Effects of prosopis juliflora on plant diversity on rangeland in Shilabo District, Somali Regional State, Ethiopia

Abstract

Shilabo District of Somali Regional State in Ethiopia is economically important for livestock production. The indigenous pasturelands are increasingly being invaded by Prosopis juliflora, thereby threatening their livestock production capacity. However, the ecological impact of Prosopis' invasion has yet to be investigated. This study was conducted to examine the effects of Prosopis juliflora on plant species abundance, diversity, and occurrence. Vegetation samples were collected from three 20-m by 20-m sub-plots nested within 100-m by 100-m main plots that were purposefully selected in target study sites and replicated three times in both invaded and uninvaded areas. The study recorded 44 plant species, of which thirteen species occurred in both invaded and uninvaded areas, 13 species in invaded areas and 24 in uninvaded areas. The collected data were analyzed using the Statistical Package for Social Sciences (SPSS) version 26, and an independent t-test was used to determine the statistical significance difference between invaded and uninvaded areas in terms of number, density, important value index, diversity, abundance, evenness, and richness of plant species. These plant parameters were recorded in invaded and uninvaded areas with species numbers of 678.33 and 1763, densities of 226.00 and 366.98/ha, abundances of 254.04 and 409.45, important value indexes of 15.00 and 9.68, Shannon diversity of 1.56 and 3.40, evenness of 0.52, and 0.99, and richness of 1.30 and 1.35, respectively. The uninvaded area had a significantly higher species diversity index (3.40) than the invaded area (1.56; P 0.00). Similarly, the number of plant species, density, abundance, important value index, species evenness, and richness were significantly higher in uninvaded areas than invaded areas (p < 0.000).

1. Introduction

Prosopis trees have been widely planted in tropical Africa, Asia, and Australia for the production of fuel wood, fodder, and land rehabilitation [22,23]. Prosopis juliflora (S·W.) D.C. is one of the species that has been introduced, naturalized, and has now become invasive in numerous locations [19,22,25,28].

P. julifora is widely spread due to its prolific planting, seed distribution by livestock, wildlife, and water, persistent accumulation of large viable seeds in soil seed banks, and coppicing [20,25]. P. juliflora can grow into a tree or a shrub, depending on the surrounding environment and genetic makeup [3,22]. However, the shrub form dominates, generating dense thickets that are impenetrable and have a negative ecological impact on the invaded areas by impeding the movement of people and cattle [18].

P. juliflora, which poses a threat to agriculture and rangeland productivity, depletes water supplies, and displaces native plants and animals, is thought to have infected about 4 million hectares in Africa alone [31]. P. Juliflora invasion is estimated to reach 171,655.67 ha in Ethiopia, mainly in the Korahe zone, with trends showing a mean increase of 14,304.64 ha/year between 1989 and 2001 [1]. Prosopis has a variety of detrimental ecological effects, including a reduction in palatable native pasture plants and a threat to wild animals [4,8,13].

Prosopis has the potential to have good ecological consequences by minimizing soil erosion, enhancing soil fertility, and being useful for reclaiming moderately salty soils and degraded fields [27]. Additionally, it's critical to combat desertification in arid areas by storing carbon dioxide and reducing climate change [3,11]. In addition to its positive ecological impacts, P. Juliflora also has positive socioeconomic impacts. Prosopis' most significant socioeconomic impact is related to its replacement of pasture fields and native trees with high browsing values, which are the only sources of feed for the animals in pastoral communities [27].

The purpose of this study is to evaluate the occurrence and abundance of indigenous species in invaded and uninvaded areas to better understand the interactions that are frequently present between P. juliflora and native species in the Korahe zone of the Somali region.

Invaded areas in this zone were estimated to be 171,655.77 ha in 2001 [1]. However, there haven't been any invasion studies done in the area to date. Therefore, the purpose of this study was to investigate how P. juliflora affected the diversity of plants in the rangelands of Shilabo district, Somali Regional State, Ethiopia.

P. juliflora is one of the most threatening woody invasive trees and shrubs in the world [3,10]. P. juliflora has just come to light as the worst weed plaguing the pastoral and agro-pastoral populations in Kenya and the rest of eastern Africa [17,19]. P. juliflora reportedly occupies over one million hectares of Ethiopia at the moment [24] and aggressively displacing native tree species and reducing the amount of grazing space [8,26]. However, insufficient attention is paid to the abundance and diversity of plant species in this study area.

2. Materials and methods

2.1. Description of the study area

The Ethiopian Somali regional state has a land area of 340,000 km², making it the second-largest region in Ethiopia [30]. The total population of this district is 107,590, of whom 67,376 are males and 34,214 are females.

The largest ethnic group identified in the Korahe zone was the Somali people (99.22%), and their economy was mostly focused on the rearing of livestock [29]. The topography of the Shilabo district is predominantly lowland plain with an average altitude of 403 m above sea level and a few foothills of higher altitude. It has a latitude and longitude of 6° 5′ 0″ north and 44° 46′ 0″ east, respectively. The Shilabo district is found at a distance of 1118 km from Addis Ababa and 487.4 km from Jigjiga. Shilabo has a tropical semiarid climate with temperatures ranging from 32 to 35 °C. The area has a bimodal rainfall pattern with two main rainy seasons. The first rainy season occurs from mid-April to the end of June, and the second season occurs from early October to late December.

2.2. Sampling techniques and data collection

The invaded and uninvaded areas were purposefully selected as study sites based on the predominance of Prosopis juliflora. 100-m x 100-m (1 ha) main plots and three 20-x-20-m sub plots per plot were laid out in three replicates across both study sites (invaded and uninvaded). Thus, in total, 20 plots (10 plots from invaded areas and 10 plots from uninvaded areas) were laid out for the study area (Fig. 1). The distance between the main plots is 100 m.

Fig. 1.

A map of Somali Region showing the study area.

Note: Invaded and uninvaded areas are represented by red and black color dots, respectively. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)

2.3. Data collection and processing

In each sub-plot, data on all types of species (herbaceous, tree, shrub, and grass species) was collected, and the geographical location of sub-plots and their altitude were determined with a hand held Global Positioning System (GPS) and recorded. Collected plant data was processed to determine abundance, density [12], and frequency, which were calculated by the formula of [15] as follows:

$$Abundance\left(AB\right)=\frac{Totalno.ofindividualofthespecies}{No.ofquadrateperunitsinwhichtheyoccur}X100$$

$$Density\left(Den\right)=\frac{Totalno.ofindividualofthespecies}{unitarea}$$

Frequency, relative frequency, relative density, relative dominance, basal area, and important value index were calculated using the methods described by Ref. [2] as follows:

$$frequency\left(Freq\text{.}\right)=\frac{No.ofquadratsinwhichspeciesoccurred}{Totalnoofquadrantsstudied}X100$$

$$Relativefrequency\left(RFreq\text{.}\right)=\frac{Frequencyofoccurrenceofspecies}{Totalfrequencyofocurrenceofallspecies}X100$$

$$RelativeDensity\left(RDen\text{.}\right)=\frac{Numberofinvididualsofspecies}{Totalnumberofindividualsinallspecies}X100$$

$$RelativeDominance\left(RD\text{.}\right)=\frac{Totalbasalareaofaspecies}{Totalbasalareaofallspecies}X100$$

$BasalArea\left(BA\text{.}\right)=\pi D^2/4$ Where D = DBH

$$Importantvalueindex=RDen+RD+RF$$

Shannon diversity index and species evenness were calculated using the methods described by Ref. [21] where:

$$Shannondiversityindex\left(H\right)=-\sum _{k=0}^{n}PiLnPi$$

$Evennes\left(E\right)=\frac{H}{\mathit{Ln}\left(n\right)}$ Where n is number of kind of different plant species in study site

$Richness\left(E\right)=\frac{ni}{SQRT\left(N\right))}$, N is total number of species

2.4. Statistical data analysis

The collected data were analyzed using the Statistical Package for Social Sciences (SPSS) version 26. The differences in number, density, important value indexes, diversity, abundance, evenness, and richness of indigenous species between invaded and uninvaded areas were determined with a t-test at a 95% confidence interval.

3. Result and discussion

3.1. Species composition of invaded and uninvaded areas

A total of 2441 plants, representing 44 species from 19 families, were recorded in the study area (Table 1a; Table 1b). Mimosaceae, Burseraceae, Tiliaceae, Malvaceae, Gramineae, Fabaceae, Salvadoraceae, Solanaceae, and Rhammceae families had 498, 388, 273, 206, 158, 151, 102, 90, and 24 species, respectively (Fig. 2). Mimosaceae had the largest number (498) of species, while Rhammceae had the lowest (24) (Fig. 2).

Table 1a.

Number and % occurrence of Species occurring in both in invaded and un-invaded areas.

S.No	Species Name	Family Name	No of plants found	No of Species in % occurrence
1	Phionopsis rotundifolia	Asteraceae	70	10.62
2	Paspalidium deserotum	Gramineae	68	10.32
3	Abutilon fruticosum	Malvaceae	172	26.1
4	Acacia bussei	Mimosaceae	77	11.68
5	Acacia senegal	Mimosaceae	96	14.57
6	Solanumm elastomoides	Solanaceae	90	13.66
7	Grewia penicillata	Tiliaceae	86	13.05
Total			659	100

Table 1b.

Number and % occurrence of Species occurring in invaded areas.

S.No	Species Name	Family name	No of plants found	No of Species in % occurrence
1	Cadaba glandulosa	Burseraceae	34	7.80
2	Balanites galabra	Cucurbitaceae	30	6.88
3	Cucumis ficifolius	Cucurbitaceae	12	2.75
4	Prosopis Juliflora	Fabaceae	89	20.41
5	Ziziphus lotus	Gramineae	30	6.88
6	Grewia bicolour	Malvaceae	34	7.80
7	Acacia horrid	Mimosaceae	23	5.28
8	Acacia Nilotica	Mimosaceae	28	6.42
9	Grewia villosa	Rhammceae	24	5.50
10	Commiphora boiviniana	Tiliaceae	35	8.03
11	Crotalaria jijigensis thulin	Tiliaceae	33	7.57
12	Grewia ferruginea	Tiliaceae	29	6.65
13	Momordica sessifolia	Tiliaceae	35	8.03

Fig. 2.

Distribution of the species encountered in the study area among families.

3.2. Plant species in invaded and non invaded areas

There are 44 individual species in the study area (Table 1a, Table 1b; Table 1c). Out of 44 plant species recorded in the study, 7 species occurred in both invaded and uninvaded areas (Table 1a), 13 species were found in invaded areas (Table 1b), and 24 species were from uninvaded areas (Table 1c).

Table 1c.

Number and % occurrence of Species occurring uninvaded areas.

S.No	Species Name	Family Name	No of plants found	No of Species in % occurrence
1	Ruellia patula	Acanthaceae	43	3.19
2	Aerva javanica	Amaranthaceae	51	3.79
3	Balanites scillin	Balaniteceae;	51	3.79
4	Cordia gharaf	Boraginaceae	47	3.49
5	Commiphora boiviniana	Burseraceae	52	3.86
6	Commiphora borensis	Burseraceae	31	2.30
7	Commiphora gawlalo	Burseraceae	57	4.23
8	Commiphora kua	Burseraceae	76	5.65
9	Commiphora playfairi	Burseraceae	62	4.61
10	Commiphora rustrata	Burseraceae	76	5.65
11	Boscia coriacea	Capparaceae	71	5.27
12	Terminalia orbicularis	Combretaceae	59	4.38
13	Ipomoea donaldsonii	Convolvulaceae	61	4.53
14	Cordeauxia edulis hemsl	Fabaceae	62	4.61
15	Enneapogon elegans	Gramineae	60	4.46
16	Acacia benaderensis	Mimosaceae	60	4.46
17	Acacia edgeworthii	Mimosaceae	43	3.19
18	Acacia mellifera	Mimosaceae	72	5.35
19	Acacia nubica	Mimosaceae	52	3.86
20	Acacia tortilis	Mimosaceae	47	3.49
21	Sesamothamus rivea	Pedaliaceae	56	4.16
22	Dobera glabra	Salvadoraceae	58	4.31
23	Salvadora persica	Salvadoraceae	44	3.27
24	Grewia tenax	Tiliaceae	55	4.09

3.3. Plant species diversity indices, evenness, and richness

There are 678.33 average species in invaded areas. In uninvaded areas, there are 1763 average species (Table 2). The average density of species in invaded areas was 226/ha, and that of univaded areas was 366.98/ha (Table 2). The average density of the species in the uninvaded area (366.98 ha) was significantly (P < 0.000) greater than the density of species in the invaded area (226.00 ha). This was due to the competition for resources with native plants.

Table 2.

Species diversity indices in invaded and un-invaded areas.

Sites		Diversity parameters
	Nav	Den.	AB	IVI	H	SE	SR
Invaded	678.33	226.00	254.04	15.00	1.56	0.52	1.30
Un-invaded	1763.00	366.98	409.45	9.68	3.40	0.99	1.35
P	0.000	0.000	0.000	0.000	0.000	0.000	0.000

P: significance level at 5%; N_av: average number of species; Den: species density (number of species per hectare).

AB: species abundance; IVI: important value index; H: Shannon diversity index; SE: species evenness; and SR: species Richness.

[9] Stated that Prosopis juliflora can suppress the growth of grasses under its canopy and increase biodiversity by delaying seed germination and reducing plant growth in terms of roots, shoots, leaf area, stem diameter, and plant height. As a result, density was lower in invaded areas than un-invaded areas. It also suppresses biodiversity by computing both resources and the natural environment [16]. Similarly [14], stated that the density of plant species decreases as the density of Prosopis Juliflora increases.

Table 2 revealed that Shannon's diversity index was significantly lower in invaded (H’ = 1.56) than in uninvaded (H’ = 3.40) areas (P < 0.000). The result was in line with [16], who reported a lower Shannon diversity index in invaded areas than that of uninvaded areas. The findings were comparable to those reported by Ref. [14], who found that high species richness was recorded under non-Prosopis juliflora sites rather than areas with Prosopis juliflora in Gujarat, India.

The species evenness and richness of plant species in invaded and uninvaded areas were 0.52 and 0.99 and 1.30 and 1.35, respectively. This indicated that species richness and evenness were significantly higher in uninvaded areas than invaded areas (Table 2). This is due to the fact that Prosopis juliflora invasion might affect the growth of native plants, inhibit light penetration, and release allelopathic chemicals [5,7].

Un-invaded areas had significantly higher average densities (366.98) and abundances (409.45) than invaded areas (N = 678; Den = 226.00/ha; AB = 254.04) (Table 2). This is because un-invaded areas have a greater number of individual species than invaded areas.

The important value index (15.00) was significantly (p = 0.000) higher in invaded areas than that of un-invaded areas (9.68) (Table 2). The higher important value index showed that there are fewer species in invaded areas than un-invaded areas. This result is in line with the result of [6], who stated that high IVI was attributed to a few species, and these species are those that are well adapted to the high pressure of disturbance, natural and environmental factors, and the effect of local communities.

4. Conclusion

The findings suggested that if the P. juliflora invasion is not addressed, there is a risk of decimation of important fodder species those that are common in uninvaded areas but completely absent in invaded areas. As a result, immediate actions on invasion and its effects on plant diversity are required.

Author contribution statement

Dagnachew Bezaredie: Conceived and designed the experiments; Performed the experiments; Wrote the paper.Zawde Tadesse: Conceived and designed the experiments; Performed the experiments; Analyzed and interpreted the data; Contributed reagents, materials, analysis tools or data; Wrote the paper.Zemenu Tadesse: Conceived and designed the experiments; Contributed reagents, materials, analysis tools or data; Wrote the paper.

Funding statement

This research did not receive any specific grant from funding agencies in the public, commercial, or not-for-profit sectors.

Declaration of competing interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.