. Among woody encroachments, Prosopis is the most jeopardy to arid and semi-arid areas in east and northeast Ethiopia particularly in Afar region.
Land use/land cover changes, competitive ecological advantages, and climate change are key factors thought to influence the probability of invasion of Prosopis. Human activities such as overgrazing, conversion of land uses from grazing lands to croplands, urbanization and construction of infrastructures have also impacts on natural resources. Currently, more than 30,000 ha of grasslands, rangelands, water points, and croplands are estimated to be occupied by Prosopis only in the Middle Awash area. Moreover, Luleseged et al (2012) estimated that expansion of woody species was at a rate of 50,000 hectares per year in between 2002 to 2012 in the region. On the other hand, Wakie et al (2014) reported that 360,500 hectares of Prosopis invasion areas only in northern parts of Afar region.
Alien plant invasions have major impacts on biodiversity, ecosystem services, agriculture, forestry, human welfare and the economy. Road sides, river courses, farmlands, irrigation canals, wetlands, grasslands, conservation areas and homesteads areas are the most severely invaded habitats in Afar region. Prosopis is dominating large areas of prime grazing land of Afar region. Consequently, nutrient rich palatable grasses, the main feed source for grazers are progressively outcompeted. In several literatures Prosopis have both positive and negative interactions with plant communities in a naturalized areas or ecosystems. However, studies conducted in many parts of the world showed that problems of Prosopis are outweighing positive ones ecologically, socio-economically and in all health aspects. For instance, there was a depressing effect of Prosopis canopy on the associated native plants. The depressive effect of Prosopis was more obvious on the annual compared with perennial plants, especially on grasses in arid lands. Studies recorded that standing biomass, frequency and cover of under storey plant species were significantly higher in the open area than under the canopies of Prosopis.
Invasive species can modify native community composition, deplete species diversity and affect ecosystem processes. For example, Prosopis canopy effects were consistently and strongly negative on species richness from associated native species. In arid grazing lands, Kumar and Mathur (2014) showed that more species richness and evenness than non-invaded sites in all land uses of Prosopis invaded sites. On the other hand, Singh et al. (2008) also depicted that species dominance and evenness were the highest under Acacia nilotica and Prosopis, whereas species diversity index and richness were the highest under native species. With respect to plant community composition, numbers of plant species recorded under Prosopis canopy were lower than under other native species. However, under Prosopis canopy, the highest total importance value index was recorded followed by other native species. Moreover, Prosopis created mono-specific stands of adult trees, locally displacing native species or limiting their recruitments. For example, in United Arab Emirates, more than 50% density of annual herbs was significantly inhibited under Prosopis canopy which was greater than the associated number of perennials. Impacts of Prosopis on biodiversity studies in the region have been discussed by many researchers such as Behanu and Tesfaye (2006), Shiferaw et al. (2004), Kebede (2009), and Getachew et al. (2012). However, little is known about the threats posed by invasive alien species Prosopis on plant diversity, composition and regeneration of woody species. So, it is imperative to quantify the impacts for future vegetation ecosystems restoration, biodiversity conservation and management.
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