Theoretically, it is widely expected that invasive plants, simply by occupying a large amount of space, impose a significant impact on the native vegetation and their associated food webs (Gerber et al., 2008). Several studies suggest that invasive plant species generally harbour smaller herbivore assemblages than native plant species (Gerber et al., 2008; Mgobozi et al., 2008). Three theories expounded by Tallamy (2004) explain this scenario. The first one predicts that specialists herbivores should be unable to grow and reproduce on plants with which they share no evolutionary history, the second one predicts that the energy stored by alien plants is not available to indigenous specialist and thus unavailable to higher trophic levels that include the insects in their diets, and the third predicts that these plants may not be palatable to most native insects. Given that there is strong association between most arthropods and native vegetation or the microhabitat it creates (Mgobozi et al., 2008; Olckers and Hulley, 1991), any decrease, extinction or alteration of the physical characteristics of some native plant species or habitats after alien plant colonization may negatively impact on species-specific herbivores (Palmer et al., 2004; Pauchard and Alback, 2004).

 

Protected areas (nature reserves) in South Africa remain critically important refugia that provide high quality habitat patches for invertebrate biodiversity conservation even though challenges resulting from their size and number do arise. Moreover, most of the country’s rich biodiversity lies outside of the approximately 6% of land area under protected area systems (Turpie, 2004; Blanchard and Holmes, 2008), with its native and semi-natural ecosystems also under increasing threats from alien plant invasions (Nel et al., 2004; Gorgens and van Wilgen, 2004; Olckers and Hulley, 1991). Healthy biological communities depend principally on interactions among small organisms (mostly invertebrates and microbes) (Hartley and Rogers, 2010). Over 4.7% of formally protected land in the Eastern Cape Province of South Africa is now biologically invaded, and this proportion is increasing (Masubelele et al., 2009; Foxcroft et al., 2011).

 

The topography of the Eastern Cape Province ranges in elevation from 0 to 1500 m a.s.l along a 200 km E-W transectin one latitude. The ecology of this area is influenced by montane climate and moist savannah vegetation type at higher elevations, and Afromontane forest with sub-tropical climate along the eastern and southern coasts of the province.

 

Reserves and non-formally protected areas/landscapes of the King Sabata Dalindyebo Municipality (KSDM) of the Eastern Cape fall within the Albany Centre of Endemism. Although these landscapes are increasingly under threat from local endemic plant extinctions resulting mainly from overgrazing, agriculture and alien plant invasions (Smith and Wilson, 2002), they are nevertheless growing in significance as  elements  of  the matrix where indigenous biodiversity conservation action must be undertaken. However, little is known about the habitat-level impact of invasive alien and native plant cover on soil-surface dwelling invertebrates within protected areas of the municipality at a local spatial scale. The overall aim of this initial study therefore was to determine the response of invertebrates to various types of vegetation cover in Luchaba Nature Reserve(LNR) under the following specific objectives: i) identify the soil-surface-dwelling invertebrate assemblage of the reserve, ii) assess the ecological impact of invasive alien and indigenous plant cover on faunal species composition.

 

Study Site

 

The study was carried out in Luchaba Nature Reserve (LNR) (460 ha) situated at 31°35’S, 28°45’E and 758 m a.s.l in the King Sabata Dalindyebo Municipality (KSDM) of the Eastern Cape Province of South Africa (Figure 1). This reserve is an un-proclaimed protected area on state land, managed as a nature reserve by the Operations Directorate of the Eastern Cape Parks Board (ECPB). Climate is characterised by mean winter and summer temperatures of 13 and 26°C respectively, with mean annual precipitation of 634 mm (DWAF, 2005). Natural forest in the reserve is made up of indigenous trees e.g. Acacia karroo (Hayne), Acacia sieberiana DC., Acacia xanthophloea B., Erythrina caffra Thunb. and Zanthoxylumcapense (Thunb.) Harv. (Moll, 1981; Palgraves, 2002). Common grass species are Eragrostis curvula (Schrad) Nees, E. Plana Nees, E. racemosa (Thumb.), Paspalum dilatatum Poir, Themeda triandra Forssk and Pennisetum Rich species while invasive alien plant species present in the reserve comprise of Eucalypt, black wattle, Lantana camara L., bugweed and inkberry. The geology of the reserve comprises predominatly of shales and sandstones of the Beaufort series of the Karoo system. These landforms are interlaced with dolerite dykes (Acocks, 1988).

 

 

(iv) Indigenous grassland (IG) patch comprising of over 70% grasses, sedges and about 20% herbaceous vegetation cover.

 

Invertebrate sampling

 

Two pitfall traps were placed in each Su to capture invertebrate specimens. Traps consisted of 250 ml blue plastic cups with rim diameter of 75 mm and were sunk into the ground with rim openings maintained at the same level with the ground surface. The cups were three-quarter filled with a mixture of soapy detergent and water as a trapping medium, and were then left open in  the  ground for 24 h to capture soil-surface dwelling invertebrates. Specimens were collected from 32 traps during each of three sampling occasions making 96 records units in all. The sampling technique was according to Southwood and Henderson (2000), and designed to maintain sample independence as soil-surface dwelling invertebrate species are much less mobile or dispersive. Specimens were sorted from other flying arthropods in the traps, preserved in vials with 70% ethanol, and transported to the laboratory for preliminary identification. Specimens were identified using a Zeiss Stereo dissecting microscope Model STEMI DV4 and a field guide by Picker at al. (2004). Spider identities were confirmed by a taxon specialist at the Agricultural Research Council (ARC) Plant Protection Research Institute (PPRI) in Pretoria, and thereafter using reference works by Dippenaar-Schoeman and Jocque (1997). Ants were identified at the Biosystematics Division of the Agricultural Research Council (ARC), Pretoria while other insects were identified using Carruthers (2008) and Picker et al. (2004). Unidentified (morphospecies) were coded, preserved in 70% alcohol for future identification by taxon specialists. Invertebrate specimen data was collected during species-rich summer months of April (weeks two and three) and May (week 1) in 2011.

 

Statistical analysis

 

Data sets were collated for each sampling unit (Su) and arranged in data matrices as proposed by Ludwig and Reynolds (1988) and Clarke and Gorley (2006). The statistical software programs DIVERSE and CLUSTER in PRIMER V6 (Clarke and Warwick, 2001) were used to determine indices of diversity and classification of species data respectively. Species-by-sample unit data matrices were 4^th root transformed to balance rarer and common species. The Bray-Curtis measure of similarity (Bray and Curtis, 1957) was then applied to the data to generate sampling unit similarity matrices that were fused successively using group average linking. Results describing patterns obtained using clusters were represented by a dendrogram. Species rank (k-dominance curves) for all four sites were calculated using the programme DOMPLOT (PRIMER V6). Curves extracted information on patterns of relative species abundances without reducing that information to a simple numeric diversity index (Lambshead et al., 1983; Clarke and Gorley, 2006). All invertebrate abundance data across sites was long-transformed to maintain normality and to satisfy the requirement for ANOVA.

Taxonomic profile of invertebrates sampled in Luchaba Nature Reserve

 

Even though flying arthropod species e.g. flies, wasps and butterflies were also collected in pitfall traps across habitat patches during the study period, these arthropod taxa were not included in the analysis as pitfall trapping was not the conventional method for sampling them. A total of 335 soil-surface dwelling invertebrate specimens in three phyla (Arthropoda, Annelida and Mollusca) were sampled. Of the nine arthropod orders sampled, four were identified to seven families and ten species while the remaining five orders and two phyla (Annelida and Mollusca) were separated into 15 morphospecies. Invertebrate species sampled across the four habitats showed varying degrees of richness and abundance patterns in response to habitat variables such as vegetation structure, composition and disturbance gradients associated with alien and indigenous cover. The highest number of invertebrate species and individuals were recorded at the mixed alien site while the Eucalypt site had the least number of specimen counts (Table 1, Figure 2). Species rank abundance (K-dominance) curves showed that the mixed alien site had a greater species  evenness  trend  than  the  other  three sites while the eucalypt patch had the highest species dominance with Pardosa crassipalpis and Pheidole spp having above 59% level of dominance (Figure 3a).

 

The dendrogram (Figure 3b) showed that the mixed alien and indigenous acacia sites had a high percentage similarity at above 80% in terms of species composition and distribution patterns. The orders with most abundant invertebrates individuals were the Araneae, and the Hymenoptera. There were no statistically significant differences (P>0.05) among habitat patches in terms of number of faunal species recorded. However, differences in total number of individuals (N) were significant (P<0.05). Also, there were statistically significant differences in abundance among sites for Araneae (P<0.01), suggesting that the population of this species-rich faunal group responded significantly to habitat patch heterogeneity. In addition to the Araneae, other non-insect arthropods sampled included morphospecies belonging to the orders Diplopoda, Opiliones, Isopoda and morphs of the phyla Annelida and Mollusca. 

 

 

 

Even though maximum invertebrate species richness is not  always  reached  at  undisturbed  sites,  each   group displays a specific pattern (Palmer et al., 2004). The mixed alien site attracted the highest number of invertebrate taxa sampled during the study, supporting the finding by Harris et al. (2004) that the impact of invasive plants on native biodiversity is not always negative. Furthermore, biodiversity estimators indicate that undisturbed habitats can be less diverse than invaded habitats (Palmer et al., 2004). The  study  clearly showed that the indigenous acacia and indigenous grassland sites harboured comparable numbers of invertebrate taxa and specimen counts that were higher than that found in the eucalypt patch, but lower than counts made at the mixed alien patch. Widespread/abundant species that occurred throughout all four habitat patches e.g. Crematogaster sp, P. crassipalpis and Pheidole sp as well as habitat-restricted species e.g. X. natalensis (Table 1) can be used as potential indicators for assessing the conservation value of habitat patches in Luchaba Nature Reserve and other protected areas of the King Sabata Dalindyebo Municipality. However, more data on a broader spatial and temporal scale is needed to support species response patterns reported in this preliminary study, since responses of soil-surface dwelling invertebrate assemblages to invasive alien and native plant cover in the reserve may likely be species-specific.

 

Most invertebrate species were sampled in the mixed alien and indigenous acacia patches that showed a high level of ecological similarity (Figure 3b) suggesting that habitat patch level management for conservation action in the short term should consider eradicating the species-poor eucalypt stands from the reserve while replacing all alien plants in the reserve area with native flora in the medium to long term. 

The authors have not declared any conflict of interest

Funding for this project was provided by the National Research Foundation (NRF) and the Directorate for Research Development at Walter Sisulu University. We thank the Eastern Cape Parks Board and the Luchaba Nature Reserve management for logistical support and granting us permission to undertake the study at the reserve.