Comparative utilisation of exotic and native <i>Solanum </i>(Solanaceae) species by <i>Chnootriba hirta</i> (Thunberg) (Coccinelidae), a native herbivorous ladybird

Invasive alien plant species have been identified as a major threat to biodiversity and the relationship with native avian dispersers may increase their invasion potential. The impact of invasive plant species needs to be quantified using comparable assessment tools across different habitats and species to allocate limited resources to high-priority species. Here, we used the Generic Impact Scoring System (GISS) to assess the impacts of 16 fleshy-fruited alien invasive plant species in South Africa generally dispersed by native avian species. The results showed that fleshy-fruited invasive species have both environmental and socio-economic impacts. The cumulated impact scores for lantana (Lantana camara) and the tree of heaven (Ailanthus altissima) were the highest, with scores of 42 and 32, respectively. Some species, such as white mulberry (Morus alba), camphor tree (Cinnamomum camphora), American bramble (Rubus cuneifolius) and Brazilian pepper tree (Schinus terebinthifolius), had low overall impact scores of 8, 18, 14 and 16, respectively, but scored the maximum impact of 5 for certain mechanisms. Environmental impacts of fleshy-fruited invasive plant species had a high impact magnitude through effects on the ecosystem and vegetation. Socio-economic impacts were mainly through effects on forest production, agriculture and human health. Species with large crop sizes, small seeds and fruit sizes had higher environmental and socio-economic impact magnitude. The information generated in this study is important for guiding resource allocation and preventing the uncontrolled introduction of invasive species in South Africa. The impact of the fleshy-fruited invasive species transcended sectors and, therefore, effective management of invasive species will require the collaboration of multiple and inter-sectoral stakeholders in South Africa.

Avian frugivores play a key role in seed dispersal of many plant species, including invasive alien plants. We assessed the effect of gut passage on the germination of selected invasive alien plant species in South Africa. Fruits of four fleshly-fruited invasive alien plant species: Solanum mauritianum, Cinnamomum camphora, Psidium guajava, and Morus alba, were fed to two species of indigenous turacos, Knysna (Tauraco corythaix) and purple-crested (Gallirex porphyreolophus) turacos, and to invasive rose-ringed parakeets (Psittacula krameri). Seed retention time was determined as this can influence both seed dispersal and germination success. Germination success of ingested seeds was compared with that of manually de-pulped seeds, as well as to seeds in whole fruit. The germination success of seeds of all the invasive plant species increased significantly after ingestion by both turaco species compared with seeds from whole fruits. Germination success of manually de-pulped seeds did not differ significantly from that of turaco ingested seeds. In contrast, seed passage through the digestive tract of rose-ringed parakeets significantly reduced germination success and viability of ingested invasive plant species. Our results suggest that Knysna and purple-crested turacos are legitimate seed dispersers of fleshy-fruited invasive plants, while rose-ringed parakeets are mainly seed predators. Although seed predation by rose-ringed parakeets negatively affects the reproductive success of these plants, it is unlikely that this will suppress the spread of these invasive alien plants in South Africa as they are already well established. Furthermore, they can facilitate dispersal by seed regurgitation and dropping uneaten fruits away from the parent plant. Similar trends could be expected for indigenous seeds that rose-ringed parakeets feed on and therefore these birds remain a negative influence within invaded ecosystems.

The growth of herbivorous insects is influenced by their host plants, which is underpinned by genetic regulations. We studied the growth performance of the grasshopper Calliptamus abbreviatus Ikonn when it fed on a suitable host plant, Artemisia frigida (Asteraceae) and unsuitable host plant, Stipa krylovii (Poaceae) in a two-year feeding trial. Additionally, the underlying transcriptomic response was also measured using RNA-seq. When C. abbreviatus fed on S. krylovii, it had a significantly reduced growth rate and large variation in its gene expression profiles. A total of 106 genes were significantly up-regulated and mainly enriched in stress resistance-related KEGG pathways. Another 161 genes were significantly down-regulated and were mainly associated with development regulation-, cuticle biosynthesis-, and nutrition metabolism-related pathways. A total of 18 candidate genes were verified by RT-qPCR and were consistent with the transcriptomic results. Stress resistance was significantly induced in grasshopper when C. abbreviatus fed on the unsuitable host. However, the capacity for cuticle biosynthesis or nutrition metabolism significantly declined. The negative effects which S. krylovii had on the grasshopper’s growth performance and gene expression indicate that this plant species was a stressor compared to A. frigida. This study enhances the knowledge on the underlying molecular mechanisms involved in plant–herbivore insect co-evolution.

The Conflicting Values and Differing Priorities of Biological Control

A recent global assessment of terrestrial alien true ferns (Polypodiophyta; hereafter alien ferns) showed that alien ferns have a high probability of becoming naturalised or invasive once introduced. We provide the first systematic assessment, based on field surveys, of the invasion status of this large taxon in South Africa. Thirteen species of alien ferns were recorded outside of cultivation and subsequently identified as invasive in South Africa. Surveys were focused in known hotspots of alien and native ferns, with invasion density peaking along the eastern coastal belt of South Africa. Within the surveyed areas, alien ferns commonly occurred with other invaders, closer to water and often in indigenous forests. The species considered in this study generally occurred in similar habitat types across their native and globally introduced ranges (including South Africa). The potential spread of alien ferns in South Africa appears limited when compared to other major invaders, however, their common occurrence in indigenous forests highlights a concern for native, co-habiting fern species (in terms of competition). Our study provided regulatory insight for 12 previously unregulated alien fern species in South Africa. We propose that eradication remains feasible for Diplazium esculentum, Doodia caudata, Lygodium japonicum, Phlebodium aureum and Platycerium bifurcatum. A risk analysis for Sphaeropteris cooperi classified the species as high risk and comprised the first quantification of risk for any alien fern species (inclusive of aquatics) in South Africa. The information put forward in this study can be used to inform risk analyses for the remaining species in South Africa, as well as countries with similar habitats and climates. Furthermore, widely traded species need to be identified as candidates for future risk assessment since horticulture is an important introduction pathway for alien ferns globally.

Simple SummaryMany spider mites are important agricultural pests in both fields and greenhouses worldwide and are diversified in their host plant range. How spider mites perceive their suitable host plants remains not completely clear. In this study, we found that spider mites cannot locate suitable host plants by volatile odours from a long distance, but they can use olfactory sensation in combination with gustatory sensation to make a precise selection for suitable host plants at a short distance. Highly polyphagous species showed strong sensitivity in sensing suitable host plants rather than the expected lowered sensitivity. We also found that the similarity among the five spider mite species in their performance in perceiving suitable host plants was highly correlated with their relative phylogenetic relationship.Spider mites belonging to the genus Tetranychus infest many important agricultural crops in both fields and greenhouses worldwide and are diversified in their host plant range. How spider mites perceive their suitable host plants remains not completely clear. Here, through two-host-choice designs (bean vs. tomato, and bean vs. eggplant), we tested the efficacies of the olfactory and gustatory systems of five spider mite species (T. urticae, T. truncatus, T. pueraricola, T. piercei, and T. evansi), which differ in host plant range in sensing their suitable host plant, by Y-tube olfactometer and two-choice disc experiments. We found that spider mites cannot locate their suitable host plants by volatile odours from a long distance, but they can use olfactory sensation in combination with gustatory sensation to select suitable host plants at a short distance. Highly polyphagous species displayed strong sensitivity in sensing suitable host plants rather than the lowered sensitivity we expected. Intriguingly, our principal component analyses (PCAs) showed that the similarity among five spider mite species in the performance of perceiving suitable host plants was highly correlated with their relative phylogenetic relationships, suggesting a close relationship between the chemosensing system and the speciation of spider mites. Our results highlight the necessity of further work on the chemosensing system in relation to host plant range and speciation of spider mites.

Cardiospermum grandiflorum

In The Origin of Species, Darwin (1859) drew attention to observations by Alphonse de Candolle (1855) that floras gain by naturalization far more species belonging to new genera than species belonging to native genera. Darwin (1859, p. 86) goes on to give a specific example: “In the last edition of Dr. Asa Gray’s ‘Manual of the Flora of the United States’ ... out of the 162 naturalised genera, no less than 100 genera are not there indigenous.” Darwin used these data to support his theory of intense competition between congeners, described only a few pages earlier: “As the species of the same genus usually have, though by no means invariably, much similarity in habits and constitution, and always in structure, the struggle will generally be more severe between them” (1859, p. 60). Darwin’s intriguing observations have recently attracted renewed interest, as comprehensive lists of naturalized plants have become available for various regions of the world. Two studies (Mack 1996; Rejmanek 1996, 1998) have concluded that naturalized floras provide some support for Darwin’s hypothesis, but only one of these studies used statistical tests. Analyses of additional floras are needed to test the generality of Darwin’s naturalization hypothesis. Mack (1996) tabulated data from six regional floras within the United States and noted that naturalized species more often belong to alien genera than native genera, with the curious exception of one region (New York). In addition to the possibility of strong competition between native and introduced congeners, Mack (1996) proposed that specialist native herbivores, or pathogens, may be

Plant responses to changes in temperature can be a key factor in predicting the presence and managing invasive plant species while conserving resident native plant species in dryland ecosystems. Climate can influence germination, establishment, and seedling biomass of both native and invasive plant species. We tested the hypothesis that common and widely distributed native and an invasive plant species in dryland ecosystems in California respond differently to increasing temperatures. To test this, we examined the effects of temperature variation on germination, establishment, and per capita seedling biomass of three native and one invasive plant species (Bromus rubens) in independent 6 week growth trial experiments in a controlled greenhouse. Higher relative temperatures reduced the germination and establishment of the tested invasive species and two tested native species, however, per capita biomass was not significantly affected. Specifically, germination and establishment of the invasive species B. rubens and the native species Phacelia tanacetifolia was significantly reduced. This invasive species can often outcompete natives, but increasing temperature could potentially shift the balance between the germination and establishment of natives. A warming climate will likely have negative impacts on native annual plant species in California tested here because increasing temperatures can co-occur with drought. This study shows that our tested native annual plant species tested here have some resilience to relatively significant increases in temperature, and this can favor at least one native species relative to at least one highly noxious invasive plant species.

Exotic plant species very often comprise a large proportion of urban floras. Because herbivorous insects depend on the presence of suitable host plants to maintain their populations, it is imperative to elucidate the relative importance of native and exotic hosts to understand the response of herbivorous guilds to urbanization. By using a plant-herbivore system composed of Asteraceae hosts and flower-head endophagous insects, we investigated whether the diversity and composition of herbivorous insects differs between native and exotic host-plant species in an urban environment. Although we found only seven exotic Asteraceae among the 30 species recorded, the overall abundance of exotics was considerably greater than that of native host plants. Overall, the exotic host species supported a small subset of the herbivore assemblage found on the native ones. The number of herbivore species per host species was significantly higher among the native plants, but we did not find a difference in herbivore abundance. Moreover, the higher taxonomic composition of herbivores on exotic Asteraceae was reduced, being composed of only three genera and two families from a total of 16 genera and six families of herbivores. These results provide support for the idea that plants outside of their original geographic distribution have lower loads of enemies than phylogenetically related native species. Our findings indicate that native host plants in urban areas play a critical role in supporting the native herbivorous insect fauna.

Plant assemblages define vegetation patterns at different scales, from plant communities at the scale of small plots to broad biomes. Species assemblages are traditionally investigated with a focus on native species, and the spatial patterns and dynamics of alien species assemblages have received much less attention. Here, we explore the biogeography of a subset of invasive alien plants (IAPs) in South Africa and derive several “alien biomes” based on the alien plant assemblages and associated environmental drivers. We propose six hypotheses (the Weed-Shaped Hole; the Biome Decides; Goldilocks; a New World Order; Something In The Way You Move; and Random Tessellation) based on different drivers (disturbance, competition, climate, global change, introduction dynamics, and null respectively) that might explain distribution patterns. In particular, we explore whether invasive plant assemblages are controlled by the same fundamental factors that define native plant assemblages and biomes. A cluster analysis of the spatial distribution of 69 invasive alien plant species revealed five clearly delineated geographic clusters, three of them significantly aligned with the distribution of vegetation biomes (fynbos, grassland and savanna). The major determinants of the distribution of IAP clusters were identified based on a classification tree analysis. We found that broad environmental variables, especially vegetation biomes, explained the distribution of IAP clusters (60% classification accuracy). We could not find a strong relationship with anthropogenic factors, such as land cover or anthromes, even at a finer scale. Our results indicate that vegetation biomes are characterised by hard environmental barriers which also constrain the distribution of IAPs in South Africa. This supports the development of biome-level strategies for the control of alien plant species in South Africa.

Pereskia aculeata Miller (Cactaceae) is an invasive alien species in South Africa that is native in Central and South America. In South Africa, P. aculeata outcompetes native plant species leading to a reduction in biodiversity at infested sites. Herbicidal and mechanical control of the plant is ineffective and unsustainable, so biological control is considered the only potential solution. Climatic matching and genotype matching indicated that the most appropriate regions in which to collect biological control agents were Santa Catarina and Rio de Janeiro provinces in Southern Brazil. Surveys throughout the native distribution resulted in 15 natural enemy species that were associated with the plant. Field host range data, as well as previous host plant records, were used to prioritise which of the species were most likely to be suitably host specific for release in South Africa. The mode of damage was used to determine which species were most likely to be damaging and effective if released. The most promising species prioritised for further study, including host specificity and impact studies, were the stem-wilter Catorhintha schaffneri Brailovsky & Garcia (Coreidae); the stem boring species Acanthodoxus machacalis Martins & Monné (Cerambycidae), Cryptorhynchus sp. (Curculionidae) and Maracayia chlorisalis (Walker) (Crambidae) and the fruit galler Asphondylia sp. (Cecidomyiidae). By prioritising the potential biological control agents that are most likely to be host-specific and damaging, the risk of conducting host specificity testing on unsuitable or ineffective biological control agents is reduced.

From an economic point of view, agriculture and tourism, along with mining production, are South Africa’s largest economic sectors. However, South Africa is becoming increasingly reliant on ecotourism as an economic growth sector, which is, in turn, reliant on maintaining the productive and aesthetic value of South Africa’s protected areas and interstitial landscapes. This, together with other important human bene¢ts gained from maintaining biodiversity (e.g., ecological services, harvesting of wild species, aesthetically and culturally), suggests that key role players responsible for policy making in the country are faced with particular challenges where conservation and economic development needs are in con®ict (see e.g., reference1). Yet, at the same time, vertebrate introductions that result in invasions have the potential to alter these valuable environments. For example, considering the anthropogenic movement of ungulate species (both alien and extralimital) in South Africa, known for their recreation and food source value, Spear and Chown2 have shown that their movement has led to a 1.34% increase in the similarity of ungulate assemblages at a quarter-degree grid-cell resolution. At least 21 invasive ¢sh species are present in South Africa. While the impact of these species on aquatic ecosystems has been profound, resulting in the extirpation or localized extinction of a number of indigenous ¢shes, amphibians, and invertebrates, alien invasive ¢shes drive a large recreational ¢shery that has signi¢cant economic value. As a result of these positive economic contributions of certain invasive vertebrates, the implementation of legislation governing the importation and movement of both alien and extralimital species in South Africa will require careful consideration of the trade-offs between economic gain and biodiversity loss.

Understanding the status and extent of spread of alien plants is crucial for effective management. We explore this issue using Australian Acacia species (wattles) in South Africa (a global hotspot for wattle introductions and tree invasions). The last detailed inventory of wattles in South Africa was based on data collated forty years ago. This paper aimed to determine: 1) how many Australian Acacia species have been introduced to South Africa; 2) which species are still present; and 3) the status of naturalised taxa that might be viable targets for eradication. All herbaria in South Africa with specimens of introduced Australian Acacia species were visited and locality records were compared with records from literature sources, various databases, and expert knowledge. For taxa not already known to be widespread invaders, field surveys were conducted to determine whether plants are still present, and detailed surveys were undertaken of all naturalised populations. To confirm the putative identities of the naturalised taxa, we also sequenced one nuclear and one chloroplast gene. We found evidence that 141 Australian Acacia species have been introduced to South Africa (approximately double the estimate from previous work), but we could only confirm the current presence of 33 species. Fifteen wattle species are invasive (13 are in category E and two in category D2 in the Unified Framework for Biological Invasions); five have naturalised (C3); and 13 are present but there was no evidence that they had produced reproductive offspring (B2 or C1). DNA barcoding provided strong support for only 23 taxa (including two species not previously recorded from South Africa), the current name ascribed was not supported for three species and, for a further three species, there was no voucher specimen on GenBank against which their identity could be checked. Given the omissions and errors found during this systematic re-evaluation of historical records, it is clear that analyses of the type conducted here are crucial if the status of even well-studied groups of alien taxa is to be accurately determined.

Summary Ceratocystis albifundus causes the disease known as wattle wilt of non‐native Acacia mearnsii trees in South Africa, Uganda and Kenya. Infection results in rapid wilt and death of susceptible trees and stem cankers on more tolerant trees. It has been suggested that C. albifundus is indigenous to southern Africa, possibly having spread from native Protea spp. to non‐native A. mearnsii and A. decurrens trees. Although C. albifundus has been collected from Protea spp., these reports are based on limited records for which only aged herbarium specimens exist. During surveys of wound‐infecting fungi on native tree species in South Africa, a fungus resembling C. albifundus was collected from Protea gaguedi, Acacia caffra, Burkea africana, Combretum molle, C. zeyheri, Faurea saligna, Ochna pulchra, Ozoroa paniculosa and Terminalia sericea. The identity of the fungus was confirmed as C. albifundus, using comparisons of DNA sequence data for the ITS and 5.8S gene of the rRNA operon. In pathogenicity trials, lesions were produced on C. molle and A. caffra, with some trees beginning to die at the termination of the experiment. This study represents the first report of C. albifundus from native tree species in South Africa and provides unequivocal evidence that the fungus occurs naturally on native Protea spp. The wide host range of C. albifundus, as well as its abundance on these indigenous hosts lends further support to the view that it is a native African pathogen.