The effects of ants nests on the physica

Geoderma 105 Ž2002. 1–20
www.elsevier.comrlocatergeoderma

The effects of ants’ nests on the physical, chemical
and hydrological properties of a rangeland soil in
semi-arid Spain
L.H. Cammeraat a,) , S.J. Willott b, S.G. Compton b, L.D. Incoll b
a

ICG, Institute for BiodiÕersity and Ecosystem Dynamics, Section Physical Geography,
UniÕersiteit Õan Amsterdam, Nieuwe Achtergracht 166, 1018 WV Amsterdam,
The Netherlands
b
Centre for BiodiÕersity and ConserÕation, School of Biology, UniÕersity of Leeds,
Leeds LS2 9JT, UK

Received 21 February 2000; received in revised form 28 January 2001; accepted 8 May 2001

Abstract
The effects of the activity of seed harvesting ants Ž Messor bouÕieri . on the fertility, rainfall
infiltration, structural properties and water repellency of top soils were investigated in a semi-arid

rangeland in SE Spain. The soil surfaces had a low vegetative cover and bare areas had a sieving
crust. Rainfall simulation experiments were carried out over ants’ nest mounds and on control
areas without surface ant activity in September 1997 and October 1998. The soils of the ants’
nests had a lower pH, higher concentrations of organic carbon and inorganic nutrients, higher
structural stability and were more water repellent than the control areas. Infiltration rates on the
control areas were comparable in both years. However, in 1997, infiltration was significantly
higher on the nests than in control areas, whereas in 1998, infiltration was lower and wetting depth
was reduced on the nests. These contrasting results are explained by a difference between the 2
years in the initial soil moisture content and the water repellency of soils and organic debris on the
ant-affected areas. It is concluded that ants’ nests can act as sinks for water under slightly humid
to wet conditions, whereas under extremely dry conditions, which prevail in summer and the
beginning of autumn, infiltration is strongly reduced. Temporal variability in the initial conditions
of the soil and spatial variability in ant activity interact to influence runoff generation at a fine
scale and should be taken into account to fully understand runoff generation at the hillslope scale.
The redistribution of materials by ants and their effects on soil properties may have consequences

)

Corresponding author. Tel.: q31-20-525-7451; fax: q31-20-525-7431.
E-mail address: l.h.cammeraat@science.uva.nl ŽL.H. Cammeraat..

0016-7061r02r$ - see front matter q 2002 Elsevier Science B.V. All rights reserved.
PII: S 0 0 1 6 - 7 0 6 1 Ž 0 1 . 0 0 0 8 5 - 4

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L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

for the development of fertile islands in semi-arid ecosystems. q 2002 Elsevier Science B.V. All
rights reserved.
Keywords: Infiltration; Mediterranean ecosystems; Messor bouÕieri; Soil structure; Soil chemistry; Water repellency

1. Introduction
There is widespread concern over desertification in the Mediterranean region
caused by changes in climate and land use Ž Brandt and Thornes, 1996. .
Abandonment of agricultural land in these areas is often associated with severe
rates of soil loss, especially in the period directly after abandonment and prior to
the establishment of vegetation cover. Understanding those processes that
govern infiltration and erosion at the local scale could inform decisions on
policies of management, which would reduce the degradation of vulnerable land.

Surface runoff and soil erosion in semi-arid areas decrease with increasing
plant cover Že.g. Lavee et al., 1998; Quinton et al., 1997; Castillo et al., 1997. ,
but where cover is low, the properties of the soil surface play a major role in
governing the processes of erosion and infiltration Ž Scoging, 1989. . Organisms
in soil such as ants affect properties of the soil surface through the construction
of nests. Nests are often, but not always, associated with higher concentrations
of nutrients and organic matter Ž Lobry de Bruyn and Conacher, 1990; Dean and
Yeaton, 1993., and there may be a corresponding change in vegetative cover or
biomass Ž Rissing, 1986; Carlson and Whitford, 1991; Whitford and DiMarco,
1995.. Ants may increase infiltration by improving porosity or decrease infiltration by producing compacted surfaces, which facilitate runoff Ž Lobry de Bruyn
and Conacher, 1990.. Many of the results are contradictory, partly because of
differences in techniques and in the biology of the species investigated, but also
due to differences in physical characteristics of sites. The effects of the nests of
the ant Pogonomyrmex rugosus on soil properties and vegetation varied with
location and soil type within a single watershed Ž Whitford and DiMarco, 1995. .
Water repellency of soil material is reported for many Mediterranean and
semi-arid ecosystems, but it is most often linked to hydrophobic substances
directly derived from plant residues ŽDoerr et al., 2000. . The activity of soil
fungi and microorganisms may also be a source of hydrophobic substances,
affecting soil hydraulic properties such as conductivity and sorptivity Ž Wallis

and Horne, 1992; Hallett and Young, 1999; Doerr et al., 2000. . However, we
have found no published work on the effects of soil macrofauna on soil
wettability as reported here.
We measured the chemistry, particle size distribution, aggregate stability and
wettability of soil associated with nests of the seed harvesting ant Messor
bouÕieri Bondroit Ž Hymenoptera: Formicidae. in Spain. We determined the area
of soil affected by nests and the rate of relocation of nest entrances over a

L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

3

12-month period. We conducted rainfall simulation experiments at different
intensities over nests and on control areas during periods of high and low soil
moisture content to determine the effects of nests on infiltration and erosion.
2. Site and methods
2.1. Site
The investigation was conducted in the Rambla Honda, the valley of an
ephemeral river leading from the Sierra de los Filabres range, Almerıa
´ Province,

SE Spain Ž37808X N, 2822X W.. The climate is semi-arid, with mean annual
rainfall of 256 mm Žbut highly variable.. Mean annual temperature is 15.8 8C,
with summer maximum temperatures exceeding 40 8C ŽPuigdefabregas
et al.,
´
.
1996, 1999 . The site varies from 630 m altitude at the valley bottom to over
800 m on boundary ridges. The bedrock on the valley sides is a Devonian–
Carboniferous slaty micaschist with quartz veins, with deep fluvial deposits on
the valley floor, and extensive alluvial fans at the valley sides. We worked on
the lower fans where the soils are Typic Torrifluvents Ž Soil Survey Staff, 1992.
or Eutric FluvisolsrHaplic Arenosols ŽFAO–UNESCO, 1988. with a 20–40-mm
thick surface layer of fine gravels over a 2–10-mm thick subsurface crust of
loamy sands. The crust is of a sieving crust type Ž Valentin and Bresson, 1992;
Puigdefabregas,
1999.. The soil is a gravelly sandy loam with very weakly
´
developed horizons. Below 30–35 cm, the soil is more gravelly with no
recognisable soil formation. The soil is moderately alkaline, with a low CaCO 3
content which declines with depths from 9.1 g kgy1 dry soil Ž A-horizon. to less

than 4.4 g kgy1 dry soil ŽC-horizon. ŽPuigdefabregas,
1995. . The very low
´
electrical conductivity reflects the low soluble salt content of the soil. Cation
exchange capacity is low Ž 25.0–36.4 mmol c kgy1 . reflecting the low content of
clay and organic matter ŽPuigdefabregas,
1995. . Soil bulk density ranges
´
between 1.52–1.65 Mg my3 for the crust and between 1.57 and 1.81 Mg my3
for the soils under the crust. Sediment yield and runoff are typically very low
ŽKosmas et al., 1997. although a rare extreme rainfall event may increase these
considerably.
On the lower fans and drainage ways, the dominant shrub is Retama
sphaerocarpa ŽL.. Boiss., a deep-rooted legume, growing to 4-m high Ž Pugnaire
et al., 1996.. There is a diverse set of other plants, mainly winter annuals,
between and beneath the canopy of these shrubs. Some lower areas of the valley
have been cultivated occasionally in the past and the land is now used for sheep
grazing, but this has been excluded from our experimental area since 1991
ŽPuigdefabregas
et al., 1996.. The conspicuous nest entrances of the seed´

harvesting ant M. bouÕieri are on the soil surface, particularly in the areas
between shrubs. This species is found in arid sites throughout the western
Mediterranean and North Africa ŽBernard, 1968. . Each entrance comprises a

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L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

hole up to 1 cm in diameter surrounded by a mound of excavated soil particles
and organic debris, the latter mostly chaff—the waste plant material from seeds
processed in the nest. Each ant colony usually uses only one entrance at a time,
although the position of these may change periodically. Attempts to excavate
nests at this site were unsuccessful, although elsewhere in the region, nests of
related species may be over 4-m deep and colonies may survive for over 40
years ŽBernard, 1968..
2.2. Rate of relocation, area and density of nest entrances
In November 1997, a set of 20 ant colonies was marked, the nests of which
were at least 10 m apart. For each colony, the mound of the nest entrance in use
at that time was permanently marked by a painted nail inserted into the soil. For
each of these mounds, the longest dimension and the length perpendicular to it

were measured and the area was then calculated by assuming that the mound
was elliptical. The nests were revisited in November 1998, when the size and
distance from the original of each new entrance mound was recorded. Several
nests were located within a 48 = 28-m2 area demarcated so that the area of soil
affected by the ants’ nests within a 12-month period could be estimated.
2.3. Chemical properties of soil
In June 1997, samples of approximately 500 g were taken from nest mounds
and from the top 5 cm of soil of a control site approximately 2 m distant. Four
replicates of these pairs of samples were taken from open areas at least 2 m from
the nearest shrub because shrubs are known to affect soil properties Ž Pugnaire et
al., 1996.. Samples were analysed Ž Phosyn Laboratories, York, UK. for pH,
electrical conductivity, and concentrations of organic carbon, nitrogen Ž as NHq
4
.
and NOy
,
potassium,
phosphorous
and
magnesium.

Chemical
analyses
fol3
lowed Anon., Ž 1986. and are summarised as follows: pH and electrical conductivity on a suspension of soil in demineralised water Ž 1:2.5 soilrwater by
volume.; organic carbon, by oxidation by potassium dichromate and sulphuric
acid followed by spectrophotometry; NHq
4 , by extraction in 2 M potassium
chloride solution followed by determination by an ammonium-ion-selective
electrode; NOy
3 , by extraction by saturated calcium sulphate solution followed
by determination by a nitrate-ion-specific electrode; K, by extraction by 1 M
ammonium nitrate followed by flame emission spectrometry; P, by extraction by
sodium hydrogen carbonate, complexing with ammonium molybdate and solution spectrophotometry Ž Olsen method. ; Mg, by extraction with 1 M ammonium
nitrate followed by determination by atomic absorption. Thus, available, rather
y
than total, N and P were measured. Data for conductivity, NHq
4 , NO 3 , potassium, phosphorous and magnesium were log-transformed and organic carbon
values Ž %. were arcsine transformed to normalise variances before statistical
analyses.

L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

5

2.4. Physical properties of soil
Samples for determination of soil aggregation and texture were taken from
three active ants’ nests and from adjacent control sites. For each nest, separate
samples were taken from the nest mound, the crust and the soil to 5 cm below
the crust Ž the AsubcrustB .. Control samples were of the crust and subcrust only.
Samples were bulked so we have no estimate of between-site variability. Each
bulked sample was divided into two equal parts for determination of texture and
soil aggregation. Soil texture was measured using the standard pipette and
sieving method Ž Gee and Bauder, 1986. .
Aggregate stability was determined by applying a drop test Ž Low, 1954. ,
which is suitable for soils with low aggregate stability Ž Imeson and Vis, 1984. .
Twenty air-dry aggregates Ž4–4.8 mm in size. were tested for each sample. The
number of drops Ž d . is used as an index of stability, indicating the number of
raindrops required to fully disintegrate a single aggregate.
Wettability of the soil was determined by the WDPT method Ž wetting depth
penetrating time; Van’t Woudt, 1959; King, 1981. on three replicates of each

sample. This test measures how long water repellency endures in the contact
area of a water droplet with soil particles. The test was carried out on a surface
of packed, air-dried soil Ž - 125 mm. for each type of sample as well as on the
organic debris left by the ants at the nest entrance. Data were log-transformed to
normalise the variance before statistical analysis.
2.5. Rainfall simulations
Rainfall of varying intensity Ž between 11 and 70 mm hy1 . was simulated on
plots measuring 1 = 0.5 m2 in late September 1997 and early October 1998 with
a portable rainfall simulator Ž ‘Amsterdam type’ dripping plate simulator;
Bowyer-Bower and Burt, 1989. . Natural rainwater was used for the simulations,
with approximately 23–25 l used in each simulation, equivalent to 46–50 mm
rainfall. Plots were on the lower alluvial fans with slopes varying from 38 to 148.
Runoff, sediment yield, infiltration and depth of wetting front were compared
between ants’ nests and control areas, with each pair of plots Ž nest and control.
receiving rainfall of similar intensity. Runoff was collected in a small gutter at
the lower end of the plot.
At the end of the experiment, a profile was dug through the centre of the long
axis of each plot, and the vertical penetration of the clearly visible wetting front
Žwetting front depth. was measured at 5-cm intervals along the profile. The
length of wetting front was also measured from this profile Ž wetting front
length.. Wetting front heterogeneity was expressed by a sinuosity index, by
calculating the ratio between the wetting front length and the length of the plot
slope surface. Gravimetric soil moisture content of bulk soil was determined
before each experiment and, following each experiment, of samples from the

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L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

Fig. 1. Number of entrance mounds Ž N . recorded over a 12-month period for 20 M. bouÕieri
colonies in the Rambla Honda Ž m indicates multiple entrances where the mounds from each had
coalesced into a larger unit..

surface crust and at intervals from the surface crust down to the wetting front
and the dry soil below it.
3. Results
3.1. Rate of relocation, area and density of nest entrances
Most colonies used two or more nest entrances with discrete mounds within
the 12-month period ŽFig. 1. . Six colonies used several entrances within a small
Table 1
Properties of soils of ants’ nest mounds and control sites in the Rambla Honda Žmean"S.E.;
ns 4., and significance of the comparison between sites by a paired t-test Ž p .
Property
pH
Conductivity ŽmS cmy1 .
y1 .
Ž
NOy
3 mg g
q Ž
y1 .
NH 4 mg g
P Žmg gy1 .
K Žmg gy1 .
Mg Žmg gy1 .
Organic C Ž%.

Site

p

Control

Nest mound

8.1"0.1
182"5
7"1
2" -1
7"1
92"8
68"4
1.2"0.1

7.4" - 0.05
548"78
26"4
15"2
47"7
227"47
156"24
2.7"0.5

0.039
0.007
0.013
0.004
0.006
0.018
0.035
0.050

L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

7

Table 2
Particle size distribution of soil samples taken from ants’ nests and control sites
Treatment

Total soil

Total fine earth

Gravel
) 2 mm
Ž%.

Fine earth
- 2 mm
Ž%.

Sand
)63 mm
Ž%.

Silt
63–2 mm
Ž%.

Clay
- 2 mm
Ž%.

Nest
Mound
Crust
Subcrust

25.9
23.0
31.6

74.1
77.0
68.4

81.5
84.7
83.5

11.9
12.5
13.9

6.7
2.8
2.6

Control
Crust
Subcrust

21.4
27.3

78.6
72.7

79.0
84.7

16.9
13.0

4.1
2.3

area, resulting in a large, diffuse mound Ž m, Fig. 1. . For those colonies which
moved their nest entrance, the mean distance between the different mounds of a
single colony was 1.7 " 0.3 m Ž mean " S.E.; n s 25, range 0.3–6.6 m.. Nest
mounds averaged 0.23 " 0.03 m2 in the area Ž mean " S.E.; n s 40, range
0.02–0.88 m 2 , with those over 0.5 m2 associated with multiple entrances within
a small area. . The total area of the 28 nest mounds of the 13 colonies within the
48 = 28-m plot was 4.4 m2 , comprising 0.33% of the total area of the plot.

Fig. 2. Boxplot of numbers of water drops Ž d . required for complete disintegration of aggregates
from four sites. Key: AC, ants’ nest crust; AS, ants’ nest subcrust; CC, control crust; CS, control
subcrust.

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L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

Fig. 3. Water drop penetration time ŽWDPT. for soil samples from various sites Žvalues are of
mean"S.E., ns 3.. Key: AC, ants’ nest crust; AS, ants’ nest subcrust; AM, ants’ nest mound
material; CC, control crust; CS, control subcrust; OD, organic debris. Values with the same letter
are not significantly different at ps 0.05 ŽTukey test..

3.2. Chemical properties of soil
Nest mounds had a significantly lower pH than bulk soil Ž Table 1. . Electrical
q
conductivity and concentrations of NOy
3 , NH 4 , P, K, Mg and organic C were
significantly greater in soil from nest mounds.

Fig. 4. Profiles of soil moisture content prior to rainfall simulation experiments in 1997 and 1998.

Table 3
Results of the rainfall simulations in 1997 and 1998 under conditions of high and low soil moisture contents, respectively

1997
A1
C1
A2
C2
A3
C3
A4
C4
1998
A5
C5
A6
C6
A7
C7

Slope
Ž8.

Duration
Žmin.

Total rainfall
applied Žmm.

Intensity
Žmm hy1 .

14
12
12
14
6
6
6
5

37.00
30.45
22.00
20.00
20.00
24.00
30.75
34.00

23.8
22.8
22.8
22.9
22.8
22.8
22.8
22.8

38.6
44.9
62.1
68.8
68.3
56.9
44.4
40.2

19.56
21.05
44.30
36.15
73.23
54.00

22.8
22.8
13.7
13.7
13.7
13.7

69.9
64.9
18.5
22.7
11.2
15.2

2.5
3
3
3
3
4

Runoff
depth Žmm.

Runoff
Ž%.

Time to
runoff Žmin.

Final infiltration
rate Žmm hy1 .

0.3
4.4
5.3
7.3
6.3
7.5
0.9
3.5

1.3
19.4
23.2
31.9
27.5
33.0
3.9
15.3

10.00
5.00
2.30
2.00
2.75
3.58
4.08
3.00

37.9
27.2
45.6
41.9
38.6
36.5
42.4
27.2

12.8
5.6
1.2
1.8
1.9
1.1

56.3
24.5
8.6
13.0
13.7
8.1

1.65
2.25
5.68
8.08
4.82
4.65

28.8
48.3
16.6
19.0
10.2
14.0

L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

Replicate

Key: A, ants’ nests; C, controls.

9

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L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

3.3. Physical properties of soil
Soil from all the samples were gravelly loamy sand, and differences between
samples were small ŽTable 2.. Soil aggregation at this site is extremely weak.
All macro-aggregates disintegrated after 35 drops of water, and usually far fewer
ŽFig. 2.. The number of drops Ž d . required to disintegrate the aggregates of ants’
nest crusts was significantly greater than for control crusts Ž Mann–Whitney
U-test; z s y2.69, n s 20, p s 0.007., but there was no significant difference
between the subcrust samples Ž z s y0.41, n s 20, p s 0.683..
During the drop test experiments, the macro-aggregates Ž 4–4.8 mm. did not
show any signs of soil hydrophobicity. For the finer material Ž- 125 mm. , there
was significant variation among samples Ž one-way ANOVA: F s 23.54, df s 5,
p - 0.001.. The soil affected by ants and their organic debris was significantly
less wettable than the control crust and subcrust soil Ž Fig. 3. .
3.4. Rainfall simulations
Prior to the experiments in 1997, there were a series of rainfall events of
which the last one Ž12 mm. occurred 5 days before the experiments. In 1998,
there were 2.4 mm of rain 13 days and 0.4 mm of rain 10 days before the

Fig. 5. The relationship between total runoff Ž%. and rainfall intensity Žmm hy1 . for all rainfall
simulation experiments; ‘wet’s low and ‘dry’s very low initial soil moisture contents in 1997
and 1998, respectively. Line 1 Žants’ nest ‘wet’.: r 2 s 0.989, regression slope ps 0.0055; lines
2q4 Žcontrol ‘wet’q‘dry’.: r 2 s 0.830; ps 0.0043; line 3 Žants’ nest ‘dry’.: r 2 s 0.889;
ps 0.216.

L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

11

experiment. Consequently, the gravimetric soil moisture content was extremely
low in 1998 Ž- 0.01 g gy1 . and low in 1997 Ž0.03–0.04 g gy1 . Ž Fig. 4. .
For the first set of experiments on soil with low moisture content Ž in 1997. ,
infiltration rates on the areas affected by ants were higher than on the control
areas for rainfall of comparable intensity Ž Table 3. . However, in 1998, on soil
with very low moisture content, infiltration rates were higher in the control areas
than on the ant-affected areas. For control areas, there were no significant

Fig. 6. Representative examples of the development of runoff with time in rainfall simulations at
different intensities Žhigh )60 mm hy1 and low - 25 mm hy1 . over ants’ nests and control soils
in 1998 when initial soil moisture content was very low.

12

L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

differences in the slopes or intercepts of the regressions of total runoff against
rainfall intensity for each year analysed separately Ž both p ) 0.1., so these data
were combined in a single regression ŽFig. 5. . In 1997, there was little runoff
from ants’ nests even at rainfall intensities of 40 mm hy1, whereas runoff was
generated from ants’ nests at rainfall intensities as low as 10 mm hy1 in 1998
ŽFig. 5.. In each case, runoff was not directly visible at the soil surface, but
occurred by flow over the crust surface under the non-embedded gravel and sand

Fig. 7. Profiles of wetting fronts showing depths of infiltration under control areas ŽA and C. and
through and around ants’ nests ŽB and D. after rainfall simulations in 1997 and 1998, respectively.
The number next to the position of each nest entrance in B and D refers to the number of the
associated wetting curve. The codes for rainfall intensity are HIs high intensity )60 mm hy1 ;
MI s middle intensity -60, ) 25 mm hy1 ; and LI s low intensity - 25 mm hy1.

L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

13

Fig. 7 Ž continued ..

layer on top of the crust. This was clearly visible at the cut made in the surface
where the runoff was collected. Sediment yield was too low to measure for any
of the experiments. The nest areas had a rougher topography due to the activity
of the ants, and the final constant infiltration rates, and thus runoff, were all
reached within 20 min, even at low rainfall intensities Ž e.g. Fig. 6. .
In 1997, the wetting fronts under the ants’ nests were significantly more
irregular Žas measured by the sinuosity index. than those of the control areas
ŽFig. 7; Table 4. . The difference in depth of infiltration between the control and
ants’ nest areas was not significant because variance of infiltration depth
increases with increasing sinuosity. In 1998, wetting depth was less under the
nests and the sinuosity index for both treatments did not differ significantly, as
infiltration on the nests was much reduced. The greater wetting depth recorded

14

L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

Table 4
Wetting front characteristics for nest and control areas in 1997 and 1998 Žmean"S.E.; ns 4
Ž1997. and ns 3 Ž1998.., and significance of the comparison between sites by a paired t-test Ž p .
Property

Site

p

Control

Nest mound

1997
Wetting front length Žcm.
Wetting front depth Žcm.
Sinuosity index

96.0"1.7
12.4"2.2
1.067"0.038

120.8"8.9
16.4"2.7
1.342"0.098

0.03
0.31
0.03

1998
Wetting front length Žcm.
Wetting front depth Žcm.
Sinuosity index

103.4"0.4
7.9"2.2
1.149"0.004

98.9"4.4
3.2"1.2
1.099"0.049

0.37
0.14
0.37

from the control areas in 1998 compared to 1997 is due to the lower intensities
of rainfall applied.

4. Discussion
At our site, the nest mounds of M. bouÕieri had concentrations of macronutrients and organic carbon two to eight times higher than in the surrounding soil.
This is consistent with the differences in nutrient concentrations and pH between
ants’ nests and their surroundings reported elsewhere, especially for larger ant
species with semi-permanent nests Ž Lobry de Bruyn and Conacher, 1990; Dean
and Yeaton, 1993; Eldridge and Myers, 1998. . As in other semi-arid environments, the relatively high pH of soil is likely to make it difficult for plants to
obtain sufficient phosphorus from the substrate Ž Louw and Seely, 1982. . The
increase in organic carbon should improve soil structure and increase waterholding capacity ŽDay and Ludeke, 1993. , and the lower pH may well be related
to higher organic matter contents. Increased water-holding capacity should
benefit the plants directly and also indirectly via an associated increase in
microbial activity, which will raise decomposition rates and result in the further
release of nutrients. Increased aggregate stability will affect soil structure in a
positive way. A more stable soil structure usually also indicates better macroporosity, which enables infiltration at low soil moisture suctions. When water
enters the soil, it may percolate more easily and deeper through the areas with
better soil structure.
By moving their nest entrances, ants may greatly enlarge the area over which
they affect soil fertility. An ant colony typically produces two or more entrances

L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

15

per year, at least doubling the area of soil affected by nest building activity, and
indicating that a single AsnapshotB survey of the number of nests may considerably underestimate the total area of soil affected.
There has been considerable interest in the effects of plant cover on infiltration of water and soil erosion in the Mediterranean region Ž e.g. Castillo et al.,
1997; Quinton et al., 1997. , and on the role of hydrophobic soil material in the
redistribution of Žsoil. water Ž e.g. Imeson et al., 1992; Ferreira et al., 2000. .
There is less work on the effects of macrofauna on these attributes. Most
research on effects of soil fauna on soil chemical and physical properties have
been carried out in Australia, North America and Africa. Results are often
contradictory, indicating both increased infiltration due to improved porosity and
soil structure, or a decrease due to the formation of compacted surfaces Ž Lobry
de Bruyn and Conacher, 1990. . In the absence of a compacted surface, increased
infiltration is what one would expect, given that an ants’ nest entrance is
effectively a hole through the surface or subsurface crust which connects to
underground tunnels. Furthermore, the tunnels and granaries of the nests of
seed-harvesting ants are thought to be plastered with an anal secretion from the
ants ŽBernard, 1968., making them relatively impermeable to water. This is
likely to increase the rate at which water is transported deeper into the soil
profile. Increased infiltration over the ants’ nests was reported for M. capensis
in South Africa ŽDean and Yeaton, 1993. , for funnel ants Ž Aphaenogaster
barbigula. in Australia ŽEldridge, 1993. , and for fire ants Ž Solenopsis inÕictsa
and S. richteri . in the southeastern United States ŽGreen et al., 1999. . However,
Wang et al. Ž1996. reported no significant effect on infiltration as entrances of
Lasius neoniger were closed during sprinkler experiments, although whether
this was due to the ants actively closing entrances or the entrances simply
becoming filled with dislocated soil particles was not clear. Similar closure of
nest entrances was observed for M. bouÕieri on limestone substrate in Spain, but
infiltration rates were still higher, indicating that infiltration is not solely through
the nest entrance itself Ž Lambregts, 1999. .
Low infiltration capacity of crusted soils is perceived as a major agricultural
problem in the warm seasonally dry tropics, and termite nests have similarly
been shown to contribute to increased infiltration and thus soil improvement
ŽMando et al., 1996.. However, it might be predicted that the mound of
excavated nest material could contribute to localised soil erosion. Our results
show that this is not the case, at least for the range of rainfall intensities in our
experiment Žup to 70 mm hy1 . . Sediment yields from our experiments were
negligible, in accordance with results from larger permanent runoff plots nearby
ŽKosmas et al., 1997; Puigdefabregas,
1999. .
´
Few studies have addressed the importance of soil moisture content on
infiltration rates over ants’ nests, despite it being well known that in general,
soil hydraulic conductivity increases with increased soil moisture content
ŽMarshall and Holmes, 1988.. Increased infiltration over ants’ nests was only

16

L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

found to be significant under conditions of ponding by Lobry de Bruyn and
Conacher Ž1994., whereas Eldridge Ž 1994. found no significant effect of soil
moisture or ponding conditions on infiltration. There was a strong contrast in the
results obtained in the 2 years of our experiments, which we attribute to the
differences in initial soil moisture content. The 1997 experiments were 5 days
after a moderate rainfall event, whereas the 1998 experiments were preceded by
a long dry period, with only two small rainfall events more than 10 days before
the simulations. In 1997, soil moisture content was approximately 0.03–0.05 g
gy1, whereas in 1998, the surface and topsoil layers were extremely dry with
moisture contents - 0.01 g gy1, which may have resulted in the strong water
repellency of the organic matter on the soil surface. As well as organic matter
incorporated into the soil, nest mounds are typically covered, at least in part, by
a layer of organic debris Žchaff.. This is mainly the discarded awns from the
seeds of the grass Stipa capensis, which formed a layer up to 4-cm thick. We
observed that even under a relatively thin layer of chaff Ž- 5 mm. , the wetting
front was far less deep than under comparable bare areas of the mound. The
control areas had less organic matter on their surface and they showed a
comparable behaviour in both years.
Water repellency is often observed in sandy soils and also for areas under
perennial plants in semi-arid to subhumid regions Ž Imeson et al., 1992. . Plantderived organic matter may act as a source of hydrophobic waxes which can
coat sand grains, thereby influencing the hydraulic behaviour of the soil Ž Franco
et al., 1995; Nicolau et al., 1996.. Water repellency of soil was positively related
to organic matter content and negatively to clay content across southwestern
Australia ŽHarper and Gilkes, 1994.. At our site, nest mound and crust material
were significantly more water repellent than crust material from the control area,
and organic debris was strongly water repellent. Subcrust material from the
control area was wettable, indicating the absence of mixing of particles with
surface organic matter. Earthworms are often responsible for mixing the soil and
incorporating organic matter, but there are no earthworms at our site Ž S.J.
Willott, unpublished data., reinforcing the important effect of ant activity on the
hydrological properties of the soil surface at this site. Microorganisms may
produce water repellent exudates, and increased microbial activity in nutrient-rich
soil may result in altered soil hydraulic properties such as sorptivity and
hydraulic conductivity ŽHallett and Young, 1999. . Concentrations of nutrients
were greater under ants’ nests ŽTable 1. and if there is an associated increase in
microbial activity, this too may contribute to the reduced wettability of subsoil
material.
Further evidence for water repellency of the soil surface of nest mounds is
that at high rainfall intensity, runoff increases rapidly in the first minutes of the
experiment, then declines by 10% after 10 min of rainfall Ž Fig. 6B, ants’ nest,
69 mm hy1 .. Our interpretation is that after 10 min, the initial water repellency
of the soil has been overcome by wetting, following which infiltration rates

L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

17

stabilise. The wetting fronts for the control areas and the ant-affected area under
very dry initial conditions show a comparable behaviour. Despite the strong
variability in wetting front depth between the individual experiments, deeper and
more heterogeneous infiltration was observed under the ant-affected areas with
slightly higher soil moisture contents. Although soil surface conditions such as
local stone cover or microrelief can be important in redirecting soil surface
water flow, the clear wetting pockets shown in Fig. 7A strongly suggest that the
preferential infiltration of water into the soil under the ants’ nests is important.
At our site, the amount of overland flow is small when measured at broader
scales, indicating that both sources and sinks for water occur at fine spatial
scales ŽPuigdefabregas
et al., 1996; Kosmas et al., 1997. . Our experiments
´
demonstrate the importance of ant activity in water redistribution at these
detailed scales, and this is the first research to document the dependence of
hydrological behaviour of ants’ nest areas on the initial conditions of soil
moisture content and repellency. With relatively wet soil, ants’ nest areas
generate less runoff than their surroundings, and the threshold for the initiation
of runoff is higher, while they may generate more runoff under dry conditions.
There is often little or no rainfall during the summer months, and the nature of
the first rainfall will dictate whether ants’ nests operate as sources or sinks for
runoff. High intensity storms may increase surface runoff due to the water-repellent character of the soil surface, while low intensity storms, resulting in slow
wetting of the soil, may lead to increased infiltration. Thus, temporal variability
in the initial conditions of the soil and spatial variability in ant activity play a
role in runoff generation at a fine scale and should be taken into account to fully
understand processes at the hillslope scale.
As well as their effect on soil physical and structural properties, the effects of
ants on nutrient translocation may also have wider ecological implications.
Semi-arid environments are often characterised by patchy perennial vegetation
under which there are higher concentrations of nutrients and greater water
availability, and consequently a greater abundance and biomass of annual plants
Že.g. Pugnaire et al., 1996.. Positive feedbacks operate to reinforce the pattern of
fertile islands with nutrient-poor and sparsely vegetated areas between them
ŽNoy-Meir, 1981; Ludwig et al., 1999., and these feedbacks are important for
the stability of these semi-arid ecosystems and their vulnerability to disturbance
ŽSchlesinger et al., 1990.. In the Rambla Honda, ants’ nest entrances are almost
always in the sparse inter-shrub areas, yet the ants forage and collect seeds from
areas of denser vegetation, and therefore, greater seed availability, under shrubs
ŽWillott et al., 2000.. Thus, material is taken away from the existing densely
covered patches to the relatively nutrient-poor bare areas, where it ultimately
contributes to increased soil fertility at the nest entrances. The ants may
therefore be moderating the positive feedback and maintaining soil fertility in
the inter-shrub areas. When nest entrances are abandoned by the ants, these
places may become favourable sites for the establishment and development of

18

L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

plant cover. We have established ant-proof enclosures to experimentally test
these predictions in the Rambla Honda.

Acknowledgements
The research for this paper was carried out as part of the MEDALUS III
ŽMediterranean Desertification and Land Use. collaborative research project
funded by the EC under its Environment and Climate Programme, contract
number ENV4-CT95-0115-PL950430. We thank the Consejo Superior de InŽ CSIC. and particularly Drs. M. Cano and J.
vestigaciones Cientıficas
´
Puigdefabregas
for allowing us to use the facilities of the Estacion
´
´ Experimental
´
de Zonas Aridas and the Rambla Honda field site. Michael Geschiere, Wouter
Mosch and Dolf van Ommneren are thanked for their help during the fieldwork
in 1998.

References
Anonymous, 1986. The Analysis of Agricultural Materials, 3rd edn. Ministry of Agriculture,
Fisheries and Food, HMSO, London.
Bernard, F., 1968. Les Fourmis d’Europe Occidentale et Septentrionale. Masson et Cie, Paris.
Bowyer-Bower, T.A.S., Burt, T.P., 1989. Rainfall simulators for investigating soil response to
rainfall. Soil Technol. 2, 1–16.
Brandt, C.J., Thornes, J.B. ŽEds.., 1996. Mediterranean Desertification and Land Use. Wiley,
Chichester, 554 pp.
Carlson, S.R., Whitford, W.G., 1991. Ant mound influence on vegetation and soils in a semiarid
mountain ecosystem. Am. Midl. Nat. 126, 125–139.
Castillo, V.M., Martinez-Mena, M., Albaladejo, J., 1997. Runoff and soil loss response to
vegetation removal in a semiarid environment. Soil Sci. Soc. Am. J. 61, 1116–1121.
Day, A.D., Ludeke, K.L., 1993. Plant Nutrients in Desert Environments. Springer-Verlag, Berlin.
Dean, W.R.J., Yeaton, R.I., 1993. The effects of harvester ant Messor capensis nest-mounds on
the physical and chemical properties of soils in the southern Karoo, South Africa. J. Arid
Environ. 25, 249–260.
Doerr, S.H., Shakesby, R.A., Walsh, R.P.D., 2000. Soil water repellency: its causes, characteristics and hydro-geomorphological significance. Earth Sci. Rev. 51, 33–65.
Eldridge, D.J., 1993. Effects of ants on sandy soils in semi-arid eastern Australia: local
distribution and their effect on infiltration of water. Aust. J. Soil Res. 31, 509–518.
Eldridge, D.J., 1994. Nests of ants and termites influence infiltration in a semi-arid woodland.
Pedobiologia 38, 481–492.
Eldridge, D.J., Myers, C.A., 1998. Enhancement of soil nutrients around nests entrances of the
funnel ant Aphaenogaster barbigula ŽMyrmicinae. in semi-arid eastern Australia. Aust. J. Soil
Res. 36, 1009–1017.
FAO–UNESCO, 1988. Soil map of the world Žrevised legend.. World Soil Resources Report, vol.
60. FAO, Rome.

L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

19

Ferreira, A.J.D., Coelho, C.O.A., Walsh, R.P.D., Shakesby, R.A., Ceballos, A., Doerr, S.H., 2000.
Hydrological implications of soil water-repellency in Eucalyptus globulus forests, north-central
Portugal. J. Hydrol. 231–232, 165–177.
Franco, C.M.M., Tate, M.E., Oades, J.M., 1995. Studies on non-wetting sands: I. The role of
intrinsic particulate organic matter in the development of water-repellency in non-wetting
sands. Aust. J. Soil Res. 33, 253–263.
Gee, G.W., Bauder, J.W., 1986. Particle size analysis. In: Klute, A. ŽEd.., Methods of Soil
Analysis: Part 1. Physical and Mineralogical Methods. Am. Soc. Agron., Madison, USA, pp.
383–411.
Green, W.P., Pettry, D.E., Switzer, R.E., 1999. Structure and hydrology of mounds of the
imported fire ants in the southeastern United States. Geoderma 93, 1–17.
Hallett, P.D., Young, I.M., 1999. Changes to water repellence of soil aggregates caused by
substrate-induced microbial activity. Eur. J. Soil Sci. 50, 35–40.
Harper, R.J., Gilkes, R.J., 1994. Soil attributes related to water repellency and the utility of soil
survey for predicting its occurrence. Aust. J. Soil Res. 32, 1109–1124.
Imeson, A.C., Vis, M., 1984. Assessing soil aggregate stability by water-drop impact and
ultrasonic dispersion. Geoderma 34, 185–200.
Imeson, A.C., Verstraten, J.M., Van Mulligen, E.J., Sevink, J., 1992. The effects of fire and water
repellency on infiltration and runoff under Mediterranean type forest. Catena 19, 345–361.
King, P.M., 1981. Comparison of methods for measuring severity of water repellence of sandy
soils and assessment of some factors that affect its measurement. Aust. J. Soil Res. 19,
275–285.
Kosmas, C., Danalatos, N., Cammeraat, L.H., Chabart, M., Diamantopuolos, J., Farand, R.,
Gutierrez, L., Jacob, A., Marques, H., Martinez-Fernandez, J., Mizara, A., Moustakas, N.,
Nicolau, J.M., Oliveros, C., Pinna, G., Puddu, R., Puigdefabregas, J., Roxo, M., Simoa, A.,
Stamou, G., Tomasi, D., Usai, D., Vacca, A., 1997. The effect of land use on soil erosion rates
and land degradation under Mediterranean conditions. Catena 29, 45–59.
Lambregts, C., 1999. The impact and assessment of soil fauna on soil surface properties of a
degraded Mediterranean ecosystem. Unpublished PhD Thesis, University of Amsterdam.
Lavee, H., Imeson, A.C., Pariente, S., 1998. The impact of climate change on geomorphology and
desertification along a mediterranean–arid transect. Land Degrad. Dev. 9, 407–422.
Lobry de Bruyn, L.A., Conacher, A.J., 1990. The role of termites and ants in soil modification: a
review. Aust. J. Soil Res. 28, 55–93.
Lobry de Bruyn, L.A., Conacher, A.J., 1994. The effect of ant biopores on water infiltration in
soils in undisturbed bushland and in farmland in a semi-arid environment. Pedobiologia 38,
193–207.
Louw, G.N., Seely, M.K., 1982. Ecology of Desert Organisms. Longman, London.
Low, A.J., 1954. The study of soil structure in the field and the laboratory. J. Soil Sci. 5, 57–74.
Ludwig, J.A., Tongway, D.J., Eager, R.W., Williams, R.J., Cook, G.D., 1999. Fine-scale
vegetation patches decline in size and cover with increasing rainfall in Australian savannas.
Landscape Ecol. 14, 557–566.
Mando, A., Stroosnijder, L., Brussard, L., 1996. Effects of termites on infiltration into crusted
soil. Geoderma 74, 107–113.
Marshall, T.J., Holmes, J.W., 1988. Soil Physics, 2nd edn. Cambridge Univ. Press, Cambridge.
Nicolau, J.M., Sole,
L., 1996. Effects of soil and vegetation on
´ A., Puigdefabregas, J., Gutierrez,
´
runoff along a catena in semi-arid Spain. Geomorphology 14, 297–309.
Noy-Meir, I., 1981. Spatial effects in modelling of arid ecosystems. In: Goodall, D.W., Perry,
R.A. ŽEds.., Arid-land Ecosystems: Structure, Functioning and Management, vol. 2. Cambridge Univ. Press, Sydney, pp. 411–432.
Pugnaire, F.I., Haase, P., Puigdefabregas,
J., Cueto, M., Clark, S.C., Incoll, L.D., 1996. Facilita´

20

L.H. Cammeraat et al.r Geoderma 105 (2002) 1–20

tion and succession under the canopy of a leguminous shrub, Retama sphaerocarpa, in a
semiarid environment in Southeast Spain. Oikos 76, 455–464.
Puigdefabregas,
J., 1995. The Almerıa
´
´ core field site. Cammeraat, L.H., Prinsen, H.A.M. ŽEds..,
The MEDALUS application on CD-ROM, DaVincirUniversity of Amsterdam, ChaumontGistouxrAmsterdam.
Puigdefabregas,
J., Alonso, J.M., Delgado, L., Domingo, F., Cueto, M., Gutierrez,
L., Lazaro,
R.,
´
´
´
Nicolau, J.M., Sanchez,
G., Sole,
´
´ A., Vidal, S., Aguilera, C., Brenner, A., Clark, S., Incoll, L.,
1996. The Rambla Honda Field site: interactions of soil and vegetation along a catena in
semi-arid southeast Spain. In: Brandt, C.J., Thornes, J.B. ŽEds.., Mediterranean Desertification
and Land Use. Wiley, Chichester, pp. 137–168.
Puigdefabregas,
J., Sole, A., Gutierrez, L., del Barrio, G., Boer, M., 1999. Scales and processes of
´
water and sediment redistribution in drylands: results from the Rambla Honda field site in
Southeast Spain. Earth Sci. Rev. 48, 39–70.
Quinton, J.N., Edwards, G.M., Morgan, R.P.C., 1997. The influence of vegetation species and
plant properties on runoff and soil erosion: results from a rainfall simulation study in southeast
Spain. Soil Use Manage. 13, 143–148.
Rissing, S.W., 1986. Indirect effects of granivory by harvester ants: plant species composition and
reproductive increase near ant nests. Oecologia 68, 231–234.
Schlesinger, W.H., Reynolds, J.F., Cunningham, G.L., Huenneke, L.F., Jarrell, W.M., Virginia,
R.A., Whitford, W.G., 1990. Biological feedbacks in global desertification. Science 247,
1043–1048.
Scoging, H., 1989. Runoff generation and sediment mobilisation by water. In: Thomas, D.S.G.
ŽEd.., Arid Zone Geomorphology. Belhaven Press, London, pp. 87–116.
Soil Survey Staff, 1992. Keys to Soil Taxonomy, 5th edn. SMSS Technical Monograph, vol. 19.
Pocahontas Press, Blacksburg, USA.
Valentin, C., Bresson, L.-M., 1992. Morphology, genesis and classification of surface crusts in
loamy and sandy soils. Geoderma 55, 225–245.
Van’t Woudt, B.D., 1959. Particle coatings affecting the wettability of soils. J. Geophys. Res. 64,
263–267.
Wallis, M.G., Horne, D.J., 1992. Soil water repellency. Adv. Soil Sci. 20, 91–146.
Wang, D., Lowery, B., Norman, J.M., McSweeny, K., 1996. Ant burrow effects on water flow
and soil hydraulic properties of Sparta sand. Soil Tillage Res. 37, 83–93.
Whitford, W.G., DiMarco, R., 1995. Variability in soils and vegetation associated with harvester
ant Ž Pogonomyrmex rugosus . nests on a Chihuahuan Desert watershed. Biol. Fertil. Soils 20,
169–173.
Willott, S.J., Compton, S.G., Incoll, L.D., 2000. Foraging, food selection and worker size in the
seed harvesting ant Messor bouÕieri. Oecologia 125, 35–44.