INTRODUCTION
Approximately 35% of the earth`s land surface is characterized as arid
to semi-arid region. Climatic conditions and grazing history are two important
factors affecting species composition and biodiversity in rangeland in
semi-arid ecosystems. Overgrazing by livestock has been considered as
a major plant cover, soil and microclimate degrading factor in woodlands
in south-western Australia (Yates et al., 2000). Overgrazing changes
vegetation structure and composition as a result of which some species
increase in abundance and others decrease. It is accepted that heavy grazing
can change the composition of plant communities (Landsberg et al.,
2002; Riginos and Hoffman, 2003). Previous studies have revealed that,
in traditional grazing ecosystems, grazing can reduce ecosystem diversity
of plants in poor soils (Anderson and Hoffman, 2007).
Overgrazing, especially around the watering points, causes destruction
of decreaser species (palatable species and mostly grasses), which these
species were replaced with low palatable grasses to unpalatable ruderal
and shrubs. Based on the studies of vegetation in semi-arid rangeland
of Damghan and Khorasan province (north-east of Iran), increase of ruderal
plant species and decrease of decreasers plant species were entirely obvious
toward the center of the critical areas, especially, in the permanent
residences of livestock and in the watering points (Hassani, 2001). Abundance
of watering points and shepherd activities are among the effective factors
for proper distribution of livestock in rangelands and consequently, decrease
of overgrazing and increase of grazing efficiency.
Environmental destruction decreases with distance from the watering points
in biosphere, a gradients of animal impact (Fernandez-Gimenez and Allen-Diaz,
2001). The results of studies in Los Croks, New Mexico, showed that watering
points had an important effect on livestock distribution in rangelands
with dry and salty plant species, but its effects is less important in
rangeland with fresh and succulent plant species (Valentine, 1947). Generally,
there are some concentric circles around the watering points, where by
increasing the distance from center forage utilization decrease (Klintenberg
and Verlinden, 2008; Sasaki et al., 2008). Therefore, awareness
of the distance from watering points and consumption value of forage is
necessary for rangeland management (Fusco et al., 1995). The results
of studies in South Australia showed that the numbers of palatable species
increased with an increase in distance from the watering points (Heshmati,
2002).
Iran`s climate is mostly arid and semi-arid, which the climatic conditions
lead to a wide variety of plant species. Despite climate conditions, overgrazing
is a main source of changes in soil and vegetation in Iran. Previous studies
were qualitative and no/little quantitative data exists on the impact
of livestock on vegetation and rangeland condition around watering points
in north east of Iran. Therefore, the objective of this study was to determine
the impacts of livestock on vegetation around watering points in this
region.
MATERIALS AND METHODS
Study site charactization: The study was conducted in semi-arid
rangeland of Northern Khorasan province in Iran in 2004. The area includes
4880 hectares of overgrazed rangeland. The area was south facing and general
slope varied between 2 to 20%. The elevation ranged between 1102 and 1345
m and annual rainfall was 235 mm. The study area had warm summer and cold
winter, which the maximum and minimum temperatures during the warmest
and coldest months were 36.67 and 4.5°C, respectively. The soil was
composed of evaporated sediments, plaster and lime salts with sandy to
sandy clay texture. Soil salinity in the northern part of the district
was higher than southern part, where, halophytes were found in the northern
part. The area experience gully and rill erosion, where these kinds of
erosion were more dominant toward the southern slops. The area included
grasslands ecosystems that were traditionally grazed by sheep and goats
for 6-7 months during summer. Percent of forage utilization for all species
was about 78%. Forage utilization was higher than allowable forage use,
where many of palatable species were not seen and some ruderal species
were dominant.
Sampling technique: In the critical area around watering points,
8 directions were defined: north, south, east, west, northeast, northwest,
southeast and southwest then transects (1000 m) were placed in each direction.
Sampling plots were established along each transect every 50 m systematically.
Data were collected from a total of 160 plots of 4 m2 along
the flowering period of dominant plants. Percentage of canopy cover, abundance,
density and species richness of the vegetation were evaluated in each
plot.
Statistical analysis: Percentage of canopy cover, abundance, density
and species richness were analyzed by using MINITAB and SPSS software.
The effects of distance from the center of the critical area on quantitative
parameters of vegetation were analyzed using one-way ANOVA. Mean values
were compared by Duncan`s multiple range test. Probability of normal distribution
of data was determined by using the Darling-Anderson Method in MINITAB
software.
Importance value of each species was calculated in various distances
from the center of the critical area by sum of relative dominance, relative
density and relative abundance of the species. Species associations were
analyzed using 2H2 contingency table and χ2 analysis.
RESULTS
Data analyses showed that percentage of canopy cover, density and species
richness, had normal distribution (Results were not shown). Average of
canopy cover was about 76.5%, where Artemisia herba-alba had the
highest with 22.31% and Peganum harmala had the lowest with
7.98%. Canopy cover of other species such as Acantholimon sp.,
Sophora sp., Hulthemia persica and Salsola sp., were
9.92, 8.15, 8.09 and 8.07% , respectively.
The results of the effects of distance from watering points on percentage
of canopy cover, density and species richness has been shown in Table
1. These results showed that distance from watering point had significant
effects on changes of percentage of canopy cover. Duncan`s multiple range
test showed that the average percentage of canopy cover in distances 50,
100, 150, 200, 250, 300 and 350 m from watering points are in the same
group and there is no significant difference between them. In other words,
350 m distance is considered as the critical area around the watering
points, where the most livestock intensity and overgrazing occurred in
this area. No other grouping was found after 350 m. These results showed
that distance from watering point had significant effects on changes of
plant density. Duncan`s multiple range test showed plant density was the
same in 200 m distance from watering points. Also, distance from watering
point had significant effects on species richness, but average of species
richness in various distances was not in a group.
| Table 1: |
p-values generated from ANOVA for percent canopy cover,
density and species richness |
 |
| Table 2: |
Associations among the 6 species in a 2H2 contingency
table |
 |
| **Significant at the 0.01 probability level, ns: Not
significant at p = 0.01, +: Positive association between different
species, -: Negative association between different species |
|
| Fig. 1: |
The effects of distance from watering points on plant
diversity of Peganum harmala (peg), Sophora sp.
(Soph) and Artemisia herba-alba (Art) |
In all directions close to the center of the critical area, abundance
of ruderal species such as Peganum harmala was more than other
species up to 500 m. Sophora sp. was accompanied with Peganum
harmala in the same distance, even after 500 m (Fig.
1). The average of importance value of Peganum harmala and
Sophora sp. in all plots was 28.53 and 9.96%, respectively.
The most relative importance of species was related to Artemisia herba-alba
with average of 34.56%, which was found at 500-1000 m distance from the
center of the critical area. The most species accompanied with Artemisia
sp. were Hulthemia persica and Acantholimon sp., which
are almost not observed among 1-500 m distance. The relative importance
of Hulthemia persica and Acantholimon sp. was 6.05 and 4.54%,
respectively.
|
| Fig. 2: |
Presence of Peganum harmala (peg), Sophora
sp. (Soph) and Artemisia herba-alba (Art) in different
geographical direction with distance from watering points |
Plant species were appeared in different distance from watering points
in different directions (Fig. 2). Peganum harmala
was appeared at 50 m distance from watering point in northeast and southeast
directions, while it was appeared in northwest direction at 200 m distance
from watering point.
Positive or negative association between different species is shown in
Table 2. The values of χ calculated for pair species
as Artemisia herba-alba-Acantholimon, Hulthemia-Sophora, Salsola-Acantholimon
and Peganum harmala-Acantholimon were 6.024, 5.114, 5.282 and 6.57
, respectively. These values are larger than the value of χ (p =
0.01). It means that occurrence of these pair species is not accidentally.
DISCUSSION
The results of this study showed that the interactions between animals
and watering points lead to changes in vegetation composition, where overgrazing
around watering points changed percentage of canopy cover, density and
species richness. High stocking rates or overgrazing in fragile ecosystems
(arid or very saline) can decrease plant diversity (Milchunas et al.,
1988; Hobbs and Huenneke, 1992; Olff and Ritchie, 1998). Brits et al.
(2002) showed a great impact of utilization near the watering points and
decreasing pressure as distance away from the watering points in Kruger
National Park, where, almost complete lack of woody individuals were found
in the immediate vicinity of the watering points, then woody plants density
increased with distance from the watering point. In another study in woodlands
of south-western Australia the same results were found where higher grazing
pressure of livestock around watering points was associated with a decline
in perennial and increase of annual cover (Yates et al., 2000).
Results of this study showed that the importance value of unpalatable
species such as Peganum harmala and Sophora was increased
in critical areas around watering points, but more palatable species such
as, Artemisia herba-alba had higher importance value in distance
from watering points (Fig. 1). Presence of unpalatable
and absence of palatable species in critical areas were reported in other
studies (Heshmati, 2002). Overgrazing, lead to absence of Artemisia
herba-alba which the most likely mechanism leading to the decline
of such species should be of the failure of reproductive organs. Todd
(2006) indicated that areas most distant from watering points contained
a greater proportion of species known to be highly palatable to livestock
in Karoo shrublands, South Africa.
Different layers of overgrazing were found in different geographical
directions (Fig. 2). Peganum harmala as the most
tolerant species to overgrazing were appeared in northeast direction more
closer to watering point than northwest direction. It may show higher
pressure of livestock on vegetation and soil in northwest, where distribution
of watering points was more abundant than other directions. These results
are in agreement with some other studies (Hassani, 2001; Jafari et
al., 2008).
Species association analyses (Table 2) show that there
were 9 positive or negative associations among 15 associations. Overall
the plant species association indicates that nearly all associations were
occur due to the intense alterations in the ecosystem. It is concluded
that some factors such as overgrazing or high stocking rate are responsible
for these patterns. Heshmati (2002) were reported the same results in
shrublands of south Australia.
The results of the Duncan`s multiple range test for percentage of canopy
cover, density and species richness show that in distance from watering
points to 350 m there were no significant differences in canopy cover,
also the average of plant density had no significant difference to the
200 m distance from watering points and was located in a same group. But
the average of species richness was different in various distances from
the watering points and wasn`t located in a same group. It is speculated
that percentage of canopy cover and plant density were shown a circle
of critical area around watering points with radius of 200-350 m, but
species richness wasn`t a suitable parameter to show the extent of critical
area around watering points.
Based upon the results of this study, it is conclude that the destructive
impact of overgrazing on vegetation around watering points in the semi-arid
rangeland of northeast of Iran was significant. Because of contribution
of watering points to deterioration of semi-arid rangeland and changes
in vegetation composition, more consider is suggested on distribution
and management of watering points in this fragile regions.
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