• Agroecosystem management;
  • Araneae;
  • arthropod conservation;
  • conventional agriculture;
  • landscape ecology;
  • organic agriculture


  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
  1. Spiders often dominate predator assemblages in vineyard agroecosystems. Land-use practices that enhance their diversity may be important for regional biodiversity conservation as well as biological control of vineyard pests.
  2. We surveyed ground-dwelling spiders in habitats at different management intensities (natural habitat remnants, organic vineyards, and conventional vineyards) in the vineyard landscape of the Cape Floristic Region (CFR) biodiversity hotspot, South Africa, to examine the effects of local land use on spider diversity.
  3. Natural habitat remnants contributed significantly to the overall diversity, and supported rare families and unique species that were not found in the agricultural matrix. Nevertheless, vineyards still supported fairly high spider diversity, with a tendency for higher species richness in organic vineyards. Environmental variables relating to plant diversity, management intensity and distance to nearest natural habitat influenced spider abundance and species richness, and changes to these factors may improve the habitability of the vineyard landscape for spiders.
  4. All habitats supported various spider families that are known to prey on South African crop pests. In addition, spiders were abundant in both sampling seasons (autumn and spring), but showed a significant increase in vineyards early in the growing season, suggesting that they may provide early-season background regulation of pests.
  5. There is considerable opportunity for spider conservation in the CFR vineyard landscape, as well as the possibility of associated benefits to biological control within vineyards.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

As in many parts of the world, spiders in South Africa are a widespread, highly diverse arthropod group (Foord et al., 2011; Dippenaar-Schoeman et al., 2012). Baseline studies, focused mostly on the north-eastern provinces, have produced much information on spider diversity and distribution in natural and agricultural systems (Foord et al., 2011; Dippenaar-Schoeman et al., 2013). Yet, surveys in the Western Cape Province, which comprises most of the Cape Floristic Region (CFR), a global biodiversity hotspot (Myers et al., 2000), have been limited. Recent surveys in the CFR suggest that the region's spider diversity may be comparable to that of the most species-rich biomes (Haddad & Dippenaar-Schoeman, 2009) and the expansion of survey efforts in the Western Cape has been recommended (Haddad & Dippenaar-Schoeman, 2009; Foord et al., 2011).

The CFR is an important agricultural area. A third of the CFR has been transformed by agriculture (Rouget et al., 2003), with vineyards being one of the dominant crops, comprising 95% of South Africa's vineyards. Future transformation of natural habitat for vineyard cultivation and its associated impacts on biodiversity is a key threat to CFR ecosystems (Fairbanks et al., 2004). Fortunately, conservation initiatives and production schemes that promote sustainable production and protection of natural habitat on CFR wine farms (CAPE, 2011; Conservation at Work, 2013; IPW, 2013; WWF, 2013) provide opportunities for biodiversity conservation in the vineyard landscape. There is considerable scope for research that will inform these practices to help make the landscape more habitable for native species.

Spiders often dominate predator assemblages in South African agroecosystems (Dippenaar-Schoeman et al., 2013), including vineyards, where, in terms of abundance, they can comprise up to 64% of the ground-living predator assemblage (Gaigher, 2008; Gaigher & Samways, 2010). Because they are such a consistent component of the natural enemy assemblage, there is increasing interest in the biological control potential of spiders in South African crops (Dippenaar-Schoeman et al., 2013). Evidence from other crops suggests that they can reduce densities of pests such as citrus psylla (Van den Berg et al., 1992), cotton bollworm, and red spider mite (Dippenaar-Schoeman et al., 1999). They may be important as part of the natural enemy complex within sustainable production systems. Investigation into their ecology in vineyards is, therefore, interesting both from a conservation and agricultural point of view.

We focus here on the diversity and distribution of spiders in the CFR vineyard landscape. We aim to contribute to the known spider distribution data in the Western Cape and to examine the effects of local land-use on spider diversity, focusing specifically on the effect of farming intensity and the contribution of natural habitat fragments. Based on these results, their potential for pest suppression in CFR vineyards is discussed.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

Study sites

Study sites were selected in Stellenbosch in the winelands of the Western Cape Province of South Africa. We selected three localities 10–20 km apart (Table 1), each containing a natural vegetation fragment (Fynbos or Renosterveld), an organic vineyard, and a conventional vineyard, to represent habitat types under various levels of management intensity. The different habitat types within a locality were separated by 1–5 km. Organic vineyards had been under certified organic management for at least 4 years and conventional vineyards operated under the Integrated Production of Wine Scheme, a system that promotes sustainable viticulture within a conventional framework (IPW, 2013).

Table 1. Study site coordinates and locality names
LocalityHabitat typeLatitudeLongitude
PolkadraaiNatural habitat−33.94738418.742603
Organic vineyard−33.95816618.753502
Conventional vineyard−33.95693718.745829
AnnandaleNatural habitat−34.00221818.876347
Organic vineyard−33.99916518.843979
Conventional vineyard−34.00358318.872713
KlapmutsNatural habitat−33.8304218.858538
Organic vineyard−33.85063418.858479
Conventional vineyard−33.84503818.869601


Spider sampling was undertaken in May and October 2006 (autumn and spring), using 250 ml pitfall traps (diameter 8 cm, depth 9 cm) half filled with 70% ethylene glycol. Ten samples of two pitfall traps per sample were taken per site (90 samples and 180 traps in total per season). The two traps within each sample were separated by 1 m, and data for each trap pair were pooled. Samples were separated by 40–50 m and were always at least 20 m from habitat edges to avoid edge effects. Traps were left open for 5 days in both seasons. Spiders were identified by a taxonomist. As immatures were present in samples, and their taxonomy is uncertain, not all specimens could be identified to species level. Specimens were deposited at the National Collection of Arachnida at the National Museum, Pretoria.

Thirteen environmental variables were obtained for each site (Table 2). These included six variables relating to vegetation structure that were measured within a 0.5 × 0.5 m quadrat at each sampling point: percentage weed cover, percentage indigenous plant cover, plant species richness, plant height, leaf litter depth, and leaf litter dry weight. Leaf litter depth was recorded as the mean of three measurements of vertical litter depth above the soil surface and litter dry weight was the total amount of litter collected within the quadrat, which was dried and weighed. Five variables relating to management intensity were obtained by calculating intensity indices from grower records for the period Jan 2006–Jan 2007 (see Gaigher & Samways, 2010): insecticide, fungicide, herbicide, fertiliser, and tillage intensity. Two landscape variables were calculated from satellite images in ArcGIS 9 (Esri, Redlands, CA, USA): percentage cover of natural habitat within a 500 m radius around each site and distance to nearest natural habitat fragment, as measured from the centre of the site.

Table 2. Mean values for environmental variables measured in the three habitat types
VariablesNatural habitatOrganic vineyardsConventional vineyards
Percentage weed cover4.5324.557.70
Percentage indigenous plant cover36.100.38
Plant species richness8.205.903.37
Plant height (cm)63.0016.104.52
Leaf litter depth (cm)0.472.300.59
Leaf litter dry weight (g)1.1410.092.00
Insecticide intensity index3.67
Fungicide intensity index4.0011.67
Herbicide intensity index5.00
Fertiliser intensity index2.676.00
Tillage intensity index2.001.83
Percentage natural habitat in 500 m radius60.4916.4925.35
Distance to nearest natural habitat (m)121.9491.68

Data analyses

Spider data for the two sampling periods were pooled for all analyses, except for the comparisons between autumn and spring sampling. Generalised linear models (GLZs) were used to test for differences in spider abundance and species richness between sites and sampling periods. GLZs extend linear models by allowing for response variables with non-normal error distributions (McCullagh & Nelder, 1989). GLZs were also used to identify environmental variables that significantly influenced spider abundance and richness. A log-link function was specified to account for Poisson distributed data in all models (McCullagh & Nelder, 1989). Akaike's information criterion was consulted in model selection, and percentage weed cover, number of plant species, leaf litter depth, insecticide intensity, tillage intensity, percentage natural vegetation, and distance from natural vegetation were included in the final models. GLZs were performed in SAS Enterprise Guide 4.1 (SAS Institute Inc., Cary, NC, USA).

To test for differences in spider assemblage structure between sites, ANOSIM procedure was performed in Primer 5.9.2 (Clarke & Gorley, 2006). ANOSIM is a non-parametric multivariate analysis of variance that uses ranked distance or dissimilarity between pairs of samples (Clarke & Warwick, 2001). An RANOSIM value closer to 0 indicates greater similarity between assemblages and closer to 1 indicates greater dissimilarity. Similarity matrices were based on square-root transformed Bray–Curtis similarities. Assemblage patterns were visualised using non-metric multidimensional scaling (nMDS).

Similarity percentage analyses (SIMPER) were performed to detect species that contributed most to differences between habitat types. The ratio of the average dissimilarity among habitats (Dis) and the associated standard deviation (SD) indicates how consistently a species contributes to differences between habitats. Species with a high Dis/SD ratio are considered to be key discriminating species (Clarke & Warwick, 2001) and, therefore, only those species with a ratio >1 were considered further.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

A total of 1586 spider individuals were collected representing 45 species from 16 families (Table 3). Three families contributed 80% of the total spider abundance, i.e. Gnaphosidae (39.5%), Lycosidae (23.5%), and Amaurobiidae (16.3%). Family diversity differed between the three habitat types, with natural sites having a higher family diversity than both vineyard types (Fig. 1). Natural habitat supported 16 families, and the overall assemblage was dominated by Gnaphosidae (33.2%), Amaurobiidae (22.4%), Lycosidae (12.7%), and Linyphiidae (10.8%). Nine families were recorded in organic vineyards and Gnaphosidae (44.3%) and Lycosidae (31.5%) dominated the assemblage. Conventional vineyards supported 11 families and were also dominated by Gnaphosidae (41.9%) and Lycosidae (27.4%) (Fig. 1). Families that are common and common-to-rare in South Africa were well-supported by all habitats, but some rare families such as Ammoxenidae and Palpimanidae were absent from vineyards (Table 3).

Table 3. Total abundance of spider species recorded pooled for the three habitat types
Family Genus and speciesNatural habitatOrganic vineyardsConventional vineyards
  1. Letters in parenthesis refer to family status (R, rare; C, common; CR, common-to-rare) and lifestyle (WE, web-dwellers; WA, wanderers). Asterisks next to family names refer to its documented role in pest suppression in South African crops (*demonstrated in the field, **demonstrated in the lab).

Amaurobiidae(R, WE)Chresiona sp.1316563
Ammoxenidae(R, WA)Ammoxenus sp.3300
Clubionidae*(C, WA)Clubiona sp.110
Genus & species undet.200
Clubiona umbilensis 010
Cyrtaucheniidae(R, WA)Ancylotrypa sp. 1003
Ancylotrypa sp. 2100
Gnaphosidae(C, WA)Camillina sp.894
Genus & species undet.131811
Zelotes sp. 3602
Genus & species undet.2150
Trachyzelotes jaxartensis 897951
Drassodes solitarius 519389
Zelotes fuligineus 984
Setaphis subtilis 403
Zelotes oneili 42612
Pterotrichia varia 723
Upognampa parvipalpa 140
Idiopidae(CR, WA)Ctenolophus sp.100
Linyphiidae(C, WE)Genus & species undet.182320
Genus & species undet.2151
Genus & species undet.2312
Genus & species undet.122
Liocranidae(R, WA)Rhaeboctesis sp.101
Lycosidae*(C, WA)Geolycosa sp.1155
Hogna sp.236041
Trabea sp.910
Pardosa crassipalpis 2667
Pardosa sp. 2294964
Palpimanidae(R, WA) Diaphorocellus biplagiatus 2900
Philodromidae*(CR, WA)Thanatus sp.142
Pisauridae(CR, WE)Rothus sp.100
Salticidae*(C, WA)Pellenes sp. 2101422
Langelurillus sp.3128
Euophrys sp.701
Pellenes sp. 3820
Stenaelurillus sp.110
Pellenes sp. 1101
Massagris sp.741
Evarcha sp.100
Scytodidae(C, WA) Scytodes testudo 322
Theridiidae**(C, WE) Steatoda capensis 101
Theridion sp.100
Steatoda sp. 2200
Thomisidae*(C, WA) Xysticus subjugalis 821
Total abundance per habitat type585574427

Figure 1. Rank abundance plot of spider families in (a) natural habitat, (b) organic vineyards, and (c) conventional vineyards, pooled for the three localities.

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Sites differed in spider abundance (χ2 = 228.72, < 0.0001; Fig. 2a). In Localities 1 and 3, natural habitat had a significantly higher abundance than organic (ZLocality1 = 4.20, < 0.0001; ZLocality3 = 2.82, < 0.005) and conventional vineyards (ZLocality1 = 3.86, < 0.0001; ZLocality3 = 7.64, < 0.0001). In Locality 2, the organic vineyard had the highest abundance, which was significantly higher than the natural habitat (ZLocality2 = 6.91, < 0.0001), and conventional vineyard (ZLocality2 = 3.85, < 0.0001).


Figure 2. Mean spider (a) abundance and (b) species richness per habitat type within the three localities (±SE). Means with letters in common are not significantly different at < 0.05.

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Sites also differed in spider species richness (χ2 = 46.84, < 0.0001; Fig. 2b). Natural habitat had the highest species richness in two of the localities, being significantly higher than the organic (ZLocality1 = 2.18, < 0.05) and conventional vineyards (ZLocality1 = 3.34, < 0.001) in Locality 1, and higher than the conventional vineyard in Locality 3 (ZLocality1 = 3.38, < 0.001). Organic vineyards had higher species richness than conventional vineyards in all localities, but these differences were non-significant.

In terms of overall assemblage structure, there was considerable overlap between sites (RANOSIM = 0.44, < 0.001). Nevertheless, nMDS revealed that natural habitat assemblages were still distinct from all vineyard assemblages (Fig. 3). Vineyards grouped according to locality rather than management class (Fig. 3). A total of 22 species were shared between all habitats, whereas natural sites had nine unique species, and organic and conventional vineyards each had one unique species.


Figure 3. nMDS ordination of Bray–Curtis similarities from square-root transformed species abundances per site, showing the grouping of sites according to habitat type (natural habitat and vineyards) and locality.

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Key discriminating species between assemblages of different habitat types were abundant species that varied in their relative abundances between habitats (Table 4). Chresiona sp. and Trachyzelotes jaxartensis contributed to differences between natural habitats and vineyards, being most abundant in natural habitats, and also more abundant in organic than conventional vineyards. Hogna sp. and Pardosa sp., both Lycosidae, a frequently encountered family in agro-ecosystems (Dippenaar-Schoeman & Jocqué, 1997), as well as Drassodes solitaries, distinguished vineyards from natural sites and were more abundant in organic than conventional vineyards. An unidentified Linyphiid (sp. 1) differed between vineyard types, being more abundant in organic vineyards (Table 4).

Table 4. Results from SIMPER analyses showing mean relative abundances of key discriminating species and their contributions to dissimilarities between natural habitats, organic vineyards, and integrated vineyards
Average dissimilarity: 65.14%Mean abundanceDis/SD% ConCum % Dis
Natural habitatOrganic vineyards
Chresiona sp.4.372.171.359.689.68
Hogna sp.0.772.001.246.6716.35
Trachyzelotes jaxartensis 2.972.631.216.6422.99
Drassodes solitarius 1.703.101.117.3130.30
Pardosa sp. 20.971.631.096.0136.31
Average dissimilarity: 68.72%Natural habitatConventional vineyards   
Chresiona sp.4.372.101.4612.7612.76
Trachyzelotes jaxartensis 2.971.701.097.3720.13
Drassodes solitarius 1.702.971.087.7527.88
Hogna sp.0.771.371.076.3934.27
Average dissimilarity: 62.65%Organic vineyardsConventional vineyards   
  1. Dis/SD, average dissimilarity/standard deviation; % Con, percentage contribution to dissimilarity; Cum % Dis, cumulative percentage dissimilarity.

  2. a

    Genus and species undetermined.

Hogna sp.2.001.371.228.218.21
Drassodes solitarius 3.102.971.157.8616.07
Trachyzelotes jaxartensis 2.631.701.138.4024.47
Chresiona sp.
Linyphiidae sp. 1a0.770.671.015.8240.27

Sites differed in terms of their environmental variables (Table 2). Natural sites generally had the highest plant species richness, plant height, indigenous plant cover, and percentage natural habitat in a 500 m radius. On average, organic vineyards had the highest weed cover and leaf litter depth and dry weight, whereas conventional vineyards had the highest intensity indices for insecticide, fungicide, herbicide, and fertiliser (Table 2). Five of the original 13 environmental variables had a significant effect on spider abundance and species richness: percentage weed cover, plant species richness, insecticide intensity, tillage intensity, and distance to nearest natural habitat (Table 5).

Table 5. Results of a generalised linear model with Poisson distribution and log-link function, indicating the influence of various biotic and abiotic variables on spider abundance and species richness
VariablesSpider abundanceSpider species richness
  1. P-values in bold indicate variables that have significant effects (< 0.05).

% Weed cover1105.18 <0.0001 118.66 <0.0001
Plant species richness148.43 <0.0001 16.01 0.01
Leaf litter depth10.000.9610.420.52
Insecticide intensity15.40 0.02 14.13 0.04
Tillage intensity162.25 <0.0001 112.66 <0.001
% Natural habitat in 500 m radius10.120.7310.050.82
Distance to nearest natural habitat172.18 <0.0001 110.71 <0.001

Season had a significant effect on spider abundance (χ2 = 23.34, < 0.0001; Fig. 4a) and species richness (χ2 = 9.79, < 0.005; Fig. 4b). Spider abundance and species richness increased from May (autumn) to October (spring) at all sites. Significant differences between seasons were seen more frequently in the vineyard sites than natural sites (Fig. 4a and b).


Figure 4. Mean spider (a) abundance and (b) species richness per site in autumn and spring (±SE). Site abbreviations: N1–N3 = natural habitats, localities 1–3, O1–O3 = organic vineyards, localities 1–3, C1–C3 = conventional vineyards, localities 1–3. * = autumn and spring samples significantly different at < 0.05.

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  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

Spider diversity and assemblage structure

Spider diversity in this study (45 species from 16 families) was comparable to that of surveys in other agro-ecosystems in South Africa. For example, 54 species from 21 families were recorded from cotton (Mellet et al., 2006) and 34 species from nine families were found in maize (Midega et al., 2008). It was also similar to that of vineyards in California where 39 species in 16 families were recorded in wine grape vineyards (Hogg & Daane, 2010) and 27 species from 15 families were recorded in wine, raisin, and table grape vineyards (Costello & Daane, 1995). Even though this is not nearly as high as spider diversity in protected CFR habitats, where family diversity estimates are as high as 54 families within reserve areas (Haddad & Dippenaar-Schoeman, 2009), our results suggest that the vineyard landscape can still support fairly high spider diversity.

Natural habitat fragments contributed considerably to the overall spider diversity in the vineyard landscape. Natural sites supported a relatively high family diversity, including some rare families and unique species not found in vineyards. In some localities, spider abundance and richness was highest in natural habitats and some of the most frequently encountered species in the study, e.g. Chresiona sp. and Trachyzelotes jaxartensis, were most abundant in natural sites. This emphasises the importance of these natural fragments as refuges for certain spiders in the landscape.

Organic vineyards were expected to be more habitable for spiders than conventional vineyards, due to their lower management intensity and more diverse plant communities. This has been shown for spiders (Feber et al., 1998), but the effects have not always been consistent for all species (Gluck & Ingrisch, 1990; Pfiffner & Luka, 2003). Our results showed a tendency for higher abundance and richness in organic vineyards. The differences were not significant, but they were consistent among localities, suggesting that reduced farming intensity may play a role in improving the agricultural matrix for spiders.

The most important factors influencing overall spider abundance and richness were related to management intensity (insecticide and tillage intensity), plant diversity (percentage weed cover and plant species richness), and distance to nearest natural habitat. Management intensity- and plant diversity-related factors have also been implicated in influencing spider assemblages in orchards in the Northern Cape (Haddad et al., 2008), and these effects have been widely demonstrated for farmland arthropods in general (Attwood et al., 2008). Arthropod mortality from pesticides is a key concern and has been shown to have consistent negative effects on biodiversity (Geiger et al., 2010). Furthermore, disruptive practices such as tillage can cause invertebrate mortality by physical disruption or by altering the microhabitat structure and resource availability (Stinner & House, 1990), and high levels of cultivation have been shown to reduce spider abundance (Sharley et al., 2008) and survival during overwintering (Horne & Edward, 1998). In contrast, increased plant diversity is known to increase prey availability and microhabitat diversity for spiders (Rypstra et al., 1999), and can increase their abundance and species richness (Costello & Daane, 1998; Feber et al., 1998; Midega et al., 2008). In addition, natural habitat patches in close proximity to vineyards can enhance spider colonisation into vineyards (Hogg & Daane, 2010). Practices within vineyards, such as a reduction in physical disturbance and an increase in plant diversity, as well as conservation of natural habitat in farmland may, therefore, benefit overall spider diversity and should be explored further.

Possible background biological control

To date, no studies have been done on spider predation in South African vineyards. Yet, in the United States and New Zealand vineyards, spiders have been observed preying on major vineyard pests such as mealybugs (Hemiptera: Pseudococcidae), leafhoppers (Hemiptera: Cicadellidae) and leafrollers (Lepidoptera: Torticidae) (Costello & Daane, 1999; Frank et al., 2007; Daane et al., 2008), and may contribute to the control of these economically important pests.

Of the families recorded in this study, Clubionidae, Lycosidae, Philodromidae, Salticidae, Thomisidae, and Theridiidae have been implicated in playing a role in pest control in other South African crops (Dippenaar-Schoeman et al., 2013), and all three habitat types supported most of these families, suggesting the possibility of there being background biological control by these groups. One notable species that occurred in all habitats is Pardosa crassipalpis (Purcell, 1903), a lycosid that dominates spider assemblages in South African strawberries (Dippenaar-Schoeman, 1976), cotton ((Dippenaar-Schoeman et al., 1999) and citrus (Van den Berg et al., 1992), and is known to prey on a variety of pest species (Van den Berg et al., 1992; Dippenaar-Schoeman et al., 1999).

That spiders can persist in vineyards throughout the year (Costello & Daane, 1995), even when prey numbers are low (Symondson et al., 2002), makes them a stable component of the natural enemy assemblage. Spiders in this study were abundant in autumn (post-harvest) and spring (early growing season), but showed a significant increase into the growing season at almost all the vineyard sites. Increased spider activity in the warmer months is also common in other South African crops e.g. (Dippenaar-Schoeman et al., 2001; Haddad et al., 2005; Haddad & Dippenaar-Schoeman, 2006), and is thought to contribute to early-season pest regulation before outbreak densities can be reached (Dippenaar-Schoeman, 1976).

Spiders are considered to provide background pest regulation as part of diverse natural enemy communities rather than as key predators of specific pests (Riechert & Lawrence, 1997; Nyffeler & Sunderland, 2003), because of their generalist feeding habits and diverse lifestyles (Symondson et al., 2002). Enhancement of spider species richness as part of overall biodiversity conservation in farmland may, therefore, contribute to providing consistent background biological control.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

Production landscapes in the CFR, if managed appropriately, have the potential to support a significant amount of regional biodiversity (Cox & Underwood, 2011) and this is certainly true for spiders in the CFR vineyard landscape. The protection of natural habitat remnants, in combination with practices that reduce physical disturbance and chemical inputs, and increase plant diversity in vineyards, may contribute to this goal. Further work that emphasises the influence of spider predation on crop pests will lend additional support to conservation efforts in CFR farmland.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References

Funding for the study was provided by the National Department of Science and Technology, National Research Foundation and Stellenbosch University. A.S. Dippenaar-Schoeman identified spider specimens. Two anonymous reviewers are thanked for their constructive input.


  1. Top of page
  2. Abstract
  3. Introduction
  4. Methods
  5. Results
  6. Discussion
  7. Conclusions
  8. Acknowledgements
  9. References
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