Changes in plant species composition and diversity along a grazing gradient from livestock watering point in Allaidege rangeland of North-eastern Ethiopia rangelands

Asheber Tegegn, Lisanwork Nigatu and Amaha Kassahun

Ethiopian Institute of Agricultural Research (EIAR), Melkassa Agricultural Research Center,
P.O. Box 436, Nazareth, Ethiopia
ashebertegegn@gmail.com   and   amahakassahun@yahoo.com

Abstract

The Allaidege open wet season grassland was reduced in to all season grazing land due to expansion of large scale irrigated crop agriculture. Development of watering points farther limited free livestock mobility and caused over utilization of the rangeland resources. In this study, four grazing intensities, namely: severely grazed, moderately to severely grazed , moderately grazed and lightly grazed sites, each with 10 sampling units were laid out along a 12.5 km gradient from a livestock watering point. Five 1 m² quadrants were randomly assigned per sampling unit, and the herbaceous layer was harvested. Accordingly, effects of watering point on plant species composition, spatial distribution and diversity were investigated.

Species composition and spatial distribution of the herbaceous layers were highly influenced by levels in grazing intensities. Moreover, reductions in grazing intensity, with increasing distance from watering point, highly favoured Decreasers (highly desirable perennial grass species), whereas, heavy grazing pressures at decreasing distances from watering point favoured Increaser species (less desirable forbs and annual grass). In terms of preference by livestock classes, most perennial grasses species (Decreasers) were highly desirable by grazers (cattle and sheep), while annuals and forbs (Increasers) were highly desired by browsers (goats and camels). Species diversity decreased in areas close to and remote from watering point. This finding suggests that the current grazing management has a negative impact on the rangeland resource and needs improvements.

Key words: Botanical composition, Decreaser, Grazing intensity, Herbaceous layer, Increaser, Livestock preference

Introduction

In arid and semi-arid rangelands, uneven grazing intensity associated with livestock watering points has effects both on vegetation and the physical environments (Todd 2006). This induces over utilization of rangeland resources (Pringle and Landsberge 2004), permanent degradation (Kidane 2005) and losses in vegetation biodiversity (Brooks et al 2006) in rangeland areas around watering points. Effects on physical environments include: depletion of soil nutrients (Brook et al 2003; Kidane 2005), diminish in nutrient recycling (Pringle and Landsberge 2004), increases in soil compaction (Brooks et al 2006) and limitations in infiltration (Trash 1997; Risch et al 2007).

High grazing pressure, around watering points, also disturbs floristic composition (Landsberg et al 2003), spatial distribution (Metzger et al 2005) and diversity of herbaceous layers (Brooks et al 2006). The situation further favour annuals (Klintenberg and Verlinden 2008) and unpalatable species (Todd 2006) than perennial grasses (Brook et al 2003 and Kidane 2005), enhance bush encroachments (Ludwig 2004), reduction in vegetation cover (Fahnestock and Detling 1999) and shrinkage of grazing lands (Bastin and James 2002), resulting declines of range forage/fodder production (Fusco et al 1995; Macopiyo 2005). To the contrary, feed mass production increased from grazing areas with increased distances from watering points (Foran 1980; James et al 1999), may be due to under utilization of the forage resources (James et al 1999).

Traditionally, the Allaidege open grassland was best used as wet season grazing land, with animals having access to surface water resources under free mobility. However, this trend was interrupted since the 1970^th, due to the expansion of irrigated large commercial scale crop agriculture (Kidane 2005). This has, thereof, limited free livestock mobility and the access to surface water resources, posing livestock and human pressures, which farther aggravated over utilization of the range bio-physical resources around watering points (Kidane 2005). On the other hand, there are too little studies under taken to assess the impacts of watering points on biophysical resources of the Ethiopian rangelands in general and the Allaidege open grassland in particular. Moreover, there exists too little information to understand the long term impacts of grazing around livestock watering points. As a result, there exists gap in knowledge to promote sustainable rangeland management strategies.

Therefore, generation of scientific information still remains to be critical in order to design and promote appropriate development interventions and management systems. Therefore, the purpose of this study was to generate information on: i) floristic composition, ii) spatial distribution, iii) desirability by livestock species vi) species richness and diversity of the herbaceous layers along the grazing gradient from a watering point, and to come up with appropriate recommendations.

Materials and Methods

Description of the study Area

The study was conducted in the semi-arid Alaidege open grassland, located at 9°35'17.170''N and 40°11'4.161''E, representing a total of 200 000 hectare, with an altitude of 820 to 840 m.a.s.l.. Mean annual minimum to maximum temperature and rainfall range between 25°C and 40°C and 300 and 560 mm, respectively. Rainfall is bimodal with main rains in July to August and short rains in March and April (WARC 2008). The vegetation types are dominated by herbaceous layers mainly Chrysopogon plumulosus, Panicum maximum as well as scattered woody plants such as Acacia nubica and Prosopis juliflora (Kidane 2005).

Site selection

A reconnaissance survey was conducted to assess, identify and select an appropriate experimental site using subjective (Van Zyl 1986) and objective (Hardy et al 1999) methods with participation of local elders and range managers. Selection criterion included: availability of watering point, grassland representation, presence of a grazing gradient, similarity in slope and soil types and pastoral livestock production systems as described by Manneteje and Jones (2000) and Reed and Dougill (2002).

Field layout and sampling techniques

The experimental site was laid out into four plots (treatments) along a 12.5 km grazing gradient from a livestock watering point. These included: i) severely grazed (SG), ii) moderately to severely grazed (MSG), iii) moderately grazed (MG) and vi) lightly grazed (LG) sites as described by Kidane (2005) and Amaha (2008). Farther, each plot was divided into ten sampling units (replicates) of 50 X 50m (0.0025km²) belt transect each. Accordingly, 5 quadrants of 1m² were randomly thrown per sampling unit.

Data Collection

The herbaceous layer in a quadrant was harvested, segregated in to their plant forms and thereafter into different species, fresh weight was taken for each species using sensitive scale and sample of each species was put in paper gabs. Species identification was done in the field by an experienced botanist using CADU (1974) guideline, while vernacular names were determined by local community members. Each species was grouped in to its relevant ecological index (i.e. Increasers, Decreasers and Invaders) as described by Tainton (1982). Desirability of each species by livestock classes was categorized based on indigenous knowledge of local pastoralists. Samples of each species were subjected to an oven at 105°C for 24 hours and percent dry matter biomass contribution calculated for each species. Species occurrence was expressed in terms of not present, present, common and dominant using values 0, <5, 5-20 and >20 percent dry matter biomass, respectively (Amsalu and Baars 2002)

Data analysis

Multivariate of correspondence and cluster analysis were applied to assess the individual and community level species response to a grazing gradient. A combination of ordination and dendogram of clustering were used to classify and group the commonly and dominantly occurred plant species with an increasing, decreasing and no change patterns in abundance at high and low grazing pressure, respectively. Averaged percent dry matter biomass of each species within the group was used to determine the community dominance of each group at each study site.

The following three indices were applied to estimate species diversity (Pielou 1969):

i) Plant richness represented by the number of species recorded in each quadrant;

ii) Shannon diversity index,

iii) Evenness index,

Where: P_i = relative importance of species i, S = plant richness

ANOVA was applied to investigate the relationship between grazing response variables against grazing gradients. SAS (1999) was used as software package

Results and Discussion

Botanical composition and related parameters of the herbaceous layers

Botanical composition of the herbaceous layer was consisted of 21 species, which included species of 14 grasses and 7 forbs, contributing 67% and 33% of the total herbaceous vegetation composition, respectively. In terms of the grass life forms, 50% were annuals and 50% perennials. In terms of their ecological index, 5(23.8%), 1(4.76%), 7(33.3%) and 8(38.1%) species were Decreasers, Increaser IIa, Increaser IIb and Increaser IIc, respectively. On the other hand, 6 grass species (28.6%) were highly desirable, 6 grass species (28.6%) were desirable, and 4 species (14.3%) were less desirable and 5species (23.8%) undesirable (Table 1).

Based on percent dry matter biomass of the herbaceous species a variation was observed in terms of occurrence along the grazing gradient. Accordingly, 5 (23.8%) were absent in the severely grazed site, while 3 (14.1%) each in the moderate to severely and moderately grazed sites were recorded as absent, whereas only 4 (19%) species were absent in the lightly grazed site. The number of species recorded as present were 9 (42.8%) in the severely grazed, 11(52.3%) each in the moderate to severely and lightly grazed sites and 12(57.1%) in the moderately grazed site. Further, 28.6% of species were commonly present in each of the severely and moderately to severely grazed sites, while 24% were also commonly present in each of the moderately and lightly grazed sites. Only one species in each of the grazing sites was found to be dominant.

According to the findings of this study, botanical composition of the herbaceous plants along the grazing gradient showed significant variations in terms of decreases in perennial species regarded as highly desirable and categorized as Decreasers in those sites close to watering point, may be due to increased pressures from livestock grazing.

Generally, there was a decline in Decreasers and an increase in Increasers along the gradient from lightly to severely grazed sites. This was in agreement to reports on the Borana rangelands (Gemedo et al 2006; Solomon et al 2006), the Central Rift Valley rangelands (Abule et al 2005), the Allaidegei rangelands (Kidane 2005) and Shinille grasslands of the Somali region of Ethiopia (Amaha 2008).

On the other hand, the proportion of highly desirable species decreased with high grazing pressure, while the abundance of herbaceous species with lower forage values tended to increase, which was in agreement to reports by Vallentine (1990) and Ameha (2008). It is therefore, likely that the abundance of species with low forage values increased as a result of livestock grazing at the expense of palatable and less grazing tolerant species (Todd 2006).

Furthermore, the results showed the abundance of forbs and annual grass species increased along the gradient from lightly to severely grazed sites, which supports the report by Fusco et al (1995); brooks et al (2006) and Todd (2006). In relation to this also, it was reported that the negative impacts of grazing on perennial grass species were greater in areas close to watering points because of livestock concentrations and trampling (Brooks et al 2006; Fongorana 2006 and Lishan 2007).

Species preference by classes of livestock

According to indigenous knowledge of pastoralists, the primary desirability by cattle and sheep was skewed towards perennials followed by annual grass species, while the forbs were highly desirable and desirable by camels and goats, respectively. For example, two perennials and four annuals, two annuals, one perennial and two forbs and three forbs were highly desirable, desirable, less desirable and undesirable by cattle, respectively. In view of sheep preference, four perennials and one annual grasses were highly desirable, three perennials and two annuals desirable, one annual and two forbs less desirable and one annual and three forbs undesirable. In relation to goats, two annuals and five forbs were desirable, two perennials and two annuals were less desirable, while two perennials and one annual were undesirable. For the camels, five forbs were highly desirable, two annuals and one forb desirable, three perennials and two annuals less desirable and only one annual undesirable (Table 2).

In terms of the interactive preference among the four livestock classes, only two annuals were highly desirable, other two annuals were desirable and one perennial was less desirable by cattle and sheep. But only one annual was desirable by cattle and goats, whereas one perennial was highly desirable and another perennial desirable by cattle and camels, respectively. In terms of the common preferences by goats and camels, one perennial, one perennial and two annuals were described as only desirable and desirable, respectively.

According to the analysis, however, neither the ruminants (cattle and sheep) nor the browsers (camels and goats) showed an equal preference for all perennial and annual grasses. In relation to this, sheep and cattle showed higher desirability for perennial grasses, except that sheep showed more preference for forbs than cattle. With regard to camels and goats, the goats showed similar preference in terms of the annuals, perennials and forbs, while camels were inclined to perennial and annual grasses than forbs. In general, most perennial and partly annual grasses were within highly desirable and desirable preference indices for cattle and sheep, while less desirable for camels and goats, but observed to be less desirable by camels and goats. On the other hand, most of forb species were within the category of highly desirable to desirable for camels and goats, and less desirable for cattle and sheep. From this context and given the higher cattle and sheep population in the area, therefore, the results indicate that, the perennial grass species are subjected to heavy grazing with subsequent opportunities to be replaced by Increaser plant species of low palatability and feeding values, which demand for well designed and systematic grazing management around watering points.

Patterns in composition of individual and community level of the herbaceous species

Two dimensions (Dimension 1 and 2) of correspondence analysis, yielded an eigen value of 180.1, which in aggregate explained about 99% of total variation (Figure 1), explaining a high association between the grazing intensities and the type of herbaceous species. In terms of individual species responses to the grazing intensities along the gazing gradient, most of the annual grasses and forbs on the left of the ordination belonged to severely grazed site. Where as most of the highly desirable and perennial grasses on the right side of the ordination belonged to the moderately and lightly grazed sites. Besides, the species at the mid of the positive side of ordination, mainly: Ericholea nubica, Ipomoea and Sporobolus ioclados were found adjacent to areas where livestock grazing pressure was moderate (Figure 1).

Figure 1: Ordination of common (% DM biomass >5% and < 15%) and dominant (% DM biomass >15%) species along the degradation gradient in the study area. (Where: Chr-plumulosus= Chrysopogon plumulosus ; P-coloratum =Panicum coloratum; Te-tenellus =Tetrapogon tenellus; Sp-ioclados =Sporobolus ioclados; Er-nubica =Eriochloa nubica; Er-procera =Eriochloa procera; Se-verticillata =Setaria verticillata; S-pumila =Setaria pumila; Tr-racemosus =Tragus racemosus ; D-aegyptium =Dactyloctenium aegyptium ; Ipomoea =Ipomoea spp.; Le-martinicensis =Leucas martinicensis; Phy-maderaspa =Phyllanthus maderaspa; Ama-dubius =Amaranthus dubius and Tri-terrestris =Tribulus terrestris)

In terms of community level responses of the plant species, combination of the ordination and cluster analysis, the herbaceous species were aggregated in to four community groups, each having similar ecological niches, that is, soil nutrient and moisture requirements (Fig. 1 and 2). The community Group 2 consisted of annual grasses and forbs, found closer to the watering point, showed a higher response to heavy grazing pressure and trampling. As opposed to Group 2, the annual grass species namely Setaria verticillata behaved differently due to it’s abundance between the severe to moderate and severely grazed sites (Group 4).On the other hand, Group 1, mainly perennial grasses, showed a higher abundance in those grazing sites remote from the watering point. Similarly, Plant community Group 3, specifically dominated by Ericholea nubica, Ipomoea and Sporobolus ioclados showed increasing patterns in abundance at moderate grazing intensity (Figure 2).

Figure 2: Cluster analysis of average linkage method and average distance between clusters

On the other hand, the average percent DM biomass contribution of the herbaceous species within the community or cluster groups in the four grazing gradients was affected by the different levels of grazing intensities. Accordingly, cluster group 1and 3 showed a remarkable increase as the level of grazing pressure decreased. In contrast, cluster group 2 showed a declining contribution as grazing pressure increased. Whereas group 4, behaved differently by contributing the highest percent DM biomass under moderate to severe grazing pressure with decreasing patterns towards the severely and lightly grazed sites (Table 3).

Generally, heavy grazing around livestock watering points was observed to encourage the succession of annuals and unpalatable species over palatable perennial species. Likewise, Metzger et al (2005) has also made similar conclusions. The findings further revealed that, the abundance of highly desirable indigenous species in the study area increased under moderately to lightly grazing pressure, where as those annuals and less palatable herbaceous species seem to be favoured by severe grazing pressures. The later also has a direct or indirect impact on the percent DM biomass contribution along the grazing gradient. This supports the findings by Harrison (2000), Del-Val and Crawley (2005) and Klintenberg and Verlinden (2008). In relation also, the pastoral elders reported, that the study area was being dominated by perennial species before some decades, but over taken by annual species over time.

This further may suggest that the current grazing management results a negative impact on the rangeland vegetation composition and seems to demand improved grazing systems with well designed distribution of watering points. Thus, range degradation has been taking place because of over grazing, which may have altered the ecosystem in favour of the annuals species and vice versa. From this context, it can be suggested that, the individual species or community groups show a difference in abundance due to their ecological niche that encourage them for certain dominance in competition for soil nutrients and moisture regimes. Therefore, high grazing intensities around watering points not only disturb the physical environment but also alter the botanical composition of the herbaceous layers, either by increasing species tolerant to heavy grazing or by reducing species regarded as highly desirable, in agreement to Harrison (2000), Metzger et al (2005) and Wu et al (2008). As a result, those individual species aggregating around similar habitats or that regrouped them selves into community groups could be explained as plant species that require similar ecological niche in terms of soil type, moisture regime, land escape and level of grazing responses.

Species richness and diversity along the grazing gradients

Species richness defined in terms of number of plant species per unit area, was influenced by the level of grazing intensity along the grazing gradient. Farther also species evenness and dominance were affected by levels in grazing intensities along the gradient from livestock watering point. Accordingly, species richness was significantly higher in the moderate to severely and moderately grazed sites, while lightly grazed site showed a higher species richness than severely grazed site at p<0.05. However, the higher grazing pressure significantly reduced the species evenness and Shannon's index in the severely grazed site, nevertheless, there was no a significant difference among the other three levels of grazing intensities at P>0.05 (Table 4).

In conclusion, the results imply that heavy and light grazing pressure reduced the species diversity, which was supported by findings of (Willoughby 1995) and (Milgo 2006). Farther, heavy grazing pressures decreased the number of less grazing resistant species and increased those grazing tolerant species, but reduced the diversity. Likewise, low grazing pressures enhanced the dominance of some species by reducing the diversity, as reported by (Willoughby and Alexander 2007). In the moderate grazing pressure, the species diversity was generally maximized, and may be induced due to a favourable macro-habitat for both heavy grazing tolerant and susceptible species to harmoniously survive, perform and reproduce (Grime 1973).

Conclusions

• Botanical compositions of the herbaceous layers were highly affected by the levels of grazing intensities along the gradient from livestock watering point. In distances closer to watering point, where grazing pressure were severe to moderately sever, forbs and annual grass species (Increasers) showed abundance in dominance, while those perennial grass species (Decreasers) were highly favoured by the moderate to lightly grazing pressures away from livestock watering point.

  
  

• From livestock preference point of view, the Decreasers were highly palatable and more preferred by the grazers (cattle and sheep), while the browsers (camels and goats) showed more preference towards the Increaser species. 

  
  

• Both extremes of sever and light grazing pressure reduced species diversity of herbaceous species, while moderate grazing pressure favoured the coexistence of annual and perennial herbaceous species, hence maximized the diversity. 

  
  

• Therefore, it can be suggested that, under pastoral extended livestock production systems, livestock watering points need to be planned, designed and utilized to enable sustainable range resource management.

Acknowledgements

The authors acknowledge the funding support of the Institute of Pastoral and Agropastoral Study of Haramaya University. The authors are also grateful the support provided by Ethiopian Institute of Agricultural Research.

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Received 14 February 2011; Accepted 10 August 2011; Published 1 September 2011

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