An Invasive Plant Parthenium hysterophorus Reduces Native Forage Cover

While some research has been conducted in sub–Saharan Africa on the alien invasive Parthenium hysterophorus , little work has assessed whether it is poses negative impact on native forages. A field survey was carried out to study the dominant co–existing plant species, and its impact on plant abundance, species, and native forage cover. We found that non–natives were the dominant co–existing plants with P. hysterophorus compared to natives. Plant species (r = -0.889, P = 0.043) and abundance (r = -0.968, P = 0.007) decreased with increasing invasive percent cover. Moreover, native forage plant ( Brachiaria reptans ; r = -0.922, P = 0.026), Cynodon dactylon ; r = -0.972, P = 0.006,


INTRODUCTION
Many exotic or non-native plant species have been accidently or purposefully introduced to other nations (Early et al., 2016;Pratt et al., 2017). In native and semi-natural habitats, several of these plants have established and some have now become significant invasive alien plant species (IAPs) (Witt & Luke, 2017). Human activities, including the introduction of plants for food, agroforestry, ornamentation, and forestry, for instance, have been reported to be the significant cause for the spread of IAPs (Dawson et al., 2008). In addition to anthropogenic activities, climate change also enhances the dispersal of alien plants outside their natural range (Taylor et al., 2012). Their invasion and dominance in new geographic regions induces serious social and ecological impacts (Ojija & Ngimba, 2021;Prass et al., 2022). For instance, they threaten human well-being, biodiversity, pollination, and ecosystem functioning and services (Laizer et al., 2021;Ojija, 2022;Uyi et al., 2021). IAPs have been referred to as ecosystem engineers because of the alterations they bring about in the recipient environments (Perkins et al., 2011). In order to guarantee rangeland sustainability, food security, human wellbeing, and overall economic prosperity, it is crucial to stop the spread of AIPs (Ngondya & Munishi, 2021;Ojija & Manyanza, 2021).
In sub-Saharan African countries such as Tanzania, most IAPs are unsafe for human, biodiversity, and ecosystem health (Ojija et al., 2019a(Ojija et al., , 2019bWitt et al., 2018). Their invasions are associated with plant community disassembly i.e., they alter the ecosystem structure, as well as farms, grazing lands, and rangeland quality (Ojija et al., 2019a(Ojija et al., , 2019b. The spread of IAPs is facilitated by their high fecundity, rapid germination, and growth rate (CABI, 2019;Eppinga et al., 2022). Parthenium hysterophorus invasion is threatening biodiversity conservation, ecosystems, livestock, and agriculture in Tanzania (Ojija et al., 2019a;Ojija & Manyanza, 2021;Ojija & Ngimba, 2021). Previous research has shown that P. hysterophorus decreases the amount of fodder on rangelands, as well as the grazing potential and the capacity to regenerate (Navie et al., 2004). The production of allelochemicals, which inhibit the growth of surrounding coexisting plant species, also displaces indigenous plant species, altering the composition of the vegetative community into communities that are dominated by Parthenium (Foxcroft et al., 2006) Despite the negative impacts associated with IAPs to native plants, limited studies have been conducted to assess the impact of P. hysterophorus on native forage species in sub-Saharan Africa, particularly in Tanzania. Similarly, little has work been done in the country to identify dominant co-exiting native plants with P. hysterophorus that could be used to suppress the invasive by increasing their density following competition experiments. Therefore, identifying native plant species that co-exist with P. hysterophorus is critical for control and management of the invasive (Ammondt & Litton, 2012;Khan et al., 2013Khan et al., , 2013Ngondya & Munishi, 2021). This is because the dominant co-coexisting native plants can be used for the restoration of invaded habitats by maintaining their abundance, density, and/or diversity in the invaded habitats (Richardson et al., 2007;Weidlich et al., 2020). The benefit of using dominant co-exiting native plants is that not only they seem to be good competitor with the invasive plants, but also they have minimum negative impacts on the environment compared to exotic species (Khan et al., 2013;Ojija & Ngimba, 2021). Hence, there is a need to determine the native forage plant species that could be sowed in rangelands and/or grazing lands to benefit livestock and ecosystem. This may contribute to achieving African Union Agenda 2063 Goals 1, 3, and 4 and the Strategy for controlling IAPs in Africa (2021-2030) (Ngondya & Munishi, 2021).
However, based on our knowledge, there are limited studies in Tanzania that have assessed the dominant co-existing plant species with P. hysterophorus, and the impact of P. hysterophorus cover on cooccurring selected native forage species. Thus, the specific objectives of the study were to assess (i) the dominant co-existing plant species with P. hysterophorus, and (ii) the impact of P. hysterophorus cover on the abundance of selected native forage plant species. We hypothesized that (i) non-natives are the dominant plants co-existing with P. hysterophorus, and (iii) the abundance of native forage plant species decreases with increasing P. hysterophorus cover.
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Characteristics of Study Species
Parthenium hysterophorus invades pastures that have been disturbed, ruined, overgrazed, and places where there is little to no grass cover (Ojija & Manyanza, 2021). It cannot, however, be established in pastures or areas with unaltered natural vegetation. It is an annual herbaceous plant (1.0 to 2.0 m tall) bearing creamy-white flowers (4 to 10 mm in diameter, Fig. 1) (Ojija et al., 2019a). It has lobed leaves, a stem that is upright and branches, a deep, robust tap root, and forked root systems (Brunel et al., 2014). Parthenium hysterophorus plants typically yield 810 flower heads, 10,000-25,000 seeds, and around 624 million grains per plant (Ojija et al., 2019a). The life cycle of the P. hysterophorus takes over 150 days to complete and takes around 42 days from seed to mature plant (Kaur et al., 2014). Its seeds can survive for four to six years and germinate all year long in the soil. Under favorable circumstances, it can complete 4-5 generations annually (Tanveer et al., 2015). For P. hysterophorus seed germination, the typical minimum and maximum temperatures are 10 o C and 25 o C, respectively (Brunel et al., 2014) The best soil pH for germinating seeds is between 5.5 and 7.0, but seeds can germinate in a wide pH range (between 2.5 and 10) (Kaur et al., 2014).

Field Sites
The fieldwork was carried out in the Arusha and Kilimanjaro regions to determine the impact of P. hysterophorus cover on other plant species and the identification of dominant co-existing plant species with the invasive. The study was conducted at KIA (S 3 o 22.453', E 37 o 59.822') and Kisongo (S 3 o 22.172', E 36 o 38.275') field sites. The mean annual temperature and rainfall in the study sites are 19.5° C and 1361 mm, respectively (Ojija et al., 2019a).

Data Collection Method
Assessing the dominant co-existing plants with P. hysterophorus and impact on native forage species. Twenty (20) plots of 1 m 2 each, 10 m apart, were established along each of the two 100 m transects at Kisongo and KIA field sites. Prior to species identification, P. hysterophorus cover was estimated in each plot based on the ACFOR abundance scale, i.e., abundant: 75-100%, common: 50-75%, frequent: 25-50%, occasional: 5-25%, rare: 1-5% (Stiers et al., 2014). Plants were East African Journal of Environment and Natural Resources, Volume 5, Issue 1, 2022 identified in each plot at species or morphospecies level. The most dominant plant species co-existing with P. hysterophorus were determined based on their relative abundance. The abundance of Brachiaria reptans, Cynodon dactylon, Digitaria milanjiana, and Indigofera spicata was used to estimate the impacts of P. hysterophorus cover because they are native forage plant species. Plant abundance was visually estimated in the plots.

Statistical Data Analysis
To investigate the associations between plant species, abundance, and native forage species cover and P. hysterophorus % cover, we performed a Pearson correlation analysis. Levene's test and the Shapiro-Wilk test, respectively, were used to confirm the homogeneity of the variance test and the normality test. All of the tests we ran had a 5% level of significance. Statistical tests were conducted with Origin (2013) version 9.0 SR1.

Dominant Plant Species Co-Occurring with P. hysterophorus
The Parthenium-dominated plots contained a total of 45 plant species from 18 different families (Table  1). Overall, non-natives (i.e., Ageratum conyzoides, Datura stramonium, Tagetes minuta, Argemone mexicana, Bidens pilosa, Senna occidentalis, Solanum incanum, and Xanthium strumarium) were the dominant plants co-existing with P. hysterophorus, except the native Digitaria milanjiana (Table 1). They had a high relative abundance compared to other plant species in the studied areas (Table 1).

Impact of P. hysterophorus Cover On Plant Abundance and Native Forage
According to a correlation analysis, plant species (n = 45, r = -0.889, P = 0.043, Figure 2) and abundance (n = 865, r = -0.968, P = 0.007, Figure 3) considerably decreased as P. hysterophorus % cover increased. The percent cover of native forage plants, Brachiaria reptans (r = -0.922, P = 0.026), Cynodon dactylon (r = -0.972, P = 0.006), Digitaria milanjiana (r = -0.938, P = 0.018), and Indigofera spicata (r = -0.977, P = 0.004), also decreased with increasing invasive cover (Fig. 4). Cynodon dactylon and I. spicata had lower cover compared to other dominant native forage spaces in plots with P. hysterophorus cover of > 70%.   Moreover, the co-existing concept demonstrates the possibility of suppressing invasive growth through competition with dominant co-existing native plants (Ammondt & Litton, 2012;Ojija & Ngimba, 2021). Previous studies show that by maintaining the abundance and diversity of coexisting native plants, the invasive could be controlled (Ammondt & Litton, 2012). Also, restoration of invaded habitats using dominant co-existing native plants could enhance pollination and ecosystem health (Arathi & Hardin, 2021;Guo et al., 2018;Weidlich et al., 2020). Therefore, it may be possible to restore P. hysterophorus-invaded habitats using dominant co-existing native plant species because they help maintain ecosystem stability by providing food for livestock and wildlife, facilitating the flow of nutrients within the ecosystem, and preserving soil fertility.

CONCLUSION
The study advances knowledge of how invasive species affect biodiversity and how they can affect forage abundance. With the increasing invasion of P. hysterophorus in Tanzanian natural and seminatural ecosystems, it is expected that more native forage species will decline. This will impact wild animals and the ecosystem beauty of these areas and eventually decrease the attractiveness of national parks to tourism. For the government, management agencies, and other parties involved in management and tourism activities, this could mean a loss of revenue. Generally speaking, this study provides baseline data for future studies to clarify P. hysterophorus' effects on biodiversity preservation.

Limitation of the Study
Time and budgetary constraints limited the study survey; therefore, it suggests additional long-term surveillance of the effects of the invasive on native plants..

CONFLICT OF INTEREST
The author affirms that there were no financial or commercial ties that might be seen as having a potential conflict of interest while the study was carried out.

FUNDING
There was no specific grant for this work