ORIGINAL PAPER
Soil compaction and surface-active arthropods in historic,
agricultural, alien, and recovering vegetation
Rembu N. Magoba
1
•
Michael J. Samways
1
•
John P. Simaika
1
Received: 19 March 2014 / Accepted: 25 March 2015 / Published online: 3 April 2015
Ó Springer International Publishing Switzerland 2015
Abstract Soil compaction is a major threat to natural
resources. However, little information is available on the
impacts of soil compaction on arthropod diversity espe-
cially relative to different types of vegetation, land use and
restoration activities. In response to this dearth of infor-
mation, we studied soil compaction, as well as percentage
soil moisture and mean leaf litter depth, associated with
four vegetation types: natural vegetation (fynbos, the his-
toric condition), agricultural land (vin eyards), invasive
alien trees, and vegetation cleared of invasive alien trees
(recovering vegetation). Our study took place in the Cape
Floristic Region, South Africa, a biodiversity hotspot,
yet also an area of intense viticulture and heavy invasion by
alien plants. We sampled soil surface-active arthropods
using pitfall traps, and compared species richness and
abundance in different vegetation types with various levels
of soil compaction and other soil variables. Overall, vine-
yards had the highest soil compaction while natural fynbos
and aliens had low and comparable compaction. For both
arthropod species richness and abundance, the order of the
four vegetation types was, from highest to lowest: natural
fynbos, alien cleared sites, vineyards, and alien infested
sites. Level of soil compaction negatively correlated with
arthropod species richness but not with abundance. Nei-
ther soil moisture nor leaf litter depth on their own sig-
nificantly affected arthropod species richness or
abundance. While alien trees overall had a strong negative
effect on both arthropod species richness and abundance,
and much more so than vineyards, the situation is re-
versible, with removal of aliens being associated with rapid
recovery of soil structure and of arthropod assemblages.
This is an encouraging sign for restoration.
Keywords Soil disturbance Invertebrates Arthropod
conservation Insect conservation Restoration
Introduction
Human-induced soil compaction (dry bulk density) is one
of most important factors threatening natural resources
(van den Akker and Soane 2005; Eudoxie and Springer
2006; Kirby 2007). It occurs when soil undergoes me-
chanical stress, mainly through use of heavy machinery or
overgrazing, especially during wet soil conditions. The
indirect effects of soil compaction may be less clear, and
are often the result of highly variable soil characteristics
(Boizard et al. 2000).
Soil compaction warrants further attention as it struc-
tures vegetation (Roberts 1987; Mitchell 1991), including
that of alien plants (Payet et al. 2001) and therefore, may
influence temperature of the soil surface directly due to
shade, and amount of leaf litter from vegetation (Farji–
Brener et al. 2008) necessary for soil fauna. In addition,
soil compaction from farming operations also reduces
water infiltration (Chan 2001; Mitchell 1991) and increases
Electronic supplementary material The online version of this
article (doi:10.1007/s10841-015-9771-8) contains supplementary
material, which is available to authorized users.
& Michael J. Samways
samways@sun.ac.za
Rembu N. Magoba
Rembu.Magoba@capetown.gov.za
John P. Simaika
simaikaj@sun.ac.za
1
Department of Conservation Ecology and Entomology,
Stellenbosch University, P/Bag X1, Matieland 7602,
South Africa
123
J Insect Conserv (2015) 19:501–508
DOI 10.1007/s10841-015-9771-8
soil erosion (Chan et al. 2006). Erosion, in turn, reduces
vegetation cover (Mitchell 1991). However, the extent of
soil compaction and its impact on surface-dwelling
arthropods under different land uses has not been investi-
gated to any extent.
Many soil organisms are important members of terres-
trial ecosystems (Smit and Van Aarde 2001). While many
arthropods spend part or all their lives in the soil (Chan and
Barchia 2007), many others feed above ground, using the
soil as a pupation or nesting site. The potential benefits
from arthropods to agro-ecosystems include improvement
of soil structure and nutrient cycling (Edwards and Bohlen
1996).
Plant litter decomposition is an important biological
process driven by ground living organisms (Dyer et al.
1990; Lawrence and Samways 2003; Pausas et al. 2004).
Knowledge of this process is important for conservation of
the soil (Chan 2001). In turn, relationships between soil
fauna and soil properties can be used to assess the impact
of landscape managem ent (Clapperton et al. 2004). Dis-
turbance of the soil profile can change, directly or indi-
rectly, the composition of arthropod diversity associated
with it (Wallwork 1970; Manzer et al. 1984; Wall and
Moore 1999; Dunxiao et al. 1999; Holland and Luff 2004;
Farji–Brener et al. 2008; Jabin 2008). Moreover, the effects
on soil structure may also affect species density through
indirect effects mediated by their potential influence on
plants (Van Dijck and Van Asch 2002; Coulouma et al.
2006).
In the Cape Floristic Region (CFR), the comparative
influence of soil compaction on ground arthropod assem-
blages under various conditions of land use has not been
investigated, despite this region being a global biodiversity
hotspot (Mittermeier et al. 2005). There are severe stressors
to the biodiversity of the region, especially from agricul-
tural conversion (especially to vineyards) and from the
extensive imp act of invasive alien plan ts (Rouget et al.
2003; Gaertner et al. 2009; Fairbanks et al. 2004).
It is unknown how infestation by alien trees and from
agricultural practices (specifically viticulture) influence
soil compaction and associated arthropod diversity. Apart
from Magoba and Samways (2012), there is little knowl-
edge on how these assemblages recover when the alien
trees are removed, a significant mitigation measure in the
region which has been assessed using aerial taxa (Samways
and Sharrat 2010). To date there is no information focusing
on the role of soil compaction in this process of recovery.
In view of this dearth of information, we pose two hy-
potheses. Firstly, we hypothesize that a change in vegeta-
tion structure will alter soil compaction. Secondly, we
hypothesize that there are significant correlations between
arthropod diversity, soil compaction, leaf litter, site loca-
tion or soil moisture content under the four main vegetation
types (natura l vegetation, agricultural produc tion, invasive
alien trees, and recovering vegetation).
Study area and methods
Study sites
Vegetation of the study area (between Stellenbosch and
Somerset West, South Africa: 18° 15
0
E, 34 °00
0
S) included
many species-rich communities, occurring on highly in-
fertile soils derived from sandstone of the Table Mountain
Group. These communities range from upland study sites
where soils are well drained and rocky to sand-loamy soil
on the foothills and in the valleys. Intrusion of shale in
some vineyards (i.e. Rustenberg) is evident. Boucher and
Moll (1981) recognize two main soil types in the area.
Lithosols, are generally shallow (\30 cm), grey sandy soils
associated with the Table Mountain Group (TMG) sand-
stone, have a weak profile differentiation and contain
coarse fragments and solid rock, and are largely the soil
type studied here. Selected sites had similar edaphic con-
ditions derived from the same parent material, TMG
sandstone. For each site, the topographical conditions of
adjacent distinct vegetation were similar.
At each of ten localities (three protected areas (PAs):
Jonkershoek, Helderberg and Hottentots Holland; and seven
wine estates: Vergelegen, Bilton, Stellenzicht, Driekoppen,
Waterford, Rustenberg and Dornier) four vegetation types
were selected: natural fynbos, alien invasive trees (IATs),
cleared of invasive alien trees (CIATs), and vineyards.
Transects (see details below) were established, so that the
four vegetation types were adjacent to each other in their
various combinations. This resulted in six different pairs of
vegetation types (Natural fynbos-IATs; Natural fynbos-
CIATs; IATs-CIATs; Natural fynbos-vineyards; vineyards-
IATs; and vineyards-CIATs). In total, there were 36
transects.
The natural fynbos sites were untransformed by human
activity and selected from the PAs and wine farms with
\10 % alien tree cover. Natural fynbos was predominantly
mountain fynbos, with common plant species being As-
palathus forbesii, A. aspalathoides, Lebeckia sepiaria,
Lotononis prostrata, Cyphia phyteuma, Chasmanthe
aethiopica, Watsonia borbonica, Gymnodiscus capillaris,
Dimorphotheca pluvialis, Hymenolepis crithmoides Protea
compacta, P. repens, P. neriifolia, and Salix species, as
well as various ericas. The IAT sites had [90 % alien tree
cover, represented mainly by Acacia mearnsii, A. longifo-
lia, A. saligna, Hakea sericea, H. drupacea, Pinus pinaster,
P. radiata, Eucalyptus lehmannii, E. diversicolor and
Populus trees, with an understorey of grasses and forbs.
The CIAT sites had been cleared of the alien trees in 2000,
502 J Insect Conserv (2015) 19:501–508
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by felling and herbicide treating of stumps, and allowing
the indigenous fynbos to recover of its own accord. After
6 years of recovery, the vegetation at these cleared sites
was comparable in dominant species to that in natural
fynbos (see above). The fourth vegetation type was organic
vineyards, where there was no application of artificial
fertilizers as the soils are relatively fertile through perma-
nent crop cover (i.e. wheat), and pesticides were only ap-
plied as a last resort when necessary. One application of the
chemical agent chlorpyrifos was applied during early Au-
gust to control the mealybug Planococcus ficus which is
the vector of a viral disease of the vines.
Sampling
Sampling of arthropods was on three occasions (August–
October 2006, May–July 2007 and November 2007–January
2008) using pitfall traps (Samways et al. 2010). Each sam-
pling occasion was for 5 days. Soil compaction and per-
centage (%) soil moisture content measurements at each
sampling station were undertaken using a Radioactive
moisture-density gauge instrument (Troxler 3411-B). Leaf
litter depth was determined by inserting a steel rod, 4 mm in
diameter, into the leaf-litter until the harder soil layer was
reached (Lawes et al. 2005). Mean leaf-litter depth was then
estimated from three random measurements in each 2 m
2
quadrat created at each sampling station around the trap set.
The 256 m transects consisted of a trap-set of two indi-
vidual pitfall traps, 1 m apart, placed at log 2 intervals: 2, 4, 8,
16, 32, 64 and 128 m on either side of the boundary between
two adjoining vegetation types. Every patch on either side of
the boundary was [100 m 9 100 m, and all but one
[250 m 9 250 m. This spacing gave equal weighting to
edges and interiors of patches. However, two transects, be-
tween alien vegetation and fynbos, were each four traps short,
owing to unavailability of extensive sites. The total was 1000
pitfall traps (two per set, fourteen sets per transect, six tran-
sects per vegetation type pair and six vegetation pairs from
four vegetation types, minus eight traps).
Pitfall traps for sampling arthropods were 500 ml plastic
honey jars, each containing a replaceable paper cup, 8 cm
diameter, 12 cm deep. Each trap was o ne-third filled with
70 % ethanediol. Traps remained closed during non-sam-
pling periods, and opened for 5 consecutive days without
rain during sampling periods (Borgelt and New 2006).
Samples were then washed in water, and later transferred to
70 % ethanol for later identification in the laboratory. The
collected surface-active arthropods were sorted and allo-
cated to families. Where possible, they were identified to
species level. Voucher specimens are in the Entomology
Museum, Stellenbosch University, although spiders are in
the National Collection of Arachnida, National Museum,
Pretoria. Identification was by keys and expert opinion.
Data analyses
One-way analysis of variance (ANOVA) using SPSS v17
software (SPSS Inc. 2006), was performed on the selected
soil factors comparing the different vegetation types. Clas-
sification trees for all the vegetation types in terms of soil
compaction, leaf litter depth, percentage soil moisture and
species richness were produced separately using CHAID
growth limits incorporated in SPSS v17 software (SPSS Inc.
2006). Significance level for splitting nodes and merging
categories was 0.05 and the significance values adjusted
using the Bonferroni method. A variety of non-parametric
species estimators were used to provide the best overall
arthropod species estimates for all the vegetation types
(Hortal et al. 2006). Incidence-based Coverage Estimator
(ICE) is a robust and accurate estimator of species richness
(Chazdon et al. 1998), whereas Chao2 and Jackknife esti-
mators provide the least biased estimates for insufficient
sampling (Colwell and Cod dington 1994). Th erefore, we
calculated these estimators using EstimateS (Colwell 2006)
for all the vegetation types separately and for a combination
of these. One-way ANOVA was performed on the species
and the log transformed abundance data comparing the dif-
ferent vegetation types with multiple comparisons of the
means, using Tamahane’s post hoc. We used SpatialPack
(version 0.2–3) in R (R Core Team 2013) to calculate the
corrected Pearson’s correlations for spatial autocorrelation
between assemblage composition and soil factors.
Results
Soil factors among the four vegetation types
Soil compaction was highest in vineyards, followed by
cleared sites and fynbos, and lowest in aliens (Table 1). In
turn, percentage moisture content was lowest in vineyards,
followed by cleared sites, aliens, and finally fynbos with
the highest. Litter depth was greatest by far in aliens, fol-
lowed by fynbos and closely by cleared sites, with vine-
yards with by far the least. ANOVA among the four
vegetation types showed that there were significant dif-
ferences in soil compaction (df = 3, f = 19.36,
p \ 0.001), litter dept h (df = 3, f = 296.6, p \ 0.001) and
percentage soil moisture (df = 3, f = 15.8, p \ 0.001).
The classification tree of significant soil compaction
values indicated simi larity between fynbos and cleared
sites (Table 1; online Figure 1). Aliens and vineyards were
significantly different from each other, but neither was
comparable to either fynbos or cleared sites. Classification
of site locations based on soil compaction resulted in three
nodes (Fig. 1; online Figure 2). All three nature reserves
had similar low soil compaction (Node 1). Some vineyards
J Insect Conserv (2015) 19:501–508 503
123
(Node 2) had significantly higher soil compaction than any
other location. Rustenberg, Bilton, and Dornier vineyards
had more comparable soil compaction (Node 3).
Classification of the different vegetation types in terms of
litter depth resulted in four separate categories (Table 1,
online Figure 3). Vineyard clustering (Node 4) showed low
leaf litter depth. Moreover, there were significant differences
between the ten site locations (df = 506, f = 35.36,
p \ 0.001) in terms of litter depth, with the highest leaf litter
depths recorded from Jonkershoek and Hottentots Holland
nature reserves respectively (online Figure 4). However,
Helderberg nature reserve was associated with relatively
lower litter depth, which was comparable to those in some
vineyards. Classification of different vegetation types using
percentage soil moisture resulted in two vegetation cate-
gories: cleared sites were comparable to vineyards, and
fynbos to aliens (online Figure 5).
Arthropod species richness among the four
vegetation types
A total of 22,255 individuals were sampled, and assigned to
198 morphospecies/species. In terms of both arthropod spe-
cies and abundance, the four vegetation types followed the
same sequence, from highest to lowest: fynbos, cleared sites,
vineyards, and aliens (Table 2). ANOVA among the four
vegetation types gave significant differences among (df = 3,
f = 41.65, p \ 0.0001), and within (df = 509, f = 41.65,
p \ 0.0001) them in terms of species richness. Any one
vegetation type shared at least 77 % of all sampled species,
with fynbos having 90 % and cleared sites 85 % (Table 2).
A Tamhane’s post hoc test after ANOVA among the four
vegetation types (Fig. 1) gave significant differences in
species richness between fynbos and aliens (p \ 0.001);
fynbos and vineyards (p \ 0.001); cleared sites and alien sites
(p \ 0.001); and cleared sites and vineyard (p \ 0.001).
However, there were no statistically significant differences
between cleared sites and fynbos (p = 1.00) in terms of spe-
cies richness (online Figure 6). Although vineyards had
relatively higher mean arthropod abundance than aliens
(Table 2), the difference was not significant (p = 1.00). All
other combinations of abundance were significant.
Correlation between soil factors and arthropod
species richness and abundance among the four
vegetation types
There was a significant negative correlation between soil
compaction and species richness, but not abundance
(Table 3). High soil compaction in vineyards did not cor-
relate with reduced species richness. However, percentage
soil moisture content and leaf litter depths were sig-
nificantly negatively correlated with the soil compaction,
and leaf litter depth was positively correlated to the per-
centage soil moisture, although neither soil moisture or
litter depth on their own significantly affected arthropod
species richness or abundance (Table 3).
Discussion
Soil factors associated with the vegetation types
We found high soil compaction in the vineyards, which
comes about from many activities associated with grape
production (Ferrero et al. 2005). At the othe r end of the
Table 1 Soil factor means
(±1SE) for fynbos, invasive
alien trees (IATs), cleared
invasive alien trees (CIATs),
and vineyard sites
Variable Vegetation
type
Mean N SE Node
Soil compaction (Kg/m
3
) CIATs 1277.1 115 12.06 1
Fynbos 1270.7 145 11.83 1
IATs 1205.4 124 15.77 2
Vineyard 1341.6 126 9.75 3
All groups 1273.8 510 6.59
Percentage soil moisture CIATs 8.78 115 0.27 1
Fynbos 9.76 145 0.28 2
IATs 9.49 124 0.30 2
Vineyard 8.36 126 0.22 1
All groups 9.13 510 0.14
Leaf litter depth (mm) CIATs 13.65 115 0.52 1
Fynbos 15.57 145 0.39 2
IATs 20.80 124 0.58 3
Vineyard 2.74 126 0.21 4
All groups 13.24 510 0.36
Values in Italic refer to the averages for the four vegetation types combined
504 J Insect Conserv (2015) 19:501–508
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spectrum, we found alien trees had significantly lower soil
compaction compared to either fynbos or the sites cleared
of alien trees, possibly due to a combination of root activity
and accumulat ion of litter. As well as having highest
compaction, vineyards also had lowest percentage moisture
content, probably due to high evaporation rates associated
with the structurally open production system, in compar-
ison with the dense fynbos or alien vegetation. Overall, soil
moisture was highly responsive to the type and structure of
associated vegetation. While fynbos and alien vegetation
Fig. 1 Boxplots comparing means among fynbos (natural), invasive
alien trees (IATs), cleared invasive alien trees and vineyard sites in
terms of species richness, species abundance, soil compaction
(kg/m
3
), percentage soil moisture and leaf litter depth (mm). Letters
at the boxplots (a–d) indicate significant differences between means,
based on the results of a Tamahane’s post hoc test after an analysis of
variance test. The dark middle line of the box is the median. Whiskers
(vertical lines) represent maximum and minimum values that are not
statistical outliers. Black dots and stars are statistical outliers
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