Invasive Milk Thistle (Silybum marianum (L.) Gaertn.) Causes Habitat Homogenization and Affects the Spatial Distribution of Vegetation in the Semi-Arid Regions of Northern Pakistan

Abstract

Global biodiversity management is of concern due to invasive plant species that dramatically disturb the native communities causing biological homogenization. Therefore, the present research investigated the impacts of Silybum marianum, an aggressive invasive alien species, on communities’ diversity and environmental variables in Khyber Pakhtunkhwa, Pakistan. Phytosociological characteristics and diversity indices of the communities were sampled in seventy-five sites using the quadrate method. These sites were categorized based on invasion intensities, i.e., fully invaded sites with a 100% importance value index of the selected species, severely invaded sites with >60% of IVI, and partially invaded sites with >30% of the chosen species. The community composition significantly changes with changes in invasion intensity. Similarly, S. marianum invasion has a pronounced impact on the community’s diversity showing significant differences among the three categorized groups (p < 0.05). The canonical correspondence analysis revealed 29.9% variance where soil texture, nutrients, and elevations were influential variables in maintaining the community’s structure and composition. The study concludes that S. marianum dominated well-established communities in the existing soil and environmental variables; therefore, it was found to be influential in disturbing the native communities and may severely harm the crop plant and agricultural system in the future.

1. Introduction

The biological invasion of alien plant species is the most critical and challenging threat presented to native biodiversity researchers [1]. The purposeful and accidental transfer of species is facilitated by human activities, the development of transportation facilities, and increased population mobility and migration [2]. Several invasive species have been introduced for various purposes, including agriculture, forestry, and ornamentation [3]. However, invasive plant species have dramatically affected native plant communities and ecological processes, having varying impacts on different habitats [4]. Although plant invasions seldom result in biodiversity loss, several studies [5] have indicated significant biodiversity losses due to invasive species establishment in an area [6]. Plant invasions may change an ecosystem’s biodiversity, speed up the nutrient cycle process, and impact the natural environment and human well-being [7]. The uniformity of plant ecosystems at different geographical scales has also been linked to species invasion [8].

Alien plant species may be found in various environments, including roads, railway lines, and urban and rural regions, resulting in community homogeneity [9]. The alteration of various populations within a region to become increasingly similar is known as biotic homogenization [10]. They are considered a valuable resource because of their ability to adapt to new environmental conditions and modify native plant populations [11]. Despite biotic and abiotic restrictions, an invasive species may be able to invade and expand in a new environment [12]. Invasive plants are successful for several reasons, including producing large numbers of viable seeds, repelling herbivores, surviving in various environments, tolerating high stress levels, and swiftly regenerating from seeds, stems, or roots. They must exhibit these features to flourish, propagate, and ultimately wreak havoc on their natural ecosystem [13].

Milk thistle (Silybum marianum (L.) Gaertn.), hereafter S. marianum, is an annual or biennial herb of Asteraceae. The species is native to the southeast coast of England and has been widely introduced outside of its native range, including North America, Iran, Australia, and New Zealand, where it is considered an invasive weed [14]. The milk thistle may be annual, winter, or biannual [15]. The plant now spreads in most of the world’s temperate regions [16] and has broad leaves that grow to 200–250 cm [17]. Even though milk thistle is commonly grown as a medical plant in Egypt, Europe, Argentina, and China, several countries have labeled it a noxious pest [18]. In Africa, North and South America, the Middle East, and Australia, S. marianum is a prevalent weed [19]. After establishing itself, the plant becomes a competitive invasive species, developing large, thick patches that shade other plants and compete for water and nutrients [20]. It is termed ruderal or weedy because it grows in dense clusters along roadsides and wastelands [21].

It has become an invasive weed in Pakistan’s northern irrigated areas, causing havoc on crops including Triticum aestivum L. (wheat), Trifolium alexandrinum L. (berseem clover), Hordeum vulgare L. (barley), Avena sativa L. (oat), and Saccharum officinarum L. (sugar cane) [22]. Due to the lack of pesticides and understanding about weed management along roadsides and irrigation systems in the area, this species is exceedingly competitive and emerges simultaneously with or before winter crops. In KP, Pakistan, wild oat (Avena fatua L.) and tiny seed canary grass (Phalaris minor L.) are the most common weeds. During the preceding decade, clodinafop and fenoxaprop-P were regularly employed to manage these grass weeds, perhaps opening a door for S. marianum invasion. Many environmental groups have also called for eliminating invasive species, such as this particular weed. Pakistan’s National Institute of Health in Islamabad has granted the construction of a ten-hectare traditional medicine park and botanical garden to lessen Pakistan’s dependence on imported medicinal plants, including S. marianum, and generate cash [23].

Invasive species S. marianum is likely to alter the native plant community’s richness, diversity, and composition due to its presence. Therefore, it was selected as a model plant for the current study. Thus, the species’ spatial distribution pattern and ecological impacts on local diversity in Khyber Pakhtunkhwa, Pakistan were assessed to evaluate its invasive behavior. The environmental conditions and soil factors that affect plants’ potential to spread were also evaluated to better understand the conditions which sustain these communities. We hypothesized that S. marianum would perform better than expected at higher elevations and steeper terrain. These assumptions were evaluated using field data, including comparing vegetation composition with decreasing degrees of S. marianum invasion intensity in terms of the Importance Value Index (IVI) and Species Diversity Indexes (SDI).

2. Materials and Methods

2.1. Study Area and Sampling Sites

Khyber Pakhtunkhwa (KP), one of Pakistan’s five administrative provinces, is situated in the country’s northwestern part. The Himalayan, Hindukush, and Karakorum mountain ranges from the province’s northern and eastern boundaries. The Hindukush mountain range stretches from the lowlands (327 m above sea level in Peshawar) to the highlands (7708 m above sea level at Tirch Mir) [24]. The sample sites range from 360 m to 1200 m above sea level and are located between 34.59 and 34.85 N° latitudes (Figure 1). The province is divided into four distinct agro-ecological zones based on the area’s geography and climate. Undulating plains and mountains surround the province, creating a climatic gradient that influences the south to the north and northwest. The climate in this area is mild all year round. There is a notable difference in temperature between the north and the south of the highlands [25]. June is the hottest month, with mean maximum and minimum temperatures of 35.01 ± 0.96 degrees Celsius and 18.20 ± 0.58 degrees Celsius. Winters are harsher since the temperature seldom rises above freezing point, and January is the coldest month with average maximum and minimum temperatures of 14.45 ± 2.10 °C and 0.83 ± 0.82 °C, respectively [26]. In terms of yearly precipitation, the range is 379–743 mm with a relative humidity of 55.32 ± 1.98 percent to 78.41 ± 2.92 percent [25]. Because the region’s economic, social, hydrological, and agricultural activities heavily depend on the climate, its evaluation is necessary in understanding plant communities [27].

Fieldwork was conducted during and just after this period, and data was collected from April to July 2021 because most plants sprout, leaf out, or bear fruit during the rainy season. The ecological implications of S. marianum were quantified in three distinct habitats (Figure 1).

The three types of habitats (

Table 1. Site conditions of Silybum marianum sampling areas distributed in Khyber Pakhtunkhwa, Pakistan.

Group-I	Group-II	Group-III
Pure/fully invaded sites 100% importance value index of S. marianum Low elevation, flat plain	Severely invaded sites <60% importance value index of S. marianum Intermediate elevation, subtropical humid conditions	Partially invaded sites ≤30% importance value index of S. marianum Higher elevation, humid temperate regions

) in the plains and hills of KP, Pakistan, with long-term populations of S. marianum include (a) relatively plain/flat areas, sites at low elevation in which S. marianum is favored by the semi-arid environmental conditions where pure communities exist (Pure sites, Group I), (b) an intermediate elevation zone, sites of relatively humid and sub-tropical condition (Severely invaded sites, Group II), and (c) sites relatively safe from human activities, having higher elevation (Partially invaded sites, Group III) as separated by clusters presented in Figure 2. The sites were selected randomly, and vegetation sampling was performed following the method in [28]. Selection and sampling of habitats in such a manner provide a reliable picture of invasive success and effect in the highland regions studied.

2.2. Vegetation Sampling

A total of 75 sites, each having 10 plots of 5 × 5 m areas (25 m²), were sampled, where phytosociological attributes such as frequency, density, and cover were measured and converted into their respective relative attributes for calculating the IVI of each species to assess the potential effects of S. marianum by following [28,29].

IVI = F₃ + D₃+ C₃/3

(1)

F₃, D₃, and C₃ represent the species’ relative frequency, density, and cover.

Plant species were identified using Flora of Pakistan and Kew Botanical Garden’s Plants of the World website (http://www.plantsoftheworldonline.org (accessed on 14 January 2022)) [30]. For accurate identification of some species, specimens were taken to the Herbarium of the University of Malakand. The sampling approach examined species composition and compared species richness and diversity. Species diversity indices were estimated using the following equations by following [31].

$${\mathrm{H}}^{\prime }={{\displaystyle \sum }}_{\mathrm{i}=1}^{\mathrm{S}}pi\mathrm{In}pi$$

(2)

$$\mathrm{E}=\frac{{\mathrm{H}}^{\prime }}{\mathrm{InS}}$$

(3)

1/D = 1/Σ (pi²)

(4)

S = St/a

(5)

where H′ is Shannon–Wiener diversity index; E is Evenness index; 1/D is Simpson’s index; S is species richness; pi is Species proportion; In is Natural logarithm; St is Number of individuals in total plots; a is Total plot numbers.

2.3. Environmental and Soil Variables

Samples from three sides of each plot were averaged to represent environmental factors accurately. The grinding process removed plant debris and tiny stones from each sample, allowing it to air-dry. After passing through a 2 mm screen, each sample’s physiochemical characteristics and nutrients were determined. Soil texture parameters were calculated using the Bouyoucos hydrometer technique, and pH was immediately measured using a pH meter (Model CON.3173) in the field after a soil suspension (1:5) had been prepared [32]. An Atomic Absorption Spectrophotometer [33] measured potassium in an unbuffered solution of NH₄OAc of IM concentration to determine organic matter (VARIAN model-AA2407, VARIAN, Palo Alto, CA, USA). An auto-analyzer was used to measure nitrogen, and an NaHCO₃ concentration of 0.5 M was used to estimate phosphorus using the calorimetric method [34,35]. A conductometer was used to measure electrical conductivity (Model CON 5). Other variables considered were altitude in meters above sea level (asl), soil pH in water, slope angle in degrees, and depth in centimeters of the soil in the test plots. The environmental characteristics are represented by an average value derived from samples obtained from three different locations on each plot.

2.4. Data Analyses

Ward’s agglomerative cluster analysis was employed to classify vegetation stands since species distribution fluctuates along an elevation and IVI gradient; this approach was favored over others and used PC-ord version 6 by measuring Euclidean distance, using Ward’s linkage method [36]. Using PC-ord version 6, Canonical correspondence analysis (CCA) was used to assess the influence of environmental and soil variables on vegetation groups. First, DCA-ordination was used to determine whether unimodal [37,38] or linear [39] response curves should be utilized in ordination analysis. CCA and redundancy analysis (RDA) was used to determine the species–environment correlation as the gradient length exceeded 4.1 on DCA-axis 1. There were 30 percent higher variances in CCA than in RDA ordination, and the stand distribution was more uniform in CCA-biplots than in RDA, and therefore preferred in analysis. SPSS version 22 was utilized for statistical analysis, and Excel 2010 was used for tabulation and visual display. Variables were assessed at p < 0.05, and the HSD test was also utilized to analyze the variance in the groups after the analysis of variance (ANOVA).

3. Results

3.1. Phytosociological/Vegetation Traits

In the 750 plots dominated or invaded by S. marianum, 22 plant species from 12 families were identified. Asteraceae, Poaceae, Amaranthaceae, and Solanaceae were the most dominant families. The species discovered were mostly annual plants belonging to the Phanerophytes and Chamaephytes Raunkiaer life-form categories (Figure 3).

The IVI of the S. marianum decreased progressively with elevation, having no co-dominant species in pure communities at low elevation. In severely invaded communities (Group II, IVI > 60%), there were three co-dominant species, i.e., Phalaris minor Retz., having an IVI of 4.47 ± 1.55, Stylosanthes humilis Kunth, having an IVI of 3.72 ± 1.56 and Fumaria indica Hausskn., having an IVI of 2.66 ± 0.99. In the partially invaded communities (Group III, IVI > 30%), the three co-dominant species were Melilotus indicus (L.) All., Solanum nigrum L., and Euphorbia helioscopia L., having IVIs of 6.75 ± 0.90, 5.81 ± 0.90, and 5.12 ± 0.91, respectively. The species importance value index varies significantly with community type variation (Groups I–III) (

Table 2. Importance value index of Silybum marianum and associated species found in the three groups separated by cluster analysis.

Species	Ac	Group I	Group II	Group III
		Mean ± SE	Mean ± SE	Mean ± SE	F	p-Value
Silybum marianum (L.) Gaernt	Sm	100 ± 0 a	62.44 ± 1.91b	30.44 ± 1.75 c	1451.3	6.61 × 10−59
Amaranthus viridis L.	Am	0 ± 0 a	1.03 ± 0.70 b	3.1 ± 1.28 c	7.11	0.001514
Artemisia scoparia Waldst. & Kit.	As	0 ± 0 a	1.58 ± 0.84 b	3.35 ± 0.91 c	13.63	9.50 × 10−6
Asphodelus tenuifolius Cav.	Ad	0 ± 0 a	1.71 ± 0.91 b	2.62 ± 0.91 c	8.59	0.0004
Avena fatua L.	Af	0 ± 0 a	1.27 ± 0.88 b	3.07 ± 0.96 c	10.16	0.00012
Avena sativa L.	As	0 ± 0 a	1.03 ± 0.70 b	0.77 ± 0.53 c	3.07	0.052573
Cannabis sativa L.	Cs	0 ± 0 a	1.64 ± 0.90 b	2.22 ± 0.87 c	6.94	0.0017
Carthamus oxyacantha M.Bieb.	Co	0 ± 0 a	2.18 ± 0.96 b	1.67 ± 0.77 c	6.99	0.0016
Chenopodium murale L.	Cm	0 ± 0 a	2.11 ± 0.92 b	5.06 ± 1.08 c	21.18	5.84 × 10−8
Cortaderia selloana (Schult. & Schult.f.) Asch. & Graebn.	Pg	0 ± 0 a	1.64 ± 0.87 b	2.85 ± 0.99 c	9.04	0.000315
Dodonaea viscosa Jacq.	Dv	0 ± 0 a	0.51 ± 0.51 b	3.26 ± 1.37 c	7.91	0.0007
Euphorbia helioscopia L.	Eh	0 ± 0 a	0 ± 0 a	5.12 ± 0.91 b	50.89	1.67 × 10−14
Fumaria indica Hausskn.	Fi	0 ± 0 a	2.66 ± 0.99 b	4.3 ± 1.04 c	17.89	4.91 × 10−7
Hypochaeris radicata L.	Hr	0 ± 0 a	2.38 ± 0.91 b	2.36 ± 0.94 c	8.78	0.000387
Lathyrus sativus L.	Ls	0 ± 0 a	1.61 ± 0.88 b	2.94 ± 1.01 c	10.77	8.08 × 10−5
Melilotus indicus (L.) All.	Mi	0 ± 0 a	0.57 ± 0.57 b	6.75 ± 0.90 c	68.9	1.90 × 10−17
Parthenium hysterophorus L.	Ph	0 ± 0 a	1.06 ± 0.73 b	1.69 ± 0.77 c	5.123	0.00831
Phalaris caroliniana Walter	Pc	0 ± 0 a	1.13 ± 0.77 b	1.85 ± 0.73 c	6.24	0.0031
Phalaris minor Retz.	Pm	0 ± 0 a	4.47 ± 1.55 b	3.41 ± 1.07 c	12.91	1.61 × 10−5
Solanum nigrum L.	Sn	0 ± 0 a	1.89 ± 1.01 b	5.81 ± 0.90 c	35.39	1.97 × 10−11
Spergula arvensis L.	Sa	0 ± 0 a	2.69 ± 1.04 b	2.97 ± 1.02 c	10.38	0.000109
Stylosanthes humilis Kunth	Sh	0 ± 0 a	3.72 ± 1.56 b	4.65 ± 1.19 c	13.78	8.52 × 10−6

Note: Ac, Species acronyms; Mean ± SE, Mean ± Standard error; Different letters indicate significant differences.

).

The species richness (p < 0.05), Shannon diversity index (p < 0.05), Evenness index (p < 0.05), Margelof index (p < 0.05), and Simpson index (p < 0.05) differ significantly between and among the purely, severely, and partially invaded sites. Furthermore, the diversity indices trends are inversely related to the S. marianum importance value index (Figure 4).

3.2. Environmental Variables

The environmental variables show that the sampling stands of S. marianum communities of Group I are situated at low altitude (340.15 ± 4.84 m ASL), are purely invaded, with low organic matter, nitrogen, lime, and potassium, and have a higher concentration of phosphorus. Group II, with an elevation of 859.38 ± 13.75 m ASL, has the highest organic matter, nitrogen, and phosphorus concentrations, whereas the potassium concentration was intermediate (

Table 3. Environmental and soil variables associated with Silybum marianum communities.

Factors	Group 1	Group II	Group III	F-Value	p-Value
Elevation	340.15 ± 4.84 a	859.38 ± 13.75 b	1548.64 ± 35.37 c	50.81	0.000
Latitude (°)	34.76 ± 0.11	34.67 ± 0.1	34.66 ± 0.86	0.365	0.69
Longitude (°)	71.86 ± 0.81	72.09 ± 0.06	71.94 ± 0.05	0.365	0.69
Aspect degree	93.79 ± 6.3	118.15 ± 13	111.6 ± 10.1	2.13	0.12
Clay (%)	31.58 ± 1.2	31.96 ± 2.9	34.55 ± 2.2	0.784	0.46
Silt %	36.59 ± 1.4	36.25 ± 2.6	32.76 ± 2.3	1.07	0.35
Sand (%)	31.94 ± 1.4	31.77 ± 2.5	32.68 ± 1.8	14.03	7.11 × 10−6
pH (1:5)	6.75 ± 0.10	5.45 ± 0.05	5.73 ± 0.04	45.64	1.57 × 10−13
Organic matter (%)	0.55 ± 0.04	0.61 ± 0.07	0.57 ± 0.05	0.793	3.124
Lime %	11.21 ± 0.5	11.63 ± 1.9	12.1 ± 0.08	0.419	0.66
Nitrogen %	2.114 ± 1.2	4.77 ± 4.3	3.39 ± 2.2	0.382	0.68
Conductivity (µs/cm)	37.74 ± 1.7	42.07 ± 2.7	57 ± 18.49	1.43	0.24
Phosphorus (mg/kg)	6.726 ± 6.3	6.6 ± 0.72	6.34 ± 0.49	0.25	0.78
Potassium (mg/kg)	137.7 ± 4.9	145.84 ± 13	157.2 ± 8.3	1.92	0.15

Different letters represent significance at p < 0.05.

). The partially invaded sites are found at higher elevations (1548.64 ± 35.37 m asl), having intermediate quantities of organic matter, nitrogen, and phosphorus, whereas they have higher quantities of potassium and lime. Among the physical parameters, % of silt particles is marginally higher than sand (31.94 ± 1.4)% and clay (31.58 ± 1.2)% in Group I, which shows that the soil is loamy in texture. Among the soil of other groups, the % mean values of sand particles are higher than silt and clay particles. The same is true for soil texture in Groups II and III. The pH of the soil was found to be a little acidic in Groups II and II (5.45 ± 0.05 and 5.73 ± 0.04) and almost neutral in Group III (6.75 ± 0.10).

The species–environment correlation was tested using Canonical correspondence analysis, which revealed the data loaded on axis 1, having an Eigenvalue of 0.34 with a % variance of 22.8, showing 88% of the Pearson’s correlation. The cumulative variance explained by all the three axes was 29.9%, in which the axis 2 and 3 contributions were 4.4 and 2.8 percent, respectively (

Table 4. Axis summary and commutative variance of the variables affecting Silybum marianum communities.

	Axis 1	Axis 2	Axis 3
Eigenvalue	0.384	0.075	0.047
Data variance
Explained variance in %	22.8	4.4	2.8
Explained cumulative %	22.8	27.2	29.9
Pearson Correlation	0.888	0.65	0.676
Kendall (Rank) Correlation	0.684	0.376	0.363

).

The ordination biplot revealed a clear-cut deflection and separation of purely invaded sites from severely and partially invaded sites. The biplot revealed that the communities are affected by soil textural (sand, silt, and clay), nutrient (lime %, potassium and phosphorus in mg/kg), and environmental (aspect degree and elevation) variables (Figure 5).

The correlation and biplot scores revealed the same loading of axis 1 with higher correlation scores for the environmental variables. The soil textural parameters were found to have a high correlation, i.e., r = 0.91, 0.77, and −0.52 for clay, silt, and sand percentages, respectively, on axis 1. In nutrient organic matter, lime, nitrogen, potassium, and phosphorus were found to have a significant negative correlation on axis 1, whereas nitrogen and organic matter have a significant positive correlation on axis 2 (

Table 5. Correlation and biplot scores of the environmental and soil variables operating on the Silybum marianum communities.

Variable		Correlation			Biplot Scores
		Axis 1	Axis 2	Axis 3	Axis 1	Axis 2	Axis 3
1	Latitude (°)	0.104	0.015	0.023	0.081	0.014	0.023
2	Longitude (°)	0.104	0.015	0.023	0.081	0.014	0.023
3	Elev.	0.638	−0.178	0.005	0.001	0.001	0.012
3	AA (°)	−0.237	−0.002	0.316	−0.186	−0.002	0.309
4	CLY (%)	0.912	−0.091	0.075	0.716	−0.088	0.073
5	SLT %	0.777	−0.086	−0.057	0.61	−0.083	−0.055
6	SND (%)	−0.528	−0.082	0.202	−0.415	−0.078	0.198
7	pH (1:5)	0.176	0.15	−0.125	0.138	0.144	−0.122
8	Organic matter (%)	−0.217	0.769	0.12	−0.17	0.74	0.118
9	Lime %	0.882	−0.02	0.068	0.692	−0.02	0.066
10	Nitrogen %	−0.41	0.646	0.098	−0.322	0.622	0.095
11	Conductivity (µs/cm)	−0.818	0.251	−0.028	−0.642	0.241	−0.027
12	Phosphorus (mg/kg)	−0.95	0.016	−0.075	−0.745	0.015	−0.074
13	Potassium (mg/kg)	−0.918	0.109	0.073	−0.72	0.105	0.071

).

4. Discussion

Plant communities were significantly homogenized by S. marianum’s presence, and its invasion significantly influenced species richness and diversity indices. As a result, the expansion and abundance of S. marianum reduced the landscape-level heterogeneity or spatial variety of the native plant groups. Invasive species have been demonstrated in alternative plant ecosystems, consistent with this study’s findings [40,41,42,43]. Solidago canadensis invasions, for example, have resulted in community homogeneity across multiple habitats and landscapes that were formerly dominated by distinct native species populations. The patterns of communities invaded by S. canadensis differed much from those in control locations and are consistent with the idea that invasive species tend to homogenize communities, which is found to be in compliance with our results.

The homogenization of communities by S. marianum is supported by communities’ physiognomic characteristics. Plant habits and biological life forms play a key role in promoting biotic homogeneity and plant community disruptions. Among the three communities studied, we observed that 64 percent of herbaceous plants, mostly of an annual character (77.7%), supported the results of [44,45]. In comparison with perennial plants, annual and biennial herbaceous species were shown to have a greater impact on promoting the homogeneity of plant communities. However, this is not the consensus among researchers, as others have found plant homogeneity in perennials, e.g., [46,47]. As an annual herb, S. marianum produces enormous numbers of viable seeds, quickly spreading and propagating across the area, disrupting the structure of the local plant communities and other trophic levels. In addition, S. marianum may be found in many different environments, including farmland, reducing crop yields. It is inedible to cattle and substitutes valuable fodder plants such as Rumex hastatus and Cenchrus ciliaris. The productivity of cereal crops, including wheat, barley, and maize, and pasture quality, may be adversely affected by the invasion of S. marianum, as reported by [22,48,49].

S. marianum’s propensity to produce homogeneous stands is a hallmark of this species, and it seems to be motivating the invasion of native species [50]. Invasions have previously been shown to impact community species richness [51]. Changes to the makeup of the invaded communities may lead to a rise in native, fast-growing species. Carthamus oxycantha, a fast-growing, semi-succulent herbaceous plant, was more common alongside S. marianum. We hypothesize that S. marianum alters anthropogenic and soil-variable patterns, affecting native diversity. For this reason, the presence of S. marianum has a discernible effect on the variety and richness of native species. S. marianum probably provides habitat for species vulnerable to herbivory and competes for space with browsing-tolerant species. According to these findings, S. marianum may have a greater influence on native plant variety and richness than previously assumed. In Kenya, [52] reported on a similar circumstance, in which the natural vegetation of Nairobi National Park was surveyed to compare invaded and un-invaded locations. In contrast to our findings, these researchers observed that invasive-species-infested ecosystems had a considerably larger variety of native species.

We found that S. marianum’s invasion of the lowlands fully homogenized the plant community and expanded to the highlands. The invasion seems to impact species richness and variety, but it also has the unintended effect of homogenizing the ecosystem. This issue will most likely be worsened in the future by climate change and the local community’s lack of understanding of the implications of S. marianum’s invasion, as successful recruitment and densification of existing S. marianum stands continue across the elevation, supporting the findings of [53]. Improving the plant’s ability to withstand drought by cultivating it for local purposes (mostly medicinal and phytochemicals) might lead to the additional spread of this invasive species, as revealed by [54], and interfere with the native vegetation’s spatial diversity and ecosystem processes. In the long run, it is expected to be the most prevalent species in the area, with an increasing influence on native flora. Raising local awareness of the problem and avoiding the future spread of the species to other natural environments is now the true task to be faced here.

S. marianum’s ability to thrive relies on its distinct biological properties and the environmental conditions it invades. Resources may fluctuate in abundance, making plant populations vulnerable to invasive species [55]. According to this idea, water availability in semi-arid areas is an example, since temporal oscillations produce a periodic shortage of resources, replenishing following rains. S. marianum, a drought-resistant invasive species, may thrive under a water-pulse regime that leaves other populations susceptible. Temperature and moisture changes in the research region are likely to boost the competitive ability of S. marianum, hence its invasiveness, which derives from even drier circumstances [56]. Controlled competition trials of S. marianum and native plants are required to properly forecast the future invasive potential of S. marianum.

The richness and composition of native plant communities are jeopardized and fragile, making the native endangered species particularly susceptible to extinction [57] due to invasive species. Their ability to affect local soils’ physical and chemical qualities is a significant aspect in disturbing the communities that mostly leads to homogenization [58]. Our findings support these generalizations, which show that S. marianum has a major influence on plant and soil ecosystems alike. With the rising IVI of S. marianum, the Shannon–Wiener diversity index, Margalef’s richness index, and evenness index decline; however, Simpson’s dominance index rises with the invasion gradient. Many other invaded parts of the globe have yet to experience the harmful ecological effects of S. marianum [59,60,61,62]. Physiochemical qualities differed significantly across locations that had been completely, substantially, or partly invaded. The soil’s build-up of secondary metabolites (especially phenolic acids) is reflected in increased acidity, electrical conductivity, and phosphorus content in strongly invaded locations [63]. S. marianum IVI may lead to increased decomposition residues and quicker decomposition rates in the soil, as reported by [64,65,66]. It has been found that decomposing plant residues alter soil properties and the biotic/abiotic composition of the soil [67,68,69] for Lantana camara L, Robinia pseudoacacia, Acacia spp., and Lonicera maackii (Rupr.) Maxim, as reported in [67,69]. Consequently, additional research is needed to confirm that S. marianum invasion has changed the characteristics of soils with high organic carbon content, which is consistent with the findings of this study.

5. Conclusions

• The findings show that S. marianum is found in low-elevation pure communities, and progressive spreading to higher elevations significantly influences local diversity and homogenizes the region.
• There was a clear correlation between the S. marianum importance value index and the diversity indices, with the latter decreasing as the former increased, and this is a possible hazard to the biodiversity of the native region.
• Communities’ responses were found to be complemented by changes in environmental variables. The results revealed that the nutrient concentrations in the severely invaded sites were more significant than those in the pure and partially invaded sites.
• The management and control of this species are thus necessary for conserving and maintaining natural vegetation.
• These findings open up many possibilities for further research. It can forecast not just the behavior of S. marianum but also the behavior of other invasive plant species with phylogenetic or morphological similarities with S. marianum.
• This research will be extremely important in increasing our understanding of the distribution, development, and spread of alien invasive species along various climatic gradients in observed and predicted fast climate change.