Termite effects on soils and plants are generally consistent along a gradient in livestock grazing

Abstract

Livestock grazing is a major driver of ecosystem functions in drylands and would be expected to influence soil biota such as termites. We examined changes in soil chemistry and plant community composition on mounds constructed by the subterranean termite Anacanthotermes ahngerianus along a gradient in grazing intensity in an arid steppe in north-eastern Iran. The grazing gradient was represented by increasing distance from an area used by resting livestock, and plant and soil attributes measured within three adjacent microsites (termite mounds, non-mound controls, intervening annular zone surrounding the mounds). Values of soil EC; pH; exchangeable Ca, Mg, and Na; and total nitrogen and organic carbon were greatest in mound soils and declined from mounds to control microsites. Mounds were completely devoid of plants. Annular zones had three-times less cover than the control sites, but there were no differences in diversity or evenness. Electrical conductivity values were ten-times greater on mounds than controls close to resting sites, but the difference diminished rapidly with distance from resting sites. For all other soil and plant variables, differences between microsites were consistent across the grazing gradient. Increased grazing intensity was associated with increasing soil pH, EC and sand content, and reduced plant cover. Overall our study shows that the effects of termites on soil chemistry and plant cover varied little across the grazing gradient. Our results suggest that termite mounds may sustain their role as sites of enhanced soil nutrients under even high levels of grazing.

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Termite effects on soils and plants are generally
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Mahsa Fallah, Mohammad Farzam, Vahid Hosseini, Gholamhossein Moravej
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ARID LAND RESEARCH AND MANAGEMENT
http://dx.doi.org/10.1080/15324982.2017.1288177
Termite effects on soils and plants are generally consistent
along a gradient in livestock grazing
Mahsa Fallaha, Mohammad Farzama, Vahid Hosseinib, Gholamhossein Moravejc, and
D. J. Eldridged
aRange & Watershed Management, Ferdowsi University of Mashhad, Mashhad, Iran; bForestry, University of
Kurdistan, Sanandaj, Iran; cPlant Protection, Ferdowsi University of Mashhad, Mashhad, Iran; dSchool of BEES,
University of NSW, Kensington, Australia
ABSTRACT
Livestock grazing is a major driver of ecosystem functions in drylands
and would be expected to influence soil biota such as termites.
We examined changes in soil chemistry and plant community
composition on mounds constructed by the subterranean termite
Anacanthotermes ahngerianus along a gradient in grazing intensity in
an arid steppe in north-eastern Iran. The grazing gradient was
represented by increasing distance from an area used by resting
livestock, and plant and soil attributes measured within three adjacent
microsites (termite mounds, non-mound controls, intervening annular
zone surrounding the mounds). Values of soil EC; pH; exchangeable
Ca, Mg, and Na; and total nitrogen and organic carbon were greatest
in mound soils and declined from mounds to control microsites.
Mounds were completely devoid of plants. Annular zones had three-
times less cover than the control sites, but there were no differences in
diversity or evenness. Electrical conductivity values were ten-times
greater on mounds than controls close to resting sites, but the
difference diminished rapidly with distance from resting sites. For all
other soil and plant variables, differences between microsites were
consistent across the grazing gradient. Increased grazing intensity was
associated with increasing soil pH, EC and sand content, and reduced
plant cover. Overall our study shows that the effects of termites on soil
chemistry and plant cover varied little across the grazing gradient. Our
results suggest that termite mounds may sustain their role as sites of
enhanced soil nutrients under even high levels of grazing.
ARTICLE HISTORY
Received 20 August 2016
Accepted 25 January 2017
KEYWORDS
Degradation; disturbance;
rangelands; sheep; soil
chemistry; soil organic
matter
Introduction
Livestock grazing is a major disturbance in drylands (arid, semi-arid and dry subhumid
environments), which occupy almost 40% of Earth’s land surface and support a similar
percentage of its human population. Globally, livestock grazing provides many cultures
with meat, milk, and transport, yet overgrazing has resulted in extreme poverty and hard-
ship and substantial negative effects on ecosystems (Steinfeld et al. 2006). In drylands
the effects of grazing are most strongly felt around watering points and resting areas
where livestock congregate and where their densities are greatest (Jankju Borzelabad
2008). Gradients in grazing become apparent where livestock move from watering points
CONTACT Mohammad Farzam mjankju@um.ac.ir Range & Watershed Management, Ferdowsi University of
Mashhad, Mashhad, Iran.
© 2017 Taylor & Francis

and resting areas to graze distant pastures. The grazing effect emanating from water is
termed the piosphere effect (Lange 1969), and studies along piospheres indicate sharp
reductions in livestock utilization with increasing distance from water (Landsberg et al.
2003). A similar phenomenon occurs in areas where livestock are corralled (Reid and Ellis
1995).
High levels of livestock grazing have been shown to influence soil physical and chemical
properties. Specifically, increases in livestock intensity are known to reduce plant compo-
sition (Milchunas and Lauenroth 1993) and have marked effects on soil cations and anions
and soil fertility (Eldridge et al. 2016). Perhaps the greatest effect of livestock grazing is
herbivory, the removal and consumption of plant biomass (Eldridge et al. 2016). However,
herbivory by invertebrates may also be substantial. Termites, for example, consume sub-
stantial quantities of plant material and are considered one of the most important insect
taxa in drylands because they are both herbivores and detritivores (Coventry, Holt, and
Sinclair 1988). Although termites could be seen as competing directly with livestock for
plant biomass, they could also indirectly enhance plant growth for vertebrate herbivores
through their positive effects on surface soils (Brody et al. 2010). Termites have been shown
to have profound positive impacts on ecosystem structure, composition, and function.
They enhance soil physical and chemical properties (Holt 1987; Holt and Lepage 2000),
increase soil porosity and therefore infiltration by constructing large macropores in the soil
(James et al. 2008), influence soil organic matter and nutrient cycling (Whitford and
Eldridge 2013), and are important prey items for reptiles small mammals (Redford and
Dorea 1984). Dramatic effects of livestock on vegetation and soil, particularly close to water
or livestock resting sites, would also be expected to influence other herbivores such as
termites.
We examined the effects of subterranean termites along a gradient in livestock grazing
intensity to ask whether termite effects on plants and soils differ across gradients in livestock
grazing. We used distance from livestock resting points as our surrogate for grazing inten-
sity, examining plants and soils on and off the mounds of subterranean termites at increas-
ing distance from resting points in an arid steppe rangeland in north-eastern Iran that is
under intense livestock grazing pressure. This rangeland is extensively colonized by the
subterranean termite Anacanthotermes ahngerianus Jacobson (Isoptera: Hodotermitidae),
which builds large surface pavements up to 3 m in diameter that generally support less
vegetation than the surrounding areas. The presence of these relatively large vegetation-free
patches suggests that termites are contributing to large-scale degradation through their
effects on surface soils. Our aim was twofold: to determine (1) the effects of termite mounds
on plants and soils and (2) whether these effects changed in relation to increasing grazing
pressure. This is important because livestock and termites are assumed to be the main
causes of land degradation and desertification in the Shorlogh rangelands, yet the combined
effects of livestock and termites on plants and soils have rarely been considered.
Methods
The study area
This research was conducted in the Shorlogh rangelands in northeastern Iran (36.32°N,
60.65E) about 578 m above sea level. The climate is cold arid and dry, the average
2 M. FALLAH ET AL.

annual temperature is 14.4°C, and average annual rainfall 202 mm (Ghayorifar and
Khalily 2005). The vegetation is dominated by ephemeral species such as Poa bulbosa
L. and Bromus tectorum L. during late winter and early spring. These species are
replaced by perennial forbs such as Diarthron vesiculosum Jaub. and Spach., Peganum
harmala L., Alhagi maurorum Medic. and Sophora secundiflora (Ortega) DC) during
summer. Soils are classified as aridisols, with their texture being sandy clay loam.
Soil depth varies from 80 to 120 cm and is limited by a calcareous hardpan (Beheshti
2015).
Soil, vegetation and termite sampling
At one livestock resting point we established three transects extending up to 720 m in
north-south, west-east, and northeast-northwesterly directions. All mounds constructed
by the subterranean termite Anacanthotermes ahngerianus within 5 m of each transect
were identified, and ten in each transect randomly selected for detailed plant and soil
measurements. At each of these thirty termite mounds we collected soil from the top
10 cm of the profile within three microsites: (1) termite mounds (hereafter ‘mound’),
(2) the zone immediately surrounding the mounds (hereafter “annular zone”) and (3) a
control, non-mound area that was located about 2–5 m from the edge of the mound away
from any influence of termites (hereafter “control”). The annular zone is the area immedi-
ately surrounding the mound, which has been shown to have higher levels of soil nutrients
and infiltration than the non-mound control areas (Eldridge 1994). Soil texture was
measured using the hydrometer method (Gee and Bauder 1985), and Na, Mg, Ca, P,
and K assessed using atomic absorption spectrophotometry. Total nitrogen (N) and carbon
(C) were measured using the Kjeldahl and Walkley-Black methods. Electrical conductivity
(EC) and pH were measured on a 1:5 soil:water extract with an electrical conductivity
meter.
In June 2014 we sampled plants within 2 m 1.5 m quadrats at a total of ten positions
on the three transects (3–4 mounds per transect) at distances ranging from 300 m to 720 m
from the livestock resting point. At each position, one quadrat was sampled in the annular
zone and the other in the control, resulting in a total of twenty quadrats. Sampling was
carried out when the dominant perennial plants (e.g., Diarthron vesiculosum, Peganum
harmala and Sophora secundiflora were at the flowering stage. We assessed total vegetation
cover and density of each plant species. We then calculated plant diversity (Shannon’s
Diversity Index), plant richness (Menhinick Index) and plant evenness (Smith and Wilson
Index; Ejtehadi, Sepehri, and Akkafi 2009).
Statistical analyses
One-way ANOVA was used to examine potential differences in exchangeable Na, K, Mg,
Ca, P, total N, electrical conductivity (EC), pH, and organic carbon (OC) among the three
microsites (mound, annular zone, control) and plant attributes between the annular zone
and control, and Tukey post-hoc tests used to determine which microsite differed signifi-
cantly. We then used regression analyses to test for the effects of livestock grazing intensity
on EC, pH, sand, silt, clay content, vegetation cover, plant richness, diversity, and evenness
using distance from livestock resting point as a proxy for grazing intensity. Analyses were
ARID LAND RESEARCH AND MANAGEMENT 3

conducted for annular zone and control microsites only. To test for potential interactions
between grazing intensity and microsite, for annular zone and control microsites, differ-
ences in slopes and intercepts for mound and control microsites were compared using F
statistics.
Results
Effects of termite activity on soil and vegetation
Values of soil EC, pH, Ca, Mg, Na, and total N and OC were greatest in mound soils and
declined with distance from the mounds, i.e., from termite mound out to the control
microsites (Table 1, P <0.05). Termite mounds had finer textures (more silt and clay) than
the other two microsites but there were no changes in soil P (P >0.05). Termite mounds
were always completely bare. Plant cover was three-times greater on the control microsites
than the annular zone around the mounds (P <0.001) and richness greater on the control
microsites (P ¼0.05; Table 1). There were no differences, however, in diversity or evenness
(P >0.09).
Effects of livestock grazing intensity on soil and vegetation
Soil pH declined along the gradient at both the annular zones and control microsites
(Figure 1a). However, this decline was significant only on the control microsites (P ¼0.045,
Table 2). Livestock grazing intensity had the greatest impact on soil EC, for both annular
zone and control microsites (P <0.001; Figure 1b, Table 2). Values of EC were 10-times
greater on termite mounds (4.98 dS m
1
) than the controls (0.47 dS m
1
), but this differ-
ence rapidly diminished rapidly with declining grazing intensity, i.e., with increasing
distance from livestock resting points (Microsite x Distance interaction: P <0.001;
Figure 1b). For the control microsites, however, EC remained relatively low but did
increase significantly with declining grazing intensity (P <0.001).
Table 1. Mean (SE) values of soil physical and chemical properties, and plant community structure
within three microsites associated with termite mounds. Within a row, different letters indicate a
significant difference at P <0.05; mounds were always devoid of plant cover.
Attribute
Termite mound Annual zone Control
Mean SE Mean SE Mean SE
EC (dS m
-1
) 6.2
a
0.039 2.7
b
0.006 0.96
c
0.005
pH 8.12
a
0.028 7.83
b
0.028 7.39
c
0.027
Ca (ppm) 859.4
a
9.80 792.5
b
2.55 494.4
c
9.46
Mg (ppm) 120.6
a
9.80 97.1
b
1.09 91.6
b
0.79
Na (ppm) 21.1
a
0.46 8.3
b
0.053 2.4
c
0.31
K (ppm) 16.5
a
0.24 12.4
b
0.32 10.6
b
1.92
N (ppm) 0.12
a
0.002 0.09
b
0.002 0.04
c
0.002
P (ppm) 4.5
a
0.07 4.6
a
0.0 4.4
a
0.105
OC (%) 1.2
a
0.04 0.70
b
0.39 0.90
c
0.21
Sand (%) 53.7
a
1.02 66.8
b
1.25 73.1
c
0.76
Silt (%) 19.0
a
0.17 11.0
b
3.33 8.9
c
1.10
Clay (%) 27.3
a
1.45 23.0
b
1.19 17.1
c
1.19
Plant cover (%) 0 0 20.2
a
3.54 57.6
b
5.16
Richness 0 0 0.37
a
0.04 0.54
b
0.07
Diversity 0 0 0.59
a
0.12 0.84
a
0.07
Evenness 0 0 0.62
a
0.14 0.93
a
0.12
Note: Data indicated by different alphabetic letters (a, b, c) are statistically significant at p < 0.05.
4 M. FALLAH ET AL.

The percentage of sand declined (Figure 1c), and clay increased, with increasing grazing
intensity, for both annular zone and control microsites (Table 2). Silt content increased
with increasing grazing intensity, but only on the annular zones (P <0.001; Figure 1d;
Table 2). Plant cover increased with declining grazing intensity (increasing distance from
resting points) for both annular zone (P <0.001) and control (P ¼0.03) microsites
(Figure 1h, Table 2) and this effect was strongest in the annular zone (R
2
¼0.89) than
control sites (R
2
¼0.47). There were no effects of increasing grazing intensity on plant
richness, diversity or evenness (P >0.10).
Figure 1. Trends in soil pH, electrical conductivity (EC), sand and silt contents, and plant richness,
diversity, evenness and cover on the annular zone of termite mounds and the control microsites
in relation to distance from livestock watering points. Linear models are presented for significant
relationships only.
ARID LAND RESEARCH AND MANAGEMENT 5

Discussion
In our study termites had marked effects on soil physical and chemical properties. Mounds
created by termites had greater levels of soil properties than soils in the annular zone sur-
rounding the nests, or the non-mound (control) interspaces. Specifically, termite mound soils
had greater pH and EC, lower levels of exchangeable cations and anions, finer surface textures,
and higher concentrations of soil organic carbon than the non-mound control surfaces. These
values were also frequently greater than the annular zone surrounding the mounds. Apart
from electrical conductivity, where values for mound and control converged at long distances
from livestock resting areas, the effects were generally consistent across the grazing gradient.
Termite mounds are higher in fine material and organic matter
In our study we found higher levels of fine soil particles (silt and clay) on the mounds,
consistent with the literature (e.g., Rajagopal 1983; Whitford and Eldridge 2013). This is
Table 2. Regression relationships for the effects of increasing grazing intensity (proximity to livestock
resting points) on soil chemical and physical properties, and vegetation. For each parameter, the
relationship was tested for Annular and Control microsites separately. Slopes and intercepts were
compared between the regression lines. R
2
values are presented only for significant effects.
Parameter Source DF F P-value R
2
EC Control 1 33.26 <0.001 0.81
Annular 1 41.33 <0.001 0.84
Slope 1 49.90 <0.001
Intercept 1 55.34 <0.001
pH Control 1 5.61 0.045 0.41
Annular 1 0.71 0.425
Slope 1 0.06 0.810
Intercept 1 392.62 <0.001
Sand Control 1 27.05 <0.001 0.77
Annular 1 12.31 0.008 60.6
Slope 1 2.06 0.170
Intercept 1 811.07 <0.001
Silt Control 1 0.10 0.763
Annular 1 24.66 0.001 0.76
Slope 1 1.52 0.235
Intercept 1 2915.25 <0.001
Clay Control 1 9.17 0.016 0.53
Annular 1 13.91 0.006 0.64
Slope 1 0.77 0.393
Intercept 1 75.37 <0.001
Plant cover Control 1 6.89 0.030 0.47
Annular 1 64.55 <0.001 0.89
Slopes 1 0.00 0.946
Intercept 1 87.29 <0.001
Plant richness Control 1 3.53 0.100
Annular 1 2.50 0.153
Slopes 1 0.44 0.520
Intercept 1 5.56 0.031
Plant diversity Control 1 1.19 0.310
Annular 1 0.87 0.378
Slopes 1 0.06 0.820
Intercept 1 3.44 0.081
Plant evenness Control 1 1.97 0.198
Annular 1 0.78 0.400
Slopes 1 0.03 0.800
Intercept 1 2.95 0.104
6 M. FALLAH ET AL.

attributed to the fact that termites use fine particles (clay) and saliva to form the structure
of their nests (Lee and Wood 1971; Lal 1987; Hulugalle and Ndi 1993; Sarcinelli et al. 2009).
The proportion of silt and clay in the mounds may depend on the ability of worker termites
to transport specific-sized soil particles (Lee and Wood 1971). Termites then use their sali-
vary secretions to bind transported organic matter and clay materials.
As central place foragers, the greatest effect of termites is to collect organic material
from the area surrounding the mounds, increasing mound soil organic matter content
(Whitford and Eldridge 2013). Levels of organic matter in some termite mounds have been
shown to be five-times greater than those on adjacent nonmound soils (Decaëns, Galvis,
and Amézquita 2001). In arid, semi-arid and some tropical ecosystems, more than half
of the potential inputs to the soil organic pool are consumed by termites (Whitford,
Ludwig, and Noble 1992). Nitrogen is highly associated with carbon, so it was not unex-
pected that our mounds also contained high levels of total nitrogen, consistent with global
studies (Brossard et al. 2007; Abdus-Salam and Itiola 2012). Higher soil temperature and
moisture in the mounds may also enhance the mineralization of N from stored organic
matter (San Jose et al. 1989).
We found higher levels of soil pH and exchangeable cations and anions in mound soils
than in the control soils. Greater levels of fine sediments and soil organic matter in termite
mounds (Whitford and Eldridge 2013) also enhance soluble and exchangeable cations that
are adsorbed onto these clay-rich particles (Rajagopal 1983; Mermut, Arshad, and St
Arnaud 1984; Sarcinelli et al. 2009; Abdus-Salam and Itiola 2012). This effect has been
widely reported from a range of environments (e.g., de Bruyn and Conacher 1990). Greater
levels of soil Ca in termite mounds has been attributed to the breakdown of carotenes in
plant tissue by termites (Lee and Wood 1971), leading to higher mound pH levels. Simi-
larly, greater levels of P and exchangeable Mg on the mounds can be attributed to the
digestion and degradation of plant tissues by termites (López-Hernández et al. 1989).
The foraging behavior of termites can also lead to increasing mound concentrations of soil
P (López-Hernández 2001), due to exposure of P-rich subsoil. In our study, however,
exchangeable P did not differ significantly among the three microsites. This may be
due to the patchy distribution of P and its lower mobility through the profile, which
makes it highly dependent upon depth and location of soil sampling (López-Hernández
et al. 1989).
Termite effects are largely consistent across a grazing gradient
Apart from changes in EC, the effects of termites were consistent across a grazing gradient,
i.e., we observed similar trends for mound and control microsites with increasing distance
from livestock resting points (Figure 1). This suggests to us that termites are largely insen-
sitive to changes in livestock intensity, consistent with observations that mounds of the
subterranean termite Drepanotermes tamminensis (Hill) were more abundant in grazed
than ungrazed plots in semi-arid rangelands in Australia (Abensperg-Traun 1992). Of
particular interest was the observation that EC values were extremely high close to, but
declined markedly with increasing distance away from, livestock resting points, with values
converging at 720 m from the resting areas (Figure 1b). Finer and more compacted soil sur-
faces under high intensity grazing could reduce soil moisture and consequently increase
soil EC (Eskandari 1995). Conversely, greater cover of vegetation under low grazing
ARID LAND RESEARCH AND MANAGEMENT 7

intensity (Figure 1h) could reduce soil evapotranspiration and hence soil EC
(Kashizenoozi, Saadat, and Namdar 2011).
We recorded greater vegetation cover under lower grazing intensity, i.e., with increasing
distance from livestock resting points, and the trends were consistent for both the annular
and control microsites. There were, however, no differences in diversity or richness.
Reductions in plant cover with increasing grazing intensity are widely reported in the litera-
ture, but there have been few studies of potential interactions between termites and grazing
(though see Abensperg-Traun 1992). Suppression of vegetation on the mounds by termites is
known from drylands, where termite removal resulted in increased total plant canopy cover
up to 20% (Bodine and Ueckert 1975). Lower levels of plant cover due to high grazing levels
would be expected to reduce potential resources for grass harvesting termites; thus, the strong
alignment between the models for plant cover on mounds and control sites (Figure 1h).
Concluding remarks
Our observations of greater organic matter, fine sediments on termite mounds is consistent
with the notion that mounds act as fertile islands, and that this fertile island effect persists
under even high levels of grazing i.e., close to livestock resting points. The degree to which
mounds might act as fertile islands, however, is probably highly termite species-specific,
and this would likely relate to mound structure (Hulugalle and Ndi 1993) i.e., whether they
shed or retain water and nutrients (convex cf. concave surfaces; see Eldridge 1994). Studies
from temperate areas have demonstrated greater vegetation cover and richness on mounds
(Levick et al. 2010), but in arid and semiarid areas, vegetation is often suppressed on the
mounds themselves but enhanced in the annular zone immediately surrounding the
mound. Active removal of plants from the surface of the mounds by termites would
reinforce this shedding of water to the area around the mounds, potentially leading to
zones of fertile soil around the mounds. Overall therefore, the effect of termite activity
was to concentrate resources around their mounds, leading to an increase in spatial hetero-
geneity, which is important for the functioning of arid systems. We showed that this effect
was generally consistent across our grazing gradient. Predicted increases in aridity during
the next century would likely result in greater livestock induced-degradation in drylands.
The extent to which the positive effect of termites might be offset by high levels of grazing
with increased levels of aridity is largely unknown.
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